Transverse section of the medulla at the level of the pyramidal decussation

A transverse section across the lower medulla oblongata (Fig. 19.4) 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. 19.8). The decussation displaces the central grey matter and 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.

Fig. 19.8 The decussation of the pyramids.

The dorsal grey column is also modified at this level as 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 (Fig. 19.5). The pyramids form two large ventral bundles flanking the ventral median fissure on the ventral surface of the medulla.

They contain corticospinal fibres of ipsilateral origin. The nucleus gracilis is prominent on the dorsal aspect, with diminishing numbers of fibres of the fasciculus gracilis located on its dorsal, medial and lateral margins. The nucleus cuneatus is well developed. Both nuclei retain continuity with the central grey matter at this level, but this is lost more rostrally. First-order afferent fibres contained within the fasciculi gracilis and cuneatus synapse upon 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 spinal tract of the trigeminal nerve and the central grey matter. The fibres decussate in the midline thereafter forming the medial lemniscus which ascends to the thalamus. The decussation of internal arcuate fibres is located dorsal to the pyramids and ventral to the central grey matter, which is therefore more dorsally displaced than in the previous section.

The medial lemniscus ascends from the decussation as a flattened tract, near the median raphe. The pyramidal tract lies ventrally, and the medial longitudinal fasciculus and the tectospinal tract lie dorsally. Decussating lemniscal fibres are rearranged as they cross, so that those derived from the nucleus gracilis come to lie ventral to those from the nucleus cuneatus. Above this level, the medial lemniscus is further rearranged so that ventral (gracile) fibres migrate laterally, whilst dorsal (cuneate) fibres migrate medially. After such rearrangement, the medial lemniscus is somatotopically organized with C1 to S4 spinal segments represented sequentially from medial to lateral.

The nucleus of the spinal tract of the trigeminal nerve is separated from the central grey matter by internal arcuate fibres, and from the lateral medullary surface by the spinal tract of the trigeminal and by some dorsal spinocerebellar tract fibres. The latter progressively incline dorsally, and enter the inferior cerebellar peduncle at a higher level.

Transverse section of the medulla at the caudal end of the fourth ventricle

A transverse section at the lower end of the fourth ventricle (Fig. 19.6) shows some new features together with most of those already described. 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.

Fig. 19.6 Transverse section through the medulla oblongata at the caudal end of the fourth ventricle.

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. 19.7).

Fig. 19.7 Some of the afferent components of the inferior cerebellar peduncles. The efferent components have been omitted.

The inferior olivary nucleus is an irregularly crenated mass of grey matter with a medially directed hilum, through which numerous fibres enter and leave the nucleus. It has prominent connections with the cerebellum and is described more fully in Chapter 20.

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

The tractus solitarius and the associated 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 portion of the tract consists of gustatory fibres from the facial, glossopharyngeal and vagal nerves; they project to the rostral pole of the nucleus solitarius, which is sometimes referred to as the gustatory nucleus.

The medial longitudinal fasciculus is a small compact tract near the midline, ventral to the hypoglossal nucleus, that 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, and is therefore 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 nucleus of the spinal tract of the trigeminal 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.

Pyramidal tract

Each pyramid contains descending corticospinal fibres, derived from the ipsilateral cerebral cortex, which have traversed the internal capsule, midbrain and pons (Fig. 19.8). Approximately 70–90% 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, such that fibres destined to end in the cervical segments cross first. Fibres continue to pass dorsally as they descend, and reach 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 the major route for discriminative aspects of tactile and 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, i.e. lower levels respond to low-threshold cutaneous stimuli, and upper reticular levels 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 upon 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 that 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. These afferents both inhibit and enhance activity and are believed to be involved in sensory modulation. The reticular zones also receive connections from the reticular formation.

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 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 sensory–motor 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 nucleus, is part of the spinocerebellar system of precerebellar nuclei (Fig. 19.7). It contains large neurones like those in the spinal thoracic nucleus and receives the lateral fibres of the fasciculus cuneatus, which carry proprioceptive impulses from the upper limb that enter the cervical spinal cord rostral to the thoracic nucleus. The accessory cuneate neurones give rise to the posterior external arcuate fibres that enter the cerebellum by the ipsilateral inferior cerebellar peduncle in the cuneocerebellar tract. A group of neurones, nucleus Z, identified in animals between the upper pole of the nucleus gracilis and the inferior vestibular nucleus, 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 towards the principal sensory nucleus, which is situated at this level (Fig. 19.9). 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, of which 90% 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 nucleus of the spinal tract of the trigeminal (spinal trigeminal nucleus; Fig. 19.4, Fig. 19.5, Fig. 19.6, Fig. 19.10, Fig. 19.11). There is a precise somatotopic organization in the tract. Fibres from the ophthalmic division of the trigeminal lie ventrolaterally, those from the mandibular division 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 of these fibres synapse in the nucleus caudalis.

Fig. 19.9 Transverse section of the pons at the level of the trigeminal nerve.

Fig. 19.10 Transverse section through the superior half of the medulla oblongata at the level of the inferior olivary nucleus.

Fig. 19.11 Transverse section through the pons at the level of the facial colliculus.

The detailed anatomy of the spinal tract of the trigeminal excited early clinical interest because it was recognized that dissociated sensory loss could occur in the trigeminal area. For example, in Wallenberg’s syndrome, 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. Neurosurgery in this region, as early as the 1890s, attempted to alleviate paroxysmal trigeminal neuralgia. The introduction of medullary tractotomy confirmed that dissociated thermoanalgesia of the face was associated with destruction of the tract.

There are conflicting opinions on the pattern of termination of the 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, whilst 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 a section 4 mm below the obex rendered the ophthalmic and maxillary areas analgesic 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 anatomico-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. Fibres from the posterior face (adjacent to C2) terminate in the lower (caudal) part, whilst 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 ‘onion-skin’ pattern of loss of pain sensation. However, in clinical practice, the progression of anaesthesia on the face is most commonly ‘divisional’ rather than onion-skin 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 subnuclei. It has a structure analogous to that of the dorsal horn of the spinal cord, with a similar arrangement of cell laminae, and 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 stimulation of 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 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, the brain stem reticular formation and the spinal cord. Fibres from lamina I project to the subnucleus medius of the medial thalamus.

Vagal nucleus

The vagal nucleus (also known as the dorsal motor nucleus of the vagus) lies dorsolateral to the hypoglossal nucleus, from which it is separated by the nucleus intercalatus. It extends caudally to the first cervical spinal segment and rostrally to the open part of the medulla under the vagal triangle of the floor of the fourth ventricle (Fig. 19.6).

The vagal nucleus is a general visceral efferent nucleus and is the largest parasympathetic nucleus in the brain stem. Most (80%) of its neurones give rise to the preganglionic parasympathetic fibres of the vagus nerve. The remainder are interneurones or project centrally. The vagal nucleus controls the non-striated muscle of the viscera of the thorax (heart, bronchi, lungs and oesophagus) and abdomen (stomach, liver, pancreas, spleen, small intestine and proximal part of the colon). Neurones within the nucleus are heterogeneous and can be classified into nine subnuclei, which are regionally grouped into rostral, intermediate and caudal divisions. Topographic maps of visceral representation in animals suggest that the heart and lungs are represented in the caudal and lateral part of the nucleus, the stomach and pancreas in intermediate regions, and the remaining abdominal organs in the rostral and medial part of the nucleus.

There may be a sparse sensory afferent supply, which arises in the nodose ganglion and projects directly to the nucleus and possibly beyond into the nucleus tractus solitarius.

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 (Fig. 19.3) and its caudal part extends into the closed part of the medulla.

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/dorsolateral subnuclei, whereas motor neurones innervating the main tongue protrusor muscle are located in ventral/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 (perihypoglossal nuclei), but none is known for certainty to be connected with 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 (sometimes through) the inferior olivary nucleus, and curve laterally to emerge as a linear series of 10–15 rootlets in the ventrolateral sulcus between the pyramid and olive.

The hypoglossal nucleus receives corticobulbar fibres from the precentral gyrus and adjacent areas of (mainly) the contralateral hemisphere. They either synapse on motor neurones of the nucleus directly 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 also with the medullary reticular formation, the trigeminal sensory nuclei and the solitary nucleus.

Inferior olivary nucleus

The olivary nuclear complex consists of the large inferior olivary nucleus and the much smaller medial accessory and dorsal accessory olivary nuclei (Fig. 19.6). 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 grey matter, to run medially and intersect the medial lemniscus (Fig. 19.6). 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).

The medial accessory olivary nucleus is a curved grey lamina, concave laterally, 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 olivary nuclei are phylogenetically older than the inferior olivary nucleus, and they are connected with the paleocerebellum. In all connections, cerebral, spinal and cerebellar, the olivary nuclei display specific topographical organization (Ch. 20).

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 both 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 and thence 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 the hypothalamic nuclei, and so mediate the route by which gustatory information may reach the limbic system and allow appropriate autonomic reactions to be made.

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. 19.12). 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 narrowed and the glottis is closed. Peristaltic activity down the oesophagus to the stomach is mediated through the pharyngeal plexus.

Fig. 19.12 Swallowing and gag reflexes.

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 (see Fig. 19.10). It extends rostrally as far as the upper end of the vagal nucleus while 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 receives corticobulbar fibres bilaterally and is connected to many brain stem centres. At its upper end, a small retrofacial nucleus intervenes between it and the facial nucleus. Although the nucleus ambiguus is generally regarded as a special visceral efferent nucleus, it is also a reputed source of general visceral efferent fibres to the vagus.

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. Forceful exhalation (coughing) occurs, caused by contraction of intercostal and abdominal wall muscles after a build-up of pressure against an initially closed glottis.

A similar mechanism underlies sneezing (Fig. 19.13) 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 the action of palatoglossus, which diverts air through the nasal cavity.

Fig. 19.13 Sneeze and cough reflexes.

Transverse sections of the pons

Transverse sections (Fig. 19.9, Fig. 19.11) reveal that the pons consists of a dorsal tegmentum, which is a continuation of the medulla (excluding the pyramids), and a ventral (basilar) part. The latter contains bundles of longitudinal descending fibres, some of which continue into the pyramids, while others end in the many pontine or medullary nuclei. It also contains numerous transverse fibres and scattered pontine nuclei.

Ventral pons

The ventral pons is similar in structure at all levels. The longitudinal fibres of the corticopontine, corticobulbar and corticospinal tracts descend from the crus cerebri of the midbrain and enter the pons compactly. They rapidly disperse into fascicles, which are separated by the pontine nuclei and transverse pontine fibres. Corticospinal fibres run uninterrupted through the pons to the medullary pyramids, where they again converge into compact tracts. Corticobulbar fibres terminate in contralateral (and some ipsilateral) nuclei of cranial nerves and other nuclei in the pontine tegmentum. Clinical evidence supports the view that the neurones of the facial nucleus, and in particular, those serving the muscles of the upper face, receive bilateral cortical control.

The pontine nuclei (Fig. 19.11) include all the neurones (in man, some 20 million) that are scattered throughout the ventral pons. They are probably all glutamatergic, and most project to the contralateral cerebellar cortex, with some input to the deep cerebellar nuclei. Efferent axons from the pontine nuclei constitute the transverse pontine (pontocerebellar) fibres which decussate and continue as the major constituent of the contralateral middle cerebellar peduncle. All pontocerebellar fibres end as mossy fibres in the cerebellar cortex, and a degree of somatotopy is maintained in these connections.

Corticopontine fibres, from the frontal, temporal, parietal and occipital cortices, end upon neurones in the pontine nuclei. Frontopontine fibres end rostral to the level of the trigeminal nerve, and connect with pontine neurones which project to the contralateral cerebellum in the more rostral transverse pontine fibres. Corticopontine fibres arise mainly from neurones in layer V of the premotor, somatosensory, posterior parietal, extrastriate visual and cingulate neocortices. Projections from prefrontal, temporal and striate cortices are sparse. The terminal fields, although divergent, form topographically segmented patterns resembling overlapping columns, slabs or lamellae within the pons.

Subcortical projections to the pontine nuclei include those from the superior colliculus to the dorsolateral pons, and the medial mammillary nucleus to the rostromedial pons and pretectal nuclei. The lateral geniculate nucleus, dorsal column nuclei, trigeminal nuclei, hypothalamus and intracerebellar nuclei also project to restricted neurones of the pons. Functionally related subcortical and cerebrocortical afferents converge, e.g. those from the somatosensory cortex and dorsal column nuclei. There is also non-specific input from the reticular formation, raphe nuclei, locus coeruleus and paraqueductal grey matter.

Pontine tegmentum

The pontine tegmentum varies in cytoarchitecture at different levels. A transverse section through the lower pontine tegmentum transects the facial colliculus in the floor of the fourth ventricle (Fig. 19.11, Fig. 19.14). The colliculus contains the nucleus of the abducens nerve and the overlying fibres of the facial nerve. More deeply placed are the facial nuclei, the nearby vestibular and cochlear nuclei and other isolated neuronal groups. The vestibular nuclei are laterally placed in the rhomboid fossa of the fourth ventricle, subjacent to the vestibular area, which spans the rostral medulla and caudal pons (Fig. 19.1, Fig. 19.3).

Fig. 19.14 Transverse section of the pons, viewed from the rostral aspect, showing the central course of fibres of the facial nerve.

The striae medullares of the fourth ventricle (Fig. 19.10) are an aberrant cerebropontocerebellar connection, in which the arcuate nuclei and external arcuate fibres are involved. Axons from arcuate nuclei spread round the medulla, above and below the inferior olive, where they are superficially visible as the circumolivary fasciculus. All these fibres, which collectively are known as the external arcuate fibres, enter the inferior cerebellar peduncle (Fig. 19.7). Some fibres from arcuate nuclei pass dorsally through the medulla near its midline, decussate near the floor of the fourth ventricle, then turn laterally under the ependyma and enter the cerebellum through the inferior peduncle.

In addition to the tracts already noted at lower levels, the lower pontine tegmentum contains the trapezoid body, lateral lemniscus and emerging fibres of the abducens and facial nerves. The medial lemniscus is ventral, its transverse outline now a flat oval. It extends laterally from the median raphe (Fig. 19.9, Fig. 19.11) and is laterally related to the lateral spinothalamic tract and trigeminal lemniscus. The fibres of the latter originate from neurones of the contralateral spinal nucleus, serving pain and thermal sensibility in facial skin and mucosae of the conjunctiva, tongue, mouth, nose, etc. Here the lemnisci together form a transverse band which is composed, in lateral order from the midline, of the medial and trigeminal lemnisci, the lateral spinothalamic tract and the lateral lemniscus.

The trapezoid body contains cochlear fibres, mainly from the ventral cochlear and trapezoid nuclei. They ascend transversely in the ventral tegmentum, pass either through or ventral to the vertical medial lemniscal fibres, and decussate with the contralateral fibres in the median raphe. Below the emerging facial axons, the trapezoid fibres turn up into the lateral lemniscus. As the lateral lemniscus ascends it lies near the dorsolateral surface of the brain stem. Above, its fibres enter the inferior colliculus and medial geniculate body. The ascending auditory pathway is described in detail in Chapter 37.

The medial longitudinal fasciculus is paramedian, ventral to the fourth ventricle, near the abducens nucleus from which it is separated by facial nerve fibres. It is the main intersegmental tract in the brain stem, particularly for interactions between nuclei of cranial nerves innervating the extraocular muscles and the vestibular system. In the lower pons it receives fibres from vestibular, and perhaps dorsal trapezoid, nuclei. Its greater part is formed by vestibular contributions.

A transverse section at an upper pontine tegmental level contains trigeminal elements (Fig. 19.9), but otherwise shows little notable alteration from a section through a lower pontine tegmental level. Its dorsolateral parts are invaded by the superior cerebellar peduncles. The small nucleus of the lateral lemniscus, a relay in the auditory pathway, lies medial to its tract in the upper pons, and receives some lemniscal terminals. Some of its efferent fibres enter the medial longitudinal fasciculus, others return to the lemniscus.

The medial lemniscus (Fig. 19.9, Fig. 19.11) retains its paramedian position in the ventral pontine tegmentum, where it lies a little lateral to the median raphe, and is joined medially by fibres from the principal trigeminal sensory nucleus. The trigeminal lemniscus, lateral spinothalamic tract, and the lateral lemniscus and its nucleus all lie dorsolaterally.

Facial nucleus

The facial (motor) nucleus lies in the caudal pontine reticular formation, posterior to the dorsal trapezoid nucleus and ventromedial to the trigeminal spinal tract and nucleus. Groups of facial neurones form columns, which innervate individual muscles or which correspond to branches of the facial nerve. Neurones innervating muscles in the scalp and upper face are dorsal, and those supplying the lower facial musculature are ventral.

Efferent fibres of the large motor neurones of the facial nucleus form the motor root of the facial nerve. The motor nucleus represents the branchial efferent column, but it lies much more deeply in the pons than might be expected, and its axons have a most unusual course (Fig. 19.14). At first they incline dorsomedially towards the fourth ventricle, below the abducens nucleus, and ascend medial to it, near the medial longitudinal fasciculus. They then curve round the upper pole of the abducens nucleus and descend ventrolaterally through the reticular formation. Finally, they pass between their own nucleus medially and the spinal trigeminal nucleus. They emerge between the olive and the inferior cerebellar peduncle at the cerebellopontine angle.

The facial nucleus receives corticobulbar fibres for volitional control. Neurones that innervate muscles in the scalp and upper face are believed to receive bilateral corticobulbar fibres, while those supplying lower facial musculature only receive a contralateral innervation. Clinically, upper and lower motor neurone lesions of the facial nerve can be differentiated because the former results in paralysis that is confined to the contralateral lower face, whilst the latter results in a complete ipsilateral paralysis (Bell’s palsy).

The facial nucleus also receives ipsilateral afferents from the nucleus solitarius and from the trigeminal sensory nucleus, establishing important reflex connections.

Some efferent fibres of the facial nerve originate from neurones in the superior salivatory nucleus, which is said to be in the reticular formation dorsolateral to the caudal end of the facial nucleus. These preganglionic parasympathetic neurones belong to the general visceral efferent column. They send fibres into the sensory root of the facial nerve. These travel via the chorda tympani to the submandibular ganglion and via the greater petrosal nerve and the nerve of the pterygoid canal to reach the pterygopalatine ganglion.

Corneal reflex

Touching the cornea or shining a bright light into the eye elicits reflex closure of both eyes. The former action stimulates nasociliary branches of the ophthalmic nerve, and the latter stimulates the retina and optic pathway. In both cases, afferent impulses enter the central nervous system and spread via interneurones to activate neurones in the facial motor nucleus in the pons (Fig. 19.15). Efferent impulses travel in the facial nerve to activate the palpebral component of orbicularis oculi, which contracts, producing a ‘blink’. The sweep of the eyelids carries lacrimal secretions across the eye helping to remove any irritating particles.

Fig. 19.15 Corneal reflex. Unilateral stimulation of the cornea elicits a bilateral corneal reflex (not illustrated).

Trigeminal sensory nucleus

On entering the pons, the fibres of the sensory root of the trigeminal nerve run dorsomedially towards the principal sensory nucleus (Fig. 19.9). About 50% of the fibres divide into ascending and descending branches; the others ascend or descend without division. The descending fibres form the spinal tract of the trigeminal, which terminates in the subjacent nucleus of the spinal tract (spinal nucleus) of the trigeminal nerve.

Some ascending trigeminal fibres, many of them heavily myelinated, synapse around the small neurones in the principal sensory nucleus (Fig. 19.16), which lies lateral to the motor nucleus and medial to the middle cerebellar peduncle, and is continuous inferiorly with the spinal nucleus. The principal nucleus is considered to be mainly concerned with tactile stimuli.

Other ascending fibres enter the mesencephalic nucleus, a column of unipolar cells whose peripheral branches may convey proprioceptive impulses from the masticatory muscles, and possibly also from the teeth and the facial and oculogyric muscles. Its neurones are unique in being the only primary sensory neurones with somata in the CNS. It is the relay for the ‘jaw jerk’, which is the only significant supraspinal monosynaptic reflex. Nerve fibres that ascend to the mesencephalic nucleus may give collaterals to the motor nucleus of the trigeminal nerve and to the cerebellum.

Jaw jerk reflex

Rapid stretching of the muscles that close the jaw (masseter, temporalis, medial pterygoid) activates muscle spindle afferents, which travel via the mandibular division of the trigeminal nerve to the brain stem (Fig. 19.17). The cell bodies of these primary afferent neurones are located in the mesencephalic nucleus of the trigeminal. Collaterals project monosynaptically to the motor nucleus of the trigeminal nerve in the pons. From here motor axons of the mandibular nerve innervate the muscles that close the jaw.

Fig. 19.17 Jaw jerk reflex.

Most fibres that arise 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). Some trigeminal nucleus efferents ascend to the nucleus ventralis posterior medialis of the ipsilateral thalamus.

Transverse sections of the midbrain

In transverse section, the cerebral peduncles are seen to be composed of dorsal and ventral regions separated by the substantia nigra (Fig. 19.18, Fig. 19.19). On each side, the dorsal region is the tegmentum and the ventral part is the crus cerebri. The tegmenti are continuous across the midline but the crura are separated.

Crus cerebri

The crus cerebri is approximately semilunar in section, being convex externally and concave internally. It contains corticobulbar and corticospinal fibres. Corticobulbar fibres terminate at various brainstem levels, some in association with the cranial nerve nuclei (corticonuclear fibres). Corticospinal fibres pass uninterrupted through the brainstem to enter the medullary pyramid and, thereafter, form the corticospinal tracts of the spinal cord (Fig. 18.18). Corticonuclear and corticospinal fibres occupy the middle two-thirds of the crus cerebri. Corticopontine fibres arise in the cerebral cortex and form two groups, both of which end in the pontine nuclei. Frontopontine fibres from the frontal lobe, principally areas 6 and 4, traverse the internal capsule and then occupy the medial sixth of the ipsilateral crus cerebri. Temporopontine fibres, which originate largely from the posterior region of the temporal lobe, traverse the internal capsule and then run in the lateral sixth of the ipsilateral crus. Parietopontine and occipitopontine fibres are also described in the crus, lying medial to the temporopontine fibres. There are few fibres from the primary sensory cortex in corticopontine projections.

Mesencephalic tegmentum

The mesencephalic tegmentum is continuous with the pontine tegmentum. At inferior collicular levels, grey matter is restricted to scattered collections of neurones in the reticular formation and a central, periaqueductal region ventral to the cerebral aqueduct. The trochlear nucleus lies in the latter region, near the midline, in a position corresponding to the abducens and hypoglossal nuclei at more caudal levels. It extends through the lower half of the midbrain, just caudal to the oculomotor nucleus and immediately dorsal to the medial longitudinal fasciculus.

The mesencephalic nucleus of the trigeminal nerve occupies a lateral position in the central grey matter (see Fig. 19.2). It ascends from the main trigeminal sensory nucleus in the pons to the level of the superior colliculus in the midbrain. Its large ovoid neurones are unipolar, like those in peripheral sensory ganglia. They are arranged in many small groups which extend as curved laminae on the lateral margins of the periaqueductal grey matter: neurones are most numerous caudally.

Apart from these nuclei, the mesencephalic tegmentum contains many other scattered neurones, most of which are included in the reticular formation.

The white matter of the mesencephalic tegmentum contains the majority of the tracts present in the pontine tegmentum. The decussation of the superior cerebellar peduncles is particularly prominent. Numerous ascending fibres enter the mesencephalic tegmentum from the cerebellum and pass ventromedially, decussating in the midline (Fig. 19.18). Thereafter, ascending fibres encapsulate and penetrate the red nucleus, in which some fibres, or their collaterals, terminate (Fig. 19.19). However, the majority of fibres ascend to terminate in the nucleus ventralis lateralis of the thalamus. A relatively small number of uncrossed fibres are believed to end in the periaqueductal grey matter and reticular formation, the interstitial nucleus and the nucleus of the posterior commissure (nucleus of Darkschewitsch). The latter nucleus may send efferent fibres to the medial longitudinal fasciculus and posterior commissure. Descending fascicles from the superior cerebellar peduncles also apparently end in the pontine and medullary reticular formation, the olivary complex and, possibly, motor cranial nerve nuclei.

The medial longitudinal fasciculus adjoins the somatic efferent cell column, dorsal to the decussating superior cerebellar peduncles (Fig. 19.18). The medial, trigeminal, lateral and spinal lemnisci form a curved band dorsolateral to the substantia nigra. Fibres in the medial, trigeminal and spinal lemnisci continue a rostral course to synapse with neurones in the lateral and medial parts of the ventral posterior nucleus of the thalamus. Some fibres of the lateral lemniscus end in the nucleus of the inferior colliculus, or send collaterals to it. The remaining lateral lemniscal fibres join with fibres originating in the inferior colliculus and enter the inferior quadrigeminal brachium, which carries them to the medial geniculate body.

At the level of the superior colliculus, the tegmentum contains the red nucleus (Fig. 19.19), which extends rostrally into the subthalamic region. The ventromedial central grey matter around the aqueduct contains the elongated oculomotor nucleus, which is adjacent ventrolaterally to the medial longitudinal fasciculus, and caudally reaches the trochlear nucleus. The oculomotor nucleus is divisible into neuronal groups which may be partially correlated with the motor distribution of the oculomotor nerve. A group of preganglionic parasympathetic neurones which control the activity of smooth muscle within the eyeball, the Edinger–Westphal nucleus, lies dorsal to the oculomotor nucleus.

Substantia nigra

The substantia nigra is a large nucleus that extends through the whole length of the midbrain (Fig. 19.18, Fig. 19.19). It has major connections with the basal ganglia and is central to the pathology of Parkinson’s disease (Ch. 22).

The substantia nigra is located between the crus cerebri ventrolaterally and ascending lemniscal fibres dorsomedially. Its medial part is traversed by oculomotor axons passing ventrally to their point of exit in the interpeduncular fossa. The substantia nigra is divided into a dorsal pars compacta and a ventral pars reticulata; the neurones in each part have quite different connections.

The pars compacta consists of darkly pigmented neurones, which contain neuromelanin granules. Their arrangement is irregular and they partially penetrate the subjacent pars reticulata. Pigmentation increases with age, is most abundant in primates, maximal in man, and present even in albinos. The pigmented pars compacta neurones synthesize dopamine as their neurotransmitter and project predominantly to the corpus striatum of the basal ganglia as well as to other sites. The pars compacta corresponds to the dopaminergic cell group A9 (Fig. 22.10). Two other dopaminergic cell groups are found in the ventral tegmentum: cell group A10 in the rostromedial region, which corresponds to the ventral tegmental area (of Tsai), and cell group A8 in the dorsolateral reticular area, which forms the nucleus parabrachialis pigmentosus. The whole ventral tegmental system of dopaminergic neurones appears to act as an integrative centre for adaptive behaviour. It projects via a number of pathways, mainly through ascending fibres in the ipsilateral medial forebrain bundle. These pathways are a mesodiencephalic system, which terminates in thalamic and hypothalamic nuclei; a mesostriatal projection to the caudate nucleus and putamen; a mesolimbic pathway to the nucleus accumbens, olfactory tubercle, lateral septum, interstitial nucleus of the stria terminalis, amygdala and entorhinal cortex; and mesocortical fibres that project to most cortical areas, particularly the prefrontal, orbitofrontal and cingulate cortex.

The pars compacta projects heavily to the caudate nucleus and putamen in a topographically organized fashion (nigrostriatal fibres). Lesser projections end in the globus pallidus and subthalamic nucleus. In Parkinson’s disease, the levels of dopamine in the substantia nigra and striatum decrease dramatically as a result of the degeneration of pars compacta neurones (Ch. 22).

The pars reticulata of the substantia nigra extends rostrally as far as the subthalamic region. It is considered to be a homologue of the medial (internal) segment of the globus pallidus, which it resembles in terms of cell type and connectivity. It consists of clusters of neurones, many of which intermingle with fibres of the crus cerebri. Most pars reticulata neurones are GABAergic and also contain high levels of iron.

There are reciprocal connections between the substantia nigra and the basal ganglia. Efferent fibres from the basal ganglia terminate largely, but not exclusively, in the pars reticulata. Topographically organized striatonigral fibres originate from the caudate nucleus and putamen and project to the pars reticulata. The head of the caudate nucleus projects to the rostral third of the substantia nigra, while the putamen projects to all parts. The fibres end in axodendritic synapses. A small number of GABAergic pallidonigral fibres from the lateral (external) segment of the globus pallidus end mostly in the pars reticulata. The subthalamic nucleus sends an important glutamatergic projection to the pars reticulata and to the globus pallidus. Subthalamonigral and subthalamopallidal projections are important in the pathophysiology of movement disorders such as Parkinson’s disease and dyskinesias (Ch. 22).

GABAergic neurones in the pars reticulata project rostrally to the ventral anterior and dorsomedial thalamic nuclei, and caudally to the pedunculopontine nucleus and reticular formation, whence impulses are relayed to spinal ventral horn neurones. A pars lateralis of the substantia nigra is recognizable, but small, in man. It projects to the ipsilateral superior colliculus, and may be involved in the control of saccadic eye movements.

Some corticonigral fibres appear to exist, passing from precentral and postcentral gyri to neurones in the pars reticulata.

Red nucleus

The red nucleus is a pink, ovoid mass, approximately 5 mm in diameter and lying dorsomedial to the substantia nigra (Fig. 19.19). The tint appears only in fresh material and is caused by a ferric iron pigment in its multipolar neurones. The latter are of varying size and their relative proportions and arrangement vary between species. In primates the magnocellular element is decreased, and there is a reciprocal increase in the size of the parvocellular component. Small multipolar neurones occur in all parts of the nucleus. In man, the larger neurones are restricted to the caudal part of the nucleus and have been estimated to be as few as 200 in number. The magnocellular element is considered phylogenetically old, which accords with the parvocellular predominance in primates. Rostrally, the red nucleus is poorly demarcated, and it blends into the reticular formation and caudal pole of the interstitial nucleus. It is traversed and surrounded by fascicles of nerve fibres, including many from the oculomotor nucleus.

The principal afferent connections of the red nucleus are corticorubral and cerebellorubral fibres. Uncrossed corticorubral fibres originate from primary somatomotor and somatosensory areas. In animals, the red nucleus receives fibres from the contralateral nucleus interpositus (which corresponds to the human globose and emboliform nuclei), and dentate nucleus, via the superior cerebellar peduncle. It has bilateral, probably reciprocal, connections with the superior colliculi. In humans, the rubrospinal tract is small and originates from the caudal magnocellular part of the red nucleus. Few fibres reach the cervical cord. The fibres decussate and then run obliquely laterally in the ventral tegmental decussation, ventral to the tectospinal decussation, and dorsal to the medial lemniscus. On reaching the grey matter ventral to the inferior cerebellar peduncle, the tract turns caudally to enter the lateral part of the lateral lemniscus. It continues descending ventral to the tract and nucleus of the trigeminal nerve throughout the medulla, and enters the upper part of the cervical cord intermingled with fibres of the lateral corticospinal tract. Some efferent axons form a rubrobulbar tract to motor nuclei of the trigeminal, facial, oculomotor, trochlear and abducens nerves.

The largest group of efferents from the red nucleus in man is a descending component of the ipsilateral, central tegmental tract. Initially, this tract lies lateral to the medial longitudinal fasciculus and dorsolateral to both the red nucleus and the decussation of the superior cerebellar peduncles (Fig. 19.9, Fig. 19.10, Fig. 19.11, Fig. 19.18). Most fibres arise from the parvocellular part of the red nucleus and pass to the ipsilateral inferior olivary nucleus in the medulla. Some tract fibres terminate in the brain stem reticular nuclei. Ascending and descending axons from the brain stem reticular formation also run in the central tegmental tract. These axons include dorsal and ventral ascending noradrenergic bundles, a ventral ascending serotoninergic bundle, and some fibres of dorsal and ventral ascending cholinergic bundles.

As mentioned elsewhere (p. 269), lesions of the corticospinal system in man result in permanent paresis or paralysis. In monkeys, although initially complete, the paralysis disappears and good recovery ensues. The explanation for this interprimate variability in recovery from corticospinal lesions appears to lie in the differential capacity of the rubrospinal system to compensate for loss of corticospinal drive. Monkeys never fully recover from combined lesions of both the corticospinal and rubrospinal tracts, which suggests that the two systems are functionally interrelated in the control of movement. Both encode force, velocity and direction parameters, but the rubrospinal system primarily directs activity both during the terminal phase of a movement and preceding a movement. There is, thus, overlap of activity in the two systems for all parameters during movements of limbs and even of individual digits. The corticospinal system is most active during the learning of new movements, whereas the rubrospinal system is most active during the execution of learnt automated movements.

The rubro-olivary projection, which travels in the central tegmental tract, connects the red nucleus indirectly to the contralateral cerebellar cortex. The cerebellum is thought to play a role in motor learning, and so the rubro-olivary system could switch the control of movements from the corticospinal to the rubrospinal system for programmed automation. The relative absence of a rubrospinal system in man could explain the poor recovery of motor function after stroke.

Oculomotor nucleus

The nuclear complex from which the efferent fibres of the oculomotor nerve arise consists of several groups of large motor neurones (collectively, the oculomotor nucleus; Fig. 19.19) and smaller preganglionic parasympathetic neurones (the Edinger–Westphal nucleus). The motor neurone groups of the oculomotor nucleus innervate, in dorsoventral order, the ipsilateral inferior rectus, inferior oblique and medial rectus. A medially placed column of cells innervates the contralateral superior rectus: the axons from this subnucleus decussate in its caudal part. The medial rectus subnucleus consists of three anatomically distinct subpopulations. The ventral portion, which contains the largest number of motor neurones, occupies the rostral two-thirds; a subpopulation of smaller motor neurones lies dorsally throughout the rostral two-thirds of the nucleus and innervates the small orbital fibres of the medial rectus; and another subpopulation lies dorsolaterally in the caudal two-thirds of the nucleus.

A median subnucleus of large neurones, the caudal central nucleus, lies at the caudal pole of the oculomotor nucleus adjacent to the superior rectus and medial rectus subnuclei. In non-human primates, approximately 30% of the motor neurones in this subnucleus innervate levator palpebrae superioris bilaterally, which is unique for paired skeletal muscles.

The Edinger–Westphal nucleus, composed of small, multipolar, preganglionic parasympathetic neurones, lies dorsal to the main oculomotor nucleus. Its neurones give rise to axons that travel in the oculomotor nerve and synapse with postganglionic neurones in the ciliary ganglion.

Fascicles of axons from all of these subnuclei course forward in the midbrain and emerge as the oculomotor nerve in the interpeduncular fossa. The fascicles are most probably arranged from medial to lateral subserving the pupil, inferior rectus, medial rectus, levator palpebrae superioris and superior rectus, and inferior oblique. The human oculomotor nerve contains approximately 15,000 axons.

Afferent inputs to the oculomotor nuclear complex include fibres from the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial nucleus of Cajal, both of which are involved in the control of vertical and torsional gaze. Other inputs come, either directly or indirectly, from the nuclei of the posterior commissure, the interstitial nucleus of Cajal, the frontal eye fields, the superior colliculus, the dentate nucleus and other cortical areas. The medial longitudinal fasciculus carries connections from the trochlear, abducens and vestibular nuclei; the medial and lateral vestibular nuclei project to the medial rectus subnucleus.

Afferent inputs to the Edinger–Westphal nucleus come primarily from the pretectal nuclei bilaterally, mediating the pupillary light reflex and from the visual cortex, mediating accommodation.

Trochlear nucleus

The trochlear nucleus lies in the grey matter in the floor of the cerebral aqueduct, level with the upper part of the inferior colliculus (Fig. 19.1, Fig. 19.18). It is in line with the ventromedial part of the oculomotor nucleus, in the position of the somatic efferent column. The medial longitudinal fasciculus is ventral and lateral to it. The oculomotor and trochlear nuclei often overlap slightly but can be distinguished by the smaller size of the trochlear neurones. The afferent inputs to the trochlear nucleus are similar to those described for the oculomotor nucleus. Trochlear efferent fibres pass laterodorsally round the central grey matter, then descend medial to the trigeminal mesencephalic nucleus to reach the upper end of the superior medullary velum, where they decussate and emerge lateral to the frenulum. A few fibres remain ipsilateral.

Medial longitudinal fasciculus

The medial longitudinal fasciculus (Fig. 19.9, Fig. 19.18, Fig. 39.12) is a heavily myelinated composite tract, lying near the midline, ventral to the periaqueductal grey matter. It ascends to the interstitial nucleus of Cajal, which lies in the lateral wall of the third ventricle, just above the cerebral aqueduct. The fasciculus retains its position relative to the central grey matter through the midbrain, pons and upper medulla, but is displaced ventrally by successive decussations of the medial lemnisci and lateral corticospinal tracts. At spinal levels it is synonymous with the medial vestibulospinal tract.

The medial longitudinal fasciculus interconnects the oculomotor, trochlear, abducens, Edinger–Westphal, vestibular, reticular and spinal accessory nuclei, coordinating conjugate eye movements and associated movements of the head and neck. Lesions cause internuclear ophthalmoplegia. All four vestibular nuclei contribute ascending fibres. Those from the superior nucleus remain uncrossed, while the others are partly crossed. Some fibres reach the interstitial and posterior commissural nuclei, and some decussate to the contralateral nuclei. Descending axons, from the medial vestibular nuclei and perhaps the lateral and inferior nuclei, partially decussate and descend in the fasciculus to form the medial vestibulospinal tract (Fig. 18.19). Fibres join the fasciculus from the dorsal trapezoid, lateral lemniscal and posterior commissural nuclei, which means that both the cochlear and vestibular components of the vestibulocochlear nerve may influence movements of the eyes and head via the medial longitudinal fasciculus. Some vestibular fibres may ascend in the medial longitudinal fasciculus as far as the thalamus.

Tectum