Ns2. The Nervous System And Some Developmental Problems. Compressed File
NS2 THE NERVOUS SYSTEM AND SOME DEVELOPMENTAL PROBLEMS by Robin O’Sullivan THE WALLED CITY OF CARCASSONNE, FRANCE
NS2 The nervous system and some developmental problems LEARNING OUTCOMES Parts of the brainstem Roots of the cranial nerves Neural tube, neural crest Fore, mid and hind-brain Spina bifida and anencephaly
BRAIN STEM The brain is divided into three regions: the FOREBRAIN, MIDBRAIN and HINDBRAIN. The forebrain is formed from the cerebral hemispheres and the diencephalon. The diencephalon lies on either side of the third ventricle. The midbrain and hindbrain constitute the brain stem.
THE BRAIN AND BRAIN STEM The MIDBRAIN lies around the cerebral aqueduct. The HINDBRAIN lies around the fourth ventricle and consists of the MEDULLA OBLONGATA , PONS (Latin, PONS = bridge… because it appears to bridge the two cerebellar hemispheres) and CEREBELLUM . The medulla oblongata is continuous with the spinal cord. MIDBRAIN PONS MEDULLA OBLONGATA CEREBELLUM The BRAIN can be defined as the part of the central nervous system that lies within the cranial cavity. The CRANIAL NERVES are attached to the brain.
SOME FEATURES ON THE VENTRAL SURFACE OF THE BRAIN STEM The PITUITARY STALK (the pituitary gland has been removed) (midline structure) The MAMMILLARY BODIES (coloured green). These contain the mammillary nuclei which are part of the limbic system. The CRURA CEREBRI (Latin, CRUS = leg) which connect the two cerebral hemispheres to the brain stem The PYRAMID , so named because it overlies the pyramidal or corticospinal tract. The OLIVE . This contains the inferior olivary nucleus, which sends fibres to the cerebellum and is involved in the control of movement
SOME FEATURES ON THE DORSAL SURFACE OF THE BRAIN STEM, AFTER REMOVAL OF THE CEREBELLUM The PINEAL GLAND in the midline. The SUPERIOR COLLICULI , involved in the visual system, especially visual reflexes. The INFERIOR COLLICULI , directly involved in the sense of hearing. SUPERIOR CEREBELLAR PEDUNCLE MIDDLE CEREBELLAR PEDUNCLE INFERIOR CEREBELLAR PEDUNCLE , not well demarcated from the middle cerebellar peduncle on this model. The cerebellum is attached to the brain stem by three distinct peduncles (=“LITTLE FEET”) on each side. The pons is continuous with the middle peduncle, the largest of the three.
THE CRANIAL NERVES There are 12 pairs of cranial nerves: OLFACTORY OPTIC OCULOMOTOR TROCHLEAR TRIGEMINAL ABDUCENS FACIAL VESTIBULOCOCHLEAR GLOSSOPHARYNGEAL VAGUS ACCESSORY HYPOGLOSSAL
CN I CN I CN II CN II The first two cranial nerves, the OLFACTORY which is associated with the sense of smell, and the OPTIC which is associated with the sense of sight, are attached to the forebrain. The remainder are attached to the brain stem.
CN III CN V CN VI CN IV The smaller picture shows a dorsal view of the brainstem and the cavity of the fourth ventricle, with the trochlear nerve which is the only nerve attached to the dorsal surface of the brainstem. The OCULOMOTOR , TROCHLEAR and ABDUCENS nerves supply motor innervation to the muscles that move the eyeball, collectively called the EXTRAOCULAR MUSCLES . The TRIGEMINAL is a mixed nerve, supplying sensory fibres to the face and motor fibres to the muscles that move the mandible, the MUSCLES OF MASTICATION .
CN VII CN VIII The FACIAL NERVE supplies motor innervation to the MUSCLES OF FACIAL EXPRESSION . This nerve also contains sensory fibres that carry taste sensation from the anterior two-thirds of the tongue and palate, and general sensation from part of the external ear. In addition, it contains parasympathetic fibres that supply the LACRIMAL GLAND in the orbit and the SUBLINGUAL and SUBMANDIBULAR SALIVARY GLANDS in the mouth. The VESTIBULOCOCHLEAR NERVE supplies sensory innervation to the inner ear. The VESTIBULAR part is concerned with movements of the head and the COCHLEAR part deals with the sense of hearing.
CN IX The GLOSSOPHARYNGEAL NERVE emerges behind the olive and contains: ● Motor fibres to the stylopharyngeus muscle, one of the muscles involved in swallowing. ● Sensory fibres that carry taste sensation from the posterior third of the tongue and pharynx, and general sensation from the same region. ● Parasympathetic fibres that supply the PAROTID SALIVARY GLAND. ● Sensory fibres from the chemoreceptors in the carotid body and the baroreceptors in the carotid sinus.
CN III CN IV CN V CN VI CN VII CN VIII CN IX CN X CN XII CN XI
The VAGUS NERVE emerges behind the olive, directly caudal to the glossopharyngeal nerve. It is distributed widely in the head, neck and trunk. It contains: ● Motor fibres to the muscles of the palate, pharynx, larynx and upper part of the oesophagus. ● Sensory fibres from the pharynx, larynx, oesophagus, ear, and the thoracic and abdominal viscera. ● Parasympathetic fibres that supply much of the gastrointestinal, cardiovascular and respiratory systems. ● Sensory fibres from the chemoreceptors in the aortic bodies and the baroreceptors in the aortic arch. CN X
The ACCESSORY NERVE has two parts. The CRANIAL ACCESSORY emerges behind the olive, directly caudal to the vagus nerve. The SPINAL ACCESSORY emerges from the lateral surface of the first five segments of the spinal cord, passes through the foramen magnum and joins the cranial accessory for a short distance. The accessory nerve contains: ● Motor fibres from the cranial part join the vagus nerve and greatly supplement the vagal fibres supplying muscles of the palate, pharynx and larynx. ● Motor fibres from the accessory part supply the sternomastoid and trapezius muscles. CN XI (CRANIAL) CN XI (SPINAL)
The HYPOGLOSSAL NERVE emerges between the pyramid and the olive. It contains: ● Motor fibres which supply the muscles of the tongue. N.B.: IT IS VERY IMPORTANT THAT YOU KNOW THE DISTRIBUTION AND FUNCTIONS OF ALL THE CRANIAL NERVES, AND THAT YOU KNOW HOW TO TEST EACH ONE OF THEM CLINICALLY . CN XII
The primitive NEURAL TUBE consists of a cavity surrounded by a wall. The wall gives rise to the various parts of the brain - the FOREBRAIN, MIDBRAIN and HINDBRAIN - and the SPINAL CORD. The cavity persists as the VENTRICULAR SYSTEM of the brain and the CENTRAL CANAL of the spinal cord.
FOREBRAIN MIDBRAIN HINDBRAIN SPINAL CORD LATERAL CEREBRAL CENTRAL VENTRICLE AQUEDUCT CANAL OF THIRD FOURTH SPINAL VENTRICLE VENTRICLE CORD
Formation of the NEURAL TUBE begins at the end of the third week after fertilisation when the folds on each side of the NEURAL GROOVE grow towards each other and fuse. Cells migrate from the neural folds, the NEURAL CREST CELLS , which will subsequently migrate and contribute to a wide variety of structures within the developing individual. NEURAL GROOVE NEURAL CREST CELLS NOTOCHORD
NEURAL CREST CELLS give rise to: ● Schwann cells in the peripheral nervous system ● Neurons in the dorsal root and autonomic ganglia ● The nerve plexuses in the gut wall ● Melanocytes in the skin ● C cells in the thyroid gland ● The medulla of the adrenal ● Part of the meninges surrounding the brain ● The mesoderm in the orofacial region from which develops cartilage, bone and dentine (one of the mineralised tissues in the teeth).
Fusion of the NEURAL TUBE commences in the region that will become the neck, and it proceeds towards either end. The open ends of the neural tube, the NEUROPORES , normally close within a few days. If they fail to close, various NEURAL TUBE DEFECTS (or NTD s) can arise. ECTODERM ENDODERM
These NTDs are very common. There are two types: 1. SPINA BIFIDA OCCULTA . The defect is a localised failure of the vertebral arches to fuse. The overlying skin often has a patch of hair known as a “FAUN’S TAIL” . It is a very common and usually a clinically insignificant abnormality. 2. SPINA BIFIDA CYSTICA . In this type, the meninges may protrude through the overlying defect as a MENINGOCELE . If neural tissue is additionally present in the protruding material, the condition is termed MENINGOMYELOCELE . FAILURE TO CLOSE AT THE CAUDAL END: SPINA BIFIDA A “FAUN’S TAIL” The source of this picture is not known
MENINGOCELE photograph reproduced by kind permission of surgical-tutor.org.uk
In our species, NTDs at the cranial end are much less common than spina bifida. They are associated with a defect in the squamous part of the occipital bone, with our without other bones being affected also, at the back of the skull. This region corresponds to the vertebral arch of a vertebra. There are four types: 1. MENINGOCELE . The bony defect is small, and only the meninges protrude through it. 2. MENINGOENCEPHALOCELE . The bony defect is larger, and some neural tissue along with the meninges protrudes through it. (The word CEPHALIC comes from the Greek KEPHALE, a head. The ancient Greeks called the brain ENCEPHALOS, meaning “within the head”). 3. MENINGOHYDROENCEPHALOCELE . The protruded material contains meninges, neural tissue, and part of the ventricular system. 4. ANENCEPHALY . The defect is extremely large and the developing brain is exposed on the surface where it degenerates. The brain stem remains intact. The infant usually dies at birth. FAILURE TO CLOSE AT THE CRANIAL END The source of this picture is not known
photographed reproduced by kind permission of surgical-tutor.org.uk MENINGOCELE
MENINGOHYDROENCEPHALOCELE ANENCEPHALY THE SOURCE OF THIS PICTURE IS NOT KNOWN