Neuroanatomy and Physiology OPT 310 PDF

Neuroanatomy. (OPT 310). Course outline. 1. Cranial nerves 2. General course of nerves 3. Distribution of nerves 4. Pathways of ne...

Neuroanatomy. (OPT 310). Course outline. 1. Cranial nerves 2. General course of nerves 3. Distribution of nerves 4. Pathways of nerve transmission. Cranial nerves. Cranial nerves are parts of the peripheral nervous system. They are typically defined as arising from the brain and passing through the holes (foramina) In the bones of the skull to exit it. Theses nerves originated from the brain connecting the brain to different parts of the head neck and trunk. They are 12 pairs usually referred to by name or by Roman numerals namely; 1. The Olfactory nerve (CN I) 2. The Optic nerve (CN II) 3. The Oculomotor nerve (CN III) 4. The Trochlear nerve (CN IV) 5. The Trigeminal nerve (CN V) 6. The Abducen nerve (CN VI) 7. The Facial nerve (CN VII) 8. The Vestibulocochlear nerve (CN VIII) 9. The Glossopharyngeal nerve (CN IX) 10. The Vagus nerve (CN X) 11. The Spinal accessory nerve (CN XI) 12. The Hypoglossal nerve (CN XII) The olfactory and the optic nerve are really not nerves at all, rather extensions of the brain (fiber tracts of the brain). The spinal accessory nerve is derived from the upper cervical segments of the spinal cord. The remaining nine (9) pairs of nerves relate to the brain stem. Origin of cranial nerve fibers. The superficial origin of a cranial nerve is that area of the brain where the nerve emerges or enters. cranial nerve fibers with motor(efferent) function arise from collection of cells that lie deep within the brain stem (motor nuclei), and are homologous to the anterior horn cells of the spinal cord. Cranial nerve fibres with sensory (afferent) function have their cells of origin (first- order nuclei) outside the brain stem, usually in ganglia homologous to the dorsal root ganglia of the spinal nerves. The second- order sensory nuclei lie within the brain stem. Names of nerves Functional components of nerves Olfactory Special sensory (SS) Optic Special sensory (SS) Oculomotor Somatic efferent (SE) and Visceral efferent (VE) Trochlear Somatic efferent (SE) Trigerminal Somatic afferent (SA) and Branchial efferent (BE) Abducen Somatic efferent (SE) Facial Branchial efferent (BE), Visceral efferents (VE), Visceral afferent (VA) and Somatic afferent (SE) Vestibulo Special cochlear sensory (SS) Branchial Glossopharyngeal efferent (BE), Visceral efferent (VE), Visceral afferent  (VE) and Somatic afferent. (SA) Vagus Branchial efferent, Visceral efferent, Visceral afferent (VA) and Somatic afferent (SA) Spinal accessory Branchial efferent (BE) Hypoglossal Somatic efferent (SE). The functional components of the cranial nerves are conveyed from or to the brainstem by six (6) types of fiber nerves. The somatic efferent fibers. They innervate striated that are derived from somites and are involved in eye movements (CN III, IV & VI) and tongue movements (CN XII). Branchial efferent fibers. They are special somatic efferent components. These fibers innervate muscles that are derived from the branchial (gill) arches and are involved in chewing (CN V), making facial expressions (CN VII), swallowing (CN IX & X), producing vocal sounds (CN X), and turning the head (CN XI). Visceral efferent fibers. (preganglionic parasympathetic components of the cranial division). It courses through CN III (smooth muscles of the inner eye), CN VII (salivary and lacrimal glands), and CN X (heart, lung and bowel muscles involved in movements and secretions). Visceral afferent fibers. They convey sensation from the alimentary tract, heart, vessels and lungs by the way of CN IX and CN X. A specialized visceral afferent component is involved with the sense of taste; fibers carrying gustatory impulses are present in CN VII, CN IX & CN X. Somatic afferent fibers. They convey sensation from skin and mucous membranes of the head. They are found mainly in CN V. A small number of afferent fibers travel with CN VII, CN IX & CN X, and these fibers terminates at the trigerminal nuclei in the brain stem. Special sensory fibers. They are found in CN I (involved in smell), CN II (involved in vision), and CN VIII (involved in hearing and equilibrium). The Olfactory Nerve. Route/course of nerve Areas of nerve distribution/ interpretation. Olfaction/ Sense of smell. They Olfactory nerve. This is the nerve of smell derived from the ectoderm of the nasal placodes. The true olfactory nerves are short connections between the olfactory mucosa (within the nose) and the olfactory bulb within the cranial cavity. It is majorly classified as special sensory nerve. It can also be classified as special somatic afferent and special visceral afferent in view of the close relationship between the sensation of vision and hearing, smell and taste respectively. They are unique among human sensory receptors because they can regenerate if damaged when their supporting layers of cells are preserved. Route/Course of nerve. The olfactory receptors are found within the cell membrane of the olfactory receptor neurons. These neurons lie in the olfactory mucosa at the upper end of the nasal cavity called the olfactory epithelium. Here their nerve fibers (axons) extend to the areas of distribution at the olfactory cortex through the olfactory bulb and tract. Their area of nerve (signal) distribution are; 1. Primary olfactory cortex 2. Entorhinal cortex 3. Amygdala  Olfaction/ Sense of Smell. When molecules from an aromatic substance enter the nasal cavity, and ascend with an inhalation; these aromatic molecules then interact with a layer of about 6 million specialized sensory receptor cells that are nestled among the supporting mucosal cells (Olfactory epithelium) in the roof of the nasal cavity on protuberances called concha. With every breath, air is forced over 10 - 50 very fine hairs called cilia Arising from each of the olfactory receptor cells that are sensitive to aromatic compounds in the air. The thin unmyelinated axons from the olfactory receptor cells coalesce into about 20 bundles, collectively referred to as the olfactory nerve and then ascends (about 30mm from the nasal cavity) to the cibriform plate (a sievelike plate/ a foramina) that allow the passage of the axons from the sensory cells to the brain. Once through the cibriform plate, the olfactory nerve axons synapse in an ovoid extrusion of the cerebral cortex that rest on the cibriform plate called the olfactory bulb which relays the signal through the olfactory tract. The olfactory tract is a white band of nerve fibers which are in bundles, connecting the olfactory bulb to several brain region in the olfactory cortex. LATERAL VIEW OF THE OLFACTORY NERVE. The optic nerve Route/ Course of nerve Areas of ner6 distribution/ interpretation Visual process/ Sense of sight.  The Optic Nerve. The optic nerve is the second cranial nerve that carries the impulse for vision. It is a sensory tract developed from the embroyologic structure of the optic cup. It contains the afferent fibers of the pupillary reflex. The optic nerve is myelinated, produced by oligodendrocytes in common with the central nervous system, as opposed to other peripheral nerves whose myelination are from Schwann cells, and for this reason, the optic nerve lack the general ability to regenerate when damaged. Route/course of Optic nerve. In the retina lies the optic nerve which is formed by the convergence of bipolar and retina ganglion cells consisting the first order and second order of visual neurons respectively. The first order of neurons lie in the bipolar cells and relays to the ganglion cell where the second order of neurons lies. Both converge to form the optic nerve at the optic disc. The optic nerve when formed leaves the orbit (eye socket) through the optic canal by the passageway of the sphenoid bone and enters the cranial cavity. As the optic nerve enters the cranial cavity it runs along the surface of the middle cranial fossa where it’s nasal halves meet to form the optic chiasma and it’s temporal halves continue their course laterally. At the optic chiasma, the nasal fibers decussate and merges with the contralateral fibers to form an optic tract. The optic tracts extends to the lateral geniculate body (where the 3rd order of visual neuron lies). Fibers from the lateral geniculate body extends to the visual cortex through the optic radiations. Areas of distribution of visual information at the cortex include; 1. Striate area 17 (Visuosensory area). 2. Peristriate area 18 ( Visuopsychic area). 3. Peristriate area 19 ( Visuopsychic area). Process of Vision/The sense of sight. Light from the retina is converted to visual signal through the receptors (rods and cones). They are processed further by the ganglion cells and transmitted to the optic nerve. The visual impulse is carried by the optic nerve through the visual canal dividing into a nasal and temporal half. The visual fibers from the nasal half of the optic nerve meets at the optic chiasma where they decussate and run contralateral, to join the ipsi- lateral (temporal) optic nerve fibers at the optic tract. (This results in each optic tract having optic nerve fibers from the temporal half of retina of same eye and nasal half of retina of the opposite eye). Visual information from each optic tract ends in the lateral geniculate body, here the visual informations are relayed through the optic radiatons to the visual cortex. SCHEMATIC ILLUSTRATION OF THE OPTIC NERVE AND THE VISUAL PATHWAY IN THE BRAIN.  Oculomotor nerve Course of nerve areas of Distribution of nerves. Pathway of light reflex Pathway of convergence and accommodation reflex. Oculomotor nerve. Oculomotor nerves are general visceral Efferent and somatic efferent nerve innervating the intrinsic (intraocular) and extrinsic (extraocular)muscles of the eye. The visceral efferent fibres (preganglionic) arise in the edingerwestiphal nucleus and terminate in the ciliary ganglion. The postganglionic fibers arising in this ganglion (ciliary) supply the intrinsic muscles which include; Sphincter pupillae muscle of the iris The ciliary muscle. The individual contraction of the ciliary and sphincter pupillae muscle are responsible for the accommodation of the crystalline lens and pupillary constriction respectively. The somatic efferent fibres supplies all the extrinsic muscles of the eyeball except the lateral rectus and superior oblique muscle. Extrinsic (Extraocular) muscles supplied by the somatic efferent fiber of Oculomotor nerve include; The superior rectus muscle. The inferior rectus muscle Mediial rectus muscle Inferior oblique muscle. Route/ Course of nerve. The oculomotor nerve originates from the oculomotor nucleus of the brain stem and emerges on the ventral side of the midbrain at the level of supervisor colliculus in the brain stem where it runs inferiorly to the posterior cerebellar artery and superiorly to the superior cerebellar artery piercing the duramater and straight down to the lateral part of the cavenous sinus where it leaves the cranial cavity via the superior orbital fissure dividing into a Superior and inferior branch as it enters the orbit.  Areas of nerve distribution. The superior branch of the Oculomotor nerve innervates the; 1. Superior rectus 2. The levator palpebrae superioris muscle. The inferior branch innervates; 1. Inferior rectus muscle 2. Medial rectus muscle 3. Inferior oblique muscle SCHEMATIC DIAGRAM OF THE OCULOMOTOR NERVE SHOWING THE INNERVATION IF FOUR EXTRAOCULAR MUSCLES.  Pathway of light reflex. Pupillary light reflex consist of both Afferent (sensory) and efferent (visceral) limb. The afferent limb has nerve fibers running within the optic nerve. The efferent has nerve fibers running along side the Oculomotor nerve. The sensory limb consist of fibers from retinal ganglion cells which receive light signals from the photoreceptors. The signal runs through the optic nerve of both eye to the optic chiasm where include from the nasal retina crosses and travel along the opposite optic tract to terminate in the pretectal nucleus. Fibers from the temporal retina runs uncrossed to the ipsi- lateral pretectal nucleus. The efferent limb begins at the pretectal nucleus, there the nasal fibers from each pretectal nucleus runs to the contralateral edingerwestiphal nucleus, the temporal fibers runs to the ipsi-lateral edingerwestiphal nucleus ( these connection forms the basis of consensual light reflex. In the edingerwestiphal nucleus, preganglionic parasympathetic nerve fibers consisting of axons of the oculomotor nerve joins with the postganglionic parasympathetic fibers in the ciliary ganglion. Postganglionic fibers travel along the short ciliary nerves (arising from the anterior surface of the ciliary ganglion) to innervate the sphincter pupillae muscle. PATHWAY OF LIGHT REFLEX  PATHWAY OF NEAR REFLEX. This pathway consist of an afferent (sensory) and efferent (visceral) limb. It is a three (3) component reflex system which includes; 1. Convergence ( the contraction of the medial rectus to adduct the eye). 2. Miosis (constriction of the pupil to increase depth of focus). 3. Accommodation (bulging of the crystalline lens for clearer vision at near). Pathway of convergence reflex. Sensory (afferent) signal from the medial rectus runs through the oculomotor to the mesenphalic nucleus of the fifth cranial nerve to a presumptive convergence center along the pretectal region. Signals are then relayed to the edingerwestiphal nucleus. Efferent from the edingerwestiphal nucleus runs along the Oculomotor nerve ( initiating the contraction of the medial rectus muscle) and relays in the accessory ganglion. Fibers from the accessory ganglion travel through the ciliary ganglion to the sphincter papillae for pupillary constriction. Pathway of accommodation reflex. Light (sensory) information from the retina runs through the optic nerve , to the optic Chiasma, optic tract, lateral geniculate body, optic radiation and to the occipital lobe (parasite cortex). Impulse from the parastriate cortex relays to the edingerwestiphal nucleus via the occpito-mesenphalic tract and pontine center. Efferent from the edingerwestiphal nucleus runs along the oculomotor nerve to reach the accessory and ciliary ganglion. From the ciliary ganglion fibers extend to the sphincter papillae muscle and ciliary muscle simultaneously causing pupillary constriction, increased depth of focus and accommodation. PATHWAY OF CONVERGENCE AND ACCOMMODATION REFLEX OCULOMOTOR NERVE NUCLEI THE OCULOMOTOR NUCLEAR COMPLEX The nuclear complex of the oculomotor nerve is located in the midbrain at the level of the superior colliculus, in the ventromedial part of the central grey matter that surrounds the cerebral aqueduct. It is a longitudinal column of about 10mm length. It extends above the level of the floor of third ventricle and below it is related to the nucleus of the trochlear nerve. The oculomotor nucleus has two major nuclei. 1. The main motor nucleus of large multipolar neurons. This nucleus is composed of small subnuclei supplying individual extraocular muscles as follows; Dorsolateral nucleus: Ipsilateral inferior rectus Intermedial nucleus: Ipsilateral inferior oblique Ventromedial nucleus: Ipsilateral medial rectus Paramedical nucleus: Contralateral Superior rectus Caudal central nucleus: Bilateral levator palpebrae. 2. The accessory motor nucleus ( Edingerwestiphal nucleus) It is situated posterior to the main oculomotor nucleus mass. It sends preganglionic parasympathetic fibers along the other oculomotor fibers. It consist of a median and two lateral components. The cranial half of the nucleus is concerned with light reflexes  and the caudal half with accommodation. The median part( nucleus of Perlia) and its role in convergence has not been fully elucidates. Connections of the nucleus The oculomotor nucleus is connected with the following 1. Cerebral cortex Motor cortex ( precentral gyrus) of both sides through the corticonuclear tract. Visual cortex through the superior colliculus and tactobulbar tract Frontal eye field(FEF) 2. Nuclei of the 4th, 5,the and 8th cranial nerves: through the medial longitudinal bundle. 3. Pretectal nucleus of both sides: for light reflexes. 4. Vertical and torsional gaze centres: through the medial longitudinal bundle. 5. Cerebellum through the vestibular nuclei. TROCHLEAR NERVE TROCHLEAR NERVE NUCLEUS The trochlear nucleus is situated in the ventromedial part of the central grey matter of the midbrain at the level of the inferior colliculus. It is caudal to and continuous with the third nerve nucleus complex. It is closely related to the medial longitudinal bundle. Connections of the nucleus 1. Cerebral cortex Motor cortex (precentral gyrus) of both sides through the corticonuclear tracts. Visual cortex through the superior colliculus and the tactobulbar tract. Frontal eye fields. 2. Nuclei of the 3rd, 6th and 8th cranial nerve s through the medial longitudinal bundle. 3. Superior colliculi through the descending predorsal bundle. 4. Vertical and torsional gaze centres. 5. Cerebellum through the vestibular nuclei. TROCHLEAR NERVE- OVERVIEW The trochlear nerve is a very long slender nerve, having a very long intracranial course. The name of the nerve was derived from the fact that the intertendon of the superior oblique muscle traverses through a connective tissue tunnel resembling a pulley. (Pulley in Latin means trochlea). The trochlear of the superior oblique muscle help to redirect the force of the muscle to allow movement of the eyeball towards the nose and provide mechanical advantage to the muscle by acting as a cleat at the point of intertendon. Without a functioning trochlear nerve, the eye will be slightly more elevated than it should be; assuming a position of rest in respect to the other trochlear functioning eye. The trochlear nerve is the only nerve that leaves the brain from its dorsal (back) surface. ANATOMICAL COURSE The left and the right trochlear nerves emanate from their respective trochlear nuclei within the midbrain) and decussates to the contralateral side from where they began and exit through the back region of the brainstem. From their respective brainstem exit, each nerve then winds laterally around the pons and continues in a forward direction and penetrates the Dura mater to enter the cavernous sinus. Once in the cavernous sinus, it runs a ventral direction and enters the orbit through the superior orbital fissure above the origin of the levator palpebral muscle and continues in a ventral and medial direction to innervate the superior oblique muscle. NB: of all nerves originating from the brainstem, trochlear nerve is the only nerve whose fibers decussate at the level of the brainstem; other cranial nerves have their fibers run ipsi-lateral at the level of the brainstem. Trochlear nerve is the longest, thinnest and the only nerve emanating from the dorso aspect of the midbrain. PATHWAY OF NERVE TRANSMISSION The trochlear nerve runs to innervate only the superior oblique muscle of the eye. RESULTANT EYE MOVEMENT The innervation of the superior rectus muscle by the trochlear nerve results in the following eye movement; Intorsion: (inward rotation of the eye)-primary movement. Abduction :( outward movement of the eye)-secondary function. Depression: (downward movement of the eye)-tertiary function. ABDUCENT NERVE ABDUCENT NERVE NUCLEUS The Abducent nucleus is situated in the lower part of the pons, close to the midline, beneath the floor of the fourth ventricle. It is closely related to the fasciculus of the facial nerve. It consists of two types of multipolar cells large and small. The large multipolar cells give rise to fibres of the Abducent nerve while the fibres of the small multipolar cells relay in the oculomotor nucleus via the medial longitudinal fasciculus. The small multipolar cells are believed to form para-abducent nucleus. Since the Abducent nucleus belongs to the group of somatic efferent nuclei, it lines in line with the nuclei of the 3rd and 4th nerves above and with the nucleus of hypoglossal nerve below. Connections of the nucleus  1. Cerebral cortex Motor cortex (precentral gyrus) through the afferent corticonuclear fibres from both cerebral hemispheres ( principally contralateral). Visual cortex through the superior colliculus and tactobulbar tract. Frontal cortex ( front eye fields) 2. Nuclei of the 3rd, 4th and 8th cranial nerves through the medial longitudinal bundle. 3. Pretectal nucleus of both sides through the tectobulbar tract. 4. Horizontal gaze centres( paramedian pontine reticular formation – PPRF) through the medial longitudinal bundle. 5. Cerebellum through vestibular nuclei. ANATOMICAL COURSE The Abducent nerve leaves the brain as a thin bundle of fibers at the lower border of the pons to enter the arachnoid space. It runs vertically and a little laterally and pierces the Dura mater a little below and medial to the trigeminal nerve to enter the cavernous sinus. Within the cavernous sinus, the abducent nerve, abducent nerve is found lateral to the internal carotid artery and medial to the ophthalmic nerve. It enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle. PATHWAY OF INNERVATION The abducent nerve runs to innervate the lateral rectus muscle. RESULTANT EYE MOVEMENT The lateral rectus muscle only functions to abduct the eye (outward movement of the eye). Thus any reduced activity in this muscle results in an eye that is more adducted than it should be, and one that does not move smoothly with the other eye when looking towards the side of the affected muscle. A CROSS SECTION OF THE CARVENOUS SINUS SHOWING THE NUCLEUS OF CN111, IV &VI EXTRAOCULAR MUSCLES AND ACTIONS DIRECTION OF EXTRAOCULAR MUSCLES ACTION DURING EYE MOVEMENT TRIGERMINAL NERVE NUCLEUS OF THE TRIGEMINAL NERVE – TRANSVERSE SECTION The trigeminal nerve has four nuclei 1. The main sensory nucleus 2. The spinal nucleus 3. The mesencephalic nucleus 4. The motor nucleus The Main sensory nucleus: It lies in the upper part of the pons lateral to the motor nucleus. It is continuous below with the spinal nucleus and above with the mesencephalic nucleus. The Spinal nucleus: It descends from the main sensory nucleus in the pons, through the whole length of the medulla oblongata and into the upper two segments of the spinal cord. The Mesencephalic nucleus: This nucleus is situated in the lateral part of the central grey matter of the midbrain and extends down in the pons up to the main sensory nucleus. The Motor nucleus: This is situated in the upper part of the pons medial to the main sensory nucleus. OVERVIEW The trigeminal nerve referred as three born together, also known as the trifacial nerve. It supplies the face and its cavities with general sensation. It also innervates the chewing muscles (muscles of mastication), a muscle that opens the Eustachian tube and one that dampens sounds so that the sensitive cells in the ears are not damaged by excessively loud noises. The trigeminal nerve may be considered the sensory branch of the purely motor cranial nerves III, 1V, VI and VII. It is the largest of the cranial nerves. The trigeminal nerve is involved in an excruciating painful condition known as trigeminal neuralgia (TN). The pain associated with this condition is episodic (paroxysmal), but when it occurs, the patient often winces which gives the condition its other name, tic douloureux (painful twitch). ANATOMICAL COURSE The trigeminal nerve exits the brainstem at about halfway between the upper and the lower borders of the pons on its ventrolateral aspect. Its fibers segregate into two bundles, a much larger lateral sensory root and a much smaller, medial motor root that variably blend as the approach the trigeminal (Gasserian,Semilunar) ganglion. The nerve takes a ventral and slightly ascending course through (piercing) the dura mater to enter a dural “cave” where the trigeminal ganglion is located. This ganglion contains the cell bodies of all the sensory fibers in the nerve. The ganglion appears flattened and is enclosed in the dural cave (Meckel’s cave), residing in a small groove on the petrous portion of the temporal bone. Three divisions of the trigeminal nerve arise from the ganglion: the first division (V1), the ophthalmic nerve; the second division (V2), the maxillary nerve; and the third division (V3), the mandibular nerve. Motor component The motor component of the fifth nerve arises from the motor nucleus in the pons and leaves the pons adjacent to the sensory root. It passes diagonally under the nerve to reach the Gasserian ganglion and then joins with the mandibular branch. The motor component innervates almost all of the muscles that concerned with chewing and grinding food (the muscles of mastication). They are the temporalis, masseter, and medial and lateral pterygoids. In addition, the mylohyoid and anterior belly of the digastric muscles are also innervated by the motor component of CN V. These two muscles are attached to the lower jaw and thus contribute to the process of mastication, but are not truly considered major muscles of mastication. Instead, their function is mostly to move the hyoid bone within the neck and may be more appropriately considered muscles of deglutition, or swallowing. The motor division also innervates two very small muscles, both of which begin with the word tensor (to tense), the tensor veli palatine and the tensor tympani. The former is a muscle of the upper throat and functions to tense the soft palate and also to open the Eustachian tube when the mouth is opened widely, as in yawning. The opening of this tube allows pressure in the middle ear to be equalized with atmospheric pressure. This is why it is recommended that we open our mouths when an airplane is ascending or descending and there are changes in cabin pressure. A second muscle that moves the soft palate, the levator veli palatini, is innervated by cranial nerve X. The tensor tympani is a small muscle that is attached to the malleus bone in the middle ear cavity. It plays a role in the tympanic reflex, which reduces the movements of the tympanic membrane (eardrum) in response to loud noises, thus protecting the delicate hair cells of the cochlea. If the motor root of the trigeminal nerve is inadvertently sectioned in a sensory root surgical resection or in removal of the ganglion (both can be done to treat TN), there will be complete and permanent paralysis of the muscles of mastication on the respective side with clearly apparent atrophy. When motor paralysis is present, upon opening the mouth, the jaw will deviate to the paralyzed side because of unopposed action of the contralateral chewing muscles. However, unilateral paralysis of the chewing muscles is generally not a serious disability. Ophthalmic division (nerve) The ophthalmic division (V1) travels forward through the cavernous sinus where it receives some fibers from the sympathetic plexus traveling with the internal carotid artery. In the sinus, the nerve is located inferior to the trochlear nerve and lateral to the abducent and oculomotor nerves. The ophthalmic nerves give off a small tentorial ramus that supplies part of the dura mater and continues through the superior orbital fissure to the orbit, where it subdivides into three terminal branches: the lacrimal, the frontal, and the nasociliary nerves). The lacrimal nerve is the most lateral of these, traveling beneath the roof of the orbit to the lacrimal gland. It receives a branch from the zygomatic nerve, which contains postganglionic parasympathetic neurons that originate in the facial nerve and innervate the lacrimal gland, which produces tear). The nerve enters the lacrimal gland and gives off several filaments, which supply sensory innervation to the gland and the conjunctiva of the eye. Then it exits the anterior aspect of the orbit and terminates in the skin of the upper eyelid, supplying sensation to this region and joining with filaments of the facial nerve. The frontal nerve is the largest of the three branches of the ophthalmic nerve. It runs ventrally beneath the roof of the orbit and divides into several branches. The largest of these is the supraorbital nerve, which traverses the foramen (or notch) of the same name to emerge on the forehead and supply the skin of the forehead. The smaller supratrochlear branch of the frontal nerve takes a more medial course to the medial part of this region and the medial angle of the eye and innervates the skin and the conjunctiva. The nasociliary nerve is the most medial branch of the ophthalmic nerve. It approaches the medial wall of the orbit and exits the orbit as the infratrochlear nerve. The infratrochlear nerve innervates the skin of the eyelids and side of the nose, the conjunctiva, lacrimal sac and tip of the nose. This latter branch is very useful clinically because if the tip of the nose is sensitive it indicates that this branch of V1 is functional. Within the orbit, the nasociliary nerve gives off some small sensory branches that penetrate the medial bony wall and supply part of the nasal sinuses, the long ciliary nerves that supply the eye with sensory, and probably some sympathetic fibers and a branch to the ciliary ganglion (sensory root) that passes through the ganglion and exits from it as short ciliary nerves that also supply sensory and sympathetic fibers to the orbit. The short ciliary nerves additionally carry postganglionic parasympathetic fibers from the oculomotor nerve. The short and long ciliary nerves take part in the corneal blink reflex. Maxillary division (nerve) The maxillary nerve (V2) travels from the cavernous sinus (where it gives off a small meningeal branch) through the foramen rotundum to the pterygopalatine fossa. The bulk of the fibers Continue as the large infraorbital nerve that runs in the infraorbital groove on the floor of the orbit and then in the infraorbital canal. It emerges through the infraorbital foramen and splits into several branches that supply the skin on the surface of the maxilla as far down as the mouth and up to the lower eyelid. A relatively large branch of the maxillary (or infraorbital) nerve is the zygomatic nerve. It runs along the lateral wall of the orbit and gives off small branches that penetrate the zygomatic bone and supply the skin covering this bone and the anterior part of the temple (zygomaticotemporal and zygomaticofacial nerves). Several superior alveolar nerves emanate from the infraorbital nerve. They supply the teeth in the maxilla; the fibers form a dental plexus above the roots of the teeth and innervate them and the surrounding gums The greater and lesser palatine nerves supply the hard and soft palates with general sensation and taste. They also carry postganglionic parasympathetic fibers that emerged from the pterygopalatine ganglion ; this ganglion may also be referred to as the (spheno palatine ganglion). The corresponding preganglionic fibers originally entered the ganglion after departing from CN VII. Some of the postganglionic fibers of the pterygopalatine ganglion are responsible for the uncomfortable symptoms associated with hay fever in the spring. Small postganglionic fibers enter the nasal cavity to supply the nasal epithelium. The allergens that cause hay fever enter the nasal cavity through the air we inspire, elicit the parasympathetic response in the nasal cavity, and we start producing mucous and have difficulty breathing. The sphenopalatine branches of V2 take a descending course; some of them traverse the pterygopalatine ganglion, but most of them bypass it. These branches supply the nasal cavityband nasal septum (nasopalatine nerve) with general sensation. The nasopalatine nerve traverses the nasal septum and then descends in the incisive canal to supply the anterior parts of the hard palate and also the upper incisor teeth. Many of the branches of the maxillary nerves are anesthetized by your dentist when you have work done on the upper teeth, the specific branches being dependent on the teeth being worked on. Mandibular division (nerve The mandibular division (V3) carries all the motor fibers of the trigeminal nerve and is the largest of the three principal trigeminal divisions. It leaves the skull through the foramen ovale in the base of the skull to emerge on the side of the face deep to the upper part of the mandible and gives off several branches. For example, there are branches that carry motor fibers to the four masticatory muscles (the temporalis, masseter, and lateral and medial pterygoids), as well as to the tensor Eli palatini (to tense the soft palate and then allow it to be raised and prevent food from entering the nasal cavity from behind during swallowing) and the tensor tympani muscles (to the ear to dampen sounds). The lingual nerve takes a ventral and descending course between the two pterygoid muscles to the base of the tongue. It gains entry to the tongue and supplies the mucous membrane of the anterior two-thirds with general sensory fibers. During its curved course, the lingual nerve receives the chorda tympani nerve, a branch of the facial nerve, which carries taste fibers from the anterior two-thirds of the tongue and (parasympathetic) visceral efferent fibers into the nerve, which stimulates two of the three large salivary glands, the submandibular and sublingual. Another branch of V3, the (long) buccal travels to the outer surface of the buccinator muscle within the cheek. It splits into many branches that penetrate the muscle and supply the mucous membrane on the inside of the cheek and the skin on the outside of the cheek. The nerve is purely sensory. (The buccinator muscle, a muscle of facial expression, is innervated by the buccal branch of the facial nerve. The inferior alveolar nerve is the largest branch of the mandibular nerve. Similar to the lingual nerve, it takes an arched course in an anterior and inferior direction, but it uniquely enters a canal within the mandible, the mandibular canal. Before it enters the canal, it gives off the mylohyoid nerve that runs under the mylohyoid muscle, under the chin, and innervates this muscle and the anterior belly of the digastric muscle. In the canal, the inferior alveolar nerve gives off numerous branches to the lower teeth. The terminal ranch of the nerve, the mental nerve, exits the bony canal through the mental foramen and supplies the skin of the lower lip and the chin. The auriculotemporal nerve is a purely sensory branch that bends laterally and enters the parotid gland, the largest of the glands that produce saliva. The auriculotemporal nerve also emerges from the foramen ovale along with the other branches of the mandibular nerve to enter the infratemporal fossa and then straddles the middle leningeal artery as the latter enters the foramen spinosum into the middle cranial fossa. After the two auriculotemporal branches come together again, the nerve ascends from the parotid gland to supply the skin of the temple and the anterior upper part of the outer ear. The nerve has a connection with the otic ganglion in the infratemporal fossa, so that postganglionic secretory fibers from this ganglion (the preganglionic fibers are from the glossopharyngeal nerve); travel to the parotid gland and cause salivation during eating. THE FACIAL NERVE It is composed of two separate nerves with many somatic and autonomic functions, including innervating all of the muscles of facial expression and causing tearing of the eye and salivation from the salivary glands beneath the tongue. NUCLEI The facial nerve has three types of nuclei: 1. Main motor nucleus- This lies deep in the reticular formation of the lower parts of the pons. 2. Parasympathetic nuclei- They are situated postero lateral to the main motor nucleus. 3. Sensory nucleus- It is situated at the upper part of the medulla oblongata in line with the other nuclei of its group at the upper part of the nucleus tractus solitaries. ANATOMICAL COURSE The facial nerve is the nerve that supplies the muscles of facial expression. A somewhat independent part of the nerve that is considered by some authorities to be a separate cranial nerve is called the intermediate nerve (nervus intermedius); it was given this name because the nerve is located “intermediate” between the facial nerve proper and the VIIIth cranial nerve as they exit the brainstem.. The fibers of the intermediate nerve typically emerge from the brainstem as a single root, although there can be more than one, and form a small bundle between the facial and the VIIIth nerve; the intermediate nerve then travels with these two nerves in their course to the opening of the internal auditory canal (IAC). The intermediate nerve has many names, that is, nervus intermedius, intermediary nerve, portio intermedia, Wrisberg’s nerve, and Sapolini’s nerve. The intermediate nerve contains parasympathetic fibers within it that, upon stimulation, result in salivary gland secretion and tearing, and also has some general sensory taste fibers. It is considered to be responsible for the sensations of taste from the anterior two-thirds of the tongue, floor of the mouth, palate, and general sensory information from a portion of the skin of the external ear (auricle), and to carry the parasympathetic fibers (motor) associated with the facial nerve. Cell bodies of the sensory neurons are located in the geniculate ganglion within the facial canal, whereas the cell bodies of the parasympathetic fibers are in the brainstem. Although the intermediate nerve is generally considered part of the facial nerve complex, there have been repeated calls for it to be considered its own cranial nerve since 1881. Sometimes, the sensory fibers associated with the facial (intermediate) nerve may cause a condition similar to trigeminal neuralgia. In 1909, Drs. Clarke and Taylor published a report titled “True tic douloureux of the sensory filaments of the facial nerve” in The Journal of the American Medical Association. The facial nerve (proper) emerges as a fairly large fiber bundle at the caudal border of the pons, in front of the VIIIth cranial nerve. The general motor fibers of the facial nerve are axons from the cells of the motor facial nucleus in the brainstem. Fibers from the motor cortex generate impulses initiating voluntary movements of the facial muscles, which reach the facial muscles via the facial nucleus in the pons. In addition, the facial nucleus may be activated by impulses from subcortical regions, which presumably play a role in the emotional innervation of the facial muscles (Reflect for a moment how effortless it is to smile without thinking about it, for example, as a response to someone else’s smile [emotional smile] but yet is difficult to produce a true smile voluntarily). Furthermore, afferent impulses entering some of the other cranial nerves may elicit reflexes in which the efferent link of the reflex is formed by the facial nerve (e.g., the blink reflex). In humans, the facial nerve contains about 7000 fibers. Three-quarters of these are myelinated and most of them are relatively thick, 7–10 μm. It is likely that the fibers from the facial nucleus in the brainstem of one side pass only to the facial muscles of that side. The initial part of the facial nerve between the brainstem and the internal auditory meatus (opening of the IAC) is called the pontine part. The meatal part of the facial nerve enters the canal, which at this point is directed laterally; as the nerve passes adjacent to the inner ear, it becomes the labyrinthine part, which continues to the geniculate ganglion where, at this so- called facial “knee” (geniculum), it makes a sharp bend in a posterior direction. The geniculate ganglion is located at this bend and this ganglion contains the cell bodies of the afferent fibers of the facial nerve (mainly those of the intermediate nerve). From the geniculum, the tympanic part of the nerve travels horizontally in a posterior direction. The canal with the nerve then makes a second bend and its final course is directed downward (mastoid part) to the stylomastoid foramen where the facial nerve leaves the skull. The majority of the fibers of the intermediate nerve leave the IAC before the main nerve exits the skull. In the first part of its course external to the skull, the facial nerve is embedded in the parotid gland within which it forms a plexus from which the nerves to the muscles of facial expression emerge. Before the facial nerve divides into its five terminal branches, it gives off a posterior auricular branch that ascends behind the auricle and innervates the extrinsic muscles of the ear and the occipital muscle. In animals that can do more than wiggle their ears as some people can, these muscles are very important for locating the source of a sound. Surprisingly, wiggling of the ears can be a medical problem because it can occur involuntarily in response to stress or other psychological problems. In another early branch of the extracranial part of the facial nerve innervates the posterior belly of the digastric muscle and the stylohyoid muscles, which are accessory chewing muscles located under the tongue. Some motor fibers emerge from the tympanic part of the nerve and enter the middle ear cavity to innervate the stapedius muscle which acts to reduce the movements of the last in the chain of the three middle ear bones, the stapes, and thus functions to dampen loud noises. The upper terminal branches of the facial nerve run from the parotid plexus anteriorly across the zygomatic arch to the frontalis, orbicularis oculi, and corrugator muscles. Other branches course horizontally to the zygomatic, orbicularis oris, and other muscles surrounding the mouth, including the buccinator muscle. A lower cervical branch supplies the platysma, which is a very superficial muscle, that tenses the skin of the neck and can “pop out” and become quite prominent when we grimace. Classically, there are five terminal branches of the facial nerve: temporal, zygomatic, buccal, marginal mandibular, and cervical. It is much easier to learn the branches of the facial nerve if the intermediate nerve is considered separately from the facial nerve proper. The fibers of the intermediate nerve are distributed primarily by two branches that leave the nerve in the facial canal. The greater petrosal nerve leaves the intermediate nerve at the geniculate ganglion and through a small opening, the hiatus of the facial canal, it gets to the anterior surface of the petrous part of the temporal bone. It then runs forward and passes below the trigeminal ganglion and lateral to the internal carotid artery as it traverses just above the foramen. In life, a thick connective tissue layer covers the opening of the foramen lacerum, and nothing with the exception of small vessels penetrates the actual foramen. The greater petrosal nerve then joins the deep petrosal nerve (from the sympathetic plexus surrounding the internal carotid artery) to form the nerve of the pterygoid canal (Vidian nerve). The Vidian nerve is named for a famous anatomist of the 16th century. Current terminology refers to the nerve as “the nerve of the pterygoid canal.” Through the pterygoid canal this nerve passes to the pterygopalatine (sphenopalatine) fossa and ends in the pterygopalatine (sphenopalatine) ganglion. In this way, preganglionic parasympathetic fibers from the intermediate nerve and sympathetic fibers surrounding the internal carotid nerve reach the pterygopalatine ganglion. Postganglionic fibers from this ganglion then stimulate tears to be produced by the lacrimal gland (via the zygomatic nerve and its connections with the lacrimal nerve). Other postganglionic fibers from the ganglion carry secretory impulses to mucous glands in the nasal cavity. Taste fibers from the palate run centrally in the greater petrosal nerve. Thus, if the greater petrosal nerve was destroyed in a person, tears would not be produced, the nasal cavity would be very dry, and taste would be lost from the palate. The other branch from the intermediate nerve, the chorda tympani leaves the nerve in the IAC just below the nerve to the stapedius. It enters and traverses the middle ear cavity adjacent to the tympanic membrane, and leaves it and the skull to enter the infratemporal fossa to join the posterior aspect of the lingual nerve. In this way, preganglionic parasympathetic fibers to the submandibular and sublingual salivary glands reach the lingual nerve. Thus, the facial nerve provides the stimulation that result in salivation from the salivary glands that are located under the tongue. The afferent taste fibers from the anterior two-thirds of the tongue run in a central direction via the lingual nerve and the chorda tympani. These fibers have their cell bodies in the geniculate ganglion. In contrast, taste fibers from the posterior one-third of the tongue travel in the glossopharyngeal nerve. Thus, if the chorda tympani is destroyed, the person would lose taste from the anterior two-thirds of the tongue and would not salivate from the glands under the tongue, but would still feel general sensation from the anterior two-thirds of the tongue if the lingual nerve remained undamaged, because general sensations are transmitted via CN V. There are some general sensory branches in the intermediate nerve that are thought to join the main branch of the facial nerve distal to the geniculate ganglion and provide sensation to a part of the auricle. These fibers are associated with the Ramsey–Hunt syndrome. As described earlier, preganglionic parasympathetic fibers from the intermediate nerve pass in part in the greater petrosal nerve to the pterigopalatine ganglion and in part in the chorda tympani to reach the lingual nerve and eventually to the submandibular ganglion. Secretion of saliva and tears from reflex or emotional stimuli is initiated by fibers that activate the cells of the salivary nucleus in the brain stem. However, we wish to emphasize that there is a definite connection between our conscious self (within the brain cortex) and that part of the autonomic system that is mundanely beyond our control and regulates the first step of digestion, salivation. Indeed, the stimulus to salivate can be visual, olfactory, or even auditory. For example, you walk into your grandmother’s kitchen and you begin salivating at the smell of garlic, olive oil, or bacon. Somehow the conscious recognition of the meaning of those odors triggers the parasympathetic response to salivate. Salivation is our way of unequivocally stating that something good is about to enter our mouths. Recently it has been demonstrated that the facial nerve sensory ganglion (classically the “geniculate ganglion”) actually consists of two components, a geniculate ganglion and a meatal ganglion. In most people, the geniculate ganglion component contains the majority of cells, but in about 10% of people the meatal component is greater. VESTIBULOCOCHLEAR NERVE (ACOUSTIC NERVE) The VIIIth cranial nerve is actually composed of two functionally different components, the vestibular nerve and the cochlear nerve; it is thus preferentially referred to as the vestibulocochlear nerve – the nerve of equilibrium and hearing. Each component primarily conducts impulses centrally from the organs of equilibrium and hearing within the inner ear. Both components of the VIIIth nerve also leave the brainstem together on the lateral surface of the medulla, just beneath the caudal border of the pons, the vestibular portion being ventral to the cochlear portion. NUCLEI The vestibular nucleus spans from the rostral medulla to the caudal pons. It is located bilaterally along the floor of the forth ventricle and it is lateral to the sulcus limitans. Its complex extends into two major columns; medial and lateral. The vestibular complex subdivides into four major nuclei; 1. The lateral vestibular nucleus (LVN). Also known as the Dieter’s nucleus extends along the lateral column of the vestibular complex. It is bordered by the superior, medial and the descending (inferior) vestibular nucleus. Lateral to the LVN is the sensory fibers of vestibulocochlear nerve. 2. The medial vestibular nucleus (MVN)- Also known as nucleus of the schwalbe. It is the largest of the nuclei in total cell volume and unlike others run in the medial column. It is bordered by the superior, inferior vestibular nucleus as well as posteriorly by the fourth ventricle and inferiorly by the dorsal motor vagal nucleus and the hypoglossal nucleus. 3. Superior vestibular nucleus (SVN)- It is known as the nucleus of Bechterew, it extends along the lateral column in an elongated , elliptical form. It is bordered ventrolateral by the LVN, dorsally by the superior cerebellar peduncle and medially by the fourth ventricle. 4. Descending/inferior vestibular nucleus- it is most caudal of the nuclei and is neigboured superiorly by the LVN, medially by the MVN, posteriorly by the lower end of the fourth ventricle and anteriorly by the reticular formation. ANATOMICAL COURSE From the lateral surface of the brain stem (medulla oblongata) the vestibulocochlear nerve (together with the facial and intermediate nerves) enters the internal auditory canal (IAC) as a large fiber bundle before splitting into its divisions. VESTIBULAR NERVE- PATHWAY OF INNERVATION The fibers of the vestibular nerve are processes (dendrites and axons) of the ganglion cells located in the vestibular (Scarpa) ganglion within the IAC. The central processes (axons) of these comprise the vestibular nerve. The peripheral ones (dendrites) travel as several smaller bundles to the crista ampullaris of the three semicircular ducts (these ducts are lined by membranes that are within the bony semicircular canals), the maculae of the utricle, and the saccule within the vestibule. The central processes (axons) of the vestibular ganglion cells enter the brainstem and most of them divide into an ascending branch and a descending branch that terminate in the four vestibular nuclei. The efferent fibers from the vestibular nuclei are so arranged as to be able to influence chiefly three other regions of the CNS: the cerebellum, the spinal cord, and the nuclei of the extraocular muscles of the eye. Furthermore, impulses from the vestibular apparatus reach the cerebral cortex. The impulses to the cerebellum and spinal cord affect posture, gait, and balance whereas those to the extraocular muscles enable fixation of the eyeball as your head turns. The vestibular nerve conducts impulses from the cristae of the semicircular ducts and from the maculae of the utricule and saccule. The stimuli are generated by currents in the endolymphatic fluid that is within the semicircular ducts, macula, and utricle, which arise from movements of the head. COCHLEAR NERVE- PATHWAY OF INNERVATION The cell bodies of the afferent fibers of the cochlear nerve are cells that comprise the spiral ganglion. This is situated in the long spiral canal of the cochlea. The dendrites of the ganglion cells arise around sensitive hair cells within the cochlea (organ of Corti), which respond to the vibrations caused by sound waves (in the endolymphatic fluid in the cochlear duct and the axons join and form smaller bundles that run toward the base of the cochlea. Through a series of small holes, they enter the IAC where they fuse to form the cochlear nerve. The fibers of the cochlear nerve enter the medulla and terminate in the dorsal and ventral cochlear nuclei. The cells in these nuclei give off axons that transmit the impulses centrally. The pathways mediating conscious perception of sound ascend in the brainstem and have relay stations there. From the latter nucleus, the last link in the pathway runs as fibers to the (primary) acoustic cortical area in the temporal lobe. The connections are both crossed and uncrossed, meaning that the impulses from one cochlea project to both sides of the brain. For each fiber of the cochlear nerve, the excitatory response is sharply tuned on one characteristic frequency. Because each fiber responds to a specific frequency, the complex of cochlear nerve fibers spanning low-to-high frequencies at variable intensities enables a total spectral analysis of them stimulating sound. Thus, one’s ability to separate complex sound into its component parts is already partially accomplished at the lowest level of the auditory system. These differently tuned fibers within the nerve each synapse in discrete regions of the cochlear nuclei and this orderly representation also occurs within the cortical acoustic area (tonotopic localization). GLOSSOPHARYNGEAL NERVE As the name implies, the nerve supplies, first and foremost, the tongue and the pharynx. It is sensory (both taste and general sensation) to the posterior one-third portion of the tongue and motor to some of the muscles of the pharynx. It also supplies parasympathetic innervation to the largest salivary gland, the parotid. Glossopharyngeal nerve makes a sympathetic appeal. It stands alone among the twelve cranial nerves, as a nerve without any very definite or important physiology and without any disease attached to its function. It has no tic or palsy or algia [pain]; it shares with the fifth and tenth nerves in supplying touch, taste and deglutition. But it is not vital to these functions. It could be resected with impunity. It receives only disregard and aloofness from surgeon and clinician (Dana, 1926). Despite Dr. Dana’s observation, clinical conditions associated with the glossopharyngeal nerve can be significant. This nerve is responsible for the gag reflex and in order for a sword swallower to overcome this reflex he must practice suppressing it. NUCLEUS-The glossopharyngeal nerve of four nuclei; 1. The Nucleus ambiguous- A common nucleus of the efferent fibres for glossopharyngeal and vagus nerve. It is located in the reticular formation in the medulla oblongata. 2. Inferior salivary nucleus- Is a cluster of neurons located in the dorsal region of the pons, just above its junction with the medulla. 3. Spinal nucleus of the trigeminal nerve- It descends from the main sensory nucleus of the trigeminal nerve in the pons, through the whole length of the medulla and into the upper two segments of the spinal cord. 4. Solitary nucleus or Nucleus tractus solitary- Is a series of sensory nuclei (clusters of nerve cell bodies) that forms a vertical column of grey matter in the dorsomedial part of the medulla oblongata. ANATOMICAL COURSE The IXth cranial nerve is formed by five–six small fiber bundles that emerge from the medulla behind oblongata immediately anterior to the vagal fibers and passes to the antero medial part of the jugular foramen. It then descends in an arch with its convexity below and the base of the tongue, where it splits into its terminal branches. The nerve then lies on the lateral wall of the pharynx, and at the approximate level at the base of the tongue it turns in a medially directed bend across the lateral side of the stylopharyngeus muscle to enter the tongue. The glossopharyngeal nerve has two sensory ganglia; the small superior ganglion is identified as a small swelling of the nerve in the jugular foramen, and below that is the somewhat larger inferior (petrosal) ganglion. The name “glossopharyngeus” refers to the tongue and pharynx. The anatomical tongue is just what you think about when you think of the tongue, although it is probably larger than you normally envision it to be (a normal adult tongue weighs 2–3 oz). The pharynx (Greek: throat) is a cylinderical-shaped conduit connecting the oral and nasal cavities (mouth and space behind the nose) to the esophagus and larynx (voice box) in the neck. The pharyngeal chambers serve both respiratory and digestive functions, and also help with phonation, the making of sound. During swallowing, circular constrictors muscles help propel food to the esophagus and longitudinal muscles lift the walls of the pharynx over the food bolus. The pharynx consists of three divisions. The upper portion is the nasal pharynx (nasopharynx), which is the posterior part of the nasal cavity. The nasal pharynx connects to the middle division, the oral pharynx (oropharynx). The oral pharynx starts at the back of the oral cavity and runs down the throat to the epiglottis, a beak-shaped piece of cartilage that covers the larynx to make sure food is directed to the esophagus. Recesses in the lateral walls of the oropharynx hold lymphatic tissue known as the palatine tonsils. These are prone to infection and are surgically removed in a tonsillectomy. The adenoids are additional masses of lymphatic tissue at the base of the skull, the uppermost part of the nasopharynx. Inflammation of the adenoids can cause a nasal voice, snoring, and mouth breathing. These are also often removed during a tonsillectomy. The continuity between the oral and nasal cavities allows us to breathe via either the nose or the mouth and, when medically required, allows food to be given by a nasogastric tube, from the nose to the stomach. It also explains why food may come out of one’s nose when vomiting, or when trying to eat or drink and laugh at the same time. The most inferior division of the pharynx is the laryngeal pharynx (laryngopharynx), which begins at the epiglottis and ends at the esophagus. Its function is to continue the process of propelling food into the esophagus and also to prevent food from entering the larynx. The muscles and mucosa of the pharynx receive their motor, sensory, and autonomic nerve supply through the pharyngeal plexus of nerves. This plexus is situated in the fascia surrounding the pharynx and is formed by pharyngeal branches of the glossopharyngeal and vagus nerves, and by sympathetic fibers. The nasopharynx receives sensory innervation primarily from the maxillary nerve (CN V2). The glossopharyngeal nerve carries general and special sensory fibers and somatic motor and visceral efferent (parasympathetic). The glossopharyngeal nerve is responsible for carrying both general sensations such as touch and taste from the back of the tongue (approximately the posterior one-third). Recall that general sensation from the anterior two-thirds of the tongue is via the lingual branch of the mandibular nerve (CN V) whereas taste from the anterior one-third is via the chorda tympanic branch of the facial nerve (CN VII), which travels with the lingual nerve for much of its course. The tongue has a very complex development and that explains why so many nerves contribute to it. One of the muscles of the pharynx, the stylopharyngeus is separately supplied by a muscular branch of the glossopharyngeal nerve and is the only muscle exclusively supplied by this otherwise primarily sensory nerve. By means of an anastomosis with the auricular branch of the vagus nerve, somatic afferent glossopharyngeal fibers take part in the innervation of the auricle of the external ear. This is important because an early symptom of throat and laryngeal cancer can be ear pain. In other words, the brain sometimes is unable to correctly ascertain the origin of the auriculotemporal nerve (a branch of the mandibular nerve; CN V3) and supply the parotid gland with secretory fibers. This route to the innervation of the parotid gland via the glossopharyngeal nerve is very complex and certainly not intuitive. The tympanic nerve, in addition to carrying fibers that supply the parotid gland, also carries sensory fibers that supply the mucous membranes of the tympanic (middle ear) cavity and the Eustachian tube. Finally, a small branch of the glossopharyngeal nerve, the carotid sinus nerve, leaves the trunk of the main nerve (less frequently, the pharyngeal rami) and runs to the bifurcation of the common carotid artery. Here, it innervates the walls of the carotid sinus, a sensor of blood pressure. Frequently, fibers can also be traced to the adjacent carotid body, a sensor of blood chemistry. This nerve commonly has small anastomoses with the vagus nerve. VAGUS NERVE The nucleus of vagus nerve consist of four nuclei located in the medulla oblongata; three main nuclei (dorso motor nucleus, nucleus ambiguous and solitary nucleus). The fourth nucleus has minor input from the vagus nerve-( the spinal trigeminal nucleus).  1. Dorsal motor nucleus of the vagus nerve- The dorsal vagal nuclei are paired nuclei located in the medial aspect of the dorsal medulla either side of the midline. It forms vagal trigone in the floor of the fourth ventricle at its most rostral extent. Caudally it extends into the cervical spinal segment. It lies dorsal and lateral to the hypoglossal nucleus and dorsomedial to the nucleus ambiguus. 2. The Nucleus ambiguous- A common nucleus of the efferent fibres for glossopharyngeal and vagus nerve. It is located in the reticular formation in the medulla oblongata. 3. Solitary nucleus or Nucleus tractus solitary- Is a series of sensory nuclei (clusters of nerve cell bodies) that forms a vertical column of grey matter in the dorsomedial part of the medulla oblongata. 4. Spinal nucleus of the trigeminal nerve- It descends from the main sensory nucleus of the trigeminal nerve in the pons, through the whole length of the medulla and inro the upper two segments of the spinal cord. The vagus nerve has a very long course and affects speech, digestion, respiration, heart function, and perhaps your body’s immune response. Vagus nerve travels from the head, through the neck and through the chest to terminate at the lower part of your abdomen, specifically near the left colic flexure. This fascinating case underscores the fundamental role that the vagus nerve serves in the regulation of phonation and swallowing. The Xth cranial nerve, the vagus nerve, was formerly known as the pneumogastric nerve because it provides innervation to the lungs and the gastrointestinal tract. Vagus means wandering in Latin, and this is the name that is used today, referring to its very long course from the brain stem to at least the large intestine. It probably descends lower than that, perhaps all the way to the uterus in women. ANATOMICAL COURSE AND INNERVATION The vagus is predominantly a visceral (autonomic; parasympathetic) nerve, with its rootlets emerging from the medulla as a series of fiber bundles just dorsal to the inferior olive. Once joined together, the vagus nerve leaves the skull via the jugular foramen where the nerve shows a small swelling, the jugular ganglion. Immediately below the jugular ganglion, the nerve again thickens to form the large, elongated nodose ganglion. The efferent (somatic and parasympathetic) vagal fibers proceed without interruption through the ganglia to continue with the sensory fibers as the trunk of the vagus nerve in the neck. The numerous visceral afferent vagal fibers, which carry sensory information from the thoracic and abdominal viscera, have their cell bodies in the large nodose ganglion, and the jugular ganglion contains the cell bodies of the limited somatic afferent fibers. In contrast to every other cranial nerve, there are some peripheral differences in the courses of the branches of the right and left vagus nerves. After the vagus nerve exits the jugular foramen, it is found close to the accessory (XI) and glossopharyngeal (IX) nerves and the internal jugular vein (which all leave the skull through the jugular foramen. The vagus nerve descends in the neck, enclosed with the carotid vessels and internal jugular vein in a common sheath of connective tissue. Both vagus nerves enter the thorax, pass posterior to the root of the respective lungs, and form plexuses on the esophagus The right vagal trunk tends to run more posteriorly on the esophagus and the left more anteriorly, a result of the rotation of the gastrointestinal tract during development. The terminal branches pierce the diaphragm with the esophagus to intermingle at the celiac ganglion. The celiac ganglion also receives preganglionic sympathetic fibers from the greater splanchnic nerve from the thorax. These latter nerves synapse at the celiac ganglion. From the ganglion, the postganglionic sympathetic nerves and the preganglionic fibers of the two original vagal trunks piggyback on to the blood supply of the gastrointestinal tract, namely the celiac arterial trunk and its derivatives, and are disseminated to all of the gastrointestinal tract and its associated organs as far as the left colic (splenic) flexure (terminal flexure of the large bowel. Thus, the vagus supplies parasympathetic innervation to the esophagus, stomach, small intestine, much of the large intestine, liver, spleen, suprarenal glands, kidneys, and to the pancreas. When punched severely in the abdomen, pressure on the celiac ganglion causes the feeling of nausea. Although, as noted in the earlier paragraph, the distal extent of the vagus’ range is classically believed to be the left splenic flexure of the large intestine, there is some evidence that its fibers actually extend to the whole colon. Furthermore, they may even go lower into the pelvis and supply some general sensation to the uterus in womb. During their courses in the neck and in the chest, the right and left vagus nerves give off many branches. The canals in the mastoid process, and supplies the dorsal wall of the external auditory meatus and part of the auricle (concha). As a result of this connection, throat pain (e.g., due to cancer) is often felt in the ear because the CNS is often unable to differentiate the location of the origin of a stimulant when multiple nerves converge upon a single shared pathway. Unfortunately, patients can thus sometimes be subjected to many ear examinations and treatments prior to finally being diagnosed with laryngeal cancer. There is a meningeal ramus of the vagus, which re-enters the skull in the jugular foramen and supplies the dura mater in the posterior cranial fossa. This may explain how stimulation of the vagus nerve has been effective in relieving some patients with intractable headache pain. Other small branches of the vagus establish anastomoses with the glossopharyngeal nerve and with the superior cervical ganglion of the sympathetic trunk. The lower fibers of CN XI are often referred to by some anatomists as the “cranial root” of CN XI whereas other anatomists consider these fibers to be part of the vagus nerve. Here, the “cranial root” of XI is considered an integral part of the vagus nerve. The vagus nerve near the jugular foramen also gives off pharyngeal rami that branch from the nodose ganglion and take a descending and anterior course on the pharynx. They take part in the formation of the pharyngeal plexus together with branches from the glossopharyngeal nerve and the cervical sympathetic nerves. Somatic efferent fibers from the plexus supply the muscular pharyngeal constrictors, the soft palate, the levator palate muscle, and the muscles of the palatal arches. Sensory fibers from the plexus innervate the mucous membranes of the pharynx, the most posterior parts of the tongue, and the upper surface of the epiglottis. The superior laryngeal nerve leaves the trunk of the vagus nerve just below the nodose ganglion and courses in an inferior and a slightly anterior direction on the lateral side of the pharynx. It then subdivides into the external laryngeal nerve, which carries chiefly somatic motor fibers to the cricothyroid muscle (a muscle of the larynx), and a larger internal laryngeal nerve.. The latter enters the larynx through the thyrohyoid membrane immediately below the greater horn of the hyoid bone and is distributed within the larynx. The internal laryngeal nerve supplies the mucous membrane of the larynx down to the vocal cords, and thus is a sensory nerve for the larynx. The recurrent laryngeal branch of the vagus is the primary motor nerve for the larynx and therefore is responsible for speech production. It leaves the trunk of the vagus nerve at a higher level on the right than on the left, because the right nerve makes a “U-turn” around the subclavian artery, whereas the left recurrent laryngeal nerve (RLN) makes a similar U-turn, but around the arch of the aorta. Both nerves then ascend (that is why they are referred to as “recurrent”) in the sulcus between the trachea and the esophagus. This strange reversal in the course of the nerve results because of the migration of neck structures during development. The terminal branch of the RLN, the inferior laryngeal nerve, enters the larynx and innervates the intrinsic laryngeal muscles and also supplies sensation to the lower part of the larynx. The RLN also sends fine branches to the trachea and esophagus and some inferior cardiac rami to the heart. Below is an Internet case of a patient who had bilateral RLN injury resulting from thyroid surgery: the most important branches of the vagus nerve to the heart are the superior cardiac rami. These leave the vagus nerve between the points of departure of the superior laryngeal nerve and the recurrent nerve and follow the common carotid artery on the left and the brachiocephalic artery to the heart on the right side. The vagal cardiac nerves (preganglionic parasympathetic) terminate with postganglionic (parasympathetic) neurons in cardiac ganglia located on the muscle of the heart. These connections explain how stimulation of the vagus nerve is used in the treatment of heart failure. The thoracic part of the vagus nerve gives off branches to the esophagus, the bronchi, and the pericardium. The bronchial branches form plexuses that continue along the branches of the bronchial tree into the lung. The postganglionic neurons in the plexuses transmit the visceral efferent impulses to the glands and the smooth muscles of the air passages in the lung. As has been mentioned, the somatic efferent fibers of the vagus nerve innervate the striated musculature of the palate and pharynx and, through the recurrent and the external laryngeal nerves, the muscles of the larynx, thereby controlling speech production. The visceral efferent (parasympathetic) fibers in the vagus nerve are found in the branches of the vagus nerve to the esophagus, trachea, bronchi, heart, stomach and intestines, pancreas, spleen and liver. In addition, a smaller number run in the superior laryngeal nerve to supply the glands of the larynx. The numerous visceral afferent vagus axons have their cell bodies in the nodose ganglion. They mediate afferent impulses from the visceral organs that are supplied by the vagus as well as taste impulses from the most posterior part of the tongue and the epiglottis. The somatic afferent vagus fibers have their cell bodies in the jugular ganglion. The majority of these fibers pass in the auricular ramus, which is purely sensory. The vagus nerve contains fibers of varying diameters and a considerable number of unmyelinated fibers. THE ACCESSORY NERVE (SPINAL ACCESSORY) The spinal accessory nucleus (also called accessory nucleus) lies within the the cervical spinal cord (C1-C5) in the postero lateral aspect of the anterior horn. The nucleus ambiguus is classically said to provide the cranial component of the accessory nerve. The spinal accessory nerve is an unusual cranial nerve in that most of it arises from the spinal cord (C1-C5), passes upward through the foramen magnum entering the skull and then leaves the skull through the jugular foramen. The nerve only functions to innervate two muscles, neither of which are in the head but both of which move the head, the sternocleidomastoid, and the trapezius. The XIth cranial nerve, the (spinal) accessory nerve, only to innervate two muscles, the sternocleidomastoid and the trapezius. Contrary, most other textbooks describe the nerve as consisting of a cranial root and a spinal root. The spinal root is what we are considering to be the true accessory nerve. The cranial root, the portion of the nerve in question here, has been considered to consist of (special) somatic efferent fibers that leave the medulla below the lowermost fibers of the vagus nerve and join the fibers of the spinal root. However, these cranial root fibers are classically reported to leave the spinal nerve trunk (as a so-called internal ramus) almost immediately to join the vagus nerve proper above the nodose ganglion. It is these fibers that are believed to innervate the muscles of the larynx and pharynx including those involved in speech production. In 2002, an article by Lachman and colleagues reported that there is really no cranial root of the accessory nerve. They reported that the fibers that emerge from the medulla do not actually join the accessory nerve but join the vagus directly and have no association with XI. Although in accordance with Lachman et al’ he said that in order for a nerve to be a “cranial nerve,” it has to originate from the brain and exit through a foramen in the skull. The “spinal root” of the accessory nerve only meets one of these criteria. Rather, it is a “spinal nerve with a cranial exit.” However, for historical reasons, we will continue to consider it a cranial nerve. ANATOMICAL COURSE AND INNERVATION The XIth nerve is formed by bundles of somatic efferent fibers that emerge from the lateral column of the spinal cord in the upper cervical segments and together make up the nerve. The fibers ascend within the vertebral canal, fuse with each other, and enter the cranial cavity through the foramen magnum. They then make an almost 180° turn and exit the skull through the jugular foramen. From the jugular foramen, the accessory nerve descends in close relation to the internal jugular vein (IJV) (usually posterior, more rarely anterior, to it). It then enters the sternocleidomastoid muscle on its medial aspect. During its further posteroinferior course, the nerve, as a rule, is embedded in this muscle and gives off motor branches to it. Approximately at the middle of the posterior aspect of the sternocleidomastoid muscle, the accessory nerve leaves the muscle and continues inferiorly and somewhat posteriorly (surrounded by cervical lymph nodes) and finally reaches the trapezius muscle, to which it gives off its terminal branches. In the neck, the accessory nerve anastomoses with nerve filaments from cervical nerves derived directly from the spinal cord, chiefly from the 3rd and 4th cervical segments. There is some indication that these fibers may supply a limited amount of motor innervation to the trapezius, as well as proprioceptive fibers to this muscle and also to the sternocleidomastoid. The reason that the accessory nerve takes such a strange course to innervate the sternocleidomastoid and trapezius muscles is based on the embryology of the growth and formation of the two muscles. These muscles are unique in their development, having elements from both the head and the trunk regions. Furthermore, these muscles are critical to moving the head in response to hearing or seeing a new object (e.g., a predator), and thus there is some admittedly anecdotal reason why these muscles are innervated by a “cranial nerve.” However, we do also admit that what the accessory nerve does is a bit “out of character” for a cranial nerve and contributes to our overall wonder and amazement. HYPOGLOSSAL NERVE Hypoglossal nucleus: The hypoglossal nuclei are located within the tegmentum of the upper medulla close to the midline, lying deep to the hypoglossal trigone or hypoglossal triangle, the medial eminence of the floor of the fourth ventricle, inferior to the stria medullaris. It is a motor nucleus found in the ventral medulla on the floor of the fourth ventricle. Overview: The hypoglossal nerve, the XIIth cranial nerve, is the motor nerve to the muscles of the tongue. The hypoglossal nerve is composed of nerve fibers that enable you to perform these movements without thought. The variability among individuals in their ability to conduct some tongue movements (e.g., whether one is able to curl one’s tongue) probably reflects variation in the connections of this nerve. ANATOMICAL COURSE AND INNERVATION The axons of the hypoglossal nucleus run in a ventral and slightly lateral direction and emerge as 10–15 fiber bundles on the ventral surface of the lower medulla. The rootlets fuse and leave the skull through the hypoglossal canal in the occipital bone. The nerve then bends behind the vagus nerve and the internal carotid artery; a course caudally, lateral to these structures, and then continues in an inferior convex arch to the root of the tongue. A little above the greater horn of the hyoid bone, it disappears between the mylohyoid and hyoglossus muscles and branches to the intrinsic and all but one of the extrinsic muscles of the tongue. The extrinsic tongue muscles innervated by CN XII are the hyoglossus, genioglossus, and styloglossus. The tongue muscle not innervated by the hypoglossal nerve, the palatoglossus, is innervated by CN X. During its peripheral course in the neck, the nerve passes below the posterior belly of the digastric muscle and lateral to the internal and external carotid arteries and the branches of the latter. In the tongue, the hypoglossal fibers anastomose (join) with branches from the lingual nerve. The fibers of the hypoglossal nerve are joined by some sympathetic (postganglionic) fibers from the superior cervical ganglion of the sympathetic trunk and by fiber bundles from the 1st and 2nd cervical nerves. The latter fibers “piggy-back” on to the nerve, but do not form part of the nerve proper. This is often a confusing point for beginning students to appreciate. These cervical fibers follow CN XII until approximately where they cross the internal carotid artery. Here they leave it again and descend as the superior root of the ansa-cervicalis, which courses in a caudal direction on the internal carotid artery and the upper part of the common carotid artery. Ansa means loop in Latin, whereas cervicalism is the adverb of the term describing its physical location in the neck. The ansa cervicalis is a prominent structure of the neck, although during cadaveric dissections it can be easily missed because it becomes embedded and often intertwined within the connective tissue fascial planes of the neck. Superficial to the carotid sheath, the superior root unites with the inferior root, which is formed by fibers from the 2nd and 3rd cervical nerves. In this way, a loop of fibers, the ansa cervicalis is formed. It gives off branches to three of the infrahyoid muscles (the sternohyoid, omohyoid, and sternothyroid muscles; while the fourth of these, the thyrohyoid muscle, is supplied by cervical fibers that follow the hypoglossal nerve proper beyond the departure of the superior root of the ansa. However, the tongue has a complex embryologic origin and therefore actually has five cranial nerves involved in its innervation: V, VII, IX, X, and XII. GENERAL NUCLEI OF CRANIAL NERVES