The hind-brain or rhombencephalon occupies the posterior fossa of the cranial cavity and lies below a fold of dura mater, the tentoriumcerebelli. It consists of (a) the myelencephalon, comprising the medulla oblongata and the lower part of the fourth ventricle; (b) the metencephalon, consisting of the pons, cerebellum, and the intermediate part of the fourth ventricle; and (c) the isthmus rhombencephali, a constricted portion immediately adjoining the mid-brain and including the superior peduncles of the cerebellum, the anterior medullary velum, and the upper part of the fourth ventricle. 1
Development of the Brain Stem & Cranial NervesThe lower part of the cranial portion of the neural tube (neuraxis) gives rise to the brain stem. The brain stem is divided into the mesencephalon and rhomben- cephalon (Fig 7–1).General Considerations and Divisions.—The brain, is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system. In its early embryonic condition it consists of three hollow vesicles, termed the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon; and the parts derived from each of these can be recognized in the adult (Fig. 677). Thus in the process of development the wall of the hind-brain undergoes modification to form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle. The mid-brain forms only a small part of the adult brain; its cavity becomes the cerebral aqueduct (aqueduct ofSylvius), which serves as a tubular communication between the third and fourth ventricles; while its walls are thickened to form the corpora quadrigemina and cerebral peduncles. The fore-brain undergoes great modification: its anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures; the cavity of the posterior part or diencephalon forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity.
The primitive central canal widens into a four-sided pyramid shape with a rhomboid floor (Fig 7–2). This becomes the fourth ventricle, which extends over the future pons and the medulla.Schematic illustration of the widening of the central cavity in the lower brain stem during development.The neural tube undergoes local enlargement and shows two permanent flexures: the cephalic flexure at the upper end and the cervical flexure at the lower end. The cephalic flexure in an adult brain is the angle between the brain stem and the horizontal plane of the brain (see Fig 1–6). The central canal in the rostral brain stem becomes the cerebral aqueduct. The roof of the rostral fourth ventricle undergoes intense cellular proliferation, and this lip produces the neurons and glia that will populate both the cerebellum and the inferior olivary nucleus.The quadrigeminal plate, the midbrain tegmentum, and the cerebral peduncles develop from the mesencephalon (midbrain; see Fig 7–1), and the cerebral aqueduct courses through it. The rhombencephalon (see Fig 7–1 A) gives rise to the metencephalon and the myelencephalon. The metencephalon forms the cerebellum and pons; it contains part of the fourth ventricle. The myelencephalon forms the medulla oblongata; the lower part of the fourth ventricle lies within this portion of the brain stem.As in the spinal cord, the embryonic brain stem has a central gray core with an alar plate (consisting mostly of sensory components) and a basal plate (composed primarily of motor components). The gray columns are not continuous in the brain stem, however, and the development of the fourth ventricle causes wide lateral displacement of the alar plate in the lower brain stem. The basal plate takes the shape of a hinge (see Fig 7–2). The process is reversed at the other end, resulting in the rhomboid shape of the floor of the fourth ventricle. In addition, long tracts, short neuronal connections, and nuclei become apposed to the brain stem. The cranial nerves, like the spinal nerves, take their origin from the basal plate cells (motor nerves) or from synapses in the alar plate cell groups (sensory nerves). Unlike spinal nerves, most cranial nerves emerge as one or more bundles of fibers from the basal or basilateral aspect of the brain stem (Figs 7–1 and 7–3). In addition, not all cranial nerves are mixed; some have only sensory components and others have only motor components (see Chapter 8: Cranial Nerves and Pathways).
Three major external divisions of the brain stem are recognizable: the medulla (medulla oblongata), the pons together with the cerebellum, and the midbrain (mesencephalon) (Figs 7–3 and 7–4). The three internal longitudinal divisions of the brain stem are the tectum (mainly in the midbrain), tegmentum, and basis (see Fig 7–4). The main external structures, seen from the dorsal aspect, are shown in Figure 7–5. The superior portion of the rhomboid fossa (which forms the floor of the fourth ventricle) extends over the pons, whereas the inferior portion covers the open portion of the medulla. The closed medulla forms the transition to the spinal cord.Three pairs of cerebellar peduncles (inferior, middle, and superior) form connections with the cerebellum. The dorsal aspect of the midbrain shows four hillocks: the two superior and the two inferior colliculi, collectively called the corpora quadrigemina or quadrigeminal plate.
PYRAMIDS-CORTICOSPINAL FIBERSThe pyramids are two elongated swellings on the ventral aspect of the medulla. Each pyramid contains approximately 1,000,000 CORTICOSPINAL AXONS. As the name suggests, these axons arise from the cerebral cortex and descend to terminate within the spinal cord. The cortical cells that give rise to corticospinal axons are called Betz cells. As corticospinal axons descend from the cortex, they course through the internal capsule, the cerebral peduncle of the midbrain, and the ventral pons (you will learn about these structures later in the course so don’t worry about them now) and onto the ventral surface of the medulla as the pyramids (see below).When corticospinal axons reach the medulla they lie within the pyramids. The pyramids are just big fiber bundles that lie on the ventral surface of the caudal medulla. The fibers in the pyramids are corticospinal. It is important to REMEMBER: THERE HAS BEEN NO CROSSING YET! in this system. The cell bodies of corticospinal axons within the pyramids lie within the IPSILATERAL cerebral cortex.At the most caudal pole of the pyramids the corticospinal axons cross over the midline and now continue their descent on the contralateral (to the cell of origin) side. This crossover point is called the PYRAMIDAL DECUSSATION. The crossing fibers enter the lateralfuniculus of the spinal cord where they are called the LATERAL CORTICOSPINAL TRACT (“corticospinal” is not good enough, you have to call them lateralcorticospinal; LCST - remember this one??). LCST axons exit the tract to terminate upon neurons in the spinal cord gray matter along its entire length. Some of the spinal cord neurons that receive direct input from lateral corticospinal fibers send axons into the ventral root to innervate striated muscles that move the arms and legs. In such instances only two neurons and one synapse are involved (monosynaptic). Lateral corticospinal tract axons also end upon spinal cord neurons that do not directly innervate muscle, but instead synapse upon lower motor neurons. Such cells are called interneurons. A pathway involving a lateral corticospinal axon, an interneuron and a lower motor neuron is disynaptic.Let’s review! The cells of origin of the corticospinal tract lie in the ipsilateral cerebral cortex. In contrast, the cells of origin of the lateralcorticospinal tract lie in the contralateral cerebral cortex. REMEMBER—the lateral corticospinal tract is the part of the corticospinal system in the spinal cord. It is important to remember that damage to the corticospinal fibers rostral to the pyramidal decussation results in contralateral motor deficits, while lesions in the spinal cord (i.e., caudal to the decussation) result in ipsilateral deficits. Clinicians refer to the motor deficit resulting from a lesion of the corticospinal tract as hemiplegia (plegia =paralysis) or hemiparesis (weakness). For instance, they would say that a lesion of the LEFTcorticospinal tract in the internal capsule (rostral to the pyramidal decussation) results in a RIGHThemiplegia (involves the right arm, trunk and leg) while a lesion of the LEFT lateral corticospinal tract at C1 (in the spinal cord and therefore caudal to the decussation) results in LEFThemiplegia. Since the motor neurons that directly innervated the muscle are OK, there is no muscle atrophy. This is important.A unilateral lesion of the lateral corticospinal tract results in motor deficits ipsilateral to the lesion. Interrupting the tract does NOT result in muscle atrophy, since the neurons innervating muscle are NOT dead. For example, if the LEFT lateral corticospinal tract is interrupted at Cl, then the LEFT arm, trunk and leg are affected. This is because the lateral corticospinal tract influences the musculature on the same (ipsilateral) side of the body. If the lesion involves the LEFT lateral corticospinal tract at T3, then only the LEFT trunk and leg are affected, since the spinal cord motor neurons that innervate the muscles of the arms still receive corticospinal input.As mentioned above, the term hemiplegia is commonly used by clinicians when discussing the effects of lesions of the corticospinal tract anywhere from the cortex to the medulla, and of the lateral corticospinal tract anywhere in the spinal cord. Cerebrovascular accidents (“strokes”) commonly damage the corticospinal tract in the motor cortex or the posterior limb of the internal capsule (a compact bundle of axons through which almost all neural traffic to and from the cortex passes). The term “stroke” is poorly defined and has different meanings to different users. It generally implies the abrupt onset of neurological deficits as the result of cerebrovascular disease. As such, it includes the manifestations of cerebral hemorrhage (Gr. blood bursting forth; may be arterial, venus or capillary) cerebral infarction (L. to stuff into; area of tissue undergoes necrosis [death] following cessation of blood supply) and intracranial and extracranialthrombosis (Gr. clot condition; when a thrombus is detached from its original site and found in another site it is called a thrombotic embolis [Gr. embole=throwing in]).As you have already learned, immediately following a lesion involving corticospinal or lateral corticospinal fibers, there is a period of flaccid paralysis (spinal shock). After a period of days to weeks, muscle tone (spasticity) and muscle reflexes return and increase. There will also be a Babinski sign, (extension of the big toe and fanning of the others in response to firmly stroking the sole of the foot; “the plantar reflexes are extensor”). There will also be clonus.You need to keep in mind that lesions of the corticospinal tract do not destroy motor neurons that DIRECTLY innervate muscle. THUS, THERE IS NO DEATH OR ATROPHY OF MUSCLE. Corticospinal tract neurons are referred to as UPPER MOTOR NEURONS since they do not innervate muscle directly. In contrast, neurons in the ventral horn that DIRECTLY innervate muscle are called LOWER MOTOR NEURONS (LMN). When these neurons die there is ATROPHY OF THE MUSCLE.In my humble opinion, the corticospinal tract is the most important pathway in clinical neurology. I will ask you about this tract every time I see you, even after you graduate! In other words, if you forget any neuroanatomy, DO NOT forget the following points:1. the pathway is crossed.2. lesions anywhere rostral to the decussation (cortex, internal capsule, cerebral peduncle, basilar pons, rostral two-thirds of pyramid) result in contralateral deficits.3. lesions involving this system in the spinal cord (i.e., the lateral corticospinal tract) result in ipsilateral deficits below the level of the lesion.4. corticospinal neurons are upper motor neurons; and therefore their death does NOT result in muscle atrophy.
ANTEROLATERAL SYSTEM (ALS)Hopefully you remember from the spinal cord module that the cells of origin of pain and temperature conveying axons in the spinal cord lie in the dorsal horn. Axons arising from these dorsal horn cells cross and ascend in the anterolateral portion of the white matter of the spinal cord (hence the name Anterolateral System; ALS). Thus the cells of origin of the ALS (or spinothalamic tract) lie in the contralateral dorsal horn. Axons in the ALS are destined for the ventral posterolateral (VPL) nucleus of the thalamus. Since the thalamus lies ROSTRAL to the midbrain, which is the most rostral part of the brain stem, the ALS is present in each of the 10 brain stem slides that you need to learn.Once the pain and temperature information traveling in the ALS reaches the VPL nucleus of the thalamus (the thalamus is the GREAT GATEWAY TO THE CORTEX), it is relayed by a thalamic neuron to the somatosensory cortex (postcentralgyrus; areas 3, 1 and 2). Don’t forget: pain and temperature information reaches cells in the dorsal horn via the central processes of dorsal root ganglion cells (delta and C fibers; neuron #1). Dorsal horn cells (neuron #2) project to the contralateral VPL via the ALS. Finally, cells in VPL (neuron #3) project to areas 3, 1 and 2 (somatosensory cortex) for perception of the pain and temperature.Interruption of the ALS anywhere in the brain stem results in loss of pain and temperature from the contralateral side of the body (arms, trunk and legs), but NOT the head. We will deal with the head later!The location of the ALS is difficult to see in fiber-stained sections, since most of these axons are either lightly myelinated or non-myelinated (i.e., slowly conducting). You must focus hard on remembering the location of the anterolateral fibers in each of the ten brain stem levels. I will include this system in many of the problem-solving questions in order to help you remember its location. YOU SHOULD NEVER FORGET THE ALS/SPINOTHALAMIC TRACT!Clinical case reports often comment that lesions involving areas of the brain stem adjacent to the anterolateral system result in an IPSILATERAL HORNER’S SYNDROME. The explanation for this finding is that descending fibers from the HYPOTHALAMUS (a major autonomic center) travel close to the ALS in the brain stem (you will not see these descending fibers in the ten brain stem levels). Their interruption means that the PREGANGLIONIC SYMPATHETIC neurons in the lateral cell column (T1-L2) of the spinal cord have lost an important “drive.” The most obvious clinical finding would be a CONSTRICTED PUPIL (MIOSIS) in the ipsilateral eye since the parasympathetic input is now in control. There would also be slight drooping (PTOSIS) of the ipsilateral upper eyelid due to the absence of sympathetic “drive” to the superior tarsal (smooth) muscle. There is also lack of sweating (anhidrosis) and vasodilatation (flushed face). Remember, sympathetics innervate sweat glands and constrict blood vessels of the face. All problems are IPSI to the brain stem lesion. For our problem solving excercises A HORNER’S WILL BE PRESENT ANY TIME THE LESION INVOLVES THE ALS ABOVE T2! REMEMBER, THE PROBLEMS RELATED TO HORNER’S WILL BE IPSI. WHILE THOSE RELATED TO THE ALS WILL BE CONTRA.
SPINAL NUCLEUS & TRACT OF THE TRIGEMINAL (C.N. V)The trigeminal nerve is the largest cranial nerve. It contains both sensory and motor fibers. The motor fibers innervate the muscles of mastication and arise from motor neurons in the pons. The general sensory fibers convey information regarding pain, temperature, touch, and conscious and unconscious proprioception. At this time I want to talk about those general sensory fibers that are conveying PAIN and TEMPERATURE because they are associated with a nucleus and tract in the caudal medulla (where we are now!). We will talk about the other general sensory nuclei and fibers (two point discrimination, conscious proprioception and vibratory sense), as well as the motor fibers associated with the trigeminal, later when we look at sections through the rostralpons.The cell bodies of the pain and temperature fibers associated with the trigeminal nerve lie in the trigeminal ganglion (located on the cerebral surface of the sphenoid bone in the middle cranial fossa). Just like cells within the dorsal root ganglia of the spinal cord, cells in the trigeminal ganglion possess peripheral and central processes. The peripheral processes of trigeminal ganglion neurons distribute to pain and temperature receptors on the face, forehead, mucous membranes of the nose, anterior two-thirds of the tongue, hard and soft palates, nasal cavities, oral cavity, teeth, and portions of cranial dura. The central processes enter the brain at the level of the pons (this is where all trigeminal sensory fibers enter the brain stem and where trigeminal motor fibers leave the brain stem). These central processes of trigeminal ganglion neurons conveying pain and temperature descend in the brain stem and comprise the SPINAL TRACT V. Fibers of spinal tract V terminate upon an adjacent cell group called the SPINAL NUCLEUS V, which forms a long cell column medial to spinal tract V (spinal tract and nucleus V form a slight elevation on the lateral surface of the caudal medulla called the tuberculum [swelling] cinereum [ashen or gray]). We are particularly concerned with the caudal-most portion of spinal nucleus V, because ALL of the PAIN and TEMPERATURE fibers from the face terminate in this caudal region of the nucleus (other portions of spinal nucleus V will not be discussed). Cells within spinal nucleus V possess axons that curve medially to CROSS the midline and course rostrally close to (but not as a part of) the medial lemniscus. These crossed fibers retain their close association with the medial lemniscus as they ascend in the brain stem and are called the trigeminothalamic tract (TTT). Neither the crossing or ascending TTT fibers can be identified in our fiberI am sorry to say that the spinal tract and spinal nucleus V are not exclusively associated with C.N. V. It is also associated with C.N.s VII (facial), IX (glossopharyngeal) and X (vagus). While this makes the story a little confusing, it also makes a lot of sense! The pain and temperature fibers associated with C.N. VII innervate the skin of the external ear, the wall of the external auditory meatus and the outer surface of the tympanic membrane (if it’s OK with you, I’ll lump these together as “EAR”). These fibers are the peripheral processes of cells that lie in the GENICULATE ganglion (located in the facial canal). The central processes of these neurons enter the brain with C.N. VII (at pontine levels, caudal to the trigeminal), travel in spinal tract V and end in spinal nucleus V. The pain and temperature information is then conveyed rostrally in the TTT (trigeminothalamic tract) to reach the VPM, from which it is relayed to somatosensory cortex (areas 3,1 and 2). Thus the pain and temperature fibers of C.N. VII don’t have their “own” central cell group, but instead use that of Like C.N. VII, C.N. IX is involved in the pain and temperature innervation of the “EAR”. In addition, C.N. IX conveys pain and temperature from the posterior one-third of the tongue, the auditory tube and the upper part of the pharynx (touching of the pharyngeal wall results in a gag reflex which we will cover in Point 9: Nucleus Ambiguus). The cell bodies of these pain and temperature axons associated with C.N. IX lie in the relatively small SUPERIOR GANGLION IX (located just outside of the jugular foramen; you will hear about the larger inferior ganglion later). The central processes of cells in the superior ganglion enter the brain with C.N. IX (at the lateral medulla), then enter our friend spinal tract V and synapse in the caudal spinal nucleus V. You know the route f FINALLY (whew!) C.N. X innervates the “EAR” (along with C.N.s VII and IX). In addition, pain and temperature from the lower pharynx, the larynx and the upper esophagus are conveyed by C.N. X. The cell bodies of these pain and temperature axons lie in the SUPERIOR GANGLION X (located just outside the jugular foramen). The central processes of cells in the superior ganglion X pass into the brain at the lateral medulla, enter our old friend spinal tract V and synapse iKEY THOUGHT: only ONE ganglion to worry about with C.N. VII (GENICULATE). SUPERIOR ganglia of BOTH IX and X=somatic afferents (inferior ganglia I As mentioned above, pain fibers end only in the caudal spinal nucleus V. You should also be aware of the fact that not much is known about the other parts of the spinal trigeminal nucleus (nucleus oralis and interpolaris). The important fact is that the great majority of the pain and temperature fibers pass to the most caudal portion of spinal nucleus V. This means that lesions anywhere along the rostrocaudal extent of the spinal tract V, as well as in the caudal spinal itself, will result in deficits in pain and temperature from the ipsilateral face etc. Finally, you have probably heard of tic douloureux or trigeminal neuralgia, which is a disorder characterized by brief at For our PROBLEM SOLVING, you must know that a lesion of spinal tract V along its entire course (the pain information never gets to the caudal spinal nucleus) results in IPSILATERAL deficits in pain and temperature from the face etc. Interruption of the trigeminothalamic tract, which is comprised of axons that have crossed the midline, results in deficits in pain and temperature on the contralateral side of the face etc.REMEMBER:1. spinal tract and nucleus V are present in the pons and medulla2. the cells of origin of spinal tract V lie in the trigeminal ganglion3. axons in spinal tract V terminate in the spinal nucleus V4. axons of cells within spinal nucleus V project to the contralateral VPM5. spinal tract and nucleus V = pain and temp. from face etc. (ear, tongue, palates, pharynx, larynx, upper esophagus)AN INTERESTING CLINICAL OBSERVATIONReports in the clinical literature note that vascular lesions that interrupt the blood supply to the spinal nucleus and tract V in the medulla (for example a thrombosis of the posterior inferior cerebellar artery) sometimes are immediately followed by sharp stabbing pain in and around the eye and on the ipsilateral face (hyperalgesia; algesia; Gr., = sense of pain). A possible explanation for this paradox is that the pain fibers are highly irritated before they die (spontaneous pain also sometimes occurs on the contralateral side of the body immediately following a lesion of the anterolateral system). Please remember that lesions of spinal tract and nucleus V can result in PAIN in the face (in addition to loss of pain and temperature later). What a paradox!
Figure 12-5 Cross-sectional diagram in which the principal structures of the midbrain at the level of the superior colliculus are depicted.Situated in a transitional position between the cruscerebri and the tegmentum is the substantianigra. It contains two groups of cells; one group is located medially and is highly compacted (called the pars compacta), and one is located laterally, reticulated in appearance, and called the pars reticulata (Fig. 12-5). Each of these regions is important because of the neurotransmitters that they synthesize as well as their projection targets. Dopamine, for example, is associated with the pars compacta and is released onto neurons of the neostriatum. In contrast, neurons of the pars reticulata project to the thalamus, using gamma-aminobutyric acid (GABA) as a neurotransmitter. Clinically, it is known that loss of dopaminergic neurons in the substantianigra results in a motor disorder called Parkinson's disease. (A more detailed analysis of the substantianigra and its functions is presented in Chapter 20.)Level of the Superior ColliculusTectumIn the rostral half of the midbrain, the tectum is formed by the superior colliculus, which has replaced the inferior colliculus (Figs. 12-5 and 12-6). The superior colliculus receives retinal inputs. Its projections to the cervical spinal cord via the tectospinal tract presumably serve to produce reflex movements of the head and neck in response to sensory inputs. It contributes to the regulation of oculomotor responses and, in particular, mediates tracking movements of objects as they move through the visual field (i.e., horizontal conjugate gaze).TegmentumThere are few overt morphological changes in the appearance of the PAG at the level of the superior colliculus relative to that of the inferior colliculus. Just below the PAG lies the oculomotor nuclear complex (CN III), which has now replaced the trochlear nucleus (Fig 12-5). The oculomotor nerve, which includes both GSE and general visceral efferent (GVE) components, passes through the red nucleus and exits in a ventromedial position within the midbrain. The somatomotor component includes nerve bundles that innervate the medial, inferior, and superior rectus muscles, as well as the inferior oblique and levatorpalpebrae muscles. Thus, the somatomotor components of this cranial nerve are essential for most vertical eye movements, medial deviation of the eyes, and elevation of the eyelid. The parasympathetic components are described in the CrusCerebri section, and the functions of this and other cranial nerves are described in detail in Chapter 14.Another major change that takes place at this level is that the red nucleus begins to replace the superior cerebellar peduncle. The red nucleus is present throughout the rostral half of the midbrain and extends into the caudal diencephalon (Fig. 12-5). It plays an important role in motor functions. The axons supply all levels of the spinal cord and facilitate the discharge of flexor motor neurons. The red nucleus also influences cerebellar activity by projecting its axons to the inferior olivary nucleus, which, in turn, supplies the contralateral cerebellar cortex.An additional new feature of the tegmentum is the presence of an important source of dopaminergic fibers that supply much of the forebrain other than the striatum. These dopaminergic cells are located in the ventromedial aspect of the tegmentum, called the ventral tegmental area, adjacent to the position occupied by the pars compacta of the substantianigra (Fig. 12-5).Other features of the tegmentum remain essentially similar to those found at the level of the inferior colliculus. Ascending sensory pathways (i.e., medial lemniscus and trigeminothalamic and spinothalamic fibers) are situated in the lateral aspect of the tegmentum. Likewise, the substantianigra remains essentially unchanged at this level. The reticular formation is also present throughout this level of the tegmentum and extends to the rostral limit of the midbrain.CrusCerebriThe cruscerebri appears similar at all levels of the midbrain, and the topographical arrangement of the descending fibers remains the same as described earlier for the level of the inferior colliculus.At the far rostral aspect of the midbrain, several morphological changes begin to appear. On the dorsal aspect of this level of the midbrain, the superior colliculus is replaced by a large mass of cells called the pretectal region. This region constitutes part of the circuit for the pupillary light reflex, which is a reflex that causes constriction of the pupil when the eye is exposed to light. This reflex involves the activation of retinal fibers (in response to light) that make synaptic contact with neurons in the pretectal region, which, in turn, project to the nucleus of CN III. Preganglionic parasympathetic neurons of the Edinger-Westphal nucleus send their axons in CN III and then synapse with postganglionic neurons in the ciliary ganglion, which innervate the pupillary constrictor muscles of the pupil. It is via this reflex pathway that, when light is shone into the eye, the pupil constricts (i.e., pupillary light reflex).This parasympathetic component also innervates the ciliary muscle, which when contracted, causes a release of the suspensory ligament of the lens; the result is an increase in the curvature of the lens. When this parasympathetic component, together with the somatic motor component of CN III, is activated, an accommodation reaction for near vision takes place.Other morphological changes that may be noted at this level include the presence of the posterior commissure, which is situated just dorsal to the PAG. It contains fibers that arise from various nuclei, including the pretectal region, which synapse with cranial nerve nuclei that control extraocular eye muscles. Such connections coordinate movements of the two eyes.Because this level of the brainstem constitutes a transitional region between the midbrain and diencephalon, it is not surprising that several structures associated with the diencephalon begin to appear in sections at this level. Three thalamic nuclei are evident that appear to sit over the midbrain like a tent. The largest of these structures is called the pulvinar, which is a massive nucleus that forms a large part of the posterior thalamus and which lies dorsolateral to the pretectal region and superior colliculus. Two other nuclei include: the medial geniculate nucleus, which is part of the auditory relay system and located lateral to the tegmentum; and the lateral geniculate nucleus, which is part of the visual relay pathway and located lateral to the medial geniculate nucleus. The functions of these structures are described in greater detail in Chap-ters 13 and 26.Clinical ConsiderationsThe most common disorders of the midbrain result mainly from vascular lesions of branches of the posterior cerebral artery but may also derive from P.200tumors such as those situated in the region of the pineal gland.Weber's SyndromeWeber's syndrome is characterized by an ipsilateral oculomotor paralysis, coupled with a contralateral upper motor neuron paralysis. The specific oculomotor deficits may include a dilated, unresponsive pupil, a drooping eyelid, and an eye that deviates downward. This disorder typically results from a vascular lesion that affects the medial aspect of the cerebral peduncle at the level of the superior colliculus and the root fibers of the oculomotor nerve.Benedikt's SyndromeIn this disorder, the patient exhibits an ipsilateral paralysis of the oculomotor nerve as well as a tremor of the opposite limb, coupled with the possible somatosensory loss in the contralateral side of the body. This disorder also results from a vascular lesion that affects root fibers of the oculomotor nerve and the region of the red nucleus, including the superior cerebellar peduncle and adjoining portions of the medial lemniscus, and possibly the spinothalamic tracts. The tremor may result from damage to the superior cerebellar peduncle or to fibers of the basal ganglia that pass close to the red nucleus or, perhaps, even damage to the red nucleus itself. Sensory loss is likely due to damage to the medial lemniscus and spinothalamic tracts.Gaze Palsy (Parinaud's Syndrome)This disorder results from a vascular lesion or a pineal tumor and involves the dorsal aspect of the midbrain, including the pretectal area and region of the posterior commissure. The patient presents with an upward gaze paralysis (see Chapter 14 for discussion of vertical gaze), possible nystagmus with downward gaze, light-near dissociation, large pupil, abnormal elevation of the upper lid, and paralysis of accommodation.
Figure 12-3 Cross-sectional diagram in which the principal structures of the midbrain at the level of the inferior colliculus are depicted. Note the basic divisions of the midbrain into the tectum, tegmentum, and cruscerebri. PAG, periaqueductal gray.CrusCerebriThe ventral aspect of the brainstem at this level contains the cruscerebri (Fig. 12-5). As described earlier, the cruscerebri contains massive bundles of descending axons from the cerebral cortex that terminate either within the brainstem (i.e., corticobulbar fibers) or within the spinal cord (i.e., corticospinal fibers). This peduncle is clearly organized in the following manner. Fibers in the lateral fifth arise from the parietal, occipital, and temporal neocortices and terminate upon deep pontine nuclei. Fibers contained in the medial fifth also terminate upon deep pontine nuclei, but they arise from the frontal lobe. Fibers contained within the medial three fifths constitute the corticobulbar and corticospinal tracts (Figs. 12-1 and 12-5). The organization of these fibers is such that fibers associated with the head region are located medially (corticobulbar fibers); whereas fibers associated with the upper limb, trunk, and lower limbs are located laterally (see Chapters 9 and 10, Fig. 9-12).
Figure 11-1 Ventral and dorsal views of the brainstem. (A) View of the ventral surface of the brainstem illustrating the position of the pons in relation to the medulla and midbrain and the loci of the cranial nerves. Note that CN VI and VII exit medially and laterally, respectively, at the level of the caudal pons, and CN V exits laterally at the level of the middle of the pons. (B) View of the dorsal surface of the brainstem looking down into the fourth ventricle illustrating the positions of the pons in relation to the medulla and midbrain. The cerebellum was removed to demonstrate the position occupied by the fourth ventricle. Note the positions occupied by the superior and middle cerebellar peduncles.PonsBasis PontisTegmentumThe tegmentum of the pons is more complex than the base. The lower pons contains the nucleus of nerve VI (abducens nucleus) and the nuclei of nerve VII (the facial, superior salivatory, and gustatory nuclei). The branchial motor component of the facial nerve loops medially around the nucleus of nerve VI. The upper half of the pons harbors the main sensory nuclei of nerve V (Figs 7–7 E and 7–8). The medial lemniscus assumes a different position (lower body, medial; upper body, lateral), and the spinothalamic tract courses even more laterally as it travels through the pons.The central tegmental tract contains descending fibers from the midbrain to the inferior olivary nucleus and ascending fibers that run from the brain stem reticular formation to the thalamus, and runs dorsolateral to the medial lemniscus. The tectospinal tract (from midbrain to cervical cord) and the medial longitudinal fasciculus are additional components of the pontine tegmentum.Middle Cerebellar PeduncleThe middle cerebellar peduncle is the largest of the three cerebellar peduncles. It contains fibers that arise from the contralateral basis pontis and end in the cerebellar hemisphere.Auditory PathwaysThe auditory system from the cochlear nuclei in the pontomedullary junction includes fibers that ascend ipsilaterally in the lateral lemniscus (see Chapter 16: The Auditory System). It also includes crossing fibers (the trapezoid body) that ascend in the opposite lateral lemniscus. A small superior olivary nucleus sends fibers into the cochlear division of nerve VIII as the olivocochlear bundle (see Fig 7–7 D); this pathway modifies the sensory input from the organ of Corti in the cochlea.Trigeminal SystemThe three divisions of the trigeminal nerve (nerve V; see Figs 7–7 D and E and Fig 7–8) all project to the brain stem. Fine touch function is relayed by the main sensory nucleus; pain and temperature are relayed into the descending spinal tract of V; and proprioceptive fibers form a mesencephalic tract and nucleus in the midbrain. The second-order neurons from the main sensory nucleus cross and ascend to the thalamus. The descending spinal tract of V sends fibers to the pars caudalis (the spinal nucleus in the medulla), the pars interpolaris (a link between trigeminal afferent components and the cerebellum), and the pars oralis. The masticatory nucleus, which is medial to the main sensory nucleus, sends branchial efferent fibers into the mandibular division of nerve V to innervate most of the muscles of mastication and the tensor tympani of the middle ear.Gross Anatomical View of the PonsIn its gross appearance, the pons serves as a connection between the medulla, which is attached to its caudal end, and the midbrain, which is attached at its rostral end (Fig. 11-1). From an anatomical perspective, the pons is typically divided into two regions, a ventral part, called the basilar pons, and a dorsal part, called the tegmentum (Fig. 11-2). The basilar pons consists of two different groups of fibers and neurons distributed throughout much of this region. One group of fibers, present throughout both the dorsoventral and rostro-caudal aspects of the basilar pons, runs in a transverse direction across the pons. This massive group of fibers is thus referred to as transverse pontine fibers (pontocerebellar) and arises from the neurons scattered throughout the basilar pons called deep pontine nuclei. The second group of fibers consists of corticospinal and corticobulbar fibers, which descend from the cerebral cortex and pass in a longitudinal manner with respect to the neuraxis of the brainstem to the spinal cord and lower brainstem, respectively, and are thus positioned perpendicular to the orientation of the transverse pontine fibers.Along the midline of the basilar pons is a groove called the basilar sulcus, which is formed by the basilar artery. Laterally, another massive fiber bundle, called the middle cerebellar peduncle, is a continuation of the transverse pontine fibers. This peduncle passes laterally and dorsally into the cerebellum from the lateral aspect of the pons, thus forming a â€œbridgeâ€ between these two structures. The dorsal aspect of the pons (tegmentum) forms the floor of the fourth ventricle and the roof of the pons. Along the floor of the fourth ventricle, a bulge can be identified caudally along its medial aspect. This is called the facial colliculus and is formed by the presence of the fibers of the facial nerve that pass over the dorsal aspect of the abducens nucleus (Fig. 11-1B). The lateral walls of the fourth ventricle are formed by the superior cerebellar peduncles that arise mainly from the cerebellum and enter the brainstem from a dorsolateral position at the level of the rostral pons.The tegmentum of the pons also consists of nuclei and fiber pathways. Nuclear groups include groups associated with cranial nerves as well as those that are unrelated to cranial nerves but that serve important sensory, motor, autonomic, and behavioral functions. The fiber pathways in this region consist of axons that may arise in the pons and that run either dorsally or caudally. In addition, this region also contains other ascending or descending fiber pathways, which originate in the brainstem, spinal cord, other parts of the brainstem, or forebrain. Details concerning the nature of these cell groups and fiber pathways are considered in the following sections.Internal Organization of the PonsFiber PathwaysMany of the same pathways present within the medulla are also present within the pons. Two of the most prominent ascending tracts include the medial lemniscus and medial longitudinal fasciculus (MLF) (Fig. 11-3). Note that, as the medial lemniscus passes in a rostral direction within the pontine tegmentum, it becomes oriented more laterally and somewhat more dorsally until it reaches its most lateral and dorsal position in the caudal diencephalon where it synapses in the ventral posterolateral nucleus (VPL) of the thalamus (see also Chapter 9). In contrast, the MLF retains its dorsomedial position as it ascends towards the midbrain. In addition, other pathways are situated in a lateral position within the pontine tegmentum as they ascend to the midbrain and more rostral levels. These pathways include the spinothalamic and trigeminothalamic pathways that mediate somatosensory information from the body and head region to the ventral posterolateral and ventral posteromedial nuclei of the thalamus, respectively. These two pathways are sandwiched between the lateral lemniscus, a part of the auditory relay circuit, and the medial lemniscus.The major descending pathways include the corticobulbar and corticospinal tracts (Fig. 11-3). These pathways maintain a position within the basilar aspect of the pons. Other pathways (not clearly discernible from normal sections of the pons) include the rubrospinal and tectospinal tracts. The rubrospinal tract occupies a position within the pons just dorsal to the medial lemniscus. The position of the tectospinal tract within the level of the pons is similar to its position within the medulla; namely, it lies dorsomedially just ventral to the MLF. In addition, the dorsolateral aspect of the tegmentum of the pons (as well as the tegmentum of the midbrain and medulla) contains descending fibers from the limbic system and hypothalamus that project to regions of the lower brainstem that mediate autonomic functions.
Figure 11-2 Sagittal view of the brainstem. (A) Midsagittal section through the neuraxis of the brain showing the relationship of the pons to the medulla, midbrain, cerebellum, and forebrain. Note that the pons can be divided into two regions: a ventral aspect called the basilar pons and a dorsal aspect called the tegmentum. (B) A cross section taken through the middle of the pons reveals the division of the pons into the basilar and tegmental regions.Major Cell Groups (Figs. 11-3,11-4,11-5,11-6)Caudal PonsA number of important nuclei, associated in part with cranial nerves, are present in the caudal pons. These include the motor nuclei of the facial (cranial nerve [CN] VII) and abducens (CN VI) nerves, the spinal nucleus of the trigeminal CN V, the superior olivary nucleus (a relay nucleus of the auditory pathway), raphe nuclei, and pontine nuclei of the basilar pons.Rostral PonsThe principal nuclei present in the rostral half of the pons include the main sensory, mesencephalic, and motor nuclei of CN V, the superior and lateral vestibular nuclei, and the locus ceruleus. Other cell groups, such as the pontine nuclei (of the basilar aspect) and the raphe nuclei (of the tegmentum), are also present at all levels of the pons.
Figure 11-6 Rostral pons. (A) Diagram of a cross section through the upper pons at the level of the locus ceruleus and mesencephalic nucleus of the trigeminal nerve. (B) Myelin-stained section through the same level of the pons as the level of the nucleus ceruleus and mesencephalic nucleus of the trigeminal nerve. ( Panel B: Reproduced with permission from Snell RS: Clinical Neuroanatomy, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2001, p. 204.
Figure 11-5 Middle pons. (A) Cross section through the middle of the pons at the level of the main sensory and motor nuclei of the trigeminal nerve. (B) Myelin-stained section through the same level of the pons at the level of the main sensory and motor nuclei of the trigeminal nerve. ( Panel B: Reproduced with permission from Snell RS: Clinical Neuroanatomy, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2001, p. 203.
Caudal pons. (A) Cross section through the caudal aspect of the pons at the level of the facial colliculus of the pons. Note that the axons of CN VII pass dorsomedially around the dorsal aspect of the nucleus of CN VI, forming the facial colliculus, and then pass ventrolaterally to exit the brainstem. In contrast, the axons of CN VI pass ventrally to exit the brainstem in a relatively medial position. (B) Myelin-stained cross section through the caudal aspect of the pons. (Panel B: Reproduced with permission from Parent A: Carpenter's Human Neuroanatomy, 9th ed. Baltimore: Williams & Wilkins, 1996, p. 496.)Nuclei and fibers of the reticular formation. As noted in Chapter 10, the reticular formation extends from the lower medulla to the rostral aspect of the midbrain. Three areas of the pontine reticular formation are noted: a midline region, containing raphe nuclei; a large-celled region situated within the medial two thirds of the reticular formation; and a small-celled region situated in the lateral aspect of the reticular formation. In brief, the raphe cells produce serotonin; these cells project to many areas of the central nervous system. The large-celled region gives rise to long ascending and descending fibers contained to a considerable extent in the central tegmental tract, and the small-celled region serves as a source of inputs to the reticular formation.
Figure 10-3 Cross-sectional diagrams of the lower medulla. (A) Level of the decussation of the pyramids. (B) Level of the decussation of the medial lemniscus.Levels of the MedullaIn describing the anatomy of the medulla, we can distinguish at least two levels. The caudal half, which includes the â€œclosedâ€ medulla, contains two decussations: (1) the pyramidal decussation and (2) a second level that contains the decussation of the medial lemniscus. The upper half of the medulla, which includes the â€œopenâ€ medulla, contains the inferior olivary nucleus. The following sections consider the neuroanatomical organization of the medulla on the basis of the structures contained at each of these levels.Level of the Pyramidal DecussationAt the level of the pyramidal decussation, the general organization of the medulla is very similar to that of the cervical spinal cord. For example, the dorsal aspect of this level of the medulla contains the fasciculus gracilis and fasciculus cuneatus (Figs. 10-3 and 10-4). The spinocerebellar and spinothalamic tracts also lie in the same general lateral position they occupy within the spinal cord. Some anterior horn cells can generally be seen at this level as well.There are major new features found at this level of medulla that are not present in the spinal cord. The most obvious change is the presence of the pyramidal decussation. As indicated earlier, about 90% of the fibers of each pyramid cross over to the contralateral side of the brain in their descent into the spinal cord as the lateral corticospinal tract. Approximately 8% of the fibers continue to descend uncrossed into the spinal cord as the anterior corticospinal tract and are situated in a ventromedial position; ultimately, these fibers cross over to the opposite side of the spinal cord. The remaining 2% of the corticospinal tract supplies the ipsilateral spinal cord grey matter. A second change is the presence of the spinal nucleus of the trigeminal nerve. It replaces the substantiagelatinosa in that it is situated in the dorsolateral aspect of the medulla, which is the approximate position occupied by the substantiagelatinosa in the dorsal horn of the spinal cord. The anatomical similarities between the substantiagelatinosa and the spinal nucleus of CN V are matched by their functional similarities as well. Both structures mediate pain and temperature modalities of sensation. The difference between the two relates to the fact that the substantiagelatinosa mediates these sensations from the body, whereas the spinal nucleus mediates sensations associated with the region of the anterior two thirds of the head, oral and nasal cavities, and the cutaneous surfaces of the ear and external auditory meatus.Level of the Decussation of the Medial LemniscusAt a level slightly rostral to that of the pyramidal decussation, a second major decussation called the decussation of the medial lemniscus is present (Figs. 10-3 and 10-5). While the pyramidal decussation involves fibers from sensorimotor cortex that are specifically involved in motor functions, the decussation of the medial lemniscus is part of an overall sensory pathway that contains fibers that arise from the nucleus gracilis and nucleus cuneatus (located immediately lateral to the nucleus gracilis) and mediates somesthetic impulses from the body to the thalamus and, ultimately, to the postcentralgyrus. Axons of the medial lemniscus initially pass ventrally in an arc (where fibers arising from the nucleus gracilis emerge prior to those of the nucleus cuneatus and collectively are called internal arcuate fibers) and then cross over to the contralateral side via the decussation of the medial lemniscus. These fibers then turn rostrally as they ascend toward the forebrain where they terminate in the ventral posterolateral (VPL) nucleus of the thalamus.At this level of the medulla, another nucleus, called the accessory cuneate nucleus, is attached to the lateral edge of the nucleus cuneatus (Fig. 10-5). This structure receives inputs from muscle spindles and Golgi tendon organs associated with the skeletal muscles of the upper limbs that are conveyed by first-order fibers passing in the fasciculus cuneatus. Axons of the accessory cuneate nucleus, called external arcuate fibers, form the cuneocerebellar tract as they pass ipsilaterally into the inferior cerebellar peduncle. Thus, they mediate unconscious proprioception from the upper limbs to the cerebellum and represent the upper limb equivalent of the dorsal spinocerebellar tract.Other structures that are prominent at the level of the decussation of the medial lemniscus are also present more caudally within the medulla. These include the pyramidal, spinocerebellar, and spino-thalamic tracts, as well as the spinal nucleus and tract of the trigeminal nerve.FIG. 692– Superior terminations of the posterior fasciculi of the medulla spinalis. 1. Posterior median sulcus. 2. Fasciculus gracilis. 3. Fasciculus cuneatus. 4. Gracile nucleus. 5. Cuneate nucleus. 6, 6’, 6’’. Sensory fibers forming the lemniscus. 7. Sensory decussation. 8. Cerebellar fibers uncrossed (in black). 9. Cerebellar fibers crossed (in black). (Testut.) (See enlarged image) FIG. 693– Transverse section passing through the sensory decussation. (Schematic.) 1. Anterior median fissure. 2. Posterior median sulcus. 3, 3. Head and base of anterior column (in red). 4. Hypoglossal nerve. 5. Bases of posterior columns. 6. Gracile nucleus. 7. Cuneate nucleus. 8, 8. Lemniscus. 9. Sensory decussation. 10. Cerebrospinal fasciculus. (Testut.) (See enlarged image) Medial LemniscusThe medial lemniscus originates from the dorsal column nuclei (described on pages 170â€“171). These fibers collectively pass ventrally for a short distance in an arc-like trajectory and are referred to as internal arcuate fibers. These second-order somatosensory fibers then cross to the opposite side of the medulla as the decussation of the medial lemniscus. The medial lemniscal fibers then pass rostrally in a medial position throughout the remainder of their trajectory through the medulla and continue into the pons. The function of the medial lemniscus is to transmit information associated mainly with conscious proprioception and vibratory stimuli to the thalamus (Fig. 10-2).
Figure 10-6 The arrangement of sensory and motor nuclei of cranial nerves. The key point is that the region derived from the basal plate is located medially and contains motor nuclei, while the region derived from the alar plate is located more laterally and contains sensory nuclei. The region in between these two regions, which is formed from the sulcuslimitans, contains neurons that relate to autonomic functions. Abbreviations: SSA, special sensory afferent neurons; GSA, general somatic afferent neurons; SVA, special visceral afferent neurons; GVA, general visceral afferent neurons; GSE, general somatic efferent fibers.Rostral Half of the MedullaAt this level of the medulla, a number of new structures make their appearance, including important cranial nerve nuclei. It is useful to recall the section of Chapter 2 (pages 22â€“26) that described the development of the brainstem. On developmental principles, the arrangement of sensory and motor nuclei, especially those of cranial nerves, is reviewed in the following paragraphs and is depicted in Fig. 10-6.
Figure 10-7 Central medulla. (A) Cross-sectional diagram of the medulla through the level of the inferior olivary nucleus. (B) Photograph of a cross section through the same level of medulla (Weigert myelin stain). Insert at bottom right indicates the level at which the section was taken. (Panel B: Reproduced with permission from Parent A: Carpenter's Human Neuroanatomy, 9th ed. Baltimore: Williams & Wilkins, 1996, p. 942.In brief, the locations of neuronal cell groups associated with cranial nerve function can be determined by knowing the following medial-to-lateral arrangement. Cell groups located in the most medial position, derived from the basal plate, are classified as general somatic efferent (GSE) neurons; they innervate skeletal muscle of somite origin. The clearest example of such a cell group that is situated at this level of the brainstem is the hypoglossal nucleus, which can be easily seen in Figures 10-7 and 10-8 and lies just above the MLF in a dorsomedial position within the medulla. Axons of the hypoglossal nucleus innervate the intrinsic and extrinsic muscles of the tongue and thus control its movements.The next column includes those cell groups that are derived from mesenchyme of the branchial arches and are classified as special visceral efferent (SVE) neurons, which also innervate skeletal muscle (Fig. 10-6). An example of this type of cell group is the nucleus ambiguus, which gives rise to axons of CN IX and X. Axons of the nucleus ambiguus travel, in part, in the vagus nerve and innervate the muscles of the larynx and pharynx, thus controlling phonation, gagging, and swallowing. Drawing a line from the dorsolateral tip of the inferior olivary nucleus to the hypoglossal nucleus can identify the position of the nucleus ambiguus. The nucleus ambiguus is located at the mid-point of this line.Continuing in a lateral direction, the third column includes those cell groups that comprise the general visceral efferent (GVE) neuronal groups (Fig 10-6). These represent preganglionic parasympathetic fibers whose axons innervate autonomic ganglia. Postganglionic parasympathetic neurons are present either in these ganglia or within the wall of the target organs, and their axons innervate smooth muscles and glands.Two groups of such neurons can be detected within the medulla. The first group of neurons includes the dorsal motor nucleus of the vagus nerve (CN X), whose axons contribute to the vagus nerve. The dorsal motor nucleus lies dorsolateral to the hypoglossal nucleus just medial to the sulcuslimitans at the rostral end of the closed medulla (see Chapter 2). The second group of neurons constitutes the inferior salivatory nucleus, whose axons form part of CN IX and which contribute to the process of salivation. Axons of these neurons form preganglionic parasympathetic fibers that innervate the otic ganglion, from which postganglionic parasympathetic fibers innervating the parotid gland arise.Immediately lateral to the sulcuslimitans lies the general visceral afferent (GVA) column (Fig. 10-6). Cells in this region, which are derived from the alar plate, receive afferent signals from visceral organs, such as blood vessels, that are mediated via branches of the glossopharyngeal (CN IX) and vagus (CN X) nerves. The principal nucleus that receives and processes such information is the caudal portion of the nucleus solitarius, which lies in a position slightly ventrolateral to the dorsal motor nucleus of the vagus nerve. Superimposed on this column is the special visceral afferent (SVA) column (Fig. 10-6), which includes the rostral half of the solitary nucleus (gustatory portion). The reason for the proximity of these two groups of separately classified cell types of neurons is that the primary neuron for both functions at this level of the brain is the solitary nucleus. Thus, different groups of cells within the solitary nucleus mediate two distinctly separate processes, one associated with GVA functions (i.e., cardiovascular and respiratory reflexes) and the other associated with SVA functions (i.e., taste).The final categories of cell groups include general sensory afferent (GSA) neurons and special sensory afferent (SSA) neurons. They both lie in a most lateral position within the medulla. The primary GSA neuronal cell group found at this level of the brainstem is the spinal nucleus of the trigeminal nerve. Within the medulla, separate groups of SSA neurons can be detected. One such group is the vestibular nuclei. There are four vestibular nuclei that receive direct inputs from the vestibular apparatus: the inferior, medial, lateral, and superior nuclei. The inferior and medial vestibular nuclei are located in the rostral medulla at the approximate level of the rostral portions of the inferior olivary nucleus. The superior and lateral vestibular nuclei lie in a more rostral position at the transition region between the medulla and pons. The medial and inferior vestibular nuclei are best seen in the rostral open medulla, while the superior and lateral vestibular nuclei are best seen in the junction between the pons and medulla just lateral to the fourth ventricle and medial to the inferior cerebellar peduncle.The other group of SSA neurons is the cochlear nuclei, which lie on the dorsolateral shoulder of the inferior cerebellar peduncle at the rostral aspect of the medulla. These neurons receive inputs from primary auditory fibers and thus constitute part of a complex auditory pathway that eventually transmits auditory signals to the cerebral cortex.This discussion has indicated how nuclei related to cranial nerve function are organized within the lower brainstem. Thus, when taken together with the analysis of the structures of the lower medulla considered earlier in this chapter, it is now possible to reconstruct the organization of the rostral half of the medulla. As is shown in Figure 10-6, the structures seen in more caudal levels of the medulla that are present at more rostral levels are again represented in the same general locations as noted in sections of the lower medulla.Structures Situated in the Medial MedullaThe arrangement of structures when going from a ventral to a dorsal level is as follows. On the ventromedial aspect of the rostral half of the medulla are the pyramids (Figs. 10-7 and 10-8). Dorsal to them, lies the medial lemniscus, and dorsal to the medial leminiscus lies two descending pathways: (1) the tectospinal tract, which contains descending fibers from the superior colliculus to the cervical cord that are presumed to mediate postural adjustments to visual stimuli; and (2) the MLF, whose descending component mediates head movements in response to postural changes in concert with the position of the eyes. Just dorsal to the position of these two pathways is the hypoglossal nucleus whose action is to protrude the tongue. Further discussion concerning functions and dysfunctions of the hypoglossal nerve is found in Structures Situated in the Lateral MedullaOn the lateral side, the inferior olivary nucleus is most conspicuous because it is situated in a ventrolateral position just lateral to the pyramids (Figs. 10-7 and 10-8). The inferior olivary nucleus consists of many groups of cells, which project to the contralateral cerebellar cortex. The inferior olivary nucleus receives inputs from the spinal cord and red nucleus. The fibers from the spinal cord reach the inferior olivary nucleus via spino-olivary fibers (see Chapter 21), whereas fibers from the red nucleus reach the inferior olivary nucleus via rubro-olivary fibers contained in a larger bundle called the central tegmental tract (see Chapter 11). The primary function of the inferior olivary nucleus appears to be that of an important relay for the transmission of afferent signals to the cerebellum that are required for smooth, coordinated movements to be made.Situated dorsally to the inferior olivary nucleus is the reticular formation, which contains many different groups of nerve cells as well as ascending and descending fibers. Within the core of the brainstem that forms the reticular formation lie several of the structures previously mentioned that are associated with cranial nerves. Two of these structures include the nucleus ambiguus and the solitary nucleus. Located just under the dorsolateral surface of the brainstem are two vestibular nuclei, the medial and inferior vestibular nuclei. The inferior vestibular nucleus can be easily recognized by its stippled appearance, which is due to the presence of descending fibers from the lateral vestibular nucleus that form the lateral vestibulospinal tract (Fig. 10-7B). The vestibular nuclei receive sensory information from the vestibular apparatus and transmit this information to the cerebellum, CN III, IV, and VI, and the spinal cord.P.176In its ventral half, the far lateral aspect of the medulla contains the ascending dorsal and ventral spinocerebellar and spinothalamic pathways as well as the descending rubrospinal tract. In a more dorsal position, the spinal nucleus and tract of CN V can be easily seen, and dorsolateral to these structures is the inferior cerebellar peduncle, which is quite prominent at this level. At the most rostral aspect of the medulla, the primary afferent fibers of CN VIII terminate upon two additional nuclear groups, the ventral and dorsal cochlear nuclei, which lie off the dorsolateral shoulder of the inferior cerebellar peduncle and constitute part of the relay system for the transmission of auditory impulses from the periphery to the cerebral cortex (Fig. 10-8). In the human brain, the dorsal cochlear nucleus is small, whereas the ventral cochlear nucleus is large, extending into the junction between the medulla and pons.
Figure 10-8 Upper medulla. (A) Cross-sectional diagram of the upper medulla just caudal to the pons. (B) Photograph of a cross section through the same level of medulla (Weigert myelin stain). Insert at bottom right indicates the level at which the section was taken. (Panel B: Reproduced with permission from Parent A: Carpenter's Human Neuroanatomy, 9th ed. Baltimore: Williams & Wilkins, 1996, p. 944
Motor ComponentsThree types of basal plate derivatives (motor nuclei) are located within the brain stem (see Table 7–2).General somatic efferent (SE or GSE)components innervate striated muscles that are derived from somites and are involved with movements of the tongue and eye, such as the hypoglossal nucleus of XII, oculomotor nucleus of III, trochlear nucleus of IV, and abducens nucleus of VI.Branchial efferent (BE)components, sometimes referred to as special visceral efferents (SVE), innervate muscles that are derived from the branchial arches and are involved in chewing, making facial expressions, swallowing, producing vocal sounds, and turning the head. Examples include the masticatory nucleus of V; facial nucleus of VII; ambiguus nucleus of IX, X, and XI; and spinal accessory nucleus of XI located in the cord.General visceral efferent (VE or GVE)components are parasympathetic preganglionic components that provide autonomic innervation of smooth muscles and the glands in the head, neck, and torso. Examples include the Edinger–Westphal nucleus of III, superior salivatory nucleus of VII, inferior salivatory nucleus of IX, and dorsal motor nucleus of X.Sensory ComponentsTwo types of alar-plate derivatives can be distinguished in the brain stem and are comparable to similar cell groups in the spinal cord (see Table 7–2).General somatic afferent (SA or GSA)components receive and relay sensory stimuli from the skin and mucosa of most of the head: main sensory, descending, and mesencephalic nuclei of V.General visceral afferent (VA or GVA)components relay sensory stimuli from the viscera and more specialized taste stimuli from the tongue and epiglottis: solitary nucleus for visceral input from IX and X and gustatory nucleus for special visceral taste fibers from VII, IX, and X.Six special sensory (SS)nuclei can also be distinguished: the four vestibular and two cochlear nuclei that receive stimuli via vestibulocochlear nerve VIII. These nuclei are derived from the primitive auditory placode in the rhombencephalon (Fig 7–7 A).
Before turning to the motor VII, you should note that the pons consists of two zones, a dorsal portion called the tegmentum of the pons and a ventral zone called the basilar pons. The tegmentum contains cranial nerve nuclei and ascending pathways such as the medial lemniscus, lateral lemniscus, ALS (spinothalamic tract), STT (solitariothalamic tract) and TTT (trigeminothalamic tract). The basilar region contains the pontine grey nuclei and massive groups of descending fibers, including the corticospinal, corticobulbar, and corticopontine tracts.The motor nucleus VII contains motor neurons (branchiomotor) that innervate the muscles of facial expression including the orbicularisoculi (CLOSES eyelid), the stapedius, the stylohyoid and the posterior belly of the digastric. Neurons comprising motor VII possess axons that pursue a rather circuitous route in order to exit the brain stem. Initially they pass dorsally and medially to loop over the abducens nucleus. The fibers then course ventrally and laterally to exit the brain stem. The bump in the floor of the fourth ventricle caused by the motor fibers of C.N. VII looping over the abducens nucleus is called the FACIAL COLLICULUS.A unilateral lesion interrupting the axons of C.N. VII results in the following: On the ipsilateral side, the forehead is immobile, the corner of the mouth sags, the nasolabial folds of the face are flattened, facial lines are lost, and saliva may drip from the corner of the mouth. The patient is unable to whistle or puff the cheek because the buccinator muscle is paralyzed. When the patient is smiling, the normal muscles draw the contralateral corner of the mouth up while the paralyzed corner continues to sag. Corneal sensitivity remains (C.N. V), but the patient is unable to blink or close the eyelid (CN VII). To protect the cornea from drying, therapeutic closure of the eyelids or other measures are taken (patient wears an eye mask, or lids are closed with sutures). Because of the paralysis of the stapedius muscle, which normally dampens the amplitude of the vibrations of the ear ossicles, the patient will experience sounds as uncomfortably loud. THIS IS CALLED HYPERACUSIS. REMEMBER, THERE WILL BE ATROPHY OF ALL OF THE ABOVE MUSCLES (i.e. Lower Motor Neuron).You will recall that the cortex sends axons to cranial nerve motor nuclei. These are called CORTICOBULBAR fibers (remember those to the hypoglossal and nucleus ambiguus?). A unilateral lesion of the corticobulbar fibers to motor VII, for example in the motor cortex, results in weakness of the muscles of expression of the face BELOW THE EYE ON THE SIDE CONTRALATERAL TO THE LESION. The frontalis muscle (wrinkles forehead) and the orbicularisoculi muscle (closes eyelid) are unaffected. The accepted explanation states that BILATERAL (crossed and uncrossed) corticobulbar projections from the cerebral cortex influence the lower motor neurons (within Motor VII) innervating the frontalis muscle and orbicularisoculi, while only crossed corticobulbar projections influence the lower motor neurons innervating the muscles of the LOWER face. Think about it like this, the lower part of Motor VII is like the hypoglossal nucleus (crossed corticobulbars), while the upper part is like nucleus ambiguus (bilateral corticobulbars).A lesion in the face representation of area 4 (motor cortex) will mean that those motor neurons in the contralateral region (ventral) of motor VII that innervate the lower facial muscles are completely deprived of cortical input. In contrast, the lower motor neurons in that part of motor VII that innervate the upper facial muscles still have cortical input from the ipsilateral motor cortex. Such muscles, therefore, contract when the patient wants to voluntarily contract them. REMEMBER, THERE IS NO MUSCLE ATROPHY FOLLOWING A LESION OF THE CORTICOBULBAR FIBERS.You should now think about the resulting neurological deficits following a lesion of the LEFT motor cortex that interrupts all CORTICOSPINAL fibers and CORTICOBULBAR fibers to motor VII, nucleus ambiguus and the hypoglossal nucleus. There will be a RIGHT hemiplegia, the tongue will deviate to the RIGHT upon protrusion, and the lower facial muscles on the RIGHT will be weak. Any problems with swallowing? Will the uvula deviate when you say ahhh? THINK! THIS IS VERY IMPORTANT.AN INTERESTING CLINICAL OBSERVATIONIt is known that following a stroke muscles of facial expression of the lower face on the opposite side are weak and the patient cannot voluntarily move these muscles. However, reflex smiling (at a joke) did result in movement of these muscles. This suggests that there are different pathways involved in moving these muscles during voluntary and reflex movements.
The cell group in the brain stem that receives VISCEROsensory information is called NUCLEUS SOLITARIUS. This sad, little, lonely nucleus lies lateral to the dorsal motor nucleus X (pregang. parasym. of C.N. X), and next to (and sometimes surrounds, but this is hard to see in our sections) the solitary fasciculus or tract (somewhat similar arrangement as spinal nuc. and tract V eh?). Only a portion of the entire rostrocaudal axis of the solitary complex can be seen in our series of 10 brain stem sections, and this is on level #3. In order to show the rostral and caudal divisions of the solitary complex (which differ functionally), I have taken some artistic license in the drawing below by schematically illustrating the solitary complex as a column extending above (rostral part) and below (caudal part) level #3. I hope this is not confusing.VISCEROSENSORY information reaches the medulla, primarily (but not exclusively) via C.N. X (vagus; the wanderer). We are generally not aware of the viscerosensory information conveyed by the vagus. Most of these messages are related to the status of the viscera (for example, information from receptors in the walls of the viscera, including the entire digestive system to the middle of the transverse colon, and in the respiratory system from the larynx to the pulmonary air sacs of the lung).The cells of origin of vagal fibers that convey messages from the viscera lie in the INFERIOR GANGLION of X. The peripheral processes of these neurons pass to viscera, while the central processes pass into the medulla to comprise FASCICULUS SOLITARIUS and synapse in the adjacent NUCLEUS SOLITARIUS. As I have mentioned above, the solitary complex is functionally divided into CAUDAL and ROSTRAL portions, and the viscerosensory information that we are talking about now synapses within the CAUDAL NUCLEUS SOLITARIUS. Neurons in the CAUDAL part of nucleus solitarius possess axons that convey information about the status of the viscera to many areas of the brain that are involved in the reflex control of the viscera. For instance, information is sent from the caudal nucleus solitarius to the dorsal motor nucleus X (preganglionic parasympathetic; increases peristalsis) and to the lateral cell column of the spinal cord (preganglionic sympathetic; decreases peristalsis). Messages are also sent to respiratory centers in the brain stem (we will not cover these areas in this course, but you will learn them in Physiology).VISCEROSENSORY information regarding blood pressure is conveyed via both C.Ns. IX and X. For example, information from the carotid sinus travels over C.N. IX (cell bodies are in INFERIOR GANGLION IX). The carotid sinus is a region near the bifurcation of the internal and external carotids. In this area the wall of the artery is thinner and contains a large number of branching, vinelike endings of C.N. IX. This area serves as a pressure receptor (baroreceptor; baros=weight). An increase in arterial pressure increases the rate of impulses in the fibers of C.N. IX that innervate the carotid sinus, above the baseline (“normal”) number of impulses, and this information passes into caudal nucleus solitarius. This results in more impulses being sent from excitatory neurons in nucleus solitarius to the DORSAL MOTOR X (C.N. X). This leads to an increase in the number of impulses sent from dorsal motor X to the heart (of course not directly). This will SLOW the heart rate. Cells in nucleus solitarius also project to the preganglionic sympathetic neurons in the upper thoracic spinal cord. An increase in blood pressure in the carotid sinus will lead to an increase in firing of the fibers of C.N. IX that reach caudal nucleus solitarius. This will result in an increase in firing of inhibitory neurons in caudal nucleus solitarius that project to preganglionic sympathetic neurons in the thoracic cord. This increase in the amount of inhibition reaching the preganglionic sympathetic neurons will lead eventually to reflex lowering of the blood pressure. While similar connections and functions are associated with the baroreceptors in the arch of the aorta, C.N. X instead of C.N. IX is involved. You should be able to figure out what happens following a decrease in blood pressure in the carotid sinus.For our PROBLEM SOLVING EXERCISES, WE WILL EQUATE A UNILATERAL LESION OF CAUDAL NUCLEUS SOLITARIUS WITH AN INCREASE IN HEART RATE (just like DORSAL MOTOR X). Loss of excitation of the dorsal motor nucleus X means dorsal motor X is firing LESS and loss of inhibition to the sympathetic neurons in the cord, means that they are firing more. This results in sympathetics dominating = heart rate UP!READ ON ONLY IF YOU ARE INTERESTED. NOT ON EXAM!There are chemoreceptors in the carotid and aortic bodies that affect respiration. The afferent information from the carotid and aortic bodies travel in C.N.s IX (carotid body; cell bodies in INFERIOR GANGLION IX) and X (aortic body; cell bodies in INFERIOR GANGLION X). The receptors in the carotid and aortic bodies respond to a decrease in arterial oxygen tension (P02) and an increase in arterial carbon dioxide (PCO2). For instance, an increase in PCO2 will result in an increase in the number of impulses traveling over C.N.s IX and X to the caudal nucleus solitarius. Neurons in nucleus solitarius project to the phrenic nucleus, which consists of a group of neurons in the ventral horn of the spinal cord from C3-C5. Axons arising from the phrenic nucleus comprise the phrenic nerve that innervates the diaphragm. Cells in nucleus solitarius also project to neurons in the spinal cord that innervate the intercostal muscles. Therefore, an increase in PCO2 will result in an increase in the depth and rate of breathing, while a decrease in PCO2 will have the opposite effect.This is NOT a course in respiratory or cardiovascular physiology! However, it is extremely important for you to remember that the brain stem, especially the medulla, is an important region for the control of respiration and cardiovascular functions. BILATERAL lesions of the caudal nucleus solitarius will result in major respiratory and cardiovascular problems that result in death.AN INTERESTING CLINICAL OBSERVATION - KNOW THIS COLDClinical case reports mention that lesions of the medulla that involve the area slightly ventral and lateral to nucleus and tractussolitarius result in HICCUP. One (of several) explanations for this finding is that such a lesion “irritates” descending information from nucleus solitarius to the phrenic nucleus. The phrenic nucleus consists of a functionally related group of cell bodies in the ventral horn from C3-C5. Axons arising from the phrenic nucleus comprise the phrenic nerve, which innervates the diaphragm. The hiccups result from spasmodic lowering of the diaphragm that causes a short, sharp inspiratory cough.I want you to remember that brain stem lesions involving the area ventral and lateral to nucleus and tractussolitarius (I have cleverly designated it the HICCUP area, but this has not been carefully studied) result in HICCUP. KEEP THIS IN MIND WHEN THE DOING PROBLEM SOLVING EXERCISES.TASTEThe ROSTRAL portion of the solitary complex is a component of the TASTE PATHWAY. The axons within the rostraltractussolitarius are the central processes of cells within THREE cranial nerve ganglia, the GENICULATE GANGLION OF C.N. VII, the INFERIOR GANGLION of C.N. IX and the INFERIOR GANGLION of C.N. X. These central processes comprise the rostraltractussolitarius and terminate within the ROSTRAL or GUSTATORY portion of the nucleus solitarius. The peripheral processes of these neurons innervate the TASTE BUDS of the tongue in the following distribution:C.N. VII=anterior twothirdsC.N. IX=posterior onethirdC.N. X=taste buds on epiglottisLike other ascending sensory pathways, taste information heads for the thalamus (the great GATEWAY to the cerebral cortex!), and in particular to the ventral posteromedial nucleus (VPM; the nucleus of the HEAD!; after all, the tongue is in the head; remember the trigeminal-VPM relationship). The third neuron in the pathway i.e., the thalamic VPM neuron, then sends its axon to the ventral lateral portion of the postcentralgyrus, areas 3, 1, and 2. UNLIKE OTHER ASCENDING SENSORY PATHWAYS, THE SOLITARIOTHALAMIC TRACT (STT) IS UNCROSSED, repeat, UNCROSSED, repeat, UNCROSSED.SOLITARIOTHALAMIC=UNCROSSEDA LESION OF THE ROSTRAL NUCLEUS AND TRACTUS SOLITARIUS WILL RESULT IN THE LOSS OF TASTE FROM THE IPSILATERAL ONE-HALF OF THE TONGUE. SO WILL A LESION OF THE SOLITARIOTHALAMIC TRACTYou should also keep in mind that the interruption of the solitariothalamic tract will not result in major problems in respiratory and cardiovascular control, since most of the pathways over which the nucleus solitarius controls these functions pass caudally in the brain stem. Let’s reserve a loss of taste from the ipsilateral side of the tongue for lesions of the solitariothalamic tract. Lesions of the rostral nucleus solitarius will also result in loss of taste from the ipsilateral side of the tongue. In contrast, a lesion of the CAUDAL portion of nucleus solitarius will result in an INCREASE IN HEART RATE.LESION of the ROSTRAL SOLITARIUS=LOSS of IPSILATERAL TASTEwhileLESION of the CAUDAL SOLITARIUS=INCREASE HEART RATE
Since the cells in the PAG contain receptors for opiate peptides, systemic injections of morphine-like substances can reduce pain by turning on the smaller inhibitory PAG neurons in the midbrain.Some of the pathways just discussed may underlie the results of classical acupuncture. Rotation of thin needles could stimulate the C and D fibers in body tissue and in turn the ALS. While you are thinking this should hurt, branches of ALS fibers (collaterals of those heading to the VPL) could end in the PAG (or NRM) and somehow turn on the descending pain modulation circuit. We do not understand the details of this circuit, but it is known that acupuncture is “opiate dependent.” That is, intravenous injection of naloxone (an opiate inhibitor) is thought to block the effects of acupuncture (and SPA and SIA).I am sorry to say that the spinal tract and spinal nucleus V are not exclusively associated with C.N. V. It is also associated with C.N.s VII (facial), IX (glossopharyngeal) and X (vagus). While this makes the story a little confusing, it also makes a lot of sense! The pain and temperature fibers associated with C.N. VII innervate the skin of the external ear, the wall of the external auditory meatus and the outer surface of the tympanic membrane (if it’s OK with you, I’ll lump these together as “EAR”). These fibers are the peripheral processes of cells that lie in the GENICULATE ganglion (located in the facial canal). The central processes of these neurons enter the brain with C.N. VII (at pontine levels, caudal to the trigeminal), travel in spinal tract V and end in spinal nucleus V. The pain and temperature information is then conveyed rostrally in the TTT (trigeminothalamic tract) to reach the VPM, from which it is relayed to somatosensory cortex (areas 3,1 and 2). Thus the pain and temperature fibers of C.N. VII don’t have their “own” central cell group, but instead use that of the trigeminal.Like C.N. VII, C.N. IX is involved in the pain and temperature innervation of the “EAR”. In addition, C.N. IX conveys pain and temperature from the posterior one-third of the tongue, the auditory tube and the upper part of the pharynx (touching of the pharyngeal wall results in a gag reflex which we will cover in Point 9: Nucleus Ambiguus). The cell bodies of these pain and temperature axons associated with C.N. IX lie in the relatively small SUPERIOR GANGLION IX (located just outside of the jugular foramen; you will hear about the larger inferior ganglion later). The central processes of cells in the superior ganglion enter the brain with C.N. IX (at the lateral medulla), then enter our friend spinal tract V and synapse in the caudal spinal nucleus V. You know the route f FINALLY (whew!) C.N. X innervates the “EAR” (along with C.N.s VII and IX). In addition, pain and temperature from the lower pharynx, the larynx and the upper esophagus are conveyed by C.N. X. The cell bodies of these pain and temperature axons lie in the SUPERIOR GANGLION X (located just outside the jugular foramen). The central processes of cells in the superior ganglion X pass into the brain at the lateral medulla, enter our old friend spinal tract V and synapse iKEY THOUGHT: only ONE ganglion to worry about with C.N. VII (GENICULATE). SUPERIOR ganglia of BOTH IX and X=somatic afferents (inferior ganglia I As mentioned above, pain fibers end only in the caudal spinal nucleus V. You should also be aware of the fact that not much is known about the other parts of the spinal trigeminal nucleus (nucleus oralis and interpolaris). The important fact is that the great majority of the pain and temperature fibers pass to the most caudal portion of spinal nucleus V. This means that lesions anywhere along the rostrocaudal extent of the spinal tract V, as well as in the caudal spinal itself, will result in deficits in pain and temperature from the ipsilateral face etc. Finally, you have probably heard of tic douloureux or trigeminal neuralgia, which is a disorder characterized by brief at For our PROBLEM SOLVING, you must know that a lesion of spinal tract V along its entire course (the pain information never gets to the caudal spinal nucleus) results in IPSILATERAL deficits in pain and temperature from the face etc. Interruption of the trigeminothalamic tract, which is comprised of axons that have crossed the midline, results in deficits in pain and temperature on the contralateral side of the face etc.REMEMBER:1. spinal tract and nucleus V are present in the pons and medulla2. the cells of origin of spinal tract V lie in the trigeminal ganglion3. axons in spinal tract V terminate in the spinal nucleus V4. axons of cells within spinal nucleus V project to the contralateral VPM5. spinal tract and nucleus V = pain and temp. from face etc. (ear, tongue, palates, pharynx, larynx, upper esophagus)AN INTERESTING CLINICAL OBSERVATIONReports in the clinical literature note that vascular lesions that interrupt the blood supply to the spinal nucleus and tract V in the medulla (for example a thrombosis of the posterior inferior cerebellar artery) sometimes are immediately followed by sharp stabbing pain in and around the eye and on the ipsilateral face (hyperalgesia; algesia; Gr., = sense of pain). A possible explanation for this paradox is that the pain fibers are highly irritated before they die (spontaneous pain also sometimes occurs on the contralateral side of the body immediately following a lesion of the anterolateral system). Please remember that lesions of spinal tract and nucleus V can result in PAIN in the face (in addition to loss of pain and temperature later). What a paradox!
DORSAL MOTOR NUCLEUS OF THE VAGUS (C.N. X)This nucleus lies slightly dorsal and lateral to the hypoglossal nucleus. Axons arising from cells within the dorsal motor X give rise to PREGANGLIONIC PARASYMPATHETIC fibers that course ventral and lateral from the nucleus to exit the brain stem dorsal and lateral to the inferior olive. These axons comprise the visceromotor component of the vagus nerve (C.N. X). Preganglionic parasympathetic fibers from the vagus wander all over the place, but eventually terminate in ganglia that contain postganglionic neurons. These terminal ganglia (NOT the sympathetic chain!) are located close to, or within the structures innervated by the short postganglionic fibers (do you remember Auerbach’s plexus [myenteric] and Meissner’s plexus [submucosal] within the gut?).Preganglionic parasympathetic visceromotor fibers from the vagus activate postganglionic neurons in ganglia associated with the pharynx, larynx and esophagus. Short postganglionic parasympathetic fibers in turn innervate GLANDS and SMOOTH MUSCLE in these structures Cardiac branches of the vagus carrying visceromotor fibers synapse on ganglia within the cardiac plexuses (superficial and deep). While some investigators have shown that stimulation of the right and left vagi have different effects upon the heart, I will leave those differences for the cardiovascular section of your physiology course. For now I want you to know that vagal stimulation slows heart rate. For our PROBLEM SOLVING exercises a unilateral lesion of either dorsal motor X (right or left) will result in an INCREASE IN HEART RATE (TACHYCARDIA). This increase in heart rate is the result of losing input from the dorsal motor nucleus, which itself slows the heart (the dorsal motor X is sometimes called the “cardioinhibitory center”). The sympathetic portion of the autonomic nervous system is left in control. The preganglionic sympathetic fibers arise from the lateral cell column of spinal cord segments T1-T5 and synapse in the three cervical sympathetic ganglia. Postganglionic sympathetic fibers arising from these ganglia pass through the cardiac plexuses and innervate sinoatrial and atrioventricular nodal tissue, conducting tissue and ventricular myocardium. Stimulation of the sympathetics increases nodal firing rate, conduction rate and ventricular force. The brain stem input to the lateral cell column will be discussed later under Point #11 (nucleus solitarius).REMEMBER:LESION OF DORSAL MOTOR X = INCREASE IN HEART RATEREAD ON ONLY IF YOU ARE INTERESTED. I WILL NOT, REPEAT, WILL NOT ASK YOU ANY QUESTIONS ABOUT WHAT FOLLOWS.The visceromotor fibers from the vagus that reach the stomach and gut end in postganglionic neurons that lie near (gastric plexus) or in (myenteric and submucosal plexuses) the organs. Stimulation of the dorsal motor nucleus results in increased peristalsis and secretion of gastric and intestinal juices, and relaxation of sphincters. You might remember from Gross Anatomy that the right and left vagus nerves exchange fibers on the outer surface of the esophagus. They then enter the abdomen as anterior (or ventral) and posterior (or dorsal) vagal trunks. There is considerable mixing of the right and left vagi as they innervate the stomach and intestine.Afferent sources (inputs) to the dorsal motor nucleus include the hypothalamus, olfactory system, autonomic centers in the reticular formation, and especially the nucleus solitarius. Most of these afferent sources you have never heard of. So, right now remember that the dorsal motor nucleus X plays an important role in various visceral reflexes. Thus, information about the “internal milieu” reaches the dorsal motor nucleus of X via visceral afferent pathways that we will soon talk about. The dorsal motor nucleus X receives these afferent messages and then sends information to the appropriate organ(s) (via the terminal ganglia).
Figure 23-3 Cross sections of the brainstem. Note the principal nuclei of the reticular formation as well as other (nonreticular) nuclei situated adjacent to them. Many of these (other) nuclei make synaptic connections with nuclei of the reticular formation. (A) Medulla at level of rostral aspect of inferior olivary nucleus. (B) Level of caudal pons at the level of the abducens and facial nerves. (C) Rostral pons at level of the nucleus locus ceruleus and superior cerebellar peduncle. (D) Oblique section in which the dorsal aspect is at the level of the inferior colliculus of the caudal midbrain and the ventral aspect is at the level of the rostral aspect of the basilar pons. (Modified with permission from Barr ML, and Kiernan JA: The Human Nervous System, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 1993, p. 153.)
Figure 23-1 Position of the reticular formation in the brainstem. Diagrams illustrate the approximate positions of the reticular formation within the (A) midbrain, (B) pons, and (C) medulla as indicated by the dotted areas. Larger-sized dots represent magnocellular (large-celled regions), and smaller-sized dots represent parvocellular (small-celled regions).Afferent ConnectionsCranial nerve nuclei and secondary sensory fibers form an outer shell that surrounds the reticular formation (and particularly the parvocellular region) on its dorsal, lateral, and ventral sides (Figs. 23-2 and 23-3). Because of this arrangement, the reticular formation is strategically positioned so that the various sensory systems can easily provide inputs into it. In this manner, the reticular formation is endowed with a capacity to modulate a wide variety of sensory processes.The results of various studies have shown that other types of sensory signals can also produce cortical excitation by virtue of their connections with the brainstem reticular formation. On the basis of these studies dating back to the period of the 1940s and 1950s, the term â€œreticular activating systemâ€ was proposed as a mechanism by which sensory and other inputs to the reticular formation can modulate cortical neurons, cortical excitability, and states of consciousness. The importance of the reticular activating system is considered later in this chapter
Figure 23-8 The ascending connections of the reticular formation and inputs into the reticular formation from lower levels of the CNS. The reticular formation receives spinoreticular fibers (shown in red). The ascending reticular fibers project either directly to the intralaminar nuclei (shown in red) or indirectly through an interneuron from the solitary nucleus to the dorsolateralpons first (shown in blue); neurons from intralaminar nuclei then project directly to the cortex (shown in red) or to specific thalamic nuclei, which then project to the cerebral cortex (not shown in this diagram). By either direct or indirect routes, inputs from the reticular formation can influence cortical activity and the transmission of sensory signals to the cortex.Other Secondary Sensory SystemsAll sensory systems contribute fibers that ultimately make their way into the reticular formation. Trigeminal (for somatosensory information), solitary (for taste information), auditory, and vestibular nuclei lie adjacent to the reticular formation. Information is thus transmitted to the reticular formation from these structures, either as a result of the dendriticarborizations of neurons whose cell bodies lie in the lateral aspect of the reticular formation but which extend into these regions or by short axons that arise from these sensory regions that pass into the lateral aspects of the reticular formation.In addition to these sensory inputs, impulses mediated from regions associated with olfactory and visual systems also contribute inputs to the reticular formation. Secondary olfactory signals are received by a number of limbic structures (amygdala, hippocampal formation, and septal area) that have known projections to the reticular formation (see page 415 and Chapters 24 and 25). Visual signals can reach the superior colliculus via optic tract fibers. From the superior colliculus, fibers project directly to the reticular formation of the midbrain and pons.Thus, it is evident that the reticular formation receives multiple sensory inputs from all sensory systems. The reticular formation cannot maintain the specificity of the information that it receives from each of these sensory systems. Therefore, information of a nonspecific nature is transmitted via ascending fibers to the thalamus; this is in contrast to the specific transmission lines characteristic of other ascending sensory systems such as the dorsal column-medial lemniscus pathway. As indicated earlier, an important function of these signals that enter the reticular formation is to allow the reticular activating system to alter excitability levels of target neurons in the thalamus.Motor SystemsIt is clear that the reticular formation plays an important role in the processing of sensory information. The likely mechanisms are discussed in the following sections. As noted in previous chapters concerning motor systems, the reticular formation also plays an important role in the regulation of motor responses. The magnocellular nuclei of the reticular formation of the medulla and pons receive significant inputs from two key regions associated with motor functions: the cerebellum and sensorimotor cortex.CerebellumThe reticular formation and cerebellum share reciprocal connections, which complete a circuit comprising feedback pathways between these two regions for the regulation of motor functions associated with each of these two structures. Cerebellar fibers that project to the reticular formation of the medulla and pons arise from the fastigial nucleus. The fibers are both crossed and uncrossed and use the uncinate fasciculus (the pathway that passes just dorsal to the superior cerebellar peduncle).Cerebral CortexFibers from the same region of the sensorimotor cortex (areas 4, 3, 1, and 2) that give rise to much of the corticospinal tract also give rise to corticoreticular fibers. Corticoreticular fibers terminate in the pons and medulla near cell groups that give rise to the reticulospinal tracts (nucleus reticularispontisoralis and nucleus reticularispontiscaudalis for the pons, and nucleus gigantocellularis for the medulla) (Fig. 23-5). In this way, corticoreticular fibers can influence both voluntary as well as reflex motor functions by acting on those neurons of the reticular formation that control extensor motoneuron activity.Autonomic (and Higher Order Visceral Regulatory) RegionsThe reticular formation receives autonomic inputs from different sources. One set of sources includes primary afferent fibers contained in cranial nerves (CN) IX and X. The other source includes fibers arising from higher order autonomic integrative regions, which include the hypothalamus, as well as parts of the limbic system.Cranial Nerve (Autonomic) NucleiAs indicated in Chapter 14, fibers of the glossopharyngeal and vagus nerves (CN IX and X) transmit both chemoreceptor and baroreceptor afferent signals to the brainstem. The signals arise from the aortic and carotid bodies (chemoreceptors) and the aortic arch and carotid sinus (baroreceptors) and terminate in the solitary nucleus. Secondary neurons located in the solitary nucleus then project to the nucleus ambiguus and the ventrolateralmedullary depressor areas located in the medullary reticular formation. As noted earlier, this information is important for reflex regulation of blood pressure and respiration. Autonomic regions of the brainstem reticular formation that provide basic mechanisms for the control of blood pressure, heart rate, and respiration were considered in Chapter 22.
Figure 23-5 Corticoreticular projections. Sagittal view of the brain depicting the principal projections of fibers from the cerebral cortex to the reticular formation (shown in red). The largest majority of fibers arise from the motor and premotor cortices. The primary targets of these projections include the nucleus reticularispontisoralis and nucleus reticularisgigantocellularis of the medulla, which give rise to the medial (green) and lateral (blue) reticulospinal tracts, respectively, and play important roles in regulating muscle tone.Forebrain Regions (Hypothalamus and Limbic System)Several different pathways that arise from both lateral and medial regions of the hypothalamus project downstream and make synaptic connections in different parts of the reticular formation, mainly the midbrain and pons. These regions of hypothalamus project most heavily to the midbrain PAG and neighboring regions of the dorsal and lateral tegmental fields of the reticular formation. Additional groups of fibers from the amygdala supply the PAG, tegmentum, and lower brainstem region (in and adjacent to the solitary nucleus). Other limbic structures also contribute fibers to the reticular formation. These include inputs from the prefrontal cortex and amygdala that are mainly directed to the midbrain PAG. These descending inputs provide a higher order control of central autonomic functions of the brainstem reticular formation.Efferent ProjectionsOrganizational ConsiderationsAs mentioned earlier, the reticular formation receives a variety of inputs from disparate regions of the central nervous system (CNS) that mediate different functions. The varied functions of the reticular formation are even more dramatically reflected by the nature of the outputs. Features concerning the basic organization of the neurons in the reticular formation should be pointed out before describing the output pathways of the reticular formation.(1) The reticular formation contains cells whose axons travel long distances. Some ascend to the forebrain, and others descend to the spinal cord or project to the cerebellum. (2) The efferent fibers that travel long distances arise from the medial two thirds of the reticular formation, and those that travel only short distances as interneurons lie mainly in the lateral third of the reticular formation. (3) The main dendritic branches of the neurons are oriented in a plane perpendicular to the long axis of the brainstem (Fig. 23-6). This arrangement increases the probability that ascending and descending fibers of the reticular formation will make synaptic contact with other regions of the CNS. (4) Cells situated in the medial two thirds of the reticular formation of the medulla and pons give rise to bifurcating axons that travel for long distances in both directions (Fig. 23-7). By virtue of axon collaterals, each of these neurons can make synaptic contact with the other, thus providing an additional (sensorimotor) integrating mechanism by which signals transmitted downstream from the reticular formation can be synchronized with those projecting upstream.Pathways to Regions Mediating Sensory Functions and Effects on Cortical Excitability LevelsThere are at least three ways in which the reticular formation can modulate sensory functions and P.416cortical excitability. The first two mechanisms involve projections from the reticular formation to the intralaminar nuclei of the thalamus, and the third mechanism involves direct projections from monoaminergic neurons of the reticular formation to the cerebral cortex.
Figure 23-6 Ascending and descending axons in reticular formation. A sagittal section from the brainstem displays large cells in the magnocellular region of the reticular formation. Shown in this illustration is a neuron that bifurcates into an ascending and a descending branch. The branches give off collaterals to the structures adjacent to the reticular formation as well as to other nuclei of the reticular formation.Consider first the projections from the reticular formation to the thalamus. Fibers from nuclei within the reticular formation project to nonspecific thalamic nuclei, including the centromedian (CM) and parts of the ventral anterior (VA) nucleus, whose properties mimic those of a nonspecific thalamic P.417nucleus (for further discussion, see Chapter 26). In turn, these nonspecific thalamic nuclei project to wide regions of cortex directly or indirectly through a synapse in the ventral anterior nucleus (Fig. 23-8). Although the projection from the ventral anterior nucleus to the cortex is extensive, its projection is directed principally to the frontal lobe. Because of the widespread projections, activation of the nonspecific thalamic nuclei by the reticular formation can lead to changes in cortical neuron excitability levels.
Figure 23-13 Descending motor pathways to the spinal cord from the pons and medulla. Note also that corticoreticular fibers from motor and premotor cortices modulate the activity neurons of the reticular formation that give rise to the reticulospinal tracts. In turn, reticulospinal tracts modulate the activity of alpha and gamma motor neurons. Activation of the lateral reticulospinal tract inhibits spinal reflexes (-), and activation of the medial reticulospinal tract facilitates (+) spinal reflexes.
Figure 23-14 Effects of stimulation of reticular formation on spinal reflexes. Stimulation of the facilitory zone (+) (shown in green) of the reticular formation causes a dramatic increase in the patellar reflex as determined by EMG measurements, whereas marked suppression of this reflex follows stimulation of the inhibitory zone (-) (shown in red) of the reticular formation.Respiratory mechanisms involve nuclear groups located in both the dorsal and ventrolateral medulla and pons. Descending reticulospinal projections to the cervical cord make synaptic contact with somatic motor neurons of the phrenic nerve that innervate the diaphragm. This provides the efferent pathway by which the reticular formation can modify respiratory responses.Pathways Modulating Functions of the Hypothalamus and Limbic SystemProjections from different regions of the reticular formation (but most notably, the midbrain PAG) and neuronal groups containing monoamine neurons to the hypothalamus and limbic structures are now well established. These efferent projections probably play significant roles in the regulation of higher order autonomic, endocrine, and behavioral functions associated with these structures.The importance of these ascending fibers is demonstrated by two types of experiments. The first is that lesions of the reticular formation disrupt functions associated with the hypothalamus and limbic system. The second is that administration of drugs that block one or more of the monoamine systems can affect mood changes that are normally associated with the hypothalamus and limbic system.Sleep and WakefulnessMany regions of the CNS mediate sleep and wakefulness. These regions include specific nuclei of the reticular formation, thalamus, hypothalamus, and basal forebrain. In this section, we will concentrate on the involvement of the reticular formation in sleep and wakefulness.Stages of SleepMost of nighttime sleep (approximately 75%) is not associated with rapid eye movements (REM) (Fig. 23-15). This activity is mediated mainly by thalamocortical circuits. These circuits are ultimately activated by the brainstem. There is an increase in gamma-aminobutyric acid (GABA)-ergic neuronal activity in the anterior hypothalamus and a decrease in noradrenergic, serotonergic, and cholinergic activity. The neuronal activity affected by these neurotransmitters is likely mediated on the thalamus and cortex and is linked to non-REM sleep.The initiation of non-REM sleep begins with brain wave (EEG) slowing and vertex waves or high-voltage slow waves recorded from the vertex of the head, called drowsiness or stage 1 sleep. As sleep deepens, stage 2 evolves from stage 1 and is characterized by slowing of the EEG and clusters of 12 to 14 Hz rhythmic waves recorded in the central head region called sleep spindles. A combination of sleep spindles and K-complexes are high-voltage, irregular, slow waves also seen during stage 2 sleep. Stage 3 is characterized by high-voltage (75 ÂµV) slow waves of frequencies in the range of 2 Hz. Sleep is moderate to deep and spindle activity declines. Stage 4 sleep is characterized by more than 50% slow wave activity. The circuitry of lighter non-REM sleep consists of rhythmic activity generated by thalamic and cortical neurons. As sleep deepens, cortical neurons generate slow (delta frequency) waves. Stage 4 sleep is the most difficult stage in which to wake someone from, although sleep walking and talking are recorded from stage 3 or 4 sleep.Rapid Eye Movement or Paradoxical Sleep and Its Anatomical LociStudies have demonstrated that the pontine reticular formation is necessary for rapid eye movement (REM) or â€œdreamingâ€ sleep. These studies also demonstrated that the pons was also instrumental in the inhibition of muscle tone during REM sleep. Projections from the pontine reticular formation to the spinal cord are essential to prevent the â€œactingoutâ€ of dreams. Ponto-geniculo-occipital (PGO) waves, as measured in electrical recordings, occur in bursts during REM sleep and originate in the nucleus pontisoralis and project to the forebrain. These waves serve as a marker for REM sleep in the cat but have never been demonstrated in humans.Two pontine reticular nuclei initiate REM sleep. Because of their functional similarity, the pedunculopontine nucleus and the lateral dorsal nucleus are often considered to be one region. These neurons contain cholinergic nuclei that project to other regions of the reticular formation, hypothalamus, basal forebrain, and thalamus. When the cholinergic regions become activated, there is a change in the activity within the thalamus and cortex. Moreover, as indicated earlier, REM sleep is associated with activation of these cholinergic neurons and a reduction of catecholaminergic activity.Multiple regions modulate REM sleep. The locus ceruleus, a major norepinephrine cell group of P.423the rostral pontine reticular formation, projects to the thalamus and cerebral and cerebellar cortices. These connections provide the anatomical bases for the regulation of sensory functions of the cerebral cortex as well as cortical activation itself, which can affect REM sleep. The serotonergic region of the reticular formation, the raphe nuclei, innervates much of the forebrain. It has been hypothesized that serotonin contributes to the process of REM sleep. Consistent with this hypothesis is the observation that destruction or pharmacological depletion of serotonin levels in the brain results in long periods of wakefulness (insomnia). Replacement therapy with 5-hydroxytryptophan (the serotonin precursor) can restore normal states of slow-wave sleep.
Brain stem 2014
Dr P.S. Deb MD, DM
GNRC Hospitals Guwahati, Assam, India
DEVELOPMENT DIVISION OF NERVOUS SYSTEM
Four stages in early development of brain and cranial nerves (times are approximate). A:
31/2 weeks. B: 41/2 weeks. C: 7 weeks. D: 11 weeks
BRAIN STEM DEVELOPMENT
of the widening of the
central cavity in the
lower brain stem during
BRAIN STEM ANATOMY
Gross anatomy of brainstem and it's division