There are twelve (XII), bilaterally paired, cranial nerves; these carry afferent and efferent fibres between the brain and peripheral structures, principally of the head and neck. The cranial nerves are individually named and numbered (Roman numerals) according to the rostrocaudal sequence in which they attach to the brain (Fig. 10.1 and Table 10.1):
Figure 10.1 The base of the brain illustrating the locations of the cranial nerves. The points of attachment are shown, except for the trochlear nerve, which arises from the dorsal aspect of the brain stem.
Table 10.1Cranial nerves
The first two cranial nerves attach directly to the forebrain, while the rest attach to the brain stem. The olfactory system is closely associated, both structurally and functionally, with parts of the forebrain collectively referred to as the limbic system; these, including cranial nerve I, are considered together in Chapter 16. The visual system and cranial nerve II are described in Chapter 15. Cranial nerves III–XII are associated with various nuclei within the brain stem, called the cranial nerve nuclei, which either receive cranial nerve afferents or contain the cell bodies of efferent neurones that have axons leaving the brain in cranial nerves. The locations of these nuclei are illustrated in Figure 10.2.
Figure 10.2 The brain stem viewed from the dorsal aspect. The diagram illustrates the locations of the afferent cranial nerve nuclei (left) and the efferent cranial nerve nuclei (right). On the right, nuclei shaded in the same colour share a common embryological origin.
Cranial nerve nuclei
Fibres carrying general sensory information (touch, pressure, pain, temperature) from the head enter the brain through the trigeminal nerve at the level of the pons and terminate in the trigeminal sensory nucleus. This is a large nucleus that runs the whole length of the brain stem and extends caudally into the cervical spinal cord. Fibres conveying the special senses of motion/positional sense and hearing run in the vestibulocochlear nerve. They terminate in the vestibular and cochlear nuclei, respectively, which are located in the medulla, in and near to the lateral part of the floor of the fourth ventricle (sometimes referred to as the vestibular area). Visceral afferents, including taste fibres, terminate in the nucleus solitarius of the medulla.
On the basis of their embryological derivation, the efferent cranial nerve nuclei can be divided into three groups, each lying in a discontinuous longitudinal column.
Nuclei of the somatic efferent cell column
The somatic efferent cell column lies near to the midline and consists of the nuclei of the III, IV, VI and XII nerves. The oculomotor nucleus lies in the ventral apex of the periaqueductal grey of the midbrain at the level of the superior colliculus. Its efferent fibres run in the oculomotor nerve to innervate the levator palpebrae superioris and all of the extraocular muscles, except the superior oblique and lateral rectus. The trochlear nucleus also lies in the midbrain, at the ventral border of the periaqueductal grey, but at the level of the inferior colliculus. Fibres leave in the trochlear nerve to innervate the superior oblique muscle of the eye. The abducens nucleus is located in the caudal pons beneath the floor of the fourth ventricle. Its efferents run in the abducens nerve and they innervate the lateral rectus muscle. In the medulla lies the hypoglossal nucleus, which innervates the intrinsic and extrinsic muscles of the tongue via the hypoglossal nerve.
Nuclei of the branchiomotor cell column
The branchiomotor cell column innervates striated muscles derived from the branchial arches. In the tegmentum of the mid-pons is located the trigeminal motor nucleus, which supplies fibres to the trigeminal nerve and innervates the muscles of mastication, tensor tympani, tensor veli palitini, mylohyoid and the anterior belly of the digastric muscle. In the caudal pontine tegmentum lies the facial motor nucleus. This innervates the muscles of facial expression and the stapedius muscle via the facial nerve. Within the medulla lies the nucleus ambiguus. This long nucleus sends motor fibres in the glossopharyngeal, vagus and cranial part of the accessory nerve to innervate the muscles of the pharynx and larynx.
Nuclei of the parasympathetic cell column
The parasympathetic cell column consists of preganglionic parasympathetic neurones that send axons into the III, VII, IX and X cranial nerves. The most rostral cell group constitutes the Edinger–Westphal nucleus, which lies in the midbrain periaqueductal grey matter adjacent to the oculomotor nucleus. Its axons leave in the oculomotor nerve and pass to the ciliary ganglion in the orbit, from which postganglionic fibres innervate the sphincter pupillae and ciliary muscles within the eye.
In the pontine tegmentum lie two cell groups, the superior and inferior salivatory nuclei. The superior salivatory nucleus supplies preganglionic fibres to the facial nerve that terminate in the pterygopalatine and submandibular ganglia. Postganglionic fibres from the pterygopalatine ganglion innervate the lacrimal gland and the nasal and oral mucous membranes. Those from the submandibular ganglion innervate the submandibular and sublingual salivary glands. The inferior salivatory nucleus sends preganglionic fibres into the glossopharyngeal nerve. These terminate in the otic ganglion, which in turn sends postganglionic axons to the parotid salivary gland.
The largest preganglionic parasympathetic cell group lies in the medulla and constitutes the dorsal motor nucleus of the vagus. Its rostral portion lies immediately beneath the floor of the fourth ventricle, lateral to the hypoglossal nucleus. Fibres leave in the vagus nerve and are widely distributed to thoracic and abdominal viscera.
III: Oculomotor nerve
The oculomotor nerve carries the majority of somatic motor neurones that innervate extraocular muscles and are responsible for moving the eye. It also contains preganglionic parasympathetic neurones that, via the intermediary of the ciliary ganglion, control the smooth muscle within the eye.
The motor neurones serving the extraocular muscles have their cell bodies in the oculomotor nucleus, which lies at the base of the periaqueductal grey of the midbrain at the level of the superior colliculus (Fig. 10.3). Preganglionic parasympathetic neurones arise from the nearby Edinger–Westphal nucleus. Fibres from both sources course ventrally through the midbrain tegmentum, many of them traversing the red nucleus, to exit on the medial aspect of the crus cerebri, within the interpeduncular fossa (Fig. 10.4). The oculomotor nerve passes between the posterior cerebral and superior cerebellar arteries, then runs anteriorly, lying in the wall of the cavernous sinus (Fig. 7.11), before gaining access to the orbit through the superior orbital fissure. The oculomotor nerve supplies all of the extraocular muscles, with the exception of the superior oblique and lateral rectus and, thus, functions to elevate, depress and adduct the eyeball (Fig. 10.5). It also innervates the striated muscle of the levator palpebrae superioris, serving to elevate the upper eyelid. Preganglionic parasympathetic neurones terminate in the ciliary ganglion, located within the orbit, behind the eyeball. From here postganglionic neurones run in the short ciliary nerves to innervate the sphincter (constrictor) pupillae muscle of the iris and the ciliary muscle contained within the ciliary body.
Figure 10.3 Transverse section through the midbrain at the level of the superior colliculus. The diagram shows the origin and course of oculomotor nerve fibres within the brain stem.
Figure 10.4 Ventral aspect of the brain showing the points of attachment of cranial nerves I, II and III.
Figure 10.5 Eye movements brought about by the extraocular muscles.
Pupillary light reflex
The amount of light entering the eye is regulated by the size of the pupil. Illumination of the retina causes constriction of the pupil through contraction of the sphincter pupillae muscle of the iris, thus reducing the amount of light reaching the retina. This is known as the direct light reflex (Fig. 10.6). Even if only one retina is illuminated (e.g. during clinical examination) the pupils of both eyes constrict. The constriction of the pupil of the non-illuminated eye is called the consensual light reflex. The afferent limb of the light reflex consists of a small contingent of optic tract fibres that pass directly from the eye to the pretectal area, just rostral to the superior colliculus, rather than to the lateral geniculate nucleus of the thalamus where the majority of visual fibres terminate (see also Ch. 15). Neurones of the pretectal area project bilaterally to the Edinger–Westphal nuclei, from which efferent fibres leave in the oculomotor nerve.
Figure 10.6 Schematic representation of a transverse section through the most rostral part of the midbrain. The diagram shows the pathways involved in the pupillary light reflex.
Fixation upon a nearby object, by convergence of the optic axes, involves concomitant contraction of the ciliary muscles to increase the convexity of the lens, thus focusing the image. It is also accompanied by pupillary constriction. The phenomenon involves the visual cortex, with corticobulbar fibres activating the parasympathetic neurones of the Edinger–Westphal nuclei bilaterally.
IV: Trochlear nerve
The trochlear nerve contains only somatic motor neurones. These arise in the trochlear nucleus, which lies in the ventral part of the midbrain periaqueductal grey, at the level of the inferior colliculus (Fig. 10.7). The efferent axons pass dorsally, around the periaqueductal grey, and decussate in the midline. The trochlear nerve emerges from the dorsal aspect of the brain stem (the only cranial nerve to do so) just caudal to the inferior colliculus (Fig. 10.8). The nerve courses round the cerebral peduncle to gain the ventral aspect of the brain (Fig. 10.9), passing between the posterior cerebral and superior cerebellar arteries, as does the oculomotor nerve. It then runs anteriorly, lying in the lateral wall of the cavernous sinus (Fig. 7.11) and enters the orbit through the superior orbital fissure. It supplies just one muscle, the superior oblique. The action of the superior oblique is complex. Depending upon the starting position of the eyeball it can act to depress, abduct or intort the eye. Importantly, with the eye adducted, the superior oblique depresses the visual axis. A lesion of the trochlear nerve may, therefore, become manifest by diplopia on looking medially and downwards (e.g. when walking downstairs).
Figure 10.7 Transverse section through the midbrain at the level of the inferior colliculus. The diagram shows the location of the trochlear nucleus and the course of trochlear nerve fibres.
Figure 10.8 Dorsal aspect of the brain stem, after removal of the cerebellum, showing the origin of the cranial nerve IV.
Figure 10.9 Ventral aspect of the brain showing cranial nerves III, IV and V.
VI: Abducens nerve
The abducens nerve, like the trochlear, contains only somatic motor neurones. The cell bodies of origin are located in the abducens nucleus, which lies beneath the floor of the fourth ventricle in the caudal pons (Fig. 10.10). Efferent axons course ventrally through the pons and emerge from the ventral surface of the brain stem at the junction between the pons and the pyramid of the medulla (Fig. 10.11). The nerve then passes anteriorly, through the cavernous sinus (Fig. 7.11) and enters the orbit through the superior orbital fissure. The abducens supplies the lateral rectus muscle, which abducts the eye.
Figure 10.10 Transverse section through the caudal pons. The diagram shows the location of the abducens nucleus and the course of abducens nerve fibres.
Figure 10.11 Ventral aspect of the brain showing the points of attachment of cranial nerves VI to IX.
Lesions of cranial nerves III, IV and VI
A third cranial nerve palsy (Fig. 10.12) is caused by a lesion of the oculomotor nucleus within the midbrain or by compression of the peripheral course of cranial nerve III by an aneurysm or tumour. It leads to drooping of the eyelid (ptosis), dilation of the pupil that is unresponsive to light and accommodation, and an inability to move the eyeball upwards, downwards or inwards (adduction).
Figure 10.12 Right oculomotor (III) nerve palsy. The upper part of the diagram shows ptosis on the right. The lower part shows that, with elevation of the eyelid, the eyeball can be seen to be abducted and the pupil dilated.
A sixth cranial nerve palsy (Fig. 10.13) is caused by a lesion of the abducens nucleus in the pons or by compression of the peripheral course of the nerve by an aneurysm or tumour. It leads to an inability to move the eye outwards (abduction).
Figure 10.13 Left abducens (VI) nerve palsy. On looking in the direction of the arrow, the left eye fails to abduct.
Combined unilateral palsies of cranial nerves III, IV and VI occur from lesions along their peripheral course where the nerves run adjacent to each other, such as within the cavernous sinus, at the entrance to the orbit (superior orbital fissure syndrome) and within the orbit. There they are vulnerable to compression by tumours and aneurysms. The effects of such unilateral lesions are:
dilation of the pupil that is unresponsive to light or accommodation
paralysis of all eye movements (ophthalmoplegia) causing double vision (diplopia).
V: Trigeminal nerve
The trigeminal nerve has both sensory and motor components. It is the main sensory nerve for the head and, additionally, it innervates the muscles of mastication. It attaches to the brain stem as two adjacent roots (a large sensory and a smaller motor root) on the ventrolateral aspect of the pons, where this merges with the middle cerebellar peduncle (Figs 10.1, 10.9, 10.11).
The sensory fibres of the trigeminal nerve are primary sensory neurones with peripheral processes distributed via the ophthalmic, maxillary and mandibular divisions of the trigeminal to numerous structures of the head (Fig. 10.14). Sensations of touch, pressure, pain and temperature are relayed from the face and scalp, the cornea, the nasal and oral cavities, including the teeth and gums and the paranasal sinuses. The trigeminal nerve also innervates the intracranial dura mater and intracranial arteries. In addition, proprioceptive fibres are carried from the muscles of mastication and the temporomandibular joint. The cell bodies of afferents in the trigeminal nerve, with the exception of those conveying proprioception, are located in the trigeminal (or semilunar) ganglion, located at the convergence of the ophthalmic, maxillary and mandibular nerves. The central processes of these cells terminate in the trigeminal sensory nucleus (Fig. 10.15).
Figure 10.14 Superficial distribution of sensory fibres in the three divisions of the trigeminal nerve.
Figure 10.15 The brain stem indicating the location of the trigeminal sensory nucleus and its major connections.
The trigeminal sensory nucleus is a large nucleus that extends throughout the length of the brain stem and into the upper cervical spinal cord. It is considered to consist of three subnuclei. The chief (or principal) sensory nucleus lies in the pontine tegmentum close to the entry of the trigeminal nerve. The mesencephalic nucleus extends rostrally into the midbrain. The spinal nucleus (or nucleus of the spinal tract of the trigeminal) extends caudally through the medulla and into the cord, where it becomes continuous with the substantia gelatinosa, of which it is considered to be the brain stem homologue.
There is a segregated distribution of afferent fibre termination in the trigeminal nucleus, depending upon the modality. Fibres conveying touch and pressure terminate in the principal nucleus. Those carrying pain and temperature end in the spinal nucleus, reaching their termination by descending in the spinal tract of the trigeminal, a fascicle of fibres lying immediately superficial to the nucleus (Figs 9.5, 9.6). In the upper cervical cord, the spinal tract of the trigeminal becomes continuous with Lissauer’s tract, which carries functionally homologous afferents of spinal nerve origin prior to their termination in the dorsal horn. Proprioceptive afferents from the muscles of mastication and the temporomandibular joint have their cell bodies not in the trigeminal ganglion, as might be expected, but in the mesencephalic nucleus of the trigeminal. They are the only primary afferents to have their cell bodies located within the CNS. Axons arising from second-order neurones in the trigeminal nucleus decussate to form the contralateral trigeminothalamic tract (trigeminal lemniscus). This terminates in the contralateral ventral posterior nucleus of the thalamus, which in turn sends fibres to the sensory cortex of the parietal lobe. In addition, the trigeminal nucleus sends fibres to the cerebellum and also establishes reflex connections with certain motor cell groups of the brain stem. Prominent among these is the facial nucleus, through which are mediated facial grimacing and eye closure (corneal reflex) in response to noxious stimulation in the territory served by the trigeminal nerve.
Lesions of the trigeminal nerve
Herpes zoster infection of the sensory roots of the trigeminal nerve (shingles) leads to pain and the eruption of vesicles localised to the dermatome supplied by one or other of the ophthalmic, maxillary and mandibular branches of the trigeminal nerve.
In syringobulbia, central cavitation of the medulla caudal to the fourth ventricle leads to preferential compressive destruction of decussating trigeminothalamic fibres, causing selective loss of pain and temperature sensibility (dissociated sensory loss) in the face (c.f. syringomyelia, p. 79).
The motor axons of the trigeminal nerve arise from cells in the trigeminal motor nucleus, which lies in the pontine tegmentum medial to the principal sensory nucleus (Fig. 10.2). Axons leave the pons in the motor root of the trigeminal and then join the mandibular division of the nerve. They innervate various muscles, the most significant of which are the muscles of mastication (masseter and temporalis, which close the jaw, and the lateral and medial pterygoids, which open the jaw). Trigeminal motor neurones also innervate tensor tympani within the middle ear, tensor veli palatini, mylohyoid and the anterior belly of digastric.
VII: Facial nerve
The facial nerve contains sensory, motor and parasympathetic components (Fig. 10.16). It joins the brain stem at the ventrolateral aspect of the caudal pons (Fig. 10.11), near the pontomedullary junction, in a region known as the cerebellopontine angle. The nerve consists of two roots, the more lateral (sometimes called the nervus intermedius) containing sensory and parasympathetic fibres, the more medial root being composed of motor axons.
Figure 10.16 Component fibres of the facial nerve and their peripheral distribution. (Red, motor; blue, sensory; purple/orange, parasympathetic.)
The sensory fibres of the facial nerve supply taste sensation from the anterior two-thirds of the tongue, the floor of the mouth and palate, and also cutaneous sensation from part of the external ear. The cell bodies of primary afferent neurones lie in the geniculate ganglion within the facial canal of the petrous temporal bone. The central processes of taste fibres terminate in the rostral part of the nucleus solitarius of the medulla. Ascending fibres from the nucleus solitarius project to the ventral posterior nucleus of the thalamus, which in turn sends fibres to the sensory cortex of the parietal lobe. Afferent facial nerve fibres that carry cutaneous sensation terminate in the trigeminal nucleus.
Motor fibres of the facial nerve originate in the facial motor nucleus of the caudal pontine tegmentum (Fig. 10.17). The axons initially pass dorsally, looping over the abducens nucleus beneath the floor of the fourth ventricle, before leaving the brain stem in the motor root of the facial nerve. Motor fibres are distributed to the muscles of facial expression, platysma, stylohyoid, the posterior belly of the digastric muscle and the stapedius muscle of the middle ear.
Figure 10.17 Transverse section through the pons. The diagram shows the origin and course of the motor fibres of the facial nerve.
The facial motor nucleus receives afferents from other brain stem areas for the mediation of certain reflexes and also from the cerebral cortex. Reflex connections are established that mediate protective eye closure in response to visual stimuli or tactile stimulation of the cornea (corneal reflex) through fibres from the superior colliculus and the trigeminal sensory nucleus, respectively. In addition, fibres from the superior olivary nucleus, a part of the central auditory pathway, subserve reflex contraction of the stapedius muscle in response to loud noise.
Corticobulbar fibres from motor cortical areas innervate the facial motor nucleus. Those that control motor neurones supplying the muscles of the upper face (frontalis, orbicularis oculi) are distributed bilaterally. Those that control the motor neurones supplying the muscles of the lower face are entirely crossed. Unilateral upper motor neurone lesions, therefore, give rise to paralysis of the lower facial muscles.
Preganglionic parasympathetic fibres of the facial nerve originate in the superior salivatory nucleus of the pons. Fibres leave the brain stem in the sensory root of the facial nerve (nervus intermedius). From here they pass to parasympathetic ganglia, namely the submandibular and pterygopalatine ganglia, where they synapse with postganglionic neurones. Postganglionic fibres from the submandibular ganglion innervate the submandibular and sublingual salivary glands. Those from the pterygopalatine ganglion innervate the lacrimal gland and the nasal and oral mucous membranes.
Bell’s palsy represents an acute unilateral inflammatory lesion of the facial nerve within its course through the skull. Pain is experienced around the ear and there is paralysis of the facial muscles unilaterally with failure to close the eye, an absent corneal reflex, hyperacusis on the affected side and loss of taste sensation on the anterior two-thirds of the tongue. When the herpes zoster virus is the inflammatory agent, a vesicular rash is apparent in the external auditory canal and the mucous membrane of the oropharynx (Ramsay Hunt syndrome).
VIII: Vestibulocochlear nerve
The vestibulocochlear nerve is a sensory nerve that conveys impulses from the inner ear. It has two component parts: the vestibular nerve, which carries information related to position and movement of the head, and the cochlear nerve, which carries auditory information. Both divisions contain the axons of first-order sensory neurones, the dendrites of which make contact with hair cells of either the vestibular or auditory apparatus of the inner ear. Both divisions pass together through the internal auditory meatus (which also contains the facial nerve) and attach to the brain stem at the junction of the medulla and pons (Fig. 10.11), in the region known as the cerebellopontine angle.
The vestibular nerve
The vestibular nerve fibres make dendritic contact with hair cells of the vestibular portions of the membranous labyrinth; their cell bodies are located in the vestibular ganglion within the internal auditory meatus. The central processes of vestibular fibres mostly end in the vestibular nuclei of the rostral medulla. There are four such nuclei (superior, inferior, medial and lateral vestibular nuclei) located close together beneath the lateral part of the floor of the fourth ventricle.
The vestibular nuclei establish contact with a number of other regions for the control of posture, maintenance of equilibrium, coordination of head and eye movements, and the conscious awareness of vestibular stimulation. Fibres from the lateral vestibular (Deiters’) nucleus descend ipsilaterally in the lateral vestibulospinal tract. The vestibular nuclei also contribute fibres to the medial longitudinal fasciculus. This extends throughout the brain stem and into the spinal cord; its descending component is also known as the medial vestibulospinal tract. Vestibulospinal fibres influence the activity of spinal motor neurones concerned with the control of body posture and balance. The ascending part of the medial longitudinal fasciculus establishes connections with the nuclei of the abducens, trochlear and oculomotor nerves for the coordination of head and eye movements. Some efferent fibres from the vestibular nuclei pass through the inferior cerebellar peduncle to the flocculonodular lobe of the cerebellum, which is concerned with the control of equilibrium. Other fibres ascend to the contralateral thalamus (ventral posterior nucleus), which in turn projects to the cerebral cortex. The cortical region responsible for conscious awareness of vestibular sensation is uncertain but is probably adjacent to the ‘head’ area of the sensory cortex in the parietal lobe or adjacent to the auditory cortex in the temporal lobe.
The cochlear nerve
The cochlear nerve fibres make dendritic contact with hair cells of the organ of Corti within the cochlear duct of the inner ear. The cell bodies of these fibres lie within the cochlea and are collectively called the spiral ganglion. The cochlear nerve joins the brain stem at the level of the rostral medulla. Its fibres bifurcate and end in the dorsal and ventral cochlear nuclei, which lie close to the inferior cerebellar peduncle. From here, the ascending auditory pathway to the thalamus and cerebral cortex (Fig. 10.18) is somewhat more complicated and variable than that for the general senses.
Figure 10.18 Principal ascending connections of the auditory component of the vestibulocochlear nerve.
There are several locations between the medulla and thalamus where axons may synapse and not all fibres in the pathway behave in the same manner. From the cochlear nuclei, second-order neurones ascend into the pons, some of them crossing to the other side of the pontine tegmentum as the trapezoid body. At this level, some fibres may terminate in the superior olivary nucleus. This nucleus is the site of origin of olivocochlear fibres, which leave the brain stem in the vestibulocochlear nerve and end in the organ of Corti. They have an inhibitory function and serve to modulate transmission of auditory information to the cochlear nerve. From the superior olivary nuclei, ascending fibres comprise the lateral lemniscus, which runs through the pontine tegmentum to end in the inferior colliculus of the midbrain. Some axons within the lateral lemniscus terminate in a small pontine nucleus called the nucleus of the lateral lemniscus. The superior olivary nucleus and the nucleus of the lateral lemniscus are thought to establish reflex connections with motor neurones of the trigeminal and facial motor nuclei, mediating contraction of the tensor tympani and stapedius muscles in response to loud noise. The inferior colliculus, in turn, sends axons to the medial geniculate nucleus of the thalamus. The final step in the ascending auditory pathway consists of axons that originate in the medial geniculate nucleus and pass through the internal capsule to the primary auditory cortex of the temporal lobe. This is located predominantly in Heschl’s gyri (Fig. 13.17), which are situated on the dorsal surface of the superior temporal gyrus and, therefore, are largely hidden within the lateral fissure. Throughout the ascending auditory projection, there exists a so-called ‘tonotopical’ representation of the cochlea, which is analogous to the ‘somatotopic’ organisation of the pathways for general sensation. Within the brain stem, some ascending fibres decussate while others do not. The representation of the cochlea is, therefore, bilateral at all levels rostral to the cochlear nuclei. For this reason, unilateral lesions of the ascending auditory pathway do not cause monaural deafness but, rather, are manifest as a loss of auditory acuity and an inability to localise the directional origin of sounds.
The region of the temporal lobe surrounding the primary auditory cortex is known as the auditory association cortex or Wernicke’s area. It is here that auditory information is interpreted and given contextual significance. Wernicke’s area is important in the processing of language by the brain (Ch. 13).
An acoustic neuroma is a benign tumour of the eighth cranial nerve which leads to compression of the nerve and adjacent structures in the cerebellopontine angle. Attacks of dizziness accompanied by profound deafness occur; with expansion of the tumour, ataxia and paralysis of the cranial nerves (especially V–VII) and the limbs follow. Unilateral and bilateral acoustic neuromas occur in the inherited disease neurofibromatosis, in which tumours of the peripheral nerves and skin (neurilemmoma and neurofibroma) can cause cosmetic blemish and deformity.
IX: Glossopharyngeal nerve
The glossopharyngeal nerve is principally a sensory nerve, although it also contains preganglionic parasympathetic fibres and a few motor fibres. It attaches to the brain stem as a linear series of small rootlets, lateral to the olive in the rostral medulla (Figs 10.11, 10.19).
Figure 10.19 Ventral aspect of the brain stem showing the points of attachment of cranial nerves VIII–XII.
The afferent fibres of the glossopharyngeal nerve convey information from:
receptors for general sensation in the pharynx, the posterior third of the tongue, Eustachian tube and middle ear
taste buds of the pharynx and the posterior third of the tongue
chemoreceptors in the carotid body and baroreceptors in the carotid sinus.
Within the brain stem, afferent fibres for general sensation end in the trigeminal sensory nucleus. Fibres carrying touch information from the pharynx and back of the tongue are important for mediating the gag reflex, through connections with the nucleus ambiguus and the hypoglossal nucleus. Visceral and taste fibres of the glossopharyngeal nerve terminate in the nucleus solitarius of the medulla.
The motor component of the glossopharyngeal nerve is very small. It arises from cells in the rostral part of the nucleus ambiguus of the medulla and innervates just one muscle, the stylopharyngeus, which is involved in swallowing.
Preganglionic parasympathetic fibres in the glossopharyngeal nerve originate in the inferior salivatory nucleus of the rostral medulla. These synapse with postganglionic neurones in the otic ganglion, which in turn innervate the parotid salivary gland.
X: Vagus nerve
Rootlets of the vagus nerve attach to the lateral aspect of the medulla immediately caudal to the glossopharyngeal nerve (Fig. 10.19). The vagus contains afferent, motor and parasympathetic fibres.
The afferent fibres of the vagus convey information from:
receptors for general sensation in the pharynx, larynx, oesophagus, tympanic membrane, external auditory meatus and part of the concha of the external ear
chemoreceptors in the aortic bodies and baroreceptors in the aortic arch
receptors widely distributed throughout the thoracic and abdominal viscera.
Within the brain stem, receptors for general sensation end in the trigeminal sensory nucleus, while visceral afferents end in the nucleus solitarius.
The motor fibres of the vagus (Fig. 10.20) arise from the nucleus ambiguus of the medulla. They innervate the muscles of the soft palate, pharynx, larynx and upper part of the oesophagus. The nucleus ambiguus is, therefore, crucially important in the control of speech and swallowing. By convention, the most caudal efferents from the nucleus ambiguus are regarded as leaving the brain stem in the cranial roots of the accessory nerve, but these transfer to the vagus nerve proper at the level of the jugular foramen (Fig. 10.20).
Figure 10.20 The caudal medulla and rostral spinal cord. The diagram illustrates the origin and course of the motor fibres of the vagus and accessory nerves.
The parasympathetic fibres of the vagus nerve originate from the dorsal motor nucleus of the vagus, which lies in the medulla immediately beneath the floor of the fourth ventricle. They are distributed widely throughout the cardiovascular, respiratory and gastrointestinal systems.
XI: Accessory nerve
The accessory nerve is purely motor in function. It consists of two parts: cranial and spinal. The cranial part emerges from the lateral aspect of the medulla as a linear series of rootlets that lie immediately caudal to the rootlets of the vagus nerve (Figs 10.19, 10.20). The cranial root of the accessory nerve carries fibres that have their origin in the caudal part of the nucleus ambiguus of the medulla. At the level of the jugular foramen these fibres join the vagus nerve and are distributed with it to the muscles of the soft palate, pharynx and larynx.
The spinal root of the accessory nerve arises from motor neurones located in the ventral horn of the spinal grey matter at levels C1–C5 (Fig. 10.20). The axons leave the cord not through the ventral roots of spinal nerves but via a series of rootlets that emerge from the lateral aspect of the cord midway between the dorsal and ventral roots. These rootlets course rostrally, coalescing as they do so, and enter the cranial cavity through the foramen magnum. At the side of the medulla, the spinal root of the accessory nerve briefly joins the cranial root, but the component fibres separate once again as the nerve leaves the cranial cavity through the jugular foramen. Here the fibres of the cranial root of the accessory, which are derived from the nucleus ambiguus, join the vagus and are distributed with it. The fibres of the spinal root pass to the sternomastoid and trapezius muscles, which serve to move the head and shoulders.
XII: Hypoglossal nerve
The hypoglossal nerve is purely motor in function. It innervates both the extrinsic and intrinsic muscles of the tongue and, therefore, serves both to move the tongue and to change its shape. The axons originate in the hypoglossal nucleus, which lies immediately beneath the floor of the fourth ventricle, near the midline (Fig. 10.21). Axons course ventrally through the medulla and emerge from its ventrolateral aspect as a linear series of rootlets located between the pyramid and the olive (Fig. 10.19). The hypoglossal nucleus receives afferents from the nucleus solitarius and the trigeminal sensory nucleus. These are involved in the control of the reflex movements of chewing, sucking and swallowing. It also receives corticobulbar fibres from the contralateral motor cortex, which subserve voluntary movements of the tongue such as occur in speech.
Figure 10.21 Transverse section through the medulla. The diagram shows the origin and course of the fibres of the hypoglossal nerve.
Motor neurone disease and lesions of cranial nerves IX–XII
Motor neurone disease is a chronic degenerative disorder seen in those aged over 50 years. The corticobulbar tracts projecting to the nucleus ambiguus and hypoglossal nucleus degenerate, leading to dysphonia (difficulty in phonation), dysphagia (difficulty in swallowing), dysarthria (difficulty in articulation) and weakness and spasticity of the tongue (pseudobulbar palsy). There is also degeneration of the nucleus ambiguus and hypoglossal nuclei themselves, leading to dysphonia, dysphagia, dysarthria and weakness, wasting and fasciculation of the tongue (bulbar palsy).
The IX, X, XI and XII nerves can be damaged by compression in their peripheral course as they exit the cranium via the foramina of the skull base. Tumours in this area lead to dysphonia and depression of the gag reflex, together with unilateral wasting of the sternomastoid and trapezius muscles (jugular foramen syndrome) and unilateral weakness, wasting and fasciculation of the tongue.