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46139954 ascending-sensory-pathways


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  • 1. Index Ascending sensory Introduction 3 pathways Sensory receptors 4 Done by: Mina Fouad Classification of receptors 4 Somatic Sensory Receptors 6 Special sensory receptors 10 Sensory pathways 12 Spinal Cord Organization 12 Reticular Formation 14 Anterolateral system 16 (ALS) References The dorsal column–medial 22 lemniscal (DCML) pathwayNeuroscience, 2nd edition by Dale Purves,George J Augustine, David Fitzpatrick, Lawrence CKatz, Anthony-Samuel LaMantia, James O McNa- The somatosensory pathways 24 to the cerebellummara, and S Mark WilliamsPrinciples of medical physiology by sabyasachisircar Trigeminal pathways 27Richer color experience in observers with multiplephotopigment opsin genes Visual Pathway 31KIMBERLY A. JAMESON and SUSAN M. HIGHNOTEUniversity of California at San Diego, La Jolla, Cali-fornia. Auditory Pathways 36A Textbook of NeuroanatomyMaria A. Patestas , Leslie P. Gartner Vestibular pathway 42Color Atlas of Neuroscience (Neuroanatomy andNeurophysiology) Ben Greenstein, Adam Green-stein Olfactory pathway 44The Human Nervous SystemStructure and Function Sixth Edition Gustatory pathway 46Charles R. Noback, Norman L. Strominger[ Ascending sensory pathways ]Review on neuroanatomy of ascending sensory pathways. 2
  • 2. IntroductionThe sensory system protects a person by detecting changes in the environment. Anenvironmental change becomes a stimulus when it initiates a nerve impulse, which then travelsto the central nervous system (CNS) by way of a sensory (afferent) neuron. A stimulus becomes asensation (something we experience) only when a specialized area of the cerebral cortexinterprets the nerve impulse it generates. Many stimuli arrive from the external environmentand are detected at or near the body surface. Others, such as stimuli from the viscera, originateinternally and help to maintain homeostasis.Classification of sensation: sensations special visceral somatic sensations sensations sensations -vision. -hearing. Cortical Deep Superficial -smell. sensations sensations sensations -taste. -tactile localization. -vibration. -Pain. -2 point -joint sense. discrimination. -muscle sense. -temperature. -stereognosis. -nerve sense. -touch. -graphosthesia. -perceptual rivalry. 3
  • 3. Sensory receptorsWhat are sensory receptors?A sensory receptor is a part of a sensory neuron or cell that receives information from astimulus in the internal or external environment of an organism and relates it to nervoussystem.Classification of receptors:By complexity: 1. Free nerve endings are dendrites whose terminal ends have little or no physical speciali- zation. 2. Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue. 3. Sense organs (such as the eyes and ears) consist of sensory neurons with receptors for the special senses (vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues.By location: 1. Exteroceptors occur at or near the surface of the skin and are sensitive to stimuli occurring outside or on the surface of the body. These receptors in- clude those for tactile sensations, such as touch, pain, and temperature, as well as those for vision, hearing, smell, and taste. 2. Interoceptors (visceroceptors) respond to stimuli occurring in the body from visceral organs and blood vessels. These receptors are the sensory neurons associated with the autonomic nervous system. 3. Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, li- gaments, and joints. These receptors collect information concerning body po- sition and the physical conditions of these locations.By type of stimulus detected: 1. Mechanoreceptors touch, pressure, vibrations, stretch. 2. Thermoreceptors sensitive to temperature changes. 3. Photoreceptors - retina of the eye. 4. Chemoreceptors- respond to chemicals in solution, molecules smelled or tasted changes in blood chemistry. 5. Nociceptors - respond to potentially damaging stimuli that result in pain. Virtually all receptors function as nociceptors at one time or another. (Excessive heat, cold, pressure and chemicals released at site of inflammation) 4
  • 4. Exteroceptors proprioceptors Interoceptors General Special General Photoreceptors rods &cones Mechanoreceptor Mechanoreceptor Mechanoreceptor hair cells in cochlea baroreceptorsSuperficial Deep Chemorecptors Chemorecptors olfactory & gustatory glucoreceptorsslowly adapting slowly adaptingmerkels disc ruffini osmoreceptorsrapidly adapting rapidly adaptingmeissener pacinian Thermoreceptors General special Nociceptors Mechanoreceptor Mechanoreceptor hair cells in semicircular canals& Golgi tendon otolith organs muscle spindle joint capsule Sensory nerve endings in the skin. 5
  • 5. Somatic Sensory Receptors Receptor Anatomical Associated Location Rate of Threshold type characteristics axons & function adaptation of activation Minimally C(paleospinothalamic -All skin specialized tract) nerve -Free nerve end- endings. ings can detect Diameter: Myelin: Velocity: temperature, me- 0.2- No 0.5-2.0 chanical stimuli 1.5 µm m/s (touch, pressure, stretch) or pain slow high (nociception). Thus, different Aδ(Neospinothalamic free nerve end- Tract) ings work asFree nerve endings (FNE) thermoreceptors, Diameter Myelin: Velocity: :1-5 µm Thin 3–30 m/s cutaneous me- chanoreceptors and nociceptors. In other words, they express po- lymodality. Encapsulated Aβ -They are distri- between buted throughout dermal Diame- Myelin: Velocity: the skin, but con- papillae ter: centrated in fin- Yes 33–75 gertips, palms, 6-12 µm m/s soles, lips, ton- Rapid Low gue, face and the skin of the male and female genit- als.Meissner corpuscle - Touch, pres- sure, low- frequency vibrations (30–50 Hz) that occur when textured objects are moved across the skin. 6
  • 6. Encapsulated; Aβ -Subcutaneous onion like tissue, interos- covering Diame- Myelin: Velocity: seous mem- ter: branes, viscera Yes 33–75 6-12 µm m/s - Deep pressure, Rapid Low vibration (high frequencies).Pacinian corpuscles Encapsulated; Aβ - All skin, hair associated follicles with peptide- Diame- Myelin: Velocity: releasing cells. ter: - Touch Yes 33–75 6-12 µm m/s Slow Low Merkel disc Encapsulated; Aβ oriented along stretch lines Diame- Myelin: Velocity: -All skin. ter: Yes 33–75 - Stretching of Slow Low 6-12 µm m/s skin.* Ruffini Endings Encapsulated Aβ -Lips, tongue, and genitals. Diame- Myelin: Velocity: ter: Yes 33–75 -Responds to 6-12 µm m/s pressure.* KRAUSECORPUSCLE 7
  • 7. Aβ - Wraps around hair follicle. Diame- Myelin: Velocity: ter: Yes 33–75 6-12 µm m/s - Responds to hair displace- Rapid ment.Hair FollicleEnding Highly specia- Type Ib -Tendons. lized. -Aα - 13-20 µm - 80–120 m/s -myelinated - Muscle ten- sion Slow LowGolgi tendon or-gans Highly specia- -Muscles. lized. -Type Ia 1ry Respond to the - Muscle length. Muscle -Aα rate of change spindle - 13-20 µm in muscle Both slow - 80–120 length, as well m/s to change in and rap- -myelinated length id Low Type II 2ry Respond only (Aβ) to changes in lengthJoint receptors Minimally Joints specialized Rapid Low Joint position 8
  • 8.  classifying axons according to their conduction velocity. Three main categories were discerned, called A, B, and C. A comprises the largest and fastest axons, C the smallest and slowest. Mechanorecep- tor axons generally fall into category A. The A group is further broken down into subgroups designated α (the fastest), β, and δ (the slowest). To make matters even more confusing, muscle afferent axons are usually classified into four additional groups—I (the fastest), II, III, and IV (the slowest)—with subgroups designated by lowercase roman letters! Touch, pressure & vibration are different form of the same sensation, pressure is felt when force applied on the skin is sufficient to reach the deep receptors whereas touch is felt when force is insufficient to reach the deep receptors. Vibration is rhythmic variation in pressure, whether the tactile receptor senses pressure or vibration depends on whether the receptor is rapidly adapting or slowly adapting. The higher the adaption rate of receptor the higher vibration frequencies it can detect. *= Skin thermoreceptors (hot and cold receptors) detect changes in environmental temper- ature. Some scientists believe that Ruffinis corpuscles (hot) and Krauses end bodies (cold) act as skin thermoreceptors. Other scientists are convinced that the receptors are naked nerve endings and that Ruffinis corpuscles and Krauses end bodies are mechanoreceptors. 9
  • 9. Special Sensory ReceptorsSpecial sensory receptors are distinct receptor cells. They are either localized within complexsensory organs such as the eyes and ears, or within epithelial structures such as the taste budsand olfactory epithelium. Receptor Location and function Comment Photo receptors Rod cell Cones are less sensitive to light than the rod cells in the retin but allow the perception of color. They Location Retina are also able to perceive finer de- tail. Because humans usually have Function Low light three kinds of cones which have different response curves and thus photoreceptor respond to variation in color in dif- ferent ways, they have trichromat- cones ic vision. Being color blind can change this, and there have been Location Retina reports of people with four types of cones, giving them tetrachro- Function Bright matic vision. light photoreceptor perception of color Hair cells in organ of Hair cells are located within corti the organ of Corti on a thin basilar membrane in the coch- lea of the inner ear. They amplify sound waves and transduce auditory informa- tion to the Brain Stem. 10
  • 10. Equilibrium Ampulla found in the semicircular canals In each ampulla is a small ele- vation called a crista. Each cris- for Dynamic equilibrium ta is made up of hair cells. Saccule : is responsible for Maculae vertical acceleration Saccule Utricle: Is responsible for horizontal acceleration Utricle Taste buds concentrated on the upper sur- There are five primary taste sensa- face of the tongue. tions:  salty  sour  sweet detect the flavor of substances  bitter  umami A single taste bud contains 50– 100 taste cells representing all 5 taste sensations (so the classic textbook pictures showing sepa- rate taste areas on the tongue are wrong)Olfactory receptor neuron Location olfactory epithelium in the nose Bipolar sensory receptor Function Detect traces of chemi- cals in inhaled air (sense of smell) 11
  • 11. Sensory pathwaysAnatomically, the ascending sensory systems consist of three distinct pathways:1- The anterolateral system (ALS)relays predominantly pain and temperature sensation, as well as nondiscriminative (crude or poorlylocalized) touch.2- The dorsal column–medial lemniscal (DCML) pathwayrelays discriminative (fine) tactile sense, vibratory sense, and position sense.3- The somatosensory pathways to the cerebellumrelay primarily proprioceptive (but also some pain and pressure) information.Spinal Cord Organization:The spinal cord is composed of a column of graymatter surrounded by a sheath of white matter.Gray matter is composed of neurons, theirprocesses, and neuroglia. It is the large number ofnerve cell bodies that is responsible for thegrayish appearance of the gray matter. Whitematter is composed of myelinated andunmyelinated processes of neurons, neuroglia,and blood vessels, and it is the white coloration ofthe myelin that gives white matter its name.The white matter consists of the ascending and descending pathways or tracts. The white matter hasbeen arbitrarily divided into three main sections, namely the dorsal, lateral, and ventral funiculi. Thewhite matter of the cord is organized into pathways that separate the transmission of differentsensations. 12
  • 12. All sensory information enters the spinal cord through the dorsal roots. Where the dorsal root fibers enter the spinal cord at the dorsal root entry zone, these separate into two divisions, the medial and lateral divisions. The medial division fibers are of relatively larger diameter than those in the lateral division (alpha-beta fibers); these transmit information of discriminative touch, pressure, vibration, and conscious proprioception originating from spinal levels C2 through S5. The lateral division of the dorsal root contains lightly myelinated delta fiber and unmyelinated axons C fiber of small diameter. These transmit pain, temperature and crude touch sensation from the body. The gray matter composed of neurons, their processes, and neuroglia, is subdivided into the ventral, dorsal, and lateral columns. Although the gray matter is completely surrounded by white matter, the dorsal horn approaches the limit of the spinal cord and is separated from the dorsolateral sulcus by a small bundle of nerve fibers, known as the dorsolateral tract (of Lissauer).The gray matter of the spinal cord can be organized into 9 layers plus the region surrounding the central canal, named Rexed laminae I–X, after the Swedish neuroanatomist who mapped out their distribution.Rexed Extent Neuronal Column Functionlamina group Marginal Dorsal receives afferent fibers carrying pain, temperature, and light I C1–S5 zone gray touch sensations. It also contributes fibers for the lateral and nucleus ventral spinothalamic tracts. C1–S5 Substantia gelati- Dorsal It relays pain, temperature and mechanical (light touch) in- II nosa of Rolando gray formation.III, C1–S5 Nucleus Dorsal receives pain, light touch, and temperature sensations andIV proprius gray provides input to the lateral and ventral spinothalamic,(V..?) tracts. VI C1–S5 ----- Dorsal This deepest layer of the dorsal horn contains neurons that gray respond to mechanical signals from joints and skin.VII C8–L3 Nucleus dorsalis Dorsal receive synapses from proprioceptive fibers, which bring (Clarke’s column) gray information from Golgi tendon organs and muscle spin- dles. Some of the axons of these large nerve cell bodies tra- vel in the dorsal spinocerebellar tracts Lateral contains preganglionic sympathetic neurons. T1–L2 Lateral nucleus gray (or L3) Sacral Lateral These preganglionic neurons of the sacral outflow of the S2–S4 parasympathetic nuc- gray parasympathetic nervous system leus (Onufrowicz) 13
  • 13. VIII C1–S5 -------------- Ventral grayIX C1–S5 motor neuron Ventral subdivided into three groups: medial, central, and lateral groups gray groups X C1–S5 -Gray commissure Peri- This represents the small neurons around the central canal. central -Substantia canal gelatinosa centralis Rexed classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups . Laminae I to IV, in general, are concerned with exteroceptive sensation. laminae V: Lamina VII are concerned primarily with proprioceptive sensations. laminae VIII-IX comprise the ventral horn and contain mainly motor neurons. Lamina X surrounds the central canal and contains neuroglia.Reticular Formation:The reticular formation consists of interconnected circuits of neurons in the tegmentum of thebrain stem, the lateral hypothalamic area, and the medial, intralaminar, and reticular nuclei ofthe thalamus.More than 100 nuclei scattered throughout the tegmentum of the midbrain, pons, and medullahave been identified as being part of the brainstem reticular formation Although the nuclei of thereticular formation have a number of diverse functions, they are classified according to thefollowing four general functions:1 -The regulation of the level of consciousness, and ultimately cortical alertness.2 -The control of somatic motor movements.3 -The regulation of visceral motor or autonomic functions.4 -The control of sensory transmission. 14
  • 14. Anatomically, the reticular formation is divided into four longitudinal zones (columns) on the basisof their mediolateral location inthe brainstem.The zones of thereticular formation are:- The unpaired median zone:also known as the median column,midline raphe ,The neurons of the median zone thatproject to higher brain centers areassociated with sleep-The paired paramedian zone:Via their connections with the cerebralcortex, cerebellum, vestibular nuclei, andspinal cord, the nuclei of the paramedianzone function in feedback systemsassociated with intricate movements.-The paired medial zone:The neurons of the medial zone influencethe ANS, level of arousal, and motorcontrol of the axial and proximal limbmusculature- The paired lateral zone: The lateral zonereceives sensory information, integratesit, and then relays it to the medial zone.The medial zone then mediates the modulation of sensory afferent input and maintenance of alertness.Some authors consider the median and paramedian zones to be one zone. 15
  • 15. Anterolateral system (ALS)The anterolateral system (ALS) transmits nociceptive, thermal, and nondiscriminatory (crude) touchinformation into higher brain centers. Crude touch Pain from the body temperatureReceptor Free nerve endings, Aδ and C Free nerve Aδ and C Free nerve Merkel’s discs, Fiber endings Fiber endings peritrichial nerve endings1st Peripheral Receive the sensation from Thinly myelinated Aδ (fast- Lightly myelinated Aδ the receptors. conducting) fibers, which fibers cold stimuliorder process relay sharp, short-term,neuron well-localized pain C fibers warm stimuli Or Unmyelinated C (slow- conducting) fibers which relay dull, persistent, poorly localized pain located in a dorsal root gan- Cell body located in a dorsal root located in a dorsal root glion. ganglion. ganglion. enter the spinal cord at the enter the spinal cord at the enter the spinal cord at the dorsal root entry zone, via dorsal root entry zone, via dorsal root entry zone, via the lateral division of the the lateral division of the the dorsal roots of the dorsal roots of the spinal dorsal roots of the spinal spinal nerves, and upon nerves, and upon nerves, and upon entry collectively form the entry collectively form the entry collectively form the dorsolateral fasciculus dorsolateral fasciculus dorsolateral fasciculus (tract of Lissauer), These (tract of Lissauer), These (tract of Lissauer), These Centeral central processes bifurcate central processes bifurcate central processes bifurcate into short ascending and into short ascending and into short ascending and process descending branches. descending branches. descending branches. These branches either as- These branches either as- These branches either as- cend or descend one to cend or descend one to cend or descend one to three spinal cord levels three spinal cord levels three spinal cord levels within this tract, to termi- within this tract, to termi- within this tract, to termi- nate in their target lami- nate in their target laminae nate in their target lami- nae of the dorsal horn, of the dorsal horn, where nae of the dorsal horn, where they synapse with they synapse with where they synapse with second order neurons (or second order neurons (or second order neurons (or with interneurons). with interneurons). with interneurons). 16
  • 16. 2nd order neuron The cell bodies of the second order neurons reside in the dorsal horn of the spinal cord Recent findings indicate that the axons of these second order neurons course in either the direct (spinothalamic) or indirect (spinoreticular) pathways of the ALS, or as three sets of fibers (the re- maining components of the ALS): the spinomesencephalic, spinotectal, or spinohypothalamic fibers. Direct pathway of the anterolateral system: Type Aδ fibers of first order neurons synapse primarily with second order neurons in lamina I (post- eromarginal nucleus) and lamina V (reticular nuc- leus) of the spinal cord gray matter. However, many first order neurons synapse with spinal cord interneurons that are associated with reflex motor activity. The axons of the second order neurons flow across the midline to the contralateral side of the spinal cord in the anterior white commissure, forming the spinothalamic tract which continues up in the brainstem as spinal lemniscus to end in : contralateral ventral posterior lateral nucleus of the thalamus.(P.L.V.N.T ) It also sends some projections to the ventral post- erior inferior (VPI), and the intralaminar nuclei of the thalamus. It also sends collaterals to the reti- cular formation. Since the spinothalamic tract (direct pathway) is phylogenetically a newer pathway, it is referred to as neospinothalamic pathway. Spinothalamic tract actually consists of two anatomically distinct tracts: the lateral spinothalamic tract (located in the lateral funiculus) and the very small anterior spinothalamic tract (located in the anterior funiculus). Earlier studies indicated that the lateral spinothalamic tract transmitted only no- ciceptive and thermal input, whereas the anterior spinothalamic tract transmitted only nondiscri- minative (crude) touch. Recent studies however, support the finding that both the anterior and later- al spinothalamic tracts (as well as the other component fibers of the ALS: spinoreticular, spinome- sencephalic, spinotectal, and spinohypothalamic), transmit nociceptive, thermal, and nondiscrimina- tive(crude) tactile signals to higher brain centers. 17
  • 17. Indirect pathway of the anterolateral system: Type C fibers of first order neurons terminate on interneurons in laminae II (substantia gelatinosa) and III of the dorsal horn. Axons of these interneurons synapse with second order neurons in lami- nae V–VIII. Many of the axons of these second order neurons ascend ipsilaterally, however a small number of axons sweep to the opposite side of the spinal cord in the anterior white commissure. These axons form the more prominent ipsilateral and smaller contralateral spinoreticular tracts. The spinoreticular tracts transmit nociceptive, thermal, and nondiscriminatory (crude) touch signals from the spinal cord to the thalamus indirectly, by forming multiple synapses in the reticular formation prior to their thalamic projections. Since the spinoreticular tract (indirect pathway) is phylogenetically an older pathway, it is referred to as the paleospinothalamic pathway. Other component fibers of the anterolateral system: The spinomesencephalic fibers terminate in the periaqueductal gray matter and the midbrain raphe nuclei, both of which are believed to give rise to fibers that modulate nociceptive transmission and are thus collectively referred toas the “descending pain-inhibiting system”. Furthermore, some spinomesencephalic fibers terminate in the parabrachial nucleus, which sends fibers to the amygdala—a component of the limbic system associated with the processing of emo- tions. Via their connections to the limbic system, the spinomesencephalic fibers play a role in the emotional component of pain. The spinotectal fibers terminate mainly in the deep layers of the superior colliculus. The superior colliculi have the reflex function of turning the upper body, head, and eyes in the direction of a pain- ful stimulus. The spinohypothalamic fibers ascend to the hypothalamus where they synapse with neurons that give rise to the hypothalamospinal tract. This pathway is associated with the autonomic and reflex responses (i.e., endocrine and cardiovascular) to nociception.3rd order neuron Cell bodies of third order neurons are housed in: the ventral posterior lateral, the ventral posterior inferior, and the intralaminar thalamic nuclei The ventral posterior lateral nucleus gives rise to fibers that course in the posterior limb of the inter- nal capsule and in the corona radiata to terminate in the postcentral gyrus (primary somatosensory cortex, S-I) of the parietal lobe of the cerebral cortex. Additionally, the ventral posterior lateral 18
  • 18. nucleus also sends some direct projections to the secondary somatosensory cortex, S-II The ventral posterior inferior nucleus projects mostly to the secondary somatosensory cortex (S-II), although some of its fibers terminate in the primary somatosensory cortex (S-I). The intralaminar nuclei send fibers to the striatum (the caudate nucleus and the putamen), the S-I and S-II, as well as to the cingulate gyrus and the prefrontal cortex.Visceral pain:Visceral pain is characterized as diffuse and poorly localized, and is often “referred to” and felt inanother somatic structure distant or near the source of visceral pain. Nociceptive signals from theviscera generally follow the same pathway as signals arising from somatic structures.General visceral afferent nociceptive information from visceral structures of the trunk is carriedmostly by type C, Aδ, or Aβ fibers. The peripheral terminals of these fibers are associated withPacinian corpuscles that respond to excessive stretching of the intestinal wall, a lesion in the wall ofthe gastrointestinal tract, or to smooth muscle spasm. The cell bodies of these sensory(pseudounipolar), first order neurons are housed in the dorsal root ganglia, and theircentralprocesses carry the information, via the dorsolateral fasciculus (tract of Lissauer), to the dorsalhorn and lateral gray matter of the spinal cord. Here, these central processes synapse with secondorder neurons as well as with neurons associated with reflex activities. The axons of the secondorder neurons join the anterolateral system to relay nociceptive signals from visceral structures tothe reticular formation and the thalamus. Fibers from the reticular formation project to theintralaminar nuclei of the thalamus, which in turn project to the cerebral cortex and thehypothalamus. Visceral pain signals relayed to the primary somatosensory cortex may beassociated with referred pain to a somatic structure. In addition to projections to thesomatosensory cortex, recent studies indicate that nociceptive signals are also relayed to theanterior cingulate and anterior insular cortices, two cortical areas implicated in the processing ofvisceral pain. 19
  • 19. Spinothalamic tract pathway 20
  • 20. Summary of ALS Receptors (free nerve endings) Peripheral processes of pseudounipolar neurons Cell bodies of type Aδ and type C pseudounipolar neurons (first order neurons) in dorsal root gangllia Central processes of pseudounipolar neurons collect to form Lateral division of dorsal root of spinal nerves enter spinal cord, at dorsal root entry zone and course in the Dorsolateral fasciculus (tract of Lissauer( as ascending and descending branches Direct pathway of the ALS Indirect pathway of the signals from Aδ fibers ALS signals from C fibers (tract of Lissauer) as as- cending and descending (substantia gelatinosa, lamina II) branches Laminae I and V Laminae II – IV and lamina III of dorsal horn of dorsal horn of dorsal horn synapse with Interneurons Second order neurons Interneurons formSpinothalamic tract )neospino- Motoneurons Second order neuronsthalamic pathway( decussate in Reflexes Spinoreticular tract)paleospino- Anterior white commissure thalamic pathway( terminate in Some fibers decussate in ante-P.L.V.N.T V.P.I Intralaminar Collaterals to rior white commissure nuclei reticular formation Many fibers ascend ipsilaterallyPosterior limb of theinternal capsule Striatum,S-I, S-II, cingu- Reticular formation late gyrus ,prefrontal Corona radiata cortex Intralaminar nuclei of the S1 S2 21 thalamus, hypothalamus, limbic cortex S2
  • 21. The dorsal column–medial lemniscal (DCML) pathwayIt relays discriminative (fine) tactile sense, vibratory sense, and position sense.Touch, pressure & vibration are different form of the same sensation, pressure is felt when force applied on theskin is sufficient to reach the deep receptors whereas touch is felt when force is insufficient to reach the deepreceptors. Vibration is rhythmic variation in pressure, whether the tactile receptor senses pressure or vibrationdepends on whether the receptor is rapidly adapting or slowly adapting. The higher the adaption rate of receptorthe higher vibration frequencies it can detect.Receptor • Free nerve endings responding to touch, pressure, and proprioception in the skin, muscles, and joint capsules. • tactile (Merkel’s) discs responding to touch and pressure in the skin; • peritrichial endings stimulated by touch of the hair follicles; • Meissner’s corpuscles activated by touch of the skin; and • Pacinian corpuscles stimulated by touch, pressure, vibration,and proprioception in the deep layers of the skin, and in visceral structures. st1 Peripheral These peripheral processes are medium-size type Aβ and large-size type Aα fibers.order processneuron Cell body Cell bodies are located in the dorsal root ganglia. Enter the spinal cord at the dorsal root entry zone via the medial division of the dorsal roots of the spinal nerves. Upon entry into the posterior funiculus of the spinal cord, the afferent fibers bifurcate into long ascending and short descending fibers. The long ascending and short descending fibers give rise to collateral branches that may synapse Centeral process with several distinct cell groups of the dorsal horn interneurons and with ventral horn motoneurons. These fibers collectively form the dorsal column pathways, either the fasciculus gracilis or the fasci- culus cuneatus, depending on the level of the spinal cord in which they enter. below level T6-gracilis include the lower thoracic, lumbar, and sacral levels that bring information from the lower limb and lower half of the trunk at level T6 and above cuneate bring information from the upper thoracic and cervical levels, that is from the upper half of the trunk and upper limb 22
  • 22. 2nd order neuron The first order fibers terminating in the nucleus gracilis and nucleus cuneatus in the medulla syn- apse with second order neurons whose cell bodies are housed in these nuclei The fibers of the second order neurons form the internal arcuate fibers as they curve ventromedially to the opposite side. These fibers ascend as the medial lemniscus in the brain stem to synapse with third order neurons in the posterior lateral ventral nucleus of the thalamus.(P.L.V.N.T) rd3 order neuron The posterior lateral ventral nucleus of the thalamus. (P.L.V.N.T) houses the cell bodies of the third order neurons of the DCML pathway. The fibers arising from the thalamus ascend in the posterior limb of the internal capsule and the corona radiate to terminate in the primary somatosensory cortex of the postcentral gyrus 23
  • 23. The somatosensory pathways to the cerebellumMost of the proprioceptive information does not reach conscious levels, and instead is transmitted directlyto the cerebellum via the ascending somatosensory cerebellar pathways without projecting to thethalamus or the cerebral cortex. These pathways, which process subconscious proprioception frommuscles, tendons, and joints, are two-neuron pathways, consisting of first order and second orderneurons.The pathways include:- dorsal (posterior) spinocerebellar tract.- The cuneocerebellar tract.- The ventral (anterior) spinocerebellar tract.- The rostral spinocerebellar tract. Dorsal Cuneocerebellar Ventral Rostral spinocerebellar tract. spinocerebellar spinocerebellar tract. tract. tract.1st Peripheral (pseudounipolar neurons) whose cell bodies are housed in the dorsal root gangliaorder process & send their peripheral processes to the skin, muscles, tendons, and joints. Here they Cell bodyneuron perceive proprioceptive information, which is then transmitted to the spinal cord by their central processes. These central processes ascend in the fasciculus transmit sensory input to synapse with 2nd join the medial division of cuneatus and terminate laminae V–VII of the order neurons the dorsal roots of the in the external lumbar, sacral, and. whose cell bodies Centeral spinal nerves to synapse in (accessory) cuneate coccygeal spinal cord reside in lamina VII process the nucleus dorsalis(Clark’s nucleus—the nucleus levels, where they of the dorsal horn column, lamina VII of spinal dorsalis of Clark terminate and synapse cord levels C8 to L2,3) at homologue at cervical with 2nd order neurons. their level of entry.Sensory levels above C8 information transmitted by spinal nerves entering below Clark’s column is relayed to the caudal extent of the nucleus dorsalis (L2,3) by ascending in the fasciculus gracilis. 24
  • 24. 2nd order neuron Clark’s column houses the The axons of the 2nd The axons of these 2nd The fibers of the 2nd cell bodies of 2nd order order neurons, whose order neurons, known as neurons form the neurons whose axons form cell bodies are housed spinal border cells, form primarily uncrossed the dorsal spinocerebellar in the accessory the ventral (anterior) spi- rostral spinocerebel- tract, which ascends ipsila- cuneate nucleus, form nocerebellar tract, which lar tract, the head terally in the lateral funicu- the cuneocerebellar decussates in the anterior and upper limb lus of the spinal cord. tract. This tract is re- white comissure and as- counterpart of the When this tract reaches the ferred to as the neck cends in the lateral funi- ventral brainstem it joins the and upper limb counter- culus of the spinal cord to spinocerebellar tract. restiform body (of the infe- part of the dorsal spino- the medulla. At pontine These fibers join the rior cerebellar pduncle), cerebellar tract. Fibers levels these fibers join restiform body (of and then passes into the of the cuneocerebellar the superior the inferior vermis of the cerebellum. tract join the restiform cerebellar peduncle to cerebellar peduncle) body (of the inferior pass into the vermis of to enter the cerebellar peduncle) the cerebellum. These cerebellum. and then enter the fibers then decussate Additionally, some anterior lobe of the ce- again to their actual side fibers pass into the rebellum ipsilaterally. of origin within the cerebellum via the cerebellum. superior cerebellar peduncle.Function 1-Relays proprioceptive 1-Relays proprioceptive 1-Relays proprioceptive 1-Relays propriocep- input from the ipsilateral information from input from the ipsilateral tive information from trunk and lower limb the ipsilateral neck and trunk and lower limb the ipsilateral head 2-Coordination of move- upper limb 2-Coordination of and upper limb ments of the lower limb 2-Movement of head movements of the lower 2-Movement of head muscles and upper limb limb muscles and upper limb 3-Posture maintenance 3-Posture maintenance 25
  • 25. The dorsalspinocerebellar tract and The cuneocerebellar tract the ventral spinocerebellar tract and the rostral spinocerebellar tract26
  • 26. Face sensation (trigeminal sensory pathway)The trigeminal nerve, the largest of the cranial nerves, provides the major general sensory innervation topart of the scalp, most of the dura mater, the conjuctiva and cornea of the eye, the face, nasal cavities,paranasal sinuses, palate, temporomandibular joint, lower jaw, oral cavity, and teeth.The trigeminal sensory pathway, which transmits touch, nociception, and thermal sensation, consists of athree neuron sequence (first, second, and third order neurons) from the periphery to the cerebral cortexrespectively.First order neuron:Cell bodies are housed in the trigeminal ganglion.The peripheral processes radiating from the trigeminal ganglion gather to form three separate nerves, thethree divisions of the trigeminal nerve whose peripheral endings terminate in sensory receptors of theorofacial region.Nearly half of the sensory fibers in the trigeminal nerve are Aβ myelinated discriminatory touch fibers. Theremaining half of the sensory fibers in the trigeminal nerve is similar to the Aδ and C nociceptive andtemperature fibers of the spinal nerves.Dvisions: ophthalmic , maxillary , and mandibular .The central processes of these neurons enter the pons and terminate in the trigeminal nuclei where theyestablish synaptic contacts with second order neurons housed in these nuclei.2nd order neuron:The trigeminal nuclei, with the exception of the mesencephalic nucleus, contain second order neurons aswell as interneurons.Trigeminal Sensory nuclei:• Main (chief, principal) nucleus: Is located in the midpons. It is homologous to the nucleus gracilis andnucleus cuneatus. It is associated with the transmission of mechanoreceptor information for discriminatory(fine) tactile and pressure sense.• Mesencephalic nucleus of the trigeminal: is unique, since it is a true “sensory ganglion” (and not anucleus). During development, neural crest cells are believed to become embedded within the CNS, 27
  • 27. instead of becoming part of the peripheral nervous system, as other sensory ganglia. This nucleus housesthe cell bodies of sensory (first order) pseudounipolar neurons, thus there are no synapses in themesencephalic nucleus. The peripheral large-diameter myelinated processes of these neurons conveygeneral proprioception input from the muscles innervated by the trigeminal nerve (and the extraocularmuscles, as well as from the periodontal ligament of the teeth. Pseudounipolar neurons of themesencephalic nucleus transmit general proprioception input to the main sensory and motor nuclei of thetrigeminal and reticular formation to mediate reflex responses.• Spinal nucleus of the trigeminal: is the largest nucleus consists of three subnucleiSubnucleus oralis: It is associated with the transmission of discriminative (fine) tactile sense from theorofacial region.Subnucleus interpolaris: is also associated with the transmission of tactile sense, as well as dental pain.Subnucleus caudalis: is associated with the transmission of nociception and thermal sensations from thehead. 28
  • 28. The trigeminal pathway for touch and pressure:-As the central processes of pseudounipolar (first order) neurons enter the pons, they bifurcate into:Short ascending fibers which synapse in the main sensory nucleusLong descending fibers which terminate and synapse mainly in the subnucleus oralis and lessfrequently in the subnucleus interpolaris-Fibers from the main sensory nucleus:Some 2nd order fibers from the main sensory nucleus cross the midline and join the ventral trigeminallemniscus to ascend and terminate in the contralateral VPM nucleus of the thalamus.Other second order fibers from the main sensory nucleus do not cross. They form the dorsal trigeminallemniscus, and then ascend and terminate in the ipsilateral VPM nucleus of the thalamus.-Fibers terminating in the subnucleus oralis or interpolaris synapse with second order neurons whosefibers cross the midline and ascend in the ventral trigeminal lemniscus to the contralateral VPM nucleus ofthe thalamus. 29
  • 29. Pain and thermal pathway: - As the central processes of pseudounipolar neurons enter the pons, they descend in the spinal tract of the trigeminal and most of them synapse in the subnucleus caudalis. Most of the second order fibers from the subnucleus caudalis cross the midline and join the contralateral ventral trigeminal lemniscus, whereas others join the ipsilateral ventral trigeminal lemniscus. All the fibers ascend to the VPM nucleus of the thalamus. 3rd order neuron: the ventral posterior medial (VPM) nucleus of the thalamus. The third order neurons then relay sensory information to the postcentral gyrus of the cerebral cortex for further processing. 30
  • 30. VISUAL PATHWAYThe visual pathway consists of photoreceptors, first order and second order neurons residing in theretina, and third order neurons in the lateral geniculate nucleus of the thalamus Photoreceptors Incoming light rays impinging on the retina cause the retinal photoreceptor cells (modified neurons), therods and cones, to become hyperpolarized. The photoreceptors then stop releasing neurotransmittersand the bipolar cells (first order neurons) are no longer inhibited. Bipolar cells )first order neurons(Bipolar cells (first order neurons) are no longer inhibited, and fire. The bipolar cells along with theinterneurons, the horizontal and amacrine cells, process, integrate, and modulate visual input. Thebipolar cells relay this sensory input to the ganglion cells (second order neurons) of the retina. Ganglion cells (second order neurons)possess nonmyelinated that course on the inner surface of the retina collect at Optic disc. Axons piercesclera in lamina cribrosa to emerge from the back of the bulb of the eye. At this point, the axons becomemyelinated and they form a large bundle, the optic nerve (CN II). Optic nerve 31
  • 31. The optic nerves of the right and left sides join superior to the body of the sphenoid bone in the middlecranial fossa to form optic chiasma. To form Optic chiasmawhere partial decussation of the optic nerve fibers (axons) of the two sides occurs. All ganglion cell axonsarising from the temporal half of the retina course in the lateral aspect of the optic chiasma withoutdecussating, to join the optic tract of the same side. All ganglion cell axons arising from the nasal half ofthe retina decussate at the optic chiasma, and enter the optic tract of the opposite side, to join thetemporal fibers. Thus, each optic tract consists of ganglion cell axons arising from both eyes (theipsilateral temporal half and the contralateral nasal half of the retina). Optic tractit courses around the cerebral peduncle to end and relay visual information primarily in the lateralgeniculate nucleus (LGN) of the thalamus, which processes visual input. The optic nerve also ends andrelays visual information in:(i) The superior colliculus, a mesencephalic relay nucleus for vision having an important function insomatic motor reflexes.(ii) The pretectal area, which mediates autonomic reflexes such as the control of pupillary constrictionand lens accommodation.(iii) The hypothalamus, which has an important function in circadian rhythms (day–night) and thereproductive cycle. Lateral geniculate nucleus (3rd order N.) 32
  • 32. The LGN houses the cell bodies of third order neurons of the visual pathway.The LGN is a laminatedstructure consisting of six distinct layers that are readily dentifiable in a horizontal section. Although eachLGN receives information from the contralateral visual hemifield, each of its layers receives input fromonly one eye.Layers 1, 4, and 6 receive ganglion cell axons arising from the contralateral retina.Layers 2, 3, and 5 receive ganglion cell axons arising from the ipsilateral retina.Layers 1 and 2 consist of large neurons and are therefore referred to as the magnocellular layers; theyreceive information from ganglion cells that are sensitive to movement and contrast but are insensitive tocolor.Layers 3–6 consist of small neurons and are referred to as the parvocellular layers; they receiveinformation from the ganglion cells responding to color and form. 33
  • 33. Axons of third order neurons originating from the LGN form the geniculocalcarine tract (optic radiations,thalamocortical projections) Geniculocalcarine Lract Join the Internal capsule Retrolenticular portion Sublenticular portion Cuneate gyrus Lingual gyrus Primary visual cortex 2ry visual cortex Tertiary visual cortex 34
  • 34. 35
  • 35. AUDITORY PATHWAYSSound waves transmitted via the Auricle (pinna) and external auditory meatus (canal) to the tympanicmembrane (eardrum) causing it to vibrate, vibrations transmitted via the Malleus (which is attached tothe tympanic membrane) Incus (which articulates with the malleus and stapes) Stapescausing it to oscillate, oscillating footplate attaches to the membrane of the oval window causing it tooscillate and in turn agitate the perilymph of the scala vestibule perilymph waves agitate thevestibular (Reissners membrane) which begins to oscillate generating waves in the Endolymph of thescala media (cochlear duct) endolymph waves cause the basilar membrane (which supports theorgan of Corti) to oscillate stimulating the Hair receptor cells which convert mechanical energy intoelectrical energy . Hair receptor cellsstimulating the Peripheral processes (dendrites) of the bipolar (first order) neurons whose cell bodies arehoused in the cochlear (spiral) ganglion . Cochlear (spiral) ganglion 1st order N.Impulses are transmitted to the central processes (axons) of the bipolar (first order) neurons which formthe root of the cochlear nerve axons leave the inner ear via the Internal auditory meatus (canal) to enterthe posterior cranial fossa then pierce the brainstem at the pontomedullary angle of the brainstem toterminate in the cochlear nuclei including:-The ventral cochlear nucleus is subdivided into a posteroventral cochlear nucleus and an anteroventralcochlear nucleus.- The dorsal cochlear nucleus Cochlear nuclei 2nd order N. 36
  • 36. Second order fibers arising from:1-anteroventral cochlear nucleus: Either -Ascend ipsilaterally to the medial and lateral superior olivary nuclei. -or decussate forming ventral acoustic striae to: • The medial nucleus of the trapezoid body, which in turn projects to the lateral superior olivary nucleus. • The medial superior olivary nucleus. • The dorsal nucleus of the lateral lemniscus and the inferior colliculus (by ascending in the contralateral lateral lemniscus)2-the posteroventral cochlear nucleus:form the intermediate acoustic stria. These fibers subsequently join the ipsilateral and contralateral laterallemniscus to ascend to, and terminate in, the ventral nucleus of the lateral lemniscus and the inferiorcolliculus, bilaterally 37
  • 37. 3-the dorsal cochlear nucleusform the dorsal acoustic stria, which decussates. These fibers join the contralateral lateral lemniscus toascend to, and terminate in, the inferior colliculus. 2nd neuron fibers and termination anteroventral cochlear nucleus: ipsilateraHy medial and lateral superior olivary nuclei. decussate ventral acoustic striae  dorsal nucleus of the lateral lemniscus and the inferior colliculus medial superior olivary nucleus lateral superior olivary nuclei via medial nucleus of trapezoid posteroventral cochlear nucleus:  intermediate acoustic stria ipsilateral and contralateral the ventral nucleus of the lateral lemniscus and the inferior colliculus. Dorsal cochlear nucleus:  dorsal acoustic striadecussate  the inferior colliculus. 38
  • 38. Superior olivary nuclei 3rd order N.The main nuclei of this complex are the medial superior olivary nucleus and the lateral superior olivarynucleus, both of which receive second order fiber terminals from the cochlear nuclei and have animportant function in sound localization in the following manner:The medial superior olivary nucleus processes auditory input by comparing the amount of time it takes fora sound to reach each ear.The lateral superior olivary nucleus processes auditory input by comparing the intensity (volume) of asound arriving at each ear.the fibers arising from the medial superior olivary nucleus join the ipsilateral lateral lemniscus, whereasthose that arise from the lateral superior olivary nucleus join the ipsilateral and contralateral laterallemniscus that terminate in the dorsal nucleus of the lateral lemniscus and in the superior colliculus. Inferior colliculus.It receives afferents ascending in the lateral lemniscus from the cochlear nuclei, the superior olivarynuclear complex, and the nuclei of the lateral lemniscus. The inferior colliculus also receives afferentsfrom the contralateral inferior colliculus.The inferior colliculus gives rise to fibers end in the ipsilateral medial geniculate nucleus, a thalamic relaystation of the auditory system. The inferior colliculus also projects to the contralateral medial geniculatenucleus and the superior colliculus (which is involved in visual reflexes). Medial geniculate nucleusFibers arising in the medial geniculate nucleus form the auditory radiations that join the sublenticularportion of the posterior limb of the internal capsule to terminate in the primary auditory cortex. Primary auditory cortex 39
  • 39. Hair receptor cells Cochlear (spiral) ganglion 1st order N.anteroventral cochlear posteroventral cochlear Dorsal cochlear nucleus cells nucleus cells nucleus cells Medial Lateral superior olivary Medial superior olivary nucleus of trapezoid nucleus nucleus dorsal nucleus of the ventral nucleus of the lateral lemniscus lateral lemniscus Inferior colliculus Ipsilateraly decussate Medial geniculate nucleus 40
  • 40. Lateral lemniscus contains the following fibers:1 -Second order fibers arising from the contralateral anteroventral cochlear nucleus (which do notsynapse in the superior olivary complex) that terminate in the dorsal nucleus of the lateral lemniscus andthe inferior colliculus.2- Second order fibers arising from the ipsilateral and contralateral posteroventral cochlear nucleus thatterminate in the ventral nucleus of the lateral lemniscus and in the inferior colliculus.3 -Second order fibers arising from the contralateral dorsal cochlear nucleus that terminate in the ventralnucleus of the lateral lemniscus and in the inferior colliculus.4- Third order fibers originating from the superior olivary nuclear complex (the fibers arising from themedial superior olivary nucleus join the ipsilateral lateral lemniscus, whereas those that arise from thelateral superior olivary nucleus join the ipsilateral and contralateral lateral lemniscus) that terminate in thedorsal nucleus of the lateral lemniscus and in the superior colliculus.5- Fibers arising from the dorsal and ventral nuclei of the lateral lemniscus that project to the ipsilateralinferior colliculus. 41
  • 41. Vestibular pathwayFirst order neuron:The cell bodies of the sensory first order bipolar neurons of the vestibular nerve reside within thevestibular ganglion of Scarpa.Their peripheral processes terminate in special receptors, the cristae in the ampullae of the semicircularducts and the maculae of the utricle and saccule.The central processes of these neurons enter the brainstem to synapse not only in the vestibular nuclearcomplex, where they synapse with second order neurons of the vestibular pathway, but also in thecerebellum. Some first order vestibular fibers, however, do not terminate in the vestibular nuclei, but takean alternate route by going around them, joining the juxtarestiform body in the inferior cerebellarpeduncle and terminating directly in the ipsilateral flocculonodular lobe of the cerebellum.The vestibular nerve is unique since it is the only cranial nerve that sends the central processes of someof its first order neurons to synapse directly in the cerebellum.Second order neuron:Vestibular nuclear complex: the vestibular nuclear complex is composed of four vestibular nuclei:1 The superior (Bechterew’s) vestibular nucleus.2 The medial (Schwalbe’s) vestibular nucleus.3 The lateral (Deiter’s) vestibular nucleus.4 The inferior (spinal, descending) vestibular nucleus.The superior and medial vestibular nuclei receive the first order neuron terminals relaying sensory inputfrom the cristae ampullares of the semicircular canals. Following the reception of this sensory input, thesenuclei then relay it via two structures:1 The medial longitudinal fasciculus (MLF) to the extraocular muscle nuclei to elicit compensatory ocularmovements triggered by movements of the head.2 The medial vestibulospinal tract to the cervical spinal cord to elicit suitable head movements.The lateral vestibular nucleus receives vestibular sensory input mainly from the maculae of the utricle, butmay also receive input from the saccule and semicircular canals. This nucleus projects via the lateralvestibulospinal tract to motoneurons or interneurons at all spinal cord levels to make posturaladjustments. 42
  • 42. The inferior vestibular nucleus receives vestibular sensory input from the semicircular canals as well asthe utricle. Most of the first order vestibular fibers terminate in this nucleus. It projects to the reticularformation and the cerebellum.3rd orden neuron:The superior and lateral vestibular nuclei give rise tosecond order fibers that join the MLF bilaterally toascend to the ventral posterior lateral and ventralposterior inferior nuclei of the thalamus.The thalamus gives rise to third order fibers thatterminate in the primary vestibular cortex (Brodmann’sarea 3a) in the parietal lobe, located next to the primarymotor area (Brodmann’s area 4). 43
  • 43. Olfactory System PathwaysThe sense of smell is mediated by the olfactory system. This is the detection of airborne chemicals byspecialized receptors in the olfactory mucosa.The olfactory system is completely neural, since the receptors are modified neurons that transduce andtransmit olfactory inputs to the brain via the olfactory bulb, the lateral olfactory tract, and from there to theolfactory cortex.The olfactory system is unique among the senses, in that receptors project directly to cortex; the othersenses relay through the thalamus.Each olfactory receptor cell gives rise to an unmyelinated centrally directed axon. They are the slowestimpulse-conducting axons of the central nervous system (CNS). The axons of these bipolar cellsconverge and assemble to form 15–20 bundles (fascicles)—the olfactory fila. The olfactory fila coursesuperiorly, traversing the sieve-like perforations of the cribriform plate of the ethmoid bone of the skull toterminate in the ventral surface of the ipsilateral olfactory bulb.The olfactory bulb is part of the forebrain, situated on its ventral surface in the olfactory sulcus, andattached to it by the olfactory tract. The olfactory tract consists mainly of fibers of the anterior olfactorynucleus, the lateral olfactory tract, and the anterior limb of the anterior commissure. This tract carriesmany centrifugal fibers from the brain to the olfactory bulb.The lateral olfactory tract (LOT), which transmits olfactory inputs to the brain, gives off collaterals to thelimbic system, to the olfactory cortex, and to the anterior olfactory nucleus. The anterior olfactory nucleusprojects mainly to both the olfactory bulbs and to its contralateral partner. The axons of the LOT travelcaudally as the lateral olfactory stria; these synapse in the piriform cortex, a major component of theolfactory cortex, and the olfactory tubercle. The LOT projects further caudally to the anterior corticalamygdaloid nucleus, the lateral entorhinal cortex and the periamygdaloid cortex, which is part of thepiriform cortex that overlies the amygdala.The main areas of the olfactory cortex are the anterior cortical amygdaloid nucleus, anterior olfactorynucleus, lateral entorhinal cortex, periamygdaloid nucleus, piriform cortex, and olfactory tubercle. Allthese areas have reciprocal intrinsic connections. The main intrinsic connections stem from the anteriorolfactory nucleus, lateral entorhinal cortex, and piriform cortex. The olfactory cortex is phylogenetically 44
  • 44. identified as paleocortex, because most of it contains three cell layers, while neocortex has six layers ofcells.The olfactory cortex projects to several other extrinsic areas. These are the olfactory bulb, which receivesfibers from all areas of the olfactory cortex except the olfactory tubercle; to the hippocampus from thelateral entorhinal cortex, and to the lateral hypothalamus, mainly from the piriform cortex and anteriorolfactory nucleus. The connections to the hippocampus mediate olfactory contribution to memory andlearning. The connections to the hypothalamus mediate feeding behavior and perhaps emotionalresponses such as food-evoked rage responses. 45
  • 45. Gustatory PathwayThe gustatory (taste) system makes possible the phenomenon of flavor perception. Modalities of tasteare sensed by taste buds in the oropharyngeal mucosa, which detects chemicals that are dissolved in thesaliva. The information is transmitted by afferent conduction to the CNS, where the modality isrecognized. Taste buds are modified oral mucosa cells, which transduce the chemical modality into anelectrical impulse; this impulse travels through first-order neurons along one of more of the cranial nervesVII, IX, and X to the solitary nucleus. From there, second-order neurons project to the thalamus Third -order neurons project to diencephalic areas involved in appetite control, food intake, and fluid and ionbalance. From the thalamus, fibers project to the orbitofrontal and insular cortex. 46