Other Motor Functions
Preganglionic Parasympathetic Fibers
General Sensory Functions
The cranial nerves, listed in the order in which numbers are assigned to them, are as follows. These numbers were introduced by von Sömmering in 1798.
An extremely thin nervus terminalis, which lies along the medial side of the olfactory bulb and tract, is sometimes numbered as cranial nerve 0. The nervus terminalis serves as the conduit along which a population of neurons migrates from the olfactory placode (a region of ectoderm of the embryonic nose) into the preoptic area and hypothalamus. These neurons are essential for reproductive function in both sexes (see Chapter 11).
Cranial nerves I, II, and VIII serve the olfactory, visual, auditory, and vestibular systems and are therefore discussed in Chapters 17, 20, 21, and 22, respectively. The special sense of taste (i.e., from the gustatory system) is dealt with in this chapter because the primary sensory neurons for taste are located in the same ganglia as sensory neurons that have other functions in cranial nerves VII, IX, and X.
This chapter has two major parts. The first is devoted to eye movements, and it includes information about the control of muscles supplied by cranial nerves III, IV, and VI. The second part of the chapter is concerned with the other cranial nerves, with the exceptions of I, II, and VIII, noted in the preceding paragraph. The central gustatory pathway is described in association with the facial nerve.
The Ocular Motor System
The control of eye movements, a complicated subject, is treated partly in this chapter and partly in Chapter 22. For the benefit of those who are unfamiliar with the anatomy of the muscles that move the eyeball, the actions of
these muscles are reviewed in Figure 8-1. The lateral rectus muscle is supplied by the abducens nerve, and the superior oblique muscle is supplied by the trochlear nerve. All the other muscles are supplied by branches of the oculomotor nerve, which also supplies the levator palpebrae superioris.
FIGURE 8-1 Muscles acting on the right eye, viewed from above. (The inferior rectus and inferior oblique are not visible.) Note that with the eye looking forward, as shown, contraction of the superior oblique muscle will cause the pupil to move downward and laterally. If the eye were looking medially, the superior oblique muscle would move the pupil downward. If the eye were looking laterally, the superior oblique muscle would rotate the eyeball but would not change the direction of gaze.
Cranial nerves III, IV, and VI provide motor innervation of the extraocular muscles. Their nuclei of origin, collectively called the ocular motor nuclei, contain motor neurons andinternuclear neurons, with axons that contact the motor neurons for muscles that move the opposite eye in the same direction. Internuclear neurons constitute part of the circuitry that coordinates the conjugate (yoked) movements of the two eyes. The oculomotor nucleus also includes a parasympathetic component. Functional impairment of any of the extraocular muscles causes misalignment of the eyes with consequent double vision (diplopia).
The oculomotor nucleus is in the periaqueductal gray matter of the midbrain, ventral to the aqueduct at the level of the superior colliculus (see Figs. 7-14, 7-15, and 8-2). Myelinated axons from each oculomotor nucleus curve ventrally through the tegmentum and emerge from the medial side of the cerebral peduncle, in the interpeduncular fossa. The nerve traverses the subarachnoid space, the cavernous sinus, and the superior orbital fissure. In the orbit, branches supply the superior, medial, and inferior rectus muscles; the inferior oblique muscle; and the levator palpebrae superioris muscle (which lifts the upper eyelid).
In the oculomotor nucleus, the motor neurons for individual muscles are localized in distinct groups or subnuclei. The small sizes of the motor units, in which about six muscle fibers are supplied by one neuron, attest to the high degree of precision required for coordinated movement of the eyes in binocular vision.
The Edinger-Westphal nucleus is situated dorsally to the main oculomotor nucleus, and its smaller cells are preganglionic parasympathetic neurons. Axons from the Edinger-Westphal nucleus accompany the other oculomotor fibers into the orbit, where they terminate in the ciliary ganglion, behind the eye. Postganglionic fibers (the axons of the neurons in the ganglion) pass through the short ciliary nerves to the eyeball, where they supply the sphincter pupillae muscle of the iris and the ciliary muscle.
FIGURE 8-2 Origins of the oculomotor nerves in the midbrain at the level of the superior colliculus. Motor neurons are red; preganglionic parasympathetic neurons are green.
The trochlear nucleus for the superior oblique muscle is immediately caudal to the oculomotor nucleus, at the level of the inferior colliculus (see Figs. 7-13 and 8-3). Trochlear nerve fibers have an unusual course, and this is the only nerve to emerge from the dorsum of the brain stem. Small bundles of fibers curve around the periaqueductal gray matter with a caudal slope and decussate in the superior medullary velum. The slender nerve emerges immediately caudal to the inferior colliculus. The function of the superior oblique muscle is to depress and inwardly rotate the eyeball (see Fig. 8-1). If the eye is initially looking forward, the superior oblique also causes abduction.
The abducens nucleus for the lateral rectus muscle is situated beneath the facial colliculus in the floor of the fourth ventricle (see Figs. 7-8 and 8-4). A bundle of facial nerve fibers (known as the internal genu) curves over the nucleus, contributing to the facial colliculus. The motor neurons in the abducens nucleus give rise to axons that pass through the pons in a ventrocaudal direction, emerging from the brain stem at the junction of the pons and the pyramid. The abducens nucleus also contains internuclear neurons whose axons cross into the contralateral medial longitudinal fasciculus and travel rostrally to the oculomotor subnucleus that supplies the medial rectus muscle (Fig. 8-5).
FIGURE 8-3 Origins of the trochlear nerves in the midbrain. The axons from the left and right trochlear nuclei are directed dorsally and caudally; they decussate in the superior medullary velum, which is below the inferior colliculi.
COORDINATED MOVEMENTS OF BOTH EYES
Voluntarily initiated conjugate movements of the eyes include those that occur when scanning a landscape or reading a printed page. These movements, known as saccadic eye movements, are rapid, with each being completed in 20 to 50 msec. A saccade serves to aim the eye at a seen or remembered object in the visual field. Frequent saccades, made when the image on the retina is continuously changing, are called optokinetic movements. Slower
conjugate movements of the eyes are possible only when tracking a moving object in the visual field. These largely involuntary smooth pursuit movements are mentioned later in connection with visual fixation. Vergence movements, in which both eyes move medially to look at a near object or laterally to look into the distance, can also occur slowly.Vestibular eye
movements, which are driven by sensory input from the vestibular apparatus of the inner ear, are described in Chapter 22.
Extraocular Muscle Weakness
All the extraocular muscles are sensitive to diseases that afflict the skeletal muscle in a general way. Myasthenia gravis is a disease in which neuromuscular transmission is inhibited (see Chapter 3). Weakness of the levator palpebrae superioris is often the first symptom, causing ptosis. Weaknesses of the other extraocular muscles follow.
Sometimes cranial nerves III, IV, and VI are all involved in a single destructive lesion. This can be caused by inflammation of unknown cause in the region of the superior orbital fissure or by compression of the nerves in the cavernous sinus (see Chapter 26).
Defective alignment of the eyes is called squint or strabismus. Most often, squint is not caused by paralysis or weakness of muscles. In such cases, both eyes can move through a full range of positions. If one eye fails to converge, it will do so if the other eye is covered. This common condition is called a concomitant squint.
A malfunction of one or more of the extraocular muscles causes a paralytic squint. If paralysis is complete, it is not usually difficult to decide which muscle or group of muscles is not working. When only weakness (paresis) is present, however, the squint may be apparent only when the eye is attempting to move in the direction of action of the affected muscle. The first symptom is diplopia (double vision), which occurs because the central foveae of the two eyes cease to receive images of the same object. With time, the brain suppresses the false image, so the symptom of diplopia disappears. The two golden rules in the diagnosis of diplopia are:
If the patient cannot be sure which eye produces which image, the uncertainty can be resolved by placing colored glass in front of one eye.
THIRD NERVE PALSY
“Palsy” is an old word for paralysis, often used for disorders of single nerves or muscles. A lesion that interrupts fibers of the oculomotor nerve causes paralysis of all extraocular muscles except the superior oblique and lateral rectus muscles. The sphincter pupillae muscle in the iris and the ciliary muscle in the ciliary body are functionally paralyzed, although they are not denervated. The consequences of such a lesion are:
The pupil no longer constricts, either in response to an increase of light intensity or in accommodation for near objects. The ciliary muscle does not contract to allow the lens to increase in thickness for focusing on near objects.
The preganglionic parasympathetic fibers run superficially in the nerve and are therefore the first axons to suffer when a nerve is affected by external pressure. Consequently, the first sign of compression of the oculomotor nerve is ipsilateral slowness of the pupillary response to light.
FOURTH NERVE PALSY
Paralysis of the superior oblique muscle, as in the rare occurrence of an isolated lesion of the trochlear nerve, causes vertical diplopia, which is maximal when the eye is directed downward and inward. A person so affected experiences difficulty in walking down stairs. The condition can occur as a manifestation of a peripheral neuropathy (e.g., in diabetes mellitus). It is an occasional persistent complication of head injury. Tiny vascular lesions in the midbrain may be the most common cause of isolated, nontraumatic oculomotor and trochlear palsies in the elderly.
SIXTH NERVE PALSY
The abducens nerve may be affected by a peripheral neuropathy, or the lateral rectus muscle itself may degenerate for an unknown reason. The consequence is a medial squint with an inability to direct the affected eye laterally because the lateral rectus is the principal muscle that abducts the eyeball. Destruction of the abducens nucleus also causes paralysis of the contralateral medial rectus muscle, so that the patient cannot direct his or her gaze to the side of the lesion. A nuclear lesion may also involve the nearby nucleus or axons of the facial nerve, causing paralysis of all the ipsilateral facial muscles.
FIGURE 8-4 Origins of the abducens nerves in the caudal part of the pons.
FIGURE 8-5 Some pathways involved in conjugate lateral movements of the eyes. Motor neurons are red, internuclear neurons are green, other preoculomotor neurons are blue, and other neurons are black.
The precise timing of contractions of extraocular muscles is determined by activity of neurons whose axons end in the nuclei of cranial nerves III, IV, and VI. An understanding of the neuroanatomical basis of ocular movements, discussed later in this chapter and in Chapter 22, is essential for the clinical analysis of impairments more complex than damage to a single muscle or cranial nerve.
Voluntary Eye Movements
The area of the cerebral cortex that controls voluntary eye movements is the frontal eye field, located anterior to the general motor cortex and known as Brodmann's area 8 (seeChapter 15). Stimulation of the frontal eye field results in conjugate deviation of the eyes to the opposite side. The voluntary control of eye movements is mediated by a polysynaptic pathway that involves the prefrontal cortex; frontal eye field; superior colliculus; various other groups of neurons in the brain stem; and the oculomotor, trochlear, and abducens nuclei (see Fig. 8-5). The “various other groups of neurons in the brain stem” include the pretectal area, superior colliculus, paramedian pontine reticular formation (PPRF), nucleus prepositus hypoglossi, rostral interstitial nucleus of the medial longitudinal fasciculus, and interstitial nucleus of Cajal. These nuclei, whose locations are illustrated in Chapter 9, are variously involved in maintaining the position of the eyes (tonic activity), generating saccades (phasic activity), and determining whether the eyes will move in the horizontal or vertical plane.
The PPRF has been called a “center for lateral gaze.” It receives afferents from the contralateral cerebral cortex (including the frontal eye field), contralateral superior colliculus, and ipsilateral vestibular nuclei. Some of its neurons send their axons into the ipsilateral abducens nucleus, and the axons terminate on both motor neurons and internuclear neurons (see Fig. 8-5). The internuclear neurons have axons that cross the midline and ascend through the contralateral medial longitudinal fasciculus to the cells of the contralateral oculomotor nucleus that supply the medial rectus muscle. The actions of the medial and lateral recti are thereby coordinated in horizontal movements of the eyes.
Neurons in the PPRF send bursts of impulses (as many as 1,000/sec) to the motor and internuclear neurons, causing rapid contraction of the lateral rectus and contralateral medial rectus muscles. Slower tonic stimulation of the ocular motor neurons, with impulses at a rate just sufficient to maintain the direction of gaze, comes from the nucleus prepositus hypoglossi, which is rostral to the hypoglossal nucleus in the medulla. The neurons in this nucleus receive afferents from the contralateral cerebral cortex and superior colliculus and have axons that project rostrally in the medial longitudinal fasciculus to all the ocular motor nuclei.
Conjugate movement of the eyes in the vertical plane is controlled by cell groups in the upper midbrain. Bursts of impulses that stimulate vertical saccades arise in the rostral interstitial nucleus of the medial longitudinal fasciculus (see Fig. 9-7), which contains neurons with axons that end in the trochlear nucleus and the oculomotor subnuclei for the superior and inferior rectus and inferior oblique muscles. The axons that go to contralateral ocular motor nuclei decussate in the posterior commissure. Tonic neurons that maintain the vertical component of the direction of gaze are located in the interstitial nucleus of Cajal (see Fig. 9-7). Some of the neural connections mediating voluntary vertical eye movements are shown in Figure 8-6.
Smooth Pursuit Movements
The eyes are normally directed toward some object in the center of the field of vision. If the object moves, both eyes will execute smooth pursuit movements to maintain visual fixation, which contributes importantly to awareness of the position of the head and, integrated with other sensory information, helps in the maintenance of the body's equilibrium. These slow eye movements are largely involuntary. They are controlled principally by the posterior parietal eye field, which is adjacent to the visual association cortex of the lateral aspect of the occipital lobe. The descending connections of this parietal eye field are essentially the same as those of the frontal eye field (see Fig. 8-5). The direct visual input from the retina to the superior colliculus is also involved in reflex eye movements for visual fixation. The neural circuitry for pursuit movements involves the cerebellum and vestibular nuclei. The connections are summarized in Figure 8-7. This diagram includes some connections of the pretectal area that mediate
the short saccades (optokinetic movements) that occur when the point of visual fixation is continuously shifting, as when looking out of the side window of a moving vehicle. Eye movements driven primarily by sensory input from the vestibular nerve are explained in Chapter 22.
FIGURE 8-6 Some pathways involved in vertical eye movements. The connections are shown for only the left eye. Motor neurons are red, preoculomotor neurons are blue, and other neurons are black.
Convergence occurs when both eyes are focused on a near object. This nonconjugate movement is accompanied by constriction of the pupil and accommodation (focusing) of the lens. The neuronal pathways of convergence are similar to those described for visual fixation. Convergence requires the integrity of the occipital cortex but not that of the frontal eye field or the PPRF. Visual guidance is also provided through the superior colliculus. This projects to the pretectal area, which contains at least one group of cells (the nucleus of the optic tract) with axons that contact medial rectus motor neurons in the oculomotor nuclei of both sides (see Fig. 8-7).
LIGHT AND ACCOMMODATION REFLEXES
The Edinger-Westphal nucleus contains preganglionic parasympathetic neurons concerned with reflex responses of the smooth muscles of the eye to light and accommodation. Thelight reflex occurs when an increased intensity of light falling on the retina causes constriction of the pupil. The afferent limb of the reflex arc involves fibers in the optic nerve and optic tract that reach one of the nuclei of the pretectal area (the olivary pretectal nucleus) by way of the superior brachium (Fig. 8-8). This part of the pretectal area projects to the Edinger-Westphal nucleus, from which fibers traverse the oculomotor nerve to the ciliary ganglion in the orbital cavity. Postganglionic fibers travel through the short ciliary nerves to the sphincter pupillae muscle of the iris. Some pretectal neurons send their axons across the midline in the posterior commissure to the contralateral Edinger-Westphal nucleus.
Both pupils constrict when a light is shone into only one eye. The response of the contralateral iris is known as the consensual light reflex. There are two reasons for the response of the other eye: (a) each optic tract contains fibers from both retinas (see Chapter 20) and (b) the pretectal area projects to the contralateral as well as to the ipsilateral Edinger-Westphal nucleus.
Accommodation of the lens accompanies ocular convergence produced by visual fixation on a near object. Both actions are triggered
by signals that originate in the retina and in the occipital cortex and are relayed through the superior colliculus to the Edinger-Westphal nucleus. The efferent part of the pathway consists of preganglionic and postganglionic fibers from the Edinger-Westphal nucleus and the ciliary ganglion, respectively. The postganglionic fibers supply the ciliary muscle, which, on contraction, allows the lens to increase in thickness and thereby increases refractive power for focusing on a near object. The sphincter pupillae muscle contracts at the same time, sharpening the image by decreasing the diameter of the pupil and reducing spherical aberration in the refractive media of the eye.
Cortical Lesions Affecting Conjugate Gaze
Destruction of the frontal eye field causes deviation of both eyes toward the side of the lesion. Voluntary (saccadic) movements of the eyes away from the side of the cortical lesion cannot be made. Commonly, this condition is caused by ischemic damage to a larger area of cerebral cortex, which also includes the motor and premotor areas, with consequent paralysis of the limbs and lower half of the face on the contralateral side. The deviated eyes look away from the paralyzed side of the body.
A destructive lesion in the posterior parietal lobe can impair the ability to make smooth pursuit movements away from the side of the lesion. Voluntary saccades are unaffected, and the attempt to pursue a target in the visual field becomes a series of small, rapid movements of the eyes.
LESIONS IN THE BRAIN STEM THAT AFFECT GAZE
Lesions that destroy the abducens nucleus have already been described and contrasted with the consequence of severing the motor axons of the abducens nerve, either in the ventral part of the pons or in the nerve itself. Foville's syndrome, which is caused by a dorsally located infarction in the caudal part of the pons, comprises ipsilateral nuclear sixth nerve palsy and lower motor neuron facial palsy, with contralateral hemiplegia. The limb paralysis recovers because most of the descending motor fibers are ventral to the infarct.
Internuclear ophthalmoplegia is caused by a tiny lesion in one medial longitudinal fasciculus at a level between the nuclei of cranial nerves III and VI. The usual cause is multiple sclerosis. Interruption of the fibers going from the abducens nucleus of the opposite side to the oculomotor nucleus of the same side causes an inability to adduct the eye on the side of the lesion. The patient will also have nystagmus of the abducting eye, which is a useful diagnostic sign even though the mechanism producing the nystagmus is not fully understood. These abnormalities are evident only when the patient is asked to gaze to the side opposite that of the lesion; contraction of the medial rectus occurs normally with convergence of the eyes for looking at a near object. A somewhat larger lesion can involve both medial longitudinal fasciculi, causing bilateral internuclear ophthalmoplegia.
A lesion that destroys the PPRF prevents saccadic contractions of the lateral rectus and the contralateral medial rectus muscles, but pursuit and vergence movements are preserved. An incomplete lesion causes abnormally small, slow saccades.
Paralysis of vertical gaze is caused by a lesion in the rostral midbrain. Causes include pressure from a nearby tumor and isolated lesions of various diseases that produce widespread changes in the brain. A tumor arising from the pineal gland compresses the posterior commissure and nearby structures and causes paralysis of upward gaze (Parinaud's syndrome). In monkeys, a tiny lesion confined to the rostral interstitial nucleus of the medial longitudinal fasciculus causes selective paralysis of downward gaze; this condition has been described in humans but is extremely rare.
Other Cranial Nerves
The trigeminal nerve is so named because it branches intracranially into three divisions. These provide general sensory innervation for most of the head (Fig. 8-9) and motor fibers to the muscles of mastication and several smaller muscles.
The cell bodies of most of the primary sensory neurons are located in the trigeminal (semilunar or gasserian) ganglion, with the remainder located in the mesencephalic trigeminal
nucleus. The peripheral processes of trigeminal ganglion cells constitute the ophthalmic and maxillary nerves and the sensory component of the mandibular nerve. The trigeminal nerve is responsible for sensation from the skin of the face and forehead, the scalp as far back as the vertex of the head, the mucosa of the oral and nasal cavities and the paranasal sinuses, and the teeth (see Fig. 8-9). The trigeminal nerve also contributes sensory fibers to most of the dura mater (see Chapter 26) and to the cerebral arteries.
Abnormal Pupillary Reflexes
No pupillary reflexes can be elicited by light shone into an eye that is blind for any reason. Both pupils constrict briskly when light is shone in the normal eye, however. If the light is then quickly moved to the blind eye (the “swinging flashlight test”), both pupils dilate. This apparently paradoxical light reflex is known as the Marcus Gunn pupil. It is seen especially in patients with optic neuritis, a condition in which a demyelinating lesion in an optic nerve causes visual failure in one eye, developing in the course of a few days. Optic neuritis is often a manifestation of multiple sclerosis, a disease in which foci of demyelination occur in scattered locations in the brain and spinal cord.
The most common abnormal visual reflex is impairment of the pupillary reaction to light in patients with deteriorating conscious level after ahead injury. The usual cause is compression of the oculomotor nerve by the uncus, which is forced over the free edge of the tentorium cerebelli as a result of pressure from a subdural or extradural hemorrhage (see Chapter 26).
An aneurysm of the posterior communicating artery can injure the nearby oculomotor nerve.
The Holmes-Adie pupil, seen most frequently in young women, responds more slowly than the other pupil to both light and accommodation. It is attributed to the death of some neurons in the ciliary ganglion, perhaps as a result of viral infection, and it may be associated (for no known reason) with sluggish stretch reflexes throughout the body. The small pupil of Horner's syndrome is explained inChapter 24.
The different pathways for pupillary responses to light and accommodation may be differently affected by disease. For example, in the Argyll Robertson pupil, constriction occurs when attention is directed to a near object, but no pupillary constriction occurs in response to light. The Argyll Robertson pupil is characteristically seen in patients with syphilitic disease of the central nervous system (CNS). Loss of the pupillary light reflex alone is probably the result of a small lesion in the pretectal or periaqueductal region, but pathological changes cannot always be found in these sites. An Argyll Robertson pupil is irregular and smaller than normal, probably because of disease in the iris itself.
Trigeminal Sensory Nuclei
The central processes of trigeminal ganglion cells make up the large sensory root of the nerve; these fibers enter the pons and terminate in the pontine and spinal trigeminal nuclei. The pontine trigeminal nucleus (also called the chief, principal, or superior sensory nucleus) is located in the dorsolateral area of the tegmentum at the level of entry of the sensory axons (see Figs. 7-11 and 8-10). Large-diameter fibers for discriminative touch terminate in the pontine trigeminal nucleus. Other entering axons divide, with one branch ending in the pontine trigeminal nucleus and the other turning caudally in the spinal tract and ending in the rostral end of the spinal trigeminal spinal nucleus. These afferents are mainly for light touch, and both nuclei, therefore, participate in this sensory modality. The pontine trigeminal nucleus also receives some branches of the axons of neurons of the mesencephalic trigeminal nucleus.
Large numbers of sensory root fibers of intermediate size and many fine, unmyelinated fibers turn caudally on entering the pons. These fibers for pain, temperature, and light touch form the spinal trigeminal tract (see Fig. 8-10). The tract also acquires incoming fibers from the facial, glossopharyngeal, and vagus nerves. These are for general somatic sensation from part of the external ear, the mucosa of the posterior part of the tongue, the pharynx, and
the larynx. Some fibers of the spinal tract descend as far as the upper two or three segments of the cord, where they intermingle with axons in the dorsolateral tract of Lissauer.
FIGURE 8-7 Some pathways involved in ocular pursuit and vergence movements. The pathways marked with green are used when the eyes converge to look at a near object.
The axons in the spinal tract terminate in the subjacent spinal trigeminal nucleus (see Fig. 8-10). The spinal nucleus extends from the pontine trigeminal nucleus to the caudal limit of the medulla, where it blends with the dorsal horn of the spinal gray matter. Based on cytoarchitecture, the spinal nucleus is divided into three parts (see Fig. 8-10). The pars caudalis, which extends from the level of the pyramidal decussation to spinal segment C3, receives fibers for pain and temperature. The integrity of the pars caudalis and of the caudal end of the spinal trigeminal tract is essential for the perception of pain that originates in the same side of the head. The pars interpolaris extends from the level of the rostral third of the inferior olivary nucleus to that of the pyramidal decussation. The pars oralis extends from the pars interpolaris rostrally to the pontine trigeminal nucleus, which it resembles in its cellular architecture and its involvement in tactile sensation.
Some efferent fibers from the sensory trigeminal nuclei terminate in motor nuclei of the trigeminal and facial nerves, the nucleus ambiguus, and the hypoglossal nucleus. These mediate reflex responses to stimuli applied to the area of distribution of the trigeminal nerve. An example is the corneal reflex: touching the cornea causes both of the eyelids to close; the afferent fibers are located in the ophthalmic nerve, and the efferent fibers of the reflex arc are located in the facial nerve. Bilateral closure (blinking) follows a noxious stimulus anywhere near the eyes. Studies of patients with small lesions in the brain stem indicate that the reflex pathway begins in the caudal part of the spinal trigeminal nucleus and passes in the lateral parts of the tegmentum to both facial motor nuclei. The projection to the contralateral facial
motor nucleus crosses the midline in the lower medulla. As a further example, irritation of the nasal mucosa causes sneezing. For this reflex, afferent impulses in the maxillary nerve are relayed to motor nuclei of the trigeminal and facial nerves, the nucleus ambiguus, the hypoglossal nucleus, and (through a reticulospinal relay) the phrenic nucleus and motor cells in the spinal cord that supply the intercostal and other respiratory muscles.
FIGURE 8-8 The pupillary light reflex. Axons from the retinas are black, central interneurons are blue, preganglionic parasympathetic neurons are red, and postganglionic neurons are green.
FIGURE 8-9 Cutaneous innervation of the head and neck. The boundaries between the territories of the three divisions of the trigeminal nerve do not overlap appreciably, as do the boundaries between spinal dermatomes.
FIGURE 8-10 Nuclei of the trigeminal nerve and their connections. Primary sensory neurons are blue, trigeminothalamic neurons are green, and motor neurons are red.
The principal pathway from the pontine and spinal trigeminal nuclei to the thalamus is the crossed ventral trigeminothalamic tract (see Fig. 8-10 and Chapter 7), which ascends close to the medial lemniscus. Smaller numbers of fibers, crossed and uncrossed, proceed from the pontine trigeminal nucleus to the thalamus in the dorsal trigeminothalamic tract.In the rostral pons and midbrain, the combined tracts are commonly called the trigeminal lemniscus. These axons end in the medial division of the VPm of the thalamus, which projects to the inferior end of the primary somatosensory area of the cerebral cortex.
The slender mesencephalic trigeminal nucleus is a strand of large unipolar neurons extending from the pontine trigeminal nucleus into the midbrain (see Fig. 8-10). These cells are primary sensory neurons in an unusual location; they are the only such cells that are incorporated
into the CNS rather than being in ganglia. Their myelinated axons constitute the mesencephalic tract of the trigeminal nerve, which runs alongside the nucleus. Each axon divides into a peripheral and a central branch. Most of the peripheral branches enter the motor root of the trigeminal nerve and are distributed within the mandibular division. These fibers end in deep proprioceptive-type receptors adjacent to the teeth of the lower jaw and in neuromuscular spindles in the muscles of mastication. Some axons from the mesencephalic nucleus enter the maxillary division and go to endings around the roots of the upper teeth. Central branches of the axons of mesencephalic trigeminal neurons end in the motor nuclei of the trigeminal nerve. This connection establishes the stretch reflex that originates in neuromuscular spindles in the masticatory muscles, together with a reflex for control of the force of the bite. Other central branches synapse with neurons in the reticular formation and the pontine trigeminal nucleus, and a few enter the cerebellum through its superior peduncle.
The trigeminal motor nucleus is medial to the pontine trigeminal nucleus (see Figs. 7-11 and 8-10). The axons of its neurons enter the motor root, which joins sensory fibers of the mandibular nerve just distal to the trigeminal ganglion. This nerve supplies the muscles of mastication (i.e., masseter, temporalis, and lateral and medial pterygoid muscles) and several smaller muscles (i.e., tensor tympani, tensor
veli palatini, anterior belly of digastric, and mylohyoid). The motor nucleus receives descending afferents from the cortex of both cerebral hemispheres by way of the corticobulbar tract.
Disorders Affecting the Trigeminal Nerve and Its Nuclei
Of the diseases that affect the trigeminal nerve, trigeminal neuralgia, or tic douloureux, is of special importance. In this disorder, demyelination of axons in the sensory root takes place, caused in most cases by pressure of a small aberrant artery. There are paroxysms of excruciating pain in the area of distribution of one of the trigeminal divisions, usually with periods of remission and exacerbation. The maxillary nerve is most frequently involved, then the mandibular nerve, and, least frequently, the ophthalmic nerve. The paroxysm is often set off by touching an especially sensitive area of skin. The abnormal signaling of pain is thought to be amplified by ephaptic (electrical) communication among the demyelinated axons, which are tightly packed, without intervening glial cytoplasm. In most patients, the symptoms are relieved by carbamazepine, a drug otherwise used to treat epilepsy. If medical treatment fails, intracranial surgery is warranted because of the severity of the pain. Moving the aberrant artery away from the sensory root of the nerve is usually curative. Other procedures interrupt the pain pathway from the affected cutaneous area to the spinal trigeminal nucleus. Lesions may be placed in the trigeminal ganglion or in the sensory root of the nerve, but these can impair corneal sensitivity, which affords protection from damage that might lead to corneal ulceration. Transection of the spinal trigeminal tract in the lower medulla abolishes the ability to feel pain in the face. The somatotopic lamination of the tract permits placement of a small lesion that restricts the analgesic area to the territory of a single division of the trigeminal nerve.
Another painful disorder that commonly affects the trigeminal nerve is herpes zoster (see Chapters 3 and 19).
The sensory and motor nuclei and the intracranial fibers of the trigeminal nerve may be included in areas damaged by vascular occlusion, trauma, tumor growth, or the presence of lesions in or near the brain stem. Interruption of the motor fibers causes paralysis and eventual atrophy of the muscles of mastication. The mandible deviates to the affected side because of the unopposed action of the contralateral lateral pterygoid muscle, which protrudes the jaw. Interruption of corticobulbar fibers does not cause paralysis of the masticatory muscles on the side opposite the lesion because the motor nucleus also receives some uncrossed fibers from the motor cortex. Laterally located lesions in the medulla interrupt the spinal trigeminal tract and impair facial and oral sensation of pain and temperature; this is part of Wallenberg's syndrome (explained in Chapter 7).
The Facial Nerve and the Middle Ear
The facial nerve is vulnerable in the middle ear, which is a region commonly invaded by bacteria and surgery. The exact site of a lesion can be determined by applying knowledge of the branches containing different functional components (Fig. 8-11).
FIGURE 8-11 Components of the peripheral parts of the facial nerve. Primary sensory neurons are blue, motor neurons are red, and preganglionic and postganglionic parasympathetic neurons are green.
Afferents for reflexes come mainly from the sensory trigeminal nuclei, including the mesencephalic nucleus. The bilateral stretch reflexes of the jaw-closing muscles are tested clinically as the jaw jerk, elicited by tapping downward on the chin; the reflex arc passes through the mandibular nerve and the mesencephalic and motor trigeminal nuclei. In thejaw-opening reflex, the contractions of the masseter, temporalis, and medial pterygoid muscles are inhibited as a result of painful pressure applied to the teeth. This reflex passes through the pars caudalis of the spinal trigeminal nucleus and the motor nucleus, with intervening neurons in the reticular formation. Cells that supply the tensor tympani muscle receive acoustic fibers from the superior
olivary nucleus. The tensor tympani, by reflex contraction, checks excessive movement of the tympanic membrane caused by loud sounds.
The facial nerve has two sensory components: one supplies taste buds, and the other contributes cutaneous fibers to part of the external ear. Motor axons in the nerve supply the muscles of facial expression, and preganglionic parasympathetic fibers go to ganglia that supply the lacrimal, submandibular, and sublingual glands. The sensory and preganglionic parasympathetic axons are located in the nervus intermedius, which is located between the motor root and the vestibulocochlear nerve (see Chapter 6).
Branches of the Facial Nerve
After traversing the internal acoustic meatus, the two roots of the facial nerve enter the facial canal and join at the geniculate ganglion, which contains the cell bodies of all the sensory fibers. The greater petrosal nerve, containing taste and preganglionic fibers, leaves the facial nerve at the level of this ganglion. Distal to the ganglion, the facial canal and its contained nerve bend sharply backward and downward, being now in the medial wall of the tympanic (middle ear) cavity, separated from the air by only the mucous membrane and a very thin layer of bone. A motor branch goes to the stapedius muscle. Near the floor of the posterior part of the tympanic cavity, the chorda tympani nerve, containing taste and preganglionic fibers, passes anteriorly beneath the mucous membrane of the inner surface of the tympanic membrane (ear drum) and then through a tiny canal in the tympanic part of the temporal bone to the infratemporal fossa. The main trunk of the facial nerve descends from the middle ear into the stylomastoid foramen, within which the single somatic sensory branch passes into the surrounding bone and joins small branches of the glossopharyngeal and vagus nerves. These three intermingled small populations of axons supply some of the skin of the tympanic membrane and external acoustic meatus as well as a small area of nearby skin behind the ear. After the departure of these sensory fibers, all the axons in the facial nerve are those of motor neurons. On emerging from the base of the skull between the styloid and mastoid processes, the facial nerve sends branches to the stylohyoid and the posterior belly of the digastric muscle and then splits into five branches (temporal, zygomatic, buccal, marginal mandibular, and cervical), which are distributed to the muscles of the scalp and face.
The cell bodies of primary sensory neurons are in the geniculate ganglion, situated at the bend of the nerve as it traverses the facial canal in the petrous temporal bone.
Gustatory Receptors and Their Innervation
The structure of the gustatory sense organ, the taste bud, is shown in Figure 8-12. Taste buds are derived from cells of the pharyngeal endoderm, and they first appear in the 8th week of intrauterine life. By 5 months, they are present throughout the oral cavity and pharynx, but the numbers then decrease. Shortly after birth, the distribution of taste buds is the same as in the adult: the soft palate; the epiglottis; and, most abundantly, certain of the papillae of the tongue. About 10 large vallate papillae, each surrounded by deep trench, are aligned across the most posterior part of the tongue. Microscopic fungiform papillae occur all over the dorsal surface, among the more numerous filiform papillae. The latter give the tongue a rough texture and do not bear taste buds. Flattened, longitudinally aligned foliate papillae support taste buds on the sides of the tongue.
The cilia of the taste cells (see Fig. 8-12) bear surface receptors that bind substances with specific flavors. Activation of the receptors results in depolarization of the cell membrane. This event activates chemical transmission: the taste cells are presynaptic to the sensory axons that innervate them. Individual taste buds respond to different kinds of chemical stimuli. Physiological, pharmacological, and biochemical studies indicate that individual taste cells respond to one of five elementary flavors: salty (e.g., sodium ions), sour (acidity), sweet (e.g., sugar), bitter (alkalinity, also many organic compounds), and umami (amino acids, especially glutamate). Taste buds that respond to sweetness are most abundant on the tip of the tongue. Sour tastes are detected especially at the lateral edges, and bitter substances at the back of the tongue. Receptors for other flavors are generally
distributed. Ordinary tastes are thought to result from mixed neural signals from at least four types of taste buds, integrated in the brain with the input from the olfactory system.
FIGURE 8-12 Structure of a taste bud. The chemical sensors are the apical microvilli of the taste cells. Chemical synapses communicate with the sensory axons. Taste cells and supporting cells are renewed every few days from the population of dividing basal cells.
The sense of taste is served by cranial nerves VII, IX, and X. Primary sensory neurons for taste account for most of the cell bodies in the geniculate ganglion. The peripheral branches of their axons enter either of two branches of the facial nerve (Fig. 8-11):
The axons of geniculate ganglion cells that subserve taste enter the brain stem in the nervus intermedius and turn caudally in the solitary tract (see Figs. 7-6 and 8-13). The facial nerve fibers in this fasciculus are joined more caudally by gustatory axons from the glossopharyngeal and vagus nerves. Fibers from all three sources terminate in the solitary nucleus, a column of cells adjacent to and partly surrounding the tract. Only the large-celled rostral part of the solitary nucleus receives taste fibers; this part is sometimes called the gustatory nucleus. (The caudal part, whose cells are smaller, receives general visceral afferents.)
Ascending Pathway for Taste
Fibers from the gustatory nucleus run rostrally in the ipsilateral central tegmental tract, through the midbrain and subthalamic region, to their site of termination in the most medial part of the ventral posterior nucleus of the thalamus. This thalamic nucleus projects to
the cortical area for taste, which is adjacent to the general sensory area for the tongue and extends onto the insula and forward to the frontal operculum. Physiological evidence has shown that in animals, gustatory stimuli influence the hypothalamus, amygdala, and cortex of the limbic system but probably not through specific ascending projections from the brain stem. Similar to the functionally related olfactory system (see Chapter 17), the pathway for taste does not cross the midline (Fig. 8-14).
FIGURE 8-13 Components of the facial nerve in the brain stem. Primary sensory neurons are blue, motor neurons are red, and preganglionic and postganglionic parasympathetic neurons are green.
Cutaneous sensory axons leave the facial nerve at the junction of the facial canal with the stylomastoid foramen (see Fig. 8-11). These fibers are distributed to the skin of the concha of the auricle, a small area behind the ear, the wall of the external acoustic meatus, and the external surface of the tympanic membrane. The central processes of the geniculate ganglion cells for cutaneous sensation enter the brain stem in the nervus intermedius. They continue into the spinal trigeminal tract (see Fig. 8-13) and terminate in the subjacent spinal trigeminal nucleus.
For Supply of Striated Muscles
The motor component is the most important part of the nerve from the clinical viewpoint. The facial motor nucleus is located in the caudal one third of the ventrolateral part of the pontine tegmentum (see Figs. 7-8 and 8-13). Axons leaving the nucleus pursue an unexpected course. Directed initially toward the floor of the fourth ventricle, these fibers loop over the caudal end of the abducens nucleus, run forward along its medial side, and loop again over the rostral end of the nucleus. The axons then proceed to the point of emergence of the motor root of the facial nerve by passing between their nucleus of origin and the spinal trigeminal nucleus. The configuration of the fiber bundle around the abducens nucleus is called the internal genu. (The external genu of the facial nerve is located in the facial canal at the level of the geniculate ganglion.)
The motor root of the facial nerve consists entirely of fibers from the motor nucleus. These fibers supply the muscles of expression (mimetic muscles), the platysma and stylohyoid muscles, and the posterior belly of the digastric muscle. The facial nerve also supplies the stapedius muscle of the middle ear; by reflex contraction in response to loud sounds, this small muscle prevents excessive movement of the stapes.
The facial motor nucleus receives afferents from several sources, including important connections for reflexes:
to intense light or a rapidly approaching object.
FIGURE 8-14 Central pathway for taste sensation, from taste buds to the ipsilateral cerebral cortex.
The salivary and lacrimal glands are supplied by parasympathetic ganglia. Neurons in the brain stem that supply these ganglia have been identified in laboratory animals, and structurally similar cells with the same histochemical properties (they contain the enzymes acetylcholinesterase and NADPH diaphorase) occur in corresponding locations in the human brain. Most are located dorsomedial and ventrolateral to the facial motor nucleus (Fig. 8-13). These groups of cells constitute the salivatory nucleus, which is the probable source of preganglionic parasympathetic fibers in the facial and glossopharyngeal nerves. (Traditionally a lacrimal nucleus and superior and inferior salivatory nuclei, in vaguely specified positions, were named as the sources of preganglionic fibers to the pterygopalatine, submandibular, and otic ganglia. The traditional notion is not supported by observations or experimental data.)
The salivatory nucleus contains the cell bodies of preganglionic parasympathetic fibers that control the submandibular and sublingual salivary glands and the lacrimal gland. Axons from the salivatory nucleus leave the brain stem in the nervus intermedius and continue in the facial nerve until branches are given off in the facial canal in the petrous temporal bone (see Fig. 8-11). The preganglionic fibers follow devious routes to their destinations, running part of the way in branches of the trigeminal nerve.
Descending Control of Facial Movements
Corticobulbar afferents are crossed, except for those that terminate on cells supplying the frontalis and orbicularis oculi muscles, which receive both crossed and uncrossed fibers. Contralateral voluntary paralysis of only the lower facial muscles is, therefore, a feature of upper motor neuron lesions. Under such circumstances, however, the facial muscles continue to respond involuntarily—and often excessively—to changing moods and emotions. In contrast, emotional changes of facial expression are typically lost in patients with Parkinson's disease (masklike face), although voluntary use of the facial muscles is retained. The neuroanatomical basis of controlling voluntary and emotionally driven facial movement is not known; it must involve different descending pathways from the cerebral hemispheres.
Fibers that control lacrimal secretion pass into the greater petrosal nerve and terminate in the pterygopalatine ganglion (also called the sphenopalatine ganglion) in the pterygopalatine fossa. Postganglionic fibers, which stimulate secretion and cause vasodilation, reach the lacrimal gland through the zygomatic branch of the maxillary nerve. Other secretomotor postganglionic fibers are distributed to mucous glands in the mucosa that lines the nasal cavity and the paranasal sinuses.
Other axons from the salivatory nucleus leave the facial nerve in the chorda tympani branch and are then carried in the lingual branch of the mandibular nerve to the floor of the oral cavity. There they terminate in the submandibular ganglion and on scattered neurons within the submandibular gland. Short postganglionic fibers are distributed to the parenchyma of the submandibular and sublingual glands, where they stimulate secretion and cause vasodilation.
The salivatory nucleus is influenced by the hypothalamus, perhaps through the dorsal longitudinal fasciculus, and by the olfactory system through relays in the reticular formation. Taste and general sensation from the mucosa of the oral cavity promote salivation through connections of the solitary nucleus and sensory trigeminal nuclei, respectively.
GLOSSOPHARYNGEAL, VAGUS, AND ACCESSORY NERVES
Cranial nerves IX, X, and XI have much in common functionally and share certain nuclei in the medulla. To avoid repetition, it is convenient to consider them together.
The glossopharyngeal and vagus nerves include sensory fibers for the special visceral sense of taste; general visceral afferents from baro- and chemoreceptors and from viscera of the thorax and abdomen; and general sensory fibers for pain, temperature, and touch from the back of the tongue, pharynx and nearby regions, the skin of part of the ear, and parts of the dura mater. The cell bodies of primary sensory neurons
are located in the superior and inferior ganglia of cranial nerves IX and X.
Facial paralysis commonly accompanies hemiplegia caused by occlusion of an artery supplying the contralateral internal capsule or the motor areas of the cerebral cortex. For reasons already stated, only the lower half of the face is affected. When a unilateral facial paralysis involves the musculature around the eyes and in the forehead in addition to that around the mouth, the lesion must involve either the cell bodies in the facial nucleus or their axons. In a common condition known as Bell's palsy, the facial nerve is affected as it traverses the facial canal in the petrous temporal bone, with rapid onset of weakness (paresis) or paralysis of all the facial muscles on the affected side. The cause is edema (perhaps caused by a viral infection) of the facial nerve and adjacent tissue in the facial canal. The signs of Bell's palsy depend not only on the severity of the axonal compression but also on where the nerve is affected in its passage through the facial canal (see Fig. 8-11). All functions of the nerve are lost if the damage is at or proximal to the geniculate ganglion. In addition to the paralysis of facial muscles, there is a loss of taste (ageusia) in the anterior two thirds of the tongue and in the palate of the affected side, together with impairment of secretion by the submandibular, sublingual, and lacrimal glands. Also, sounds seem abnormally loud (hyperacusis) because of paralysis of the stapedius muscle. In contrast, compression near the stylomastoid foramen affects only the motor fibers of the nerve.
In mild cases of Bell's palsy, the axons are not damaged severely enough to result in Wallerian degeneration, and the prognosis is favorable. Recovery is slow and frequently incomplete when it must rely on axonal regeneration. There is no regeneration into the brain stem of sensory fibers that have been interrupted on the central side of the geniculate ganglion. In the case of such a lesion in the proximal part of the nerve, some regenerating salivatory fibers may find their way into the greater petrosal nerve and reach the pterygopalatine ganglion. This results in lacrimation (crocodile tears) instead of salivation when aromas and taste sensations cause stimulation of cells in the superior salivatory nucleus.
The unipolar cell bodies for the gustatory fibers are situated in the two glossopharyngeal ganglia (a tiny superior ganglion and a larger inferior ganglion) and in the inferior ganglion of the vagus nerve. The last of these is frequently called the nodose ganglion. The distal axonal branches are distributed through the glossopharyngeal nerve to taste buds on the posterior two thirds of the tongue as well as to the few that occur in the pharyngeal mucosa. Vagal fibers supply taste buds present on the epiglottis. Central processes of the ganglion cells join the solitary tract and terminate in the rostral portion of the solitary nucleus, the gustatory nucleus (see Fig. 7-6 and 8-15). The ascending pathway for taste is described and illustrated in Figure 8-14 in conjunction with the visceral afferent component of the facial nerve.
General visceral afferent neurons receive signals used for reflex regulation of cardiovascular, respiratory, and alimentary function. Their cell bodies are located in the glossopharyngeal and inferior vagal ganglia, together with the neurons for taste. These fibers in the glossopharyngeal nerve supply the carotid sinus at the bifurcation of the common carotid artery and the adjacent carotid body. Sensory endings in the wall of the carotid sinus function as baroreceptors, which monitor arterial blood pressure. The carotid bodycontains chemoreceptors, which monitor the concentration of oxygen in the circulating blood. Vagal fibers similarly supply baroreceptors in the aortic arch and chemoreceptors in the small aortic bodies adjacent to the aortic arch. The vagus nerve also contains many afferent fibers that are distributed to the viscera of the thorax and abdomen; impulses conveyed centrally are important in reflex control of cardiovascular, respiratory, and alimentary functions. The central branches of the axons of the general visceral afferent neurons descend in the solitary tract and end in the more caudal part of the solitary nucleus (see Figs. 8-15 and 8-16). Connections from the latter site are established bilaterally with several regions of the reticular formation. Reticulobulbar and reticulospinal projections, together with a small solitariospinal tract, provide pathways for reflex responses mediated by the parasympathetic and sympathetic nervous systems and by somatic motor neurons that supply the muscles of respiration.
FIGURE 8-15 Components of the glossopharyngeal nerve in the medulla. Primary sensory neurons are blue, motor neurons are red, and preganglionic parasympathetic neurons are green.
FIGURE 8-16 Components of the vagus nerve in the medulla. Primary sensory neurons are blue, motor neurons are red, and preganglionic parasympathetic neurons are green.
Some axons from the solitary nucleus proceed rostrally to the hypothalamus. Others probably go to the ventral posteromedial nucleus of the thalamus, providing for conscious sensations other than pain, such as fullness or emptiness of the stomach.
Somatic Afferent Fibers
The glossopharyngeal nerve includes fibers for the general sensations of pain, temperature, and touch in the mucosa of the posterior one third of the tongue, upper part of the pharynx (including the tonsillar area), auditory or eustachian tube, and middle ear. The vagus nerve carries fibers with the same functions to the lower part of the pharynx, the larynx, and the esophagus. The cell bodies of these sensory neurons are located in the glossopharyngeal ganglia and in the superior ganglion of the vagus nerve, which is also called the jugular ganglion. The central branches of their axons enter the spinal trigeminal tract and terminate in the spinal trigeminal nucleus (see Figs. 8-15 and 8-16). The afferents for touch from the pharynx are important in the gag reflex: touching the pharynx causes elevation of the soft palate and movement of the tongue through a pathway that includes the nucleus ambiguus and the hypoglossal nucleus.
The vagus nerve sends general sensory (pain) fibers to the dura that lines the posterior fossa of the cranial cavity. Through its auricular branch, it contributes sensory fibers to the concha of the external ear, a small area behind the ear, the wall of the external acoustic meatus, and the tympanic membrane. The cell bodies are located in the superior ganglion of the nerve, and the central processes join the spinal trigeminal tract. The area of skin and tympanic membrane supplied by the auricular branch of the vagus nerve is coextensive with that supplied by the facial nerve. The vagus nerve also sends general sensory fibers to the larynx, trachea, bronchi, and esophagus.
Cranial nerves IX, X, and XI include motor fibers for striated muscles, and cranial nerves IX and X contain parasympathetic efferents.
For Supply of Striated Muscles
The nucleus ambiguus is a slender column of motor neurons situated dorsal to the inferior olivary nucleus (see Figs. 7-5, 7-6, and 7-7
and 8-15, 8-16, and 8-17). Axons from this nucleus are directed dorsally at first. They then turn sharply to mingle with other fibers in the glossopharyngeal and vagus nerves, and some of them constitute the entire cranial root of the accessory nerve. The nucleus ambiguus supplies muscles of the soft palate, pharynx, and larynx, together with striated muscle fibers in the upper part of the esophagus. (The only muscle in these regions not supplied by this nucleus is the tensor veli palatini, which is innervated by the trigeminal nerve.)
A small group of cells in the rostral end of the nucleus ambiguus supplies the stylopharyngeus muscle through the glossopharyngeal nerve (see Fig. 8-15). A large region of the nucleus supplies the remaining pharyngeal muscles, the cricothyroid (an external muscle of the larynx), and the striated muscle of the esophagus, through the vagus nerve (see Fig. 8-16). Fibers from the caudal part of the nucleus ambiguus leave the brain stem in the cranial root of the accessory nerve (see Fig. 8-17). These fibers temporarily join the spinal root of the accessory nerve and then constitute the internal ramus of the nerve, which passes over to the vagus nerve in the region of the jugular foramen. These fibers supply muscles of the soft palate and the intrinsic muscles of the larynx.
The nucleus ambiguus receives afferents from sensory nuclei of the brain stem, most importantly from the spinal trigeminal and solitary nuclei. These connections establish reflexes for coughing, gagging, and vomiting, with the stimuli arising in the mucosa of the respiratory and alimentary passages. Corticobulbar afferents are both crossed and uncrossed;muscles supplied by the nucleus ambiguus are, therefore, not paralyzed in the event of a unilateral lesion of the upper motor neuron type. The nucleus ambiguus is
not composed solely of motor neurons. As described later, some of its cells are preganglionic parasympathetic neurons for control of the heart rate.
FIGURE 8-17 Spinal and cranial roots of the accessory nerve.
Motor neurons for the sternocleidomastoid and trapezius muscles are located in the spinal cord (segments C1 to C5) and constitute the accessory nucleus in the ventral gray horn. Arising as a series of rootlets along the side of the spinal cord, just dorsal to the denticulate ligament, the spinal root of the accessory nerve ascends next to the spinal cord (seeFig. 8-17). On reaching the side of the medulla by passing through the foramen magnum, the spinal and cranial roots unite and continue as the accessory nerve, but only as far as the jugular foramen. Fibers from the nucleus ambiguus then join the vagus nerve, as already noted. Those of spinal origin proceed through the posterior triangle of the neck and supply the sternocleidomastoid and trapezius muscles.
Preganglionic parasympathetic fibers are present in the glossopharyngeal and vagus nerves. The salivatory nucleus consists of groups of neurons located lateral and medial to the facial motor nucleus and is the source of preganglionic fibers in the facial and glossopharyngeal nerves. (There is no evidence for the existence of distinct superior and inferior salivatory nuclei.) Axons from the salivatory nucleus pass into the tympanic branch of the glossopharyngeal nerve, the tympanic plexus, and the lesser petrosal nerve to the otic ganglion, which lies beneath the foramen ovale, close to the mandibular division of the trigeminal nerve. The neurons in the otic ganglion have axons (i.e., postganglionic fibers) that join the auriculotemporal branch of the mandibular nerve and thus reach the parotid gland. The parasympathetic supply to the parotid gland stimulates secretion and vasodilation. The salivatory nucleus is influenced by stimuli from the hypothalamus, olfactory system, solitary nucleus, and sensory trigeminal nuclei.
Accessory Nerve Palsy
If the accessory nerve is injured (typically by an object falling onto the back of the shoulder or neck), the sternocleidomastoid and trapezius muscles will be paralyzed or weakened ipsilaterally.
Corticospinal fibers that control the spinal accessory neurons are both crossed and uncrossed. Those for the trapezius are from the contralateral cerebrum. Those for the sternocleidomastoid are from the ipsilateral cerebrum, an arrangement consistent with the action of this muscle, which turns the head toward the opposite side. An upper motor neuron lesion, therefore, causes weakness paresis) of the contralateral trapezius and of the ipsilateral sternocleidomastoid muscle.
The largest parasympathetic nucleus is the dorsal nucleus of the vagus nerve (also called dorsal motor nucleus, but it does not directly innervate muscles). This column of cells is situated in the gray matter lateral to the central canal and extending beneath the vagal triangle in the floor of the fourth ventricle (see Figs. 7-4, 7-5, 7-6, and 7-7). The axons of the cells in the dorsal nucleus constitute the majority of the preganglionic parasympathetic fibers of the vagus nerve. They end in tiny ganglia in the pulmonary plexus and in abdominal viscera, notably the stomach. For details, see Chapter 24.
Other vagal parasympathetic neurons have their cell bodies near and among the motor neurons of the nucleus ambiguus. The axons of these neurons terminate in small ganglia associated with the heart. In some laboratory animals, about 10% of the cardioinhibitory neurons are located in the dorsal nucleus of the vagus. In others, the cardiac ganglia receive all their afferent fibers from the nucleus ambiguus and none from the dorsal nucleus. It seems likely that the nucleus ambiguus contains most or all of the vagal neurons that control the human heart.
The dorsal nucleus of the vagus nerve and the visceral efferent neurons of the nucleus ambiguus
are influenced, directly or indirectly, by the solitary nucleus, hypothalamus, olfactory system, and autonomic “centers” in the reticular formation (see Chapter 9). Despite the functional importance of the visceral afferent and preganglionic parasympathetic fibers, transection of the vagus nerve does not cause cardiovascular symptoms. Vagal denervation of the stomach suppresses acid secretion there and causes gastric distention caused by inadequate emptying through the pylorus.
Hypoglossal Nerve Palsy
Destruction of the hypoglossal nucleus or interruption of the motor axons in the medulla or in the nerve is followed by paralysis and eventual atrophy of the affected muscles. The tongue deviates to the weak side on protrusion because of the unopposed action of the contralateral genioglossus muscle.
Corticobulbar afferents to the hypoglossal nucleus are predominantly but not exclusively crossed. A unilateral upper motor neuron lesion causes paresis of the opposite side of the tongue, which usually recovers quite quickly as the ipsilateral cerebral hemisphere assumes the functions of the damaged descending pathway.
The hypoglossal nucleus lies between the dorsal nucleus of the vagus nerve and the midline of the medulla (see Figs. 7-4, 7-5, 7-6, and 7-7 and 8-18). The hypoglossal triangle in the floor of the fourth ventricle marks the position of the rostral part of the nucleus. The axons of hypoglossal neurons course ventrally on the lateral side of the medial lemniscus and emerge along the sulcus between the pyramid and the olive. The hypoglossal nerve supplies the intrinsic muscles of the tongue and the three extrinsic muscles (genioglossus, styloglossus, and hyoglossus). The nucleus receives afferents from the solitary nucleus and the sensory trigeminal nuclei for reflex movements of the tongue in swallowing, chewing, and sucking in response to gustatory and other stimuli from the oral and pharyngeal mucosae.
Summary of Cranial Nerve Nuclei and Components
Distinct functions are associated with the nuclei of origin or termination of the component fibers of cranial nerves. Table 8-1 summarizes the functions of the nuclei and emphasizes the sharing of nuclei by different cranial nerves.
FIGURE 8-18 Right hypoglossal nerve and origin of the cranial root of the left accessory nerve in the medulla.
TABLE 8-1 Cranial Nerve Nuclei, Associated Ganglia, and Their Functions
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