Brain Stem

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Chapter 10 Brain Stem

The brain stem consists of the medulla oblongata, pons and midbrain. It is sited in the posterior cranial fossa, and its ventral surface lies on the clivus. It contains numerous intrinsic neurone cell bodies and their processes, some of which are the brain stem homologues of spinal neuronal groups. These include the sites of termination and cells of origin of axons that enter or leave the brain stem through the cranial nerves. They provide the sensory, motor and autonomic innervation of structures that are mostly in the head and neck. Autonomic fibres, which arise from the brain stem, are distributed more widely. Additional groups of neurones receive input related to the special senses of hearing, vestibular function and taste (Ch. 12). The reticular formation is an extensive and often ill-defined network of neurones that extends throughout the length of the brain stem and is continuous caudally with its spinal counterpart. Some of its nuclei are concerned with cardiac, respiratory and alimentary control; some are involved in aspects of many neural activities, and others provide or receive massive afferent and efferent cerebellar projections.

The brain stem is the site of termination of numerous ascending and descending fibres and is traversed by many others. The spinothalamic (spinal lemniscal), medial lemniscal and trigeminal systems ascend through the brain stem to reach the thalamus (see Figs 8.32, 10.22). Prominent corticospinal projections descend through the brain stem, and corticobulbar projections end within it (see Fig. 8.41).

Clinically, damage to the brain stem is often devastating and life threatening. This is because it is a structurally and functionally compact region, where even small lesions can destroy vital cardiac and respiratory centres, disconnect forebrain motor areas from brain stem and spinal motor neurones and sever incoming sensory fibres from higher centres of consciousness, perception and cognition. Irreversible cardiac and respiratory arrest follows complete destruction of the neural respiratory and cardiac centres in the medulla.

This chapter starts with a brief systematic overview of the cranial nerves that attach to the brain stem, their central origins and their connections within the cranial nerve nuclei. The major subdivisions of the brain stem are then described. Many structures, including nuclei and tracts, extend longitudinally across their boundaries. The structure and function of the most notable of these are discussed in detail at the most appropriate point in the text. As is customary, transverse sections of the brain stem are included to illustrate the relationships between structures and the regional variation that occurs at different levels.

Overview of Cranial Nerves and Cranial Nerve Nuclei

The cranial nerves are the conduits by which the brain receives information directly from, and controls the functions of, structures that are mainly, but not exclusively, within the head and neck. All but 2 of the 12 pairs of cranial nerves attach to the brain stem; this chapter is therefore an appropriate place to describe their structure and function.

The cranial nerves are individually named and numbered (using roman numerals) in a rostrocaudal sequence (see Table 1.1). Cranial nerve I (olfactory) terminates directly in cortical and subcortical areas of the frontal and temporal lobes. It is closely associated functionally with the limbic system and is described in that context (Ch. 16). The fibres of cranial nerve II (optic) pass into the optic chiasma and emerge as the optic tract, which terminates in the lateral geniculate nucleus of the thalamus. Cranial nerves III (oculomotor) and IV (trochlear) attach to the midbrain. Cranial nerve V (trigeminal) attaches to the pons, medial to the middle cerebellar peduncle. Cranial nerves VI (abducens), VII (facial) and VIII (vestibulocochlear) attach to the brain stem at or close to the junction of the pons and the medulla. Cranial nerves IX (glossopharyngeal) and X (vagus), the cranial part of cranial nerve XI (accessory) and cranial nerve XII (hypoglossal) all attach to the medulla.

Cranial nerves III to XII, which attach to the brain stem, are associated with a number of cell groupings of varying size, referred to collectively as the cranial nerve nuclei (Fig. 10.1). The nuclei are either the origin of efferent cranial nerve fibres or the site of termination of cranial nerve afferents. For convenience, they are considered to be organized into six discontinuous, longitudinal cell columns that correspond to the columns that can be identified in the embryo (see Fig. 1.4). Three columns are ‘sensory’ and three are ‘motor’ in function.

The trigeminal sensory nucleus, which extends throughout the length of the brain stem and into the cervical spinal cord, represents a general somatic afferent cell column. Its principal afferents are carried in the trigeminal nerve. General visceral afferents carried by the facial, glossopharyngeal and vagus nerves end in the nucleus solitarius of the medulla. The special visceral afferent column corresponds to the vestibular and cochlear nuclei, which are located beneath the vestibular area of the floor of the fourth ventricle.

The general somatic efferent cell column consists of four nuclei that lie near the midline and give rise to motor fibres that run in nerves of the same name. From rostral to caudal, these are the oculomotor, trochlear and abducens nuclei, which innervate the extraocular muscles, and the hypoglossal nucleus, which innervates all but one of the muscles of the tongue. The general visceral efferent, or parasympathetic, cell column is made up of the Edinger–Westphal nucleus of the midbrain, salivary nuclei of the pons and vagal nucleus of the medulla. Cells in the special visceral efferent column innervate muscles derived from the branchial arches and lie in the trigeminal motor nucleus, facial nucleus and nucleus ambiguus.

Medulla Oblongata

External Features and Relations

The medulla oblongata extends from the lower pontine margin to a transverse plane that is above the first pair of cervical spinal nerves and intersects the upper border of the atlas dorsally and the centre of the dens ventrally (Fig. 10.2). It is approximately 3 cm long and 2 cm in diameter at its widest. The ventral surface of the medulla is separated from the basilar part of the occipital bone and apex of the dens by the meninges and occipito-axial ligaments. Caudally, the dorsal surface of the medulla occupies the midline notch between the cerebellar hemispheres.

The ventral and dorsal surfaces of the medulla (Fig. 10.3; see also Fig. 5.11) possess a longitudinal median fissure and sulcus, respectively, which are continuous with their spinal counterparts. Caudally, the ventral median fissure is interrupted by the obliquely crossing fascicles of the pyramidal decussation. Rostrally, it ends at the pontine border in a diminutive depression, the foramen caecum. Immediately lateral to the ventral median fissure is a prominent elongated ridge called the pyramid, which contains descending pyramidal, or corticospinal, axons (see Fig. 10.3). The lateral margin of the pyramid is indicated by a shallow ventrolateral sulcus. From this emerges, in line with the ventral spinal nerve roots, a linear series of rootlets that constitute the hypoglossal nerve. The abducens nerve emerges at the slightly narrowed rostral end of the pyramid, where it adjoins the pons. Caudally, the pyramid tapers into the spinal ventral funiculus. Lateral to the pyramid and the ventrolateral sulcus is an oval prominence, the olive (Figs 10.3, 10.4), which contains the inferior olivary nucleus. Lateral to the olive is the posterolateral sulcus. The glossopharyngeal, vagus and accessory nerves join the brain stem along the line of this sulcus, in line with the dorsal spinal nerve roots.

The spinal central canal extends into the caudal half of the medulla, migrating progressively more dorsally until it opens out into the lumen of the fourth ventricle. This divides the medulla into a closed part, which contains the central canal, and an open part, which contains the caudal half of the fourth ventricle (see Figs 10.2, 5.11).

In the closed part of the medulla, a shallow posterointermediate sulcus on either side of the dorsal median sulcus, continuous with its cervical spinal counterpart, indicates the location of the ascending dorsal columns (fasciculus gracilis and fasciculus cuneatus). The ascending fasciculi are at first parallel to each other, but at the caudal end of the fourth ventricle they diverge, and each develops an elongated swelling, the gracile and cuneate tubercles, produced by the subjacent nuclei gracilis and cuneatus, respectively (Figs 10.5, 10.6). Most fibres in the fasciculi synapse with neurones in their respective nuclei, and these project to the contralateral thalamus, which in turn projects to the primary somaesthetic cortex (see Fig. 8.32). The inferior cerebellar peduncle forms a rounded ridge between the caudal part of the fourth ventricle and the glossopharyngeal and vagal rootlets. The two peduncles diverge and incline to enter the cerebellar hemispheres, where they are crossed by the striae medullares, which run to the median ventricular sulcus (see Fig. 5.11). Here also the peduncles form the anterior and rostral boundaries of the lateral recess of the fourth ventricle. This becomes continuous with the subarachnoid space through the lateral apertures of the fourth ventricle, the foramina of Luschka. A tuft of choroid plexus, continuous with that of the fourth ventricle, protrudes from the foramina on either side. The fibre composition of the inferior cerebellar peduncle is described in Chapter 13.

CASE 1 Downbeat Nystagmus and Arnold–Chiari Malformation

A 24-year-old woman presents with a long history of increasing headache, blurred vision when attempting to read and an increasingly unsteady gait with intermittent falls. Neurological examination reveals downbeat nystagmus with the eyes in the primary position, amplified by down-gaze; dysmetria of the lower extremities with heel-to-shin testing; and hyperreflexia in both lower extremities.

Magnetic resonance imaging (MRI) shows ‘beaking’ of the dorsal midbrain and enlargement of the lateral and third ventricles, with herniation of the cerebellar tonsils through the foramen magnum. See Figure 10.7.


Fig. 10.7 Arnold–Chiari malformation. MRI demonstrates downward displacement of the cerebellar tonsil (arrow) below the plane of the foramen magnum.

(© 2010 Thomas Jefferson University. All rights reserved. Reproduced with the permission of Thomas Jefferson University.)

A ventriculoperitoneal shunt is placed, with marked symptomatic improvement.

Discussion: Downbeat nystagmus consists of a rapid downbeat motion of the eyes followed by a slower upward movement. This is usually present with the eyes in the primary position, but at times it is so subtle that it can be seen only with ophthalmoscopy. The amplitude of the movements is usually increased by down-gaze and sometimes by horizontal gaze to either side. It is characteristically associated with conditions involving the medulla oblongata, particularly at the level of the craniocervical junction. These conditions include Arnold–Chiari malformation, as is the case in this woman. It has also been reported with drug toxicity involving lithium and phenytoin.

Internal Structure

Transverse Section of the Medulla at the Level of the Pyramidal Decussation

A transverse section across the lower medulla oblongata (see Fig. 10.5) intersects the dorsal, lateral and ventral funiculi, which are continuous with their counterparts in the spinal cord. The ventral funiculi are separated from the central grey matter by corticospinal fibres, which cross in the pyramidal decussation to reach the contralateral lateral funiculi (see Fig. 10.11). The decussation displaces the ventral intersegmental tract, the central grey matter and the central canal dorsally. Continuity between the ventral grey column and central grey matter, which is maintained throughout the spinal cord, is lost. The column subdivides into the supraspinal nucleus (continuous above with that of the hypoglossal nerve), which is the efferent source of the first cervical nerve, and the spinal nucleus of the accessory nerve, which provides some spinal accessory fibres and merges rostrally with the nucleus ambiguus.

The dorsal grey column is also modified at this level where the nucleus gracilis appears as a grey lamina in the ventral part of the fasciculus gracilis. The nucleus begins caudal to the nucleus cuneatus, which invades the fasciculus cuneatus from its ventral aspect in similar fashion.

The spinal nucleus and spinal tract of the trigeminal nerve are visible ventrolateral to the dorsal columns. They are continuous with the substantia gelatinosa and tract of Lissauer of the spinal cord.

Transverse Section of the Medulla at the Level of the Decussation of the Medial Lemniscus

The medullary white matter is rearranged above the level of the pyramidal decussation (see Fig. 10.6). The pyramids contain ipsilateral corticospinal and corticonuclear fibres, the latter distributed to nuclei of cranial nerves and other medullary nuclei. At this level, they form two large ventral bundles flanking the ventral median fissure. The accessory olivary nuclei and lemniscal decussation are dorsal.

The nucleus gracilis is broader at this level, and the fibres of its fasciculus are located on its dorsal, medial and lateral surfaces. The nucleus cuneatus is well developed. Both nuclei retain continuity with the central grey matter at this level, but this is subsequently lost. First-order gracile and cuneate fascicular fibres, which have ascended ipsilaterally and uninterrupted from their origin in the spinal cord, synapse on neurones in their respective nuclei. Second-order axons emerge from the nuclei as internal arcuate fibres, at first curving ventrolaterally around the central grey matter and then ventromedially between the trigeminal spinal tract and the central grey matter. They decussate to form an ascending contralateral tract, the medial lemniscus. The lemniscal decussation is located dorsal to the pyramids and ventral to the central grey matter. The latter is therefore more dorsally displaced than in the previous section.

The medial lemniscus ascends from the lemniscal decussation on each side as a flattened tract near the median raphe. As the tracts ascend, they increase in size because fibres join from upper levels of the decussation. Corticospinal fibres are ventral, and the medial longitudinal fasciculus and tectospinal tract are dorsal. Fibres are rearranged in the decussation, so that those from the nucleus gracilis come to lie ventral to those from the nucleus cuneatus. Above this, the medial lemniscus is also rearranged, with ventral (gracile) fibres becoming lateral and dorsal (cuneate) fibres medial. At this level, medial lemniscal fibres show a laminar somatotopy on a segmental basis, in that fibres from C1 to S4 spinal segments are segregated sequentially from medial to lateral.

The nucleus of the spinal tract of the trigeminal nerve (see Fig. 10.22) is separated from the central grey matter by internal arcuate fibres; it is separated from the lateral medullary surface by the trigeminal spinal tract, which ends in it, and by some dorsal spinocerebellar tract fibres. The latter progressively incline dorsally and enter the inferior cerebellar peduncle at a higher level.

Two other nuclei occur at this level. One is dorsolateral to the pyramid, and the other is medial to it and near the median plane. These are parts of the precerebellar medial accessory olivary nucleus, described with the inferior olivary nuclear complex. Precerebellar nuclei of the vestibular, pontine and reticular system are described in Chapter 13.

Transverse Section of the Medulla at the Caudal End of the Fourth Ventricle

A transverse section level with the lower end of the fourth ventricle shows some new features, along with most of those already described (Fig. 10.8). The total area of grey matter is increased by the presence of the large olivary nuclear complex and nuclei of the vestibulocochlear, glossopharyngeal, vagus and accessory nerves.

A smooth, oval elevation—the olive—lies between the ventrolateral and dorsolateral sulci of the medulla. It is formed by the underlying inferior olivary complex of nuclei and lies lateral to the pyramid, separated from it by the ventrolateral sulcus and emerging hypoglossal nerve fibres. The roots of the facial nerve emerge between its rostral end and the lower pontine border, in the cerebellopontine angle. The arcuate nuclei are curved, interrupted bands, ventral to the pyramids, and are said to be displaced pontine nuclei. Anterior external arcuate fibres and those of the striae medullares are derived from them. They project mainly to the contralateral cerebellum through the inferior cerebellar peduncle (Fig. 10.9).

The inferior olivary nucleus is a hollow, irregularly crenated grey mass. It has a longitudinal medial hilum and is surrounded by myelinated fibres that form the olivary amiculum. Dorsolateral to the pyramid, it underlies the olive but ascends within the pons.

The central grey matter at this level constitutes the ventricular floor. It contains (sequentially from medial to lateral) the hypoglossal nucleus, dorsal vagal nucleus, nucleus solitarius and caudal ends of the inferior and medial vestibular nuclei.

The tractus solitarius and its associated circumferential nucleus solitarius extend throughout the length of the medulla. The tract is composed of general visceral afferents from the vagus and glossopharyngeal nerves. The nucleus and its central connections with the reticular formation subserve the reflex control of cardiovascular, respiratory and cardiac functions. The rostral fibres of the tract consist of gustatory fibres from the facial, glossopharyngeal and vagal nerves that project to the rostral pole of the nucleus solitarius, which is sometimes referred to as the gustatory nucleus.

The medial longitudinal fasciculus, a small, compact tract near the midline and ventral to the hypoglossal nucleus, is continuous with the ventral vestibulospinal tract. At this medullary level it is displaced dorsally by the pyramidal and lemniscal decussations. It ascends in the pons and midbrain, maintaining its relationship to the central grey matter and midline, so it is near the somatic efferent nuclear column. Fibres from a variety of sources course for short distances in the tract.

The spinocerebellar, spinotectal, vestibulospinal, rubrospinal and lateral spinothalamic (spinal lemniscal) tracts all lie in the ventrolateral area of the medulla at this level. The tracts are limited dorsally by the spinal trigeminal nucleus and ventrally by the pyramid.

Numerous islets of grey matter are scattered centrally in the ventrolateral medulla, an area intersected by nerve fibres that run in all directions. This is the reticular formation, which exists throughout the medulla and extends into the pontine tegmentum and midbrain.

CASE 2 Avellis’ Syndrome

A 47 year old man, previously well, suddenly developed numbness of the left hand. Within 2 days, the sensory loss spread to involve the entire left arm, then the left leg; at that point he developed an increasingly severe left hemiparesis. Speech was described as occasionally slurred. Examination demonstrated a flaccid left hemiparesis with exaggerated reflex activity bilaterally. Plantar response on the left arm was extensor. There was reduction in vibratory sense on the left side; sensation was otherwise normal. There was mild wasting on the left side of the tongue, with fasciculation and the left sternomastoid muscle was slightly atrophic.

MRI demonstrated an acute infarction in the right medial lowermost medulla involving the pyramid, the medial lemniscus, and the hypoglossal nerve in its course through the medulla.

COMMENT: The patient demonstrated the classic features of Avellis syndrome due to a lesion (infarction) in the medial aspect of the lower medulla involving to variable extents. The neuro-anatomic structures involved include the corticospinal tracts causing contralateral hemiparesis, the medial lemniscus leading to impaired posterior column sensibility, the accessory nerve causing mild atrophy of the sternomastoid muscle, and the hypoglossal nerve producing atrophy of the tongue with fasciculations. This syndrome is rare, and variably described; in a number of cases, palatal weakness has been observed. See Figure 10.10.

Pyramidal Tract

Each pyramid contains descending corticospinal fibres, derived from the ipsilateral cerebral cortex, which have traversed the internal capsule, midbrain and pons (Fig. 10.11). Approximately 70% to 90% of the axons leave the pyramids in successive bundles, crossing in and deep to the ventral median fissure as the pyramidal decussation. In the rostral medulla, fibres cross by inclining ventromedially, whereas more caudally, they pass dorsally, decussating ventral to the central grey matter. The decussation is orderly, with fibres destined to end in the cervical segments crossing first. They continue to pass dorsally as they descend, reaching the contralateral spinal lateral funiculus as the crossed lateral corticospinal tract. Most uncrossed corticospinal fibres descend ventromedially in the ipsilateral ventral funiculus, as the ventral corticospinal tract. A minority run dorsolaterally to join the lateral corticospinal tracts as a small uncrossed component. The corticospinal tracts display somatotopy at almost all levels. In the pyramids the arrangement is like that at higher levels, in that the most lateral fibres subserve the most medial arm and neck movements. Similar somatotopy is ascribed to the lateral corticospinal tracts within the spinal cord.

Dorsal Column Nuclei

The nuclei gracilis and cuneatus are part of the pathway that is considered the major route for discriminative aspects of tactile and locomotor (proprioceptive) sensation. The upper regions of both nuclei are reticular and contain small and large multipolar neurones with long dendrites. The lower regions contain clusters of large, round neurones with short and profusely branching dendrites. Upper and lower zones differ in their connections, but both receive terminals from the dorsal spinal roots at all levels. Dorsal funicular fibres from neurones in the spinal grey matter terminate only in the superior, reticular zone. Variable ordering and overlap of terminals, on the basis of spinal root levels, occur in both zones. The lower extremity is represented medially, the trunk ventrally and the digits dorsally. There is modal specificity; that is, lower levels respond to low-threshold cutaneous stimuli, and upper reticular levels respond to inputs from fibres serving receptors in the skin, joints and muscles. The cuneate nucleus is divided into several parts. Its middle zone contains a large pars rotunda, in which rostrocaudally elongated, medium-sized neurones are clustered between bundles of densely myelinated fibres. The reticular poles of its rostral and caudal zones contain scattered but evenly distributed neurones of various sizes. The pars triangularis is smaller and laterally placed. There is a somatotopic pattern of termination of cutaneous inputs from the upper limb on the cell clusters of the pars rotunda. Terminations are diffuse in the reticular poles.

The gracile and cuneate nuclei serve as relays between the spinal cord and higher levels. Primary spinal afferents synapse with multipolar neurones in the nuclei to form the major nuclear efferent projection. The nuclei also contain interneurones, many of which are inhibitory. Descending afferents from the somatosensory cortex reach the nuclei through the corticobulbar tracts and appear to be restricted to the upper, reticular zones. Because these afferents both inhibit and enhance activity, the nuclear region is clearly one of sensory modulation. The reticular zones also receive connections from the reticular formation. Feedback from the gracile and cuneate nuclei to the spinal cord probably occurs.

Neurones of dorsal column nuclei receive terminals of long, uncrossed, primary afferent fibres of the fasciculi gracilis and cuneatus, which carry information concerning deformation of the skin, movement of hairs, joint movement and vibration. Unit recording of the neurones in dorsal column nuclei shows that their tactile receptive fields (i.e. the skin area in which a response can be elicited) vary in size, although they are mostly small and are smallest for the digits. Some fields have excitatory centres and inhibitory surrounds, which means that stimulation just outside its excitatory field inhibits the neurone. Neurones in the nuclei are spatially organized into a somatotopic map of the periphery (in accord with the similar localization in the dorsal columns). In general, specificity is high. Many cells receive input from one or a few specific receptor types (e.g. hair, type I and II slowly adapting receptors and Pacinian corpuscles), and some cells respond to Ia muscle spindle input. However, some neurones receive convergent input from tactile pressure and hair follicle receptors.

A variety of control mechanisms can modulate the transmission of impulses through the dorsal column–medial lemniscus pathway. Concomitant activity in adjacent dorsal column fibres may result in presynaptic inhibition by depolarization of the presynaptic terminals of one of them. Stimulation of the sensorimotor cortex also modulates the transmission of impulses by both pre- and postsynaptic inhibitory mechanisms, and sometimes by facilitation. These descending influences are mediated by the corticospinal tract. Modulation of transmission by inhibition also results from stimulation of the reticular formation, raphe nuclei and other sites.

The accessory cuneate nucleus, dorsolateral to the cuneate, is part of the spinocerebellar system of precerebellar nuclei (see Fig. 10.9); it contains large neurones like those in the spinal thoracic nucleus. These form the posterior external arcuate fibres, which enter the cerebellum by the ipsilateral inferior peduncle. The nucleus receives the lateral fibres of the fasciculus cuneatus, carrying proprioceptive impulses from the upper limb (which enter the cervical spinal cord rostral to the thoracic nucleus). Its efferent fibres form the cuneocerebellar tract. A group of neurones, called nucleus Z, has been identified in animals between the upper pole of the nucleus gracilis and the inferior vestibular nucleus and is said to be present in the human medulla. Its input is probably from the dorsal spinocerebellar tract, which carries proprioceptive information from the ipsilateral lower limb, and it projects through internal arcuate fibres to the contralateral medial lemniscus.

Trigeminal Sensory Nucleus

The trigeminal sensory nucleus receives the primary afferents of the trigeminal nerve. It is a large nucleus and extends caudally into the cervical spinal cord and rostrally into the midbrain. The principal and largest division of the nucleus is located in the pontine tegmentum.

On entering the pons, the fibres of the sensory root of the trigeminal nerve run dorsomedially toward the principal sensory nucleus, which is situated at this level (Fig. 10.12). Before reaching the nucleus, approximately 50% of the fibres divide into ascending and descending branches; the others ascend or descend without division. The descending fibres, 90% of which are less than 4 µm in diameter, form the spinal tract of the trigeminal nerve, which reaches the upper cervical spinal cord. The tract embraces the spinal trigeminal nucleus (Figs 10.5, 10.6, 10.8, 10.13, 10.14). There is a precise somatotopic organization in the tract. Fibres from the ophthalmic root lie ventrolaterally, those from the mandibular root lie dorsomedially and the maxillary fibres lie between them. The tract is completed on its dorsal rim by fibres from the sensory roots of the facial, glossopharyngeal and vagus nerves. All these fibres synapse in the nucleus caudalis.

The detailed anatomy of the trigeminospinal tract excited early clinical interest because it was recognized that dissociated sensory loss could occur in the trigeminal area. For example, in Wallenberg’s syndrome (see Case 3), occlusion of the posterior inferior cerebellar branch of the vertebral artery leads to loss of pain and temperature sensation in the ipsilateral half of the face, with retention of common sensation.

There are conflicting opinions about the termination pattern of fibres in the spinal nucleus. It has long been held that fibres are organized rostrocaudally within the tract. According to this view, ophthalmic fibres are ventral and descend to the lower limit of the first cervical spinal segment, maxillary fibres are central and do not extend below the medulla oblongata and mandibular fibres are dorsal and do not extend much below the mid-medullary level. The results of section of the spinal tract in cases of severe trigeminal neuralgia support this distribution. It was found that sectioning 4 mm below the obex produced analgesia in the ophthalmic and maxillary areas, but tactile sensibility, apart from the abolition of ‘tickle,’ was much less affected. To include the mandibular area, it was necessary to section at the level of the obex. More recently, it has been proposed that fibres are arranged dorsoventrally within the spinal tract. There appear to be sound anatomical, physiological and clinical reasons for believing that all divisions terminate throughout the whole nucleus, although the ophthalmic division may not project fibres as far caudally as the maxillary and mandibular divisions do. Fibres from the posterior face (adjacent to C2) terminate in the lower (caudal) part, whereas those from the upper lip, mouth and nasal tip terminate at a higher level. This can give rise to a segmental (cross-divisional) sensory loss in syringobulbia. Tractotomy of the spinal tract, if carried out at a lower level, can spare the perioral region, a finding that would accord with the ‘onionskin’ pattern of loss of pain sensation. However, in clinical practice, the progression of anaesthesia on the face is most commonly ‘divisional’ rather than ‘onionskin’ in distribution.

Fibres of the glossopharyngeal, vagus and facial nerves subserving common sensation (general visceral afferent) form a column dorsally within the spinal tract of the trigeminal nerve and synapse with cells in the lowest part of the spinal trigeminal nucleus. Consequently, operative section of the dorsal part of the spinal tract results in analgesia that extends to the mucosa of the tonsillar sinus, the posterior third of the tongue and adjoining parts of the pharyngeal wall (glossopharyngeal nerve) and the cutaneous area supplied by the auricular branch of the vagus.

Other afferents that reach the spinal nucleus are from the dorsal roots of the upper cervical nerves and from the sensorimotor cortex.

The spinal nucleus is considered to consist of three parts: the subnucleus oralis (which is most rostral and adjoins the principal sensory nucleus), the subnucleus interpolaris and the subnucleus caudalis (which is the most caudal part and is continuous below with the dorsal grey column of the spinal cord). The structure of the subnucleus caudalis is different from that of the other trigeminal sensory nuclei. It has a structure analogous to that of the dorsal horn of the spinal cord, with a similar arrangement of cell laminae, and it is involved in trigeminal pain perception. Cutaneous nociceptive afferents and small-diameter muscle afferents terminate in layers I, II, V and VI of the subnucleus caudalis. Low-threshold mechanosensitive afferents of Aβ neurones terminate in layers III and IV of the subnucleus caudalis and rostral (interpolaris, oralis and main sensory) nuclei.

Many of the neurones in the subnucleus caudalis that respond to cutaneous or tooth pulp stimulation are also excited by noxious electrical, mechanical or chemical stimuli derived from the jaw or tongue muscles. This indicates that convergence of superficial and deep afferent inputs via wide dynamic range or nociceptive-specific neurones occurs in the nucleus. Similar convergence of superficial and deep inputs occurs in the rostral nuclei and may account for the poor localization of trigeminal pain and for the spread of pain, which often makes diagnosis difficult.

There are distinct subtypes of cells in lamina II. Afferents from ‘higher centres’ arborize within it, as do axons from nociceptive and low-threshold afferents. Descending influences from these higher centres include fibres from the periaqueductal grey matter and from the nucleus raphe magnus and associated reticular formation.

The nucleus raphe magnus projects directly to the subnucleus caudalis, probably via enkephalin-, noradrenaline- and 5-HT–containing terminals. These fibres directly or indirectly (through local interneurones) influence pain perception. Stimulation of the periaqueductal grey matter or nucleus raphe magnus inhibits the jaw opening reflex to nociception and may induce primary afferent depolarization in tooth pulp afferents and other nociceptive facial afferents. Neurones in the subnucleus caudalis can be suppressed by stimuli applied outside their receptive field, particularly by noxious stimuli. The subnucleus caudalis is an important site for relay of nociceptive input and functions as part of the pain ‘gate control.’ However, rostral nuclei also have a nociceptive role. Tooth pulp afferents via wide dynamic range and nociceptive-specific neurones may terminate in rostral nuclei, which all project to the subnucleus caudalis.

Most fibres arising in the trigeminal sensory nuclei cross the midline and ascend in the trigeminal lemniscus. They end in the contralateral thalamic nucleus ventralis posterior medialis, from which third-order neurones project to the cortical postcentral gyrus (areas 1, 2 and 3). However, some trigeminal nucleus efferents ascend to the nucleus ventralis posterior medialis of the ipsilateral thalamus.

Fibres from the subnucleus caudalis, especially from laminae I, V and VI, also project to the rostral trigeminal nuclei, cerebellum, periaqueductal grey of the midbrain, parabrachial area of the pons, brain stem reticular formation and spinal cord. Fibres from lamina I project to the subnucleus medius of the medial thalamus.

Hypoglossal Nucleus

The prominent hypoglossal nucleus lies near the midline in the dorsal medullary grey matter. It is approximately 2 cm long. Its rostral part lies beneath the hypoglossal triangle in the floor of the fourth ventricle, and its caudal part extends into the closed part of the medulla (see Figs 10.2, 10.6, 5.11).

The hypoglossal nucleus consists of large motor neurones interspersed with myelinated fibres. It is organized into dorsal and ventral nuclear tiers, each divisible into medial and lateral subnuclei. There is a musculotopic organization of motor neurones within the nuclei that corresponds to the structural and functional divisions of tongue musculature. Thus, motor neurones innervating tongue retrusor muscles are located in dorsal and dorsolateral nuclei, whereas motor neurones innervating the main tongue protrusor muscle are located in ventral and ventromedial regions of the nucleus. Although relatively little is known about the organization of motor neurones innervating the intrinsic muscles of the tongue, experimental evidence suggests that motor neurones of the medial division of the hypoglossal nucleus innervate tongue muscles that are oriented in planes transverse to the long axis of the tongue (transverse and vertical intrinsics and genioglossus), whereas motor neurones of the lateral division innervate tongue muscles that are oriented parallel to this axis (styloglossus, hyoglossus, superior and inferior longitudinal).

Several smaller groups of cells lie near the hypoglossal nucleus. They are perhaps misnamed the ‘perihypoglossal complex’ or ‘perihypoglossal grey,’ for none is known with certainty to be connected to the hypoglossal nerve or nucleus. They include the nucleus intercalatus, sublingual nucleus, nucleus prepositus hypoglossi and nucleus paramedianus dorsalis (reticularis). Gustatory and visceral connections are attributed to the nucleus intercalatus.

Hypoglossal fibres emerge ventrally from their nucleus, traverse the reticular formation lateral to the medial lemniscus, pass medial to (or sometimes through) the inferior olivary nucleus and curve laterally to emerge superficially as a linear series of 10 to 15 rootlets in the ventrolateral sulcus between the pyramid and olive (see Fig. 10.3).

The hypoglossal nucleus receives corticonuclear fibres from the precentral gyrus and adjacent areas of mainly the contralateral hemisphere. They synapse either directly on motor neurones of the nucleus or on interneurones. Evidence indicates that the most medial hypoglossal subnuclei receive projections from both hemispheres. The nucleus may connect with the cerebellum via adjacent perihypoglossal nuclei and perhaps with the medullary reticular formation, trigeminal sensory nuclei and solitary nucleus.

Inferior Olivary Nucleus

The olivary nuclear complex consists of the large inferior olivary nucleus and the much smaller medial and dorsal accessory olivary nuclei. They are the so-called precerebellar nuclei, a group that also includes the pontine, arcuate, vestibular, reticulocerebellar and spinocerebellar nuclei, all of which receive afferents from specific sources and project to the cerebellum. The inferior olivary nucleus contains small neurones, most of which form the olivocerebellar tract, which emerges either from the hilum or through the adjacent wall to run medially and intersect the medial lemniscus (see Fig. 10.9). Its fibres cross the midline and sweep either dorsal to or through the opposite olivary nucleus. They intersect the lateral spinothalamic and rubrospinal tracts and the spinal trigeminal nucleus and enter the contralateral inferior cerebellar peduncle, where they constitute its major component. Fibres from the contralateral inferior olivary complex terminate on Purkinje cells in the cerebellum as climbing fibres; there is a one-to-one relationship between Purkinje cells and neurones in the complex. Afferent connections to the inferior olivary nucleus are both ascending and descending. Ascending fibres, mainly crossed, arrive from all spinal levels in the spino-olivary tracts and via the dorsal columns. Descending ipsilateral fibres come from the cerebral cortex, thalamus, red nucleus and central grey of the midbrain. In part, the two latter projections make up the central tegmental tract (fasciculus) that forms the olivary amiculum.

The medial accessory olivary nucleus is a curved grey lamina that is concave laterally and located between the medial lemniscus and pyramid and the ventromedial aspect of the inferior olivary nucleus. The dorsal accessory olivary nucleus is a similar lamina dorsomedial to the inferior olivary nucleus. Both nuclei are connected to the cerebellum. The accessory nuclei are phylogenetically older than the inferior and are connected with the palaeocerebellum. In all connections—cerebral, spinal and cerebellar—the olivary nuclei sometimes display very specific somatotopy, particularly in their cerebellar connections, which are described in detail in Chapter 13.

Nucleus Solitarius

The nucleus solitarius (solitary nucleus, nucleus of the solitary tract) lies ventrolateral to the vagal nucleus and is almost coextensive with it. A neuronal group ventrolateral to the nucleus solitarius has been termed the nucleus parasolitarius. The nucleus solitarius is intimately related to, and receives fibres from, the tractus solitarius, which carries afferent fibres from the facial, glossopharyngeal and vagus nerves. These fibres enter the tract in descending order and convey gustatory information from the lingual and palatal mucosa. They may also convey visceral impulses from the pharynx (glossopharyngeal and vagus) and from the oesophagus and abdominal alimentary canal (vagus). There is some overlap in this vertical representation.

Termination of special visceral gustatory afferents within the nucleus shows a viscerotopic pattern, predominantly in the rostral region. Experimental evidence suggests that fibres from the anterior two-thirds of the tongue and the roof of the oral cavity (which travel via the chorda tympani and greater petrosal branches of the facial nerve) terminate in the extreme rostral part of the solitary complex. Those from the circumvallate and foliate papillae of the posterior third of the tongue, tonsils, palate and pharynx (which travel via the lingual branch of the glossopharyngeal nerve) are distributed throughout the rostrocaudal extent of the nucleus, predominantly rostral to the obex. Gustatory afferents from the larynx and epiglottis (which travel via the superior laryngeal branch of the vagus) have a more caudal and lateral distribution. The nucleus solitarius may also receive fibres from the spinal cord, cerebral cortex and cerebellum.

Medial and commissural subnuclei in the caudal part of the nucleus appear to be the primary site of termination for gastrointestinal afferents. Ventral and interstitial subnuclei probably receive tracheal, laryngeal and pulmonary afferents and play an important role in respiratory control and possibly rhythm generation. The carotid sinus and aortic body nerves terminate in the dorsal and dorsolateral region of the nucleus solitarius, which may be involved in cardiovascular regulation.

The nucleus solitarius is thought to project to the sensory thalamus with a relay to the cerebral cortex. It may also project to the upper levels of the spinal cord through a solitariospinal tract. Secondary gustatory axons cross the midline. Many subsequently ascend the brain stem in the dorsomedial part of the medial lemniscus and synapse on the most medial neurones of the thalamic nucleus ventralis posterior medialis (in a region sometimes termed the accessory arcuate nucleus). Axons from the nucleus ventralis posterior medialis radiate through the internal capsule to the anteroinferior area of the sensorimotor cortex and the insula. It is thought that other ascending paths end in a number of hypothalamic nuclei and thus mediate the route by which gustatory information reaches the limbic system, allowing appropriate autonomic reactions.

Swallowing and Gag Reflexes

During the normal processes of eating and drinking, passage of material to the rear of the mouth stimulates branches of the glossopharyngeal nerve in the oropharynx (Fig. 10.15). This information is relayed via the nucleus solitarius to the nucleus ambiguus, which contains the motor neurones innervating the muscles of the palate, pharynx and larynx. The nasopharynx is closed off from the oropharynx by elevation of the soft palate. The larynx is raised, its entrance narrows and the glottis is closed. Peristaltic activity down the oesophagus to the stomach is mediated through the pharyngeal plexus.


Fig. 10.15 Swallowing and gag reflexes.

(Redrawn from MacKinnon, P., Morris, J. (Eds.), 1990. Oxford Textbook of Functional Anatomy, vol 3, Head and Neck. Oxford University Press, Oxford. By permission of Oxford University Press.)

If stimulation of the oropharynx occurs other than during swallowing, the gag reflex may be initiated. There is a reflex contraction of the muscles of the pharynx, soft palate and fauces that, if extreme, may result in retching and vomiting.

Nucleus Ambiguus

The nucleus ambiguus is a group of large motor neurones situated deep in the medullary reticular formation. It extends rostrally as far as the upper end of the vagal nucleus; caudally, it is continuous with the nucleus of the spinal accessory nerve. Fibres emerging from it pass dorsomedially, then curve laterally. Rostral fibres join the glossopharyngeal nerve. Caudal fibres join the vagus and cranial accessory nerves and are distributed to the pharyngeal constrictors, intrinsic laryngeal muscles and striated muscles of the palate and upper oesophagus.

The nucleus ambiguus contains several cellular subgroups, and some topographical representation of the muscles innervated has been established. Individual laryngeal muscles are innervated by relatively discrete groups of cells in more caudal zones. Neurones that innervate the pharynx lie in the intermediate area, and neurones that innervate the oesophagus and soft palate are rostral.

The nucleus ambiguus is connected to corticonuclear tracts bilaterally and to many brain stem centres. At its upper end, a small retrofacial nucleus intervenes between it and the facial nucleus. Although the nucleus ambiguus lies in line with the special visceral efferent nuclei, it is a reputed source of general visceral efferent vagal fibres.

Cough and Sneeze Reflexes

Irritation of the larynx or trachea is conveyed via laryngeal branches of the vagus nerve to the trigeminal sensory nucleus of the brain stem. Impulses are relayed to medullary respiratory centres and to the nucleus ambiguus. More or less energetic exhalation (coughing) occurs, caused by the contraction of intercostal and abdominal wall muscles after a buildup of pressure against a closed glottis.

A similar mechanism underlies sneezing (Fig. 10.16), except that the stimulus arises from the nasal mucosa, and afferent impulses are conveyed by the ophthalmic or maxillary divisions of the trigeminal nerve to the trigeminal sensory nucleus. After sharp inhalation, explosive exhalation occurs, with closure of the oropharyngeal isthmus by action of the palatoglossus, which diverts air through the nasal cavity and expels the irritant.


Fig. 10.16 Sneeze and cough reflexes.

(Redrawn from MacKinnon, P., Morris, J. (Eds.), 1990. Oxford Textbook of Functional Anatomy, vol 3, Head and Neck. Oxford University Press, Oxford. By permission of Oxford University Press.)

CASE 3 Wallenberg’s Syndrome

A 60-year-old retired teacher with known hypertension and diabetes presents with the acute onset of difficulty walking, along with incoordination of his left arm and leg. He has noticed that a soda can does not feel cold in his left hand, and the beverage does not feel cold in the right side of his mouth. He also complains of nausea and hiccups and of a change in his speech.

On examination, he has a mild left eyelid ptosis and impaired elevation of the soft palate on the left. His speech is nasal, and the left pupil fails to dilate in the dark. Cranial nerve functions are otherwise intact. Motor power is normal throughout, as is reflex activity; the plantar responses are flexor. Sensory testing shows reduced pain and temperature sensations on his left face and throughout his right arm and leg; proprioception is normal. He has incoordination and ataxia with finger–nose–finger and heel–shin testing on the left.

MRI shows an infarct in the left lateral medullary tegmentum (Fig. 10.17).

Discussion: Lateral medullary (or Wallenberg’s) syndrome is due to infarction in the distribution of the posterior inferior cerebellar artery. This vessel arises from the vertebral artery and supplies the tegmentum of the lateral medulla (the so-called lateral medullary plate) and the inferior cerebellum. Symptoms of lateral medullary syndrome include ipsilateral facial sensory loss due to involvement of the descending spinal trigeminal nucleus and tract; ispsilateral ataxia, reflecting the lesion in the inferior cerebellar peduncle (restiform body); contralateral pain and temperature loss due to involvement of the lateral spinothalamic tract; and Horner’s syndrome due to involvement of the descending sympathetic tracts in the tegmentum. Patients may also have vertigo and nystagmus, indicating that the vestibular complex is affected, and hoarseness due to involvement of the vagal nerve nucleus. There is no motor involvement because the descending corticospinal tracts lie medial and inferior to this vascular supply.


External Features and Relations

The pons lies rostral to the medulla and caudal to the midbrain. Ventrally, the site of transition with the medulla is demarcated superficially by a transverse sulcus. Laterally, in a region known as the cerebellopontine angle (see Figs 10.2, 10.3), the facial, vestibulocochlear and glossopharyngeal roots and the nervus intermedius all lie on the choroid plexus of the fourth ventricle (which protrudes from the foramen of Luschka into the subarachnoid space). The ventral surface of the pons (see Fig. 10.3) is separated from the clivus (basisphenoid and dorsum sellae) by the cisterna pontis. It is markedly convex transversely and less so vertically; it grooves the petrous part of the temporal bone laterally up to the internal acoustic meatus. The surface has a shallow vertical median sulcus in which the basilar artery runs, bounded bilaterally by prominences that are formed partly by underlying corticospinal fibres as they descend through the pons. Bundles of transverse fibres, bridging the midline and originating from nuclei in the basal pons (nuclei pontis), converge on each side into the large middle cerebellar peduncle and project to the cerebellum. The trigeminal nerve emerges near the mid-pontine level. It has a small superomedial motor root and a large inferolateral sensory root.

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