Arcuate nuclei number of uncrossed corticospinal fibres descend in the ipsilateral
Cuneocerebellar tract
ventral funiculus, as the ventral corticospinal tract, and a smaller
Nucleus gracilis Accessory cuneate nucleus uncrossed component joins the lateral corticospinal tracts (see Fig.
20.16). These small, uncrossed fascicles have no significant clinical
Nucleus cuneatus
relevance. The corticospinal tracts display somatotopy at almost all
levels. In the pyramids the arrangement is like that at higher brainstem
levels in that the lower extremity is represented laterally and the upper
extremity is represented medially (Haines 2013, Nieuwenhuys et al
2008). Similar somatotopy is ascribed to the lateral corticospinal tracts
Dorsal spinocerebellar tract within the spinal cord.
Dorsal column nuclei
The nuclei gracilis and cuneatus, the dorsal column nuclei, are part of
Dorsal root of spinal nerve
the pathway, commonly called the dorsal column–medial lemniscus
system, that is the major route for discriminative aspects of tactile and
Posterior thoracic nucleus proprioceptive sensation (Haines 2015, Millar and Basbaum 1975,
(of Clarke)
Nieuwenhuys et al 2008). The upper regions of both nuclei are reticular
and contain small and large multipolar neurones with long dendrites.
Fig. 21.9 Some of the afferent components of the restiform body. The The lower regions contain clusters of large, round neurones with short
efferent components have been omitted. 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
central grey matter and midline, and is therefore near the somatic effer- grey matter are components of the postsynaptic dorsal column system
ent nuclear column. Fibres from a variety of sources course for variable and terminate only in the superior, reticular zone. Variable ordering and
distances in this tract. overlap of terminals, on the basis of spinal root levels, occur in both


The spinocerebellar, spinotectal, vestibulospinal and rubrospinal zones. The lower extremity is represented medially, the trunk ventrally
tracts and the anterolateral system (spinal lemniscal) all lie in the ven- and the digits dorsally. There is modal specificity, i.e. lower levels
trolateral area of the medulla at this level (Nathan and Smith 1982). respond to low-threshold cutaneous stimuli, and upper reticular levels
These tracts are limited dorsally by the spinal tract and nucleus of the to inputs from fibres serving receptors in the skin, joints and muscles.
trigeminal nerve, and ventrally by the pyramid. The nucleus cuneatus is divided into several parts. Its middle zone
Numerous islets of grey matter are scattered centrally in the ventro- contains a large pars rotunda, in which rostrocaudally elongated,
lateral medulla, an area intersected by nerve fibres that run in all medium-sized neurones are clustered between bundles of densely


medulla oblongata
315
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myelinated fibres. The reticular poles of its rostral and caudal zones (cuneocerebellar) fibres that enter the cerebellum by the ipsilateral
contain scattered, but evenly distributed, neurones of various sizes. The inferior cerebellar peduncle. A group of neurones, nucleus Z, identified
pars triangularis is smaller and laterally placed. in animals between the upper pole of the nucleus gracilis and the infe-
There is a somatotopic pattern of termination of cutaneous inputs rior vestibular nucleus, is said to be present in the human medulla. Its
from the upper limb on the cell clusters of the pars rotunda. Termina- input is probably from the dorsal spinocerebellar tract, which carries
tions are diffuse in the reticular poles. proprioceptive information from the ipsilateral lower limb, and it
The nucleus gracilis and nucleus cuneatus serve as relays between projects through internal arcuate fibres to the contralateral medial
the spinal cord and higher levels. Primary spinal afferents synapse with lemniscus.
multipolar neurones in the nuclei that form the major nuclear efferent
Sensory nucleus of the trigeminal nerve
projection. The nuclei also contain interneurones, many of which are
inhibitory. Descending afferents from the somatosensory cortex reach The sensory nucleus of the trigeminal nerve is a large cell group
the nuclei through the corticonuclear tracts, and appear to be restricted that receives the primary afferents of the trigeminal nerve. It extends
to the upper, reticular zones. These afferents both inhibit and enhance caudally into the cervical spinal cord and rostrally into the midbrain;
activity and are believed to be involved in sensory modulation. The its principal divisions are the spinal trigeminal, principal sensory and
reticular zones also receive connections from the reticular formation. mesencephalic nuclei (see Figs 21.10–21.12) (Nieuwenhuys et al 2008;
Neurones of the dorsal column nuclei receive terminals of long, Olszewski 1950).
uncrossed, primary afferent fibres of the fasciculi gracilis and cuneatus, On entering the pons, the fibres of the sensory root of the trigeminal


which carry information concerning deformation of skin, movement of nerve run dorsomedially towards the principal sensory nucleus, which
hairs, joint movement and vibration. Unit recording of the neurones in is situated at this level. Before reaching the nucleus, approximately 50%
the dorsal column nuclei shows that their tactile receptive fields (i.e. of the fibres divide into ascending and descending branches; the others
the skin area in which a response can be elicited) vary in size, although ascend or descend without division. The descending fibres, of which
they are especially small for the digits and the face and lips. Some fields 90% are less than 4 µm in diameter, form the spinal tract of the trigemi-
have excitatory centres and inhibitory surrounds, which means that nal nerve, which embraces the spinal nucleus of the trigeminal nerve
stimulation just outside its excitatory field inhibits the neurone. Neu- and reaches the upper cervical spinal cord (see Figs 21.6–21.8; Fig.
rones in the nuclei are spatially organized into a somatotopic map of 21.10). There is a precise somatotopic organization within the tract (see
the periphery (in accord with the similar localization in the dorsal Fig. 21.1). Fibres from the ophthalmic division of the trigeminal nerve
columns). In general, specificity is high. Many cells receive input from lie ventrally, those from the mandibular division lie dorsally, and those
one or a few specific receptor types, e.g. hair, type I and II slowly adapt- from the maxillary division lie between. The tract is completed on its
ing receptors and Pacinian corpuscles, and some cells respond to Ia dorsal rim by fibres from the sensory roots of the facial, glossopharyn-
muscle spindle input. However, some neurones receive convergent geal and vagus nerves. All of these fibres synapse in the pars caudalis of
input from tactile pressure and hair follicle receptors. the spinal nucleus of the trigeminal nerve.
A variety of control mechanisms can modulate the transmission of The detailed anatomy of the spinal tract of the trigeminal nerve
impulses through the dorsal column–medial lemniscus pathway. Con- excited early clinical interest because it was recognized that dissociated


comitant activity in adjacent dorsal column fibres may result in presy- sensory loss could occur in the trigeminal area. For example, in Wall-
naptic inhibition by depolarization of the presynaptic terminals of one enberg’s syndrome (lateral medullary syndrome), occlusion of the pos-
of them. Stimulation of the sensory–motor cortex also modulates the terior inferior cerebellar artery (a branch of the vertebral artery) leads
transmission of impulses by both pre- and postsynaptic inhibitory to loss of pain and thermal sense on the ipsilateral half of the face with
mechanisms, and sometimes by facilitation. These descending influ- retention of common sensation (Haines 2013). Neurosurgery in this
ences are mediated by the corticospinal tract. Modulation of transmis- region, as early as the 1890s, attempted to alleviate paroxysmal trigemi-
sion by inhibition also results from stimulation of the reticular nal neuralgia. The introduction of medullary tractotomy confirmed that
formation, raphe nuclei and other sites. dissociated thermoanalgesia of the face was associated with destruction
The accessory cuneate nucleus, dorsolateral to the cuneate nucleus, of the tract.
is part of the spinocerebellar system of precerebellar relay nuclei (see There are conflicting opinions on the pattern of termination of the
Fig. 21.9). It contains large neurones like those in the posterior thoracic fibres in the spinal nucleus. It has long been held that fibres are organ-
nucleus (Clarke’s column) and receives the lateral fibres of the fascicu- ized rostrocaudally within the tract. According to this view, ophthalmic
lus cuneatus, which carry proprioceptive impulses from the upper limb fibres are ventral and descend to the lower limit of the first cervical
that enter the cervical spinal cord rostral to the thoracic nucleus. The spinal segment, and maxillary fibres are central and do not extend below
accessory cuneate neurones give rise to the posterior external arcuate the medulla oblongata, whilst mandibular fibres are dorsal and do not
Dorsal cochlear
nucleus Striae medullares
Fourth ventricle Inferior salivatory nucleus
Inferior vestibular Tectospinal tract
nucleus
Restiform body
Medial longitudinal Reticular formation
fasciculus


Ventral cochlear nucleus
Spinal nucleus of
trigeminal nerve
Nucleus ambiguus
Vestibular nerve
Lateral spinothalamic tract
Ventral spinocerebellar tract Olivocerebellar fibres
Amiculum olivare,
central tegmental tract Medial accessory olivary nucleus
Principal olivary nucleus
Anterior internal arcuate fibres
Medial lemniscus
Arcuate nuclei
Pyramid
Fig. 21.10 A transverse section through the superior half of the medulla oblongata at the level of the inferior olivary nucleus.


Brainstem
316
3
nOitCes
extend much below the mid-medullary level. The results of section of Vagal nucleus
the spinal tract in cases of severe trigeminal neuralgia support this
The vagal nucleus (the dorsal motor nucleus of the vagus) lies slightly
distribution. It was found that a section 4 mm below the obex rendered
dorsolateral to the hypoglossal nucleus, from which it is separated by
the ophthalmic and maxillary areas analgesic, but tactile sensibility,
the nucleus intercalatus. It extends caudally to the first cervical spinal
apart from the abolition of ‘tickle’, was much less affected. To include
segment and rostrally to the open part of the medulla under the vagal
the mandibular area it was necessary to section at the level of the obex.
trigone in the floor of the fourth ventricle (see Fig. 21.8).
More recently, it has been proposed that fibres are arranged dorso-
The vagal nucleus is a general visceral efferent nucleus and is the
ventrally within the spinal tract. There appear to be sound anatomico-
largest parasympathetic nucleus in the brainstem. Most (80%) of its
physiological and clinical reasons for believing that all divisions
neurones give rise to the preganglionic parasympathetic fibres of the
terminate throughout the whole nucleus, although the ophthalmic divi-
vagus nerve. The remainder are interneurones or project centrally. The
sion may not project fibres as far caudally as the maxillary and man-
vagal nucleus innervates the non-striated (smooth, cardiac) muscle of
dibular divisions. Fibres from the posterior face (adjacent to C2)
the viscera of the thorax (heart, bronchi, lungs and oesophagus) and
terminate in the lower (caudal) part, whilst those from the upper lip,
abdomen (stomach, liver, pancreas, spleen, small intestine and proxi-
mouth and nasal tip terminate at a higher level. This can give rise to a
mal part of the colon), and glandular epithelium. Neurones within the
segmental (cross-divisional) sensory loss in syringobulbia. Tractotomy
nucleus are heterogeneous and can be classified into nine subnuclei,


of the spinal tract, if carried out at a lower level, can spare the perioral
which are regionally grouped into rostral, intermediate and caudal divi-
region, a finding that would accord with the ‘onion-skin’ pattern of loss
sions. Topographic maps of visceral representation in animals suggest
of pain sensation. However, in clinical practice, the progression of
that the heart and lungs are represented in the caudal and lateral part
anaesthesia on the face is commonly ‘divisional’ rather than strictly
of the nucleus, the stomach and pancreas in intermediate regions, and
‘onion-skin’ in distribution.
the remaining abdominal organs in the rostral and medial part of the
Fibres of the glossopharyngeal, vagus and facial nerves subserving
nucleus.
common sensation (general somatic afferent) enter the dorsal region
There is a sparse sensory afferent supply, which arises in the nodose
of the spinal tract of the trigeminal nerve and synapse with cells in the
ganglion and projects directly to the nucleus and possibly beyond into
caudal part of the spinal nucleus. Consequently, operative section of
the nucleus tractus solitarius.
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 Hypoglossal nucleus
adjoining parts of the pharyngeal wall (supplied by the glossopharyn- The prominent hypoglossal nucleus lies near the midline in the dorsal
geal nerve), and the cutaneous areas of the ear. Other afferents that medullary grey matter. It is approximately 2 cm long. Its rostral part lies
reach the spinal nucleus are from the dorsal roots of the upper cervical beneath the hypoglossal trigone in the floor of the fourth ventricle (see
nerves and from the sensory–motor cortex. Fig. 21.5) and its caudal part extends into the closed part of the medulla.
The spinal nucleus of the trigeminal nerve consists of three parts: The hypoglossal nucleus consists of large motor neurones inter-
the pars oralis (which adjoins the principal sensory nucleus); the pars spersed with myelinated fibres. It is organized into dorsal and ventral


interpolaris; and the pars caudalis (which is continuous with the dorsal nuclear tiers, each divisible into medial and lateral subnuclei. There is
horn of the spinal cord). The pars caudalis is different from the other a musculotopic organization of motor neurones within the nuclei that
parts because it has a structure analogous to that of the dorsal horn of corresponds to the structural and functional divisions of tongue mus-
the spinal cord, with a similar arrangement of cell laminae (subnuclei culature. Thus, motor neurones innervating tongue retrusor muscles are
zonalis, gelatinosus and magnocellularis), and is involved in trigeminal located in dorsal/dorsolateral subnuclei, whereas motor neurones
pain perception. Cutaneous nociceptive afferents and small-diameter innervating the main tongue protrusor muscle are located in ventral/
muscle afferents terminate in layers I, II, V and VI of the pars caudalis ventromedial regions of the nucleus. Although relatively little is known
(see Fig. 21.1). Low-threshold mechanosensitive afferents of Aβ neu- about the organization of motor neurones innervating the intrinsic
rones terminate in layers III and IV of the pars caudalis and in the rostral muscles of the tongue, experimental evidence suggests that motor neu-
(interpolaris, oralis, principal sensory) nuclei. rones of the medial division of the hypoglossal nucleus innervate
Many of the neurones in the pars caudalis that respond to cutaneous tongue muscles that are orientated in planes transverse to the long axis
or tooth-pulp stimulation are also excited by stimulation of jaw or of the tongue (transverse and vertical intrinsics and genioglossus),
tongue muscles. This indicates that convergence of superficial and deep whereas motor neurones of the lateral division innervate tongue
afferent inputs via wide-dynamic-range or nociceptive-specific neurones muscles that are orientated parallel to this axis (styloglossus, hyoglos-
occurs in this nucleus. Similar convergence of superficial and deep sus, superior and inferior longitudinal).
inputs occurs in the rostral nuclei and may account for the poor locali- Hypoglossal fibres emerge ventrally from their nucleus, traverse the


zation of trigeminal pain, and for the spread of pain, which often makes reticular formation lateral to the medial lemniscus, pass medial to the
diagnosis difficult. inferior olivary nuclei, and curve laterally to emerge as a linear series of
There are distinct subtypes of cells in lamina II. Afferents from 10–15 rootlets in the ventrolateral sulcus between the pyramid and
‘higher centres’ arborize within it, as do axons from nociceptive and olivary eminence (see Fig. 21.8).
low-threshold afferents. Descending influences from these higher The hypoglossal nucleus receives corticonuclear fibres from the pre-
centres include fibres from the periaqueductal grey matter and from the central gyrus and adjacent areas of predominately the contralateral
nucleus raphe magnus and associated reticular formation. hemisphere. They synapse either on motor neurones of the nucleus
The nucleus raphe magnus projects directly to the pars caudalis, directly or on interneurones. Evidence indicates that the most medial
probably via enkephalin, noradrenaline (norepinephrine) and 5-HT hypoglossal subnuclei may receive projections from both hemispheres.
(5-hydroxytryptamine, serotonin)-containing terminals. These fibres The nucleus may connect with the cerebellum via adjacent perihy-
directly, or indirectly through local interneurones, influence pain per- poglossal nuclei, and perhaps also with the medullary reticular forma-
ception. Stimulation of periaqueductal grey matter or nucleus raphe tion, the trigeminal sensory nuclei and the nucleus solitarius.
magnus inhibits the jaw-opening reflex to nociception, and may induce
primary afferent depolarization in tooth-pulp afferents and other noci- Inferior olivary nucleus
ceptive facial afferents. Neurones in the pars caudalis can be suppressed The olivary nuclear complex consists of a large principal olivary nucleus
by stimuli applied outside their receptive field, particularly by noxious and smaller medial accessory and dorsal accessory olivary nuclei (see


stimuli. The pars caudalis is an important site for relay of nociceptive Figs 21.8, 21.10). They are also precerebellar nuclei, a group that
input and functions as part of the pain ‘gate control’. However, rostral includes the pontine, arcuate, vestibular, reticulocerebellar and spinoc-
nuclei may also have a nociceptive role. Tooth-pulp afferents via wide- erebellar nuclei, all of which receive afferents from specific sources and
dynamic-range and nociceptive-specific neurones may terminate in project to the cerebellum. The inferior olivary nucleus contains small
rostral nuclei, which all project to the subnucleus caudalis. neurones, most of which form the olivocerebellar tract, which emerges
Most fibres arising in the trigeminal sensory nuclei cross the midline primarily from the hilum to run medially and intersect the medial
and ascend as trigeminothalamic fibres (trigeminal lemniscus). They lemniscus (see Fig. 21.8). Its fibres cross the midline and sweep either
end in the contralateral ventral posteromedial thalamic nucleus, from dorsal to, or through, the opposite olivary nuclei. They intersect the
which third-order neurones project to the cortical postcentral gyrus lateral spinothalamic and rubrospinal tracts and the spinal nucleus of
(areas 3, 1, 2). However, some trigeminal nuclear efferents ascend to the trigeminal nerve, and enter the contralateral restiform body (and
the ipsilateral ventral posteromedial nucleus. eventually the inferior cerebellar peduncle), where they constitute its
Fibres from the pars caudalis, especially from laminae I, V and VI, major component. Fibres from the contralateral inferior olivary complex
also project to the rostral trigeminal nuclei, cerebellum, periaqueductal terminate on Purkinje cells in the cerebellum as climbing fibres; there
grey of the midbrain, parabrachial area of the pons, the brainstem is a one-to-one relationship between Purkinje cells and neurones in the


reticular formation and the spinal cord. Fibres from lamina I project to complex (Nieuwenhuys et al 2008). Afferent connections to the inferior
the subnucleus medius of the medial thalamus. olivary nuclei are both ascending and descending. Ascending fibres,


Brainstem
316.e1
12
retPaHC
Several smaller groups of cells lie near the hypoglossal nucleus (peri-
hypoglossal nuclei), but none is known for certainty to be connected
with the hypoglossal nerve or nucleus. They include the nucleus inter-
calatus, sublingual nucleus, nucleus prepositus hypoglossi and nucleus
paramedianus dorsalis (reticularis). Gustatory and visceral connections
are attributed to the nucleus intercalatus.


Pons
317
12
retPaHC
mainly crossed, arrive from all spinal levels in the spino-olivary tracts Stimulation of nasal mucosa by physical or chemical irritants initi-
and via the dorsal columns. Descending ipsilateral fibres come from ates a sneezing reflex. Afferent impulses travel via the ethmoidal and
the cerebral cortex, cerebellar nuclei, thalamus, red nucleus and central maxillary nerves to the spinal nucleus of the trigeminal nerve. Interneu-
grey of the midbrain. In part, the two latter projections make up the rones project to the nucleus ambiguus and a putative ‘sneezing centre’
central tegmental tract. in the rostral dorsolateral medulla (Seijo-Martínez et al 2006). Recruit-
ment of a critical number of inspiratory and expiratory neurones initi-
Nucleus solitarius
ates a sneeze, which involves eye closing and deep inspiration, followed
The nucleus solitarius (solitary nucleus, nucleus of the solitary tract) by explosive exhalation as described above. If the oropharyngeal
lies lateral or ventrolateral to the vagal nucleus (see Fig. 21.8). A neu- isthmus is closed by the action of palatoglossus, the air flow is diverted
ronal group ventrolateral to the nucleus solitarius has been termed the through the nasal cavity; otherwise the stream of expelled air flows
nucleus parasolitarius. The nucleus solitarius is intimately related to, through both oral and nasal cavities.
and receives fibres from, the tractus solitarius, which carries afferent
fibres from the facial, glossopharyngeal and vagus nerves (Ciriello 1983,
Haines 2013, Hamilton and Norgren 1984). These fibres enter the tract PONS
in descending order and convey gustatory information from the lingual
and palatal mucosa. They may also convey visceral impulses from the EXTERNAL FEATURES AND RELATIONS
pharynx (glossopharyngeal and vagus) and from the oesophagus and
abdominal alimentary canal (vagus). There is some overlap in this verti-
The pons lies rostral to the medulla and caudal to the midbrain (see
cal representation.


Fig. 21.4). On the ventral surface of the brainstem, the transition
The nucleus solitarius is thought to project to the sensory thalamus
between medulla and pons is clearly demarcated by a transverse sulcus.
and thence to the cerebral cortex (Hamilton and Norgren 1984). It may
Laterally, in a region known as the cerebellopontine angle, the facial,
also project to the upper levels of the spinal cord through a solitario-
vestibulocochlear and glossopharyngeal nerves emerge. The ventral
spinal tract. Secondary gustatory axons cross the midline. Many subse-
surface of the pons is separated from the clivus (basisphenoid and
quently ascend the brainstem in association with the medial lemniscus
dorsum sellae) by the pontine cistern (cisterna pontis) (see Fig. 18.15).
and synapse on the most medial neurones of the ventral posteromedial
It is markedly convex transversely but less so vertically, and abuts the
thalamic nucleus (in a region sometimes termed the accessory arcuate
petrous part of the temporal bone laterally up to the internal acoustic
nucleus). Axons from the ventral posteromedial nucleus radiate through
meatus. The surface of the basilar pons has a shallow median sulcus,
the internal capsule to the anteroinferior area of the sensory–motor
in which the basilar artery runs, bounded bilaterally by prominences
cortex and the insula. It is thought that other ascending paths end in a
formed partly by descending corticospinal fibres. Bundles of transverse
number of the hypothalamic nuclei, and so mediate the route by which
fibres, bridging the midline and originating from nuclei in the basal
gustatory information may reach the limbic system and allow appropri-
pons (nuclei pontis, pontine nuclei), converge on each side into the
ate autonomic reactions to be made.
large middle cerebellar peduncle and project to the cerebellum (Brodal
Nucleus ambiguus and Bjaalie 1992, Haines 2013, Nieuwenhuys et al 2008). The trigemi-


The nucleus ambiguus is a group of large motor neurones, situated deep nal nerve emerges near the mid-pontine level; the basilar pons is infe-
in the medullary reticular formation (see Fig. 21.10). It extends rostrally rior to this exit and the middle cerebellar peduncle superior and caudal
as far as the upper end of the vagal nucleus, while caudally it is in line to it. It has a small superomedial motor root (portio minor) and a large
with, but is not continuous with, the nucleus of the accessory nerve. inferolateral sensory root (portio major).
Fibres emerging from it pass dorsomedially, then curve laterally. Rostral The dorsal surface of the pons is hidden by the cerebellum, which
fibres join the glossopharyngeal nerve. Caudal fibres join the vagus and covers the rostral half of the rhomboid fossa, into which the cerebral
are distributed to the pharyngeal constrictors, intrinsic laryngeal muscles aqueduct empties. The roof of the fossa is formed by a thin sheet of
and striated muscles of the palate and upper oesophagus. tissue, the superior medullary velum, which is overlain by the lingula
The nucleus ambiguus receives corticonuclear fibres bilaterally with of the cerebellar vermis. The velum is attached on each side to the
a contralateral preponderance and is connected to many brainstem superior cerebellar peduncles and is enclosed by pia mater above and
centres. At its upper end, a small retrofacial nucleus intervenes between ependyma below (see Fig. 21.5). The trochlear nerves decussate in the
it and the facial nucleus. Although the nucleus ambiguus is generally rostral aspect of the velum.
regarded as a special visceral efferent nucleus, it is also a reputed source
of general visceral efferent fibres to the vagus.
INTERNAL ORGANIZATION
swallowing and gag reflexes
Swallowing is initiated when food or liquid stimulates sensory nerves Transverse sections of the pons
in the oropharynx and is usually regarded as programmed motor behav-
iour rather than a reflex. The patterning and timing of striated muscle


Transverse sections (Figs 21.11–21.12) reveal that the pons consists of
contraction that occur in the pharynx, larynx and oesophagus during
a tegmental part, which is a continuation of the medulla (excluding the
swallowing are generated in the brainstem in a network of neural cir-
pyramids), and a basilar part. The latter contains bundles of longitudi-
cuits. The afferent limb is the glossopharyngeal nerve: information is
nal descending fibres, some of which continue into the pyramids, while
relayed via the nucleus solitarius to the nucleus ambiguus, which con-
others end in the many pontine or medullary nuclei. It also contains
tains the motor neurones innervating the muscles of the palate, pharynx
numerous transverse fibres and pontine nuclei.
and larynx. If stimulation of the oropharynx occurs other than during
swallowing, a gag reflex may be initiated. There is a reflex contraction
Basilar pons
of the muscles of the pharynx, soft palate and fauces that, if extreme,
may result in retching and vomiting.
The basilar pons is similar in structure at all levels. The longitudinal
Cough and sneeze reflexes fibres of the corticopontine, corticonuclear and corticospinal tracts
The cough reflex is normally initiated by irritation of tracheal or laryn- descend from the crus cerebri of the midbrain and enter the pons com-
geal mucosae: there is evidence of both mechanosensing and chemo- pactly. They rapidly disperse into fascicles, which are separated by the
sensing cough receptors (McGarvey 2014). Coughing involves a pontine nuclei and transverse pontine fibres. Corticospinal fibres run
sequence of coordinated events that produce the profound change in uninterrupted through the pons to the medullary pyramids, where they
breathing pattern needed to expel an irritant from the lower airway. again converge into compact tracts. Corticonuclear fibres terminate in
Rapid inspiration is followed by an expiratory effort against a closed contralateral (and some ipsilateral) nuclei of cranial nerves and other


glottis, the rapid generation of intrapulmonary pressure and the sudden nuclei in the pontine tegmentum. Clinical evidence supports the view
opening of the glottis and contraction of intercostal and abdominal that the neurones of the facial nucleus, and, in particular, those serving
wall muscles, collectively producing a high-velocity flow of expired air the muscles of the upper face, receive bilateral cortical control. The
that sweeps irritant material up towards the pharynx in a forceful exha- lower half of the face is controlled predominately by the contralateral
lation (cough). Laryngeal branches of the vagus nerve carrying general cortex.
visceral afferent information (with cell bodies in the inferior vagal The pontine nuclei (see Fig. 21.11) include all the neurones (in
ganglion) terminate in the nucleus of the solitary tract. Second-order humans, some 20 million) that are scattered throughout the basilar
neurones project to medullary respiratory centres (including a putative pons. They are probably all glutamatergic, and most project to the
cough centre) and to the nucleus ambiguus, recruiting motor neurones contralateral cerebellar cortex, with collaterals to the cerebellar nuclei.
innervating pharyngeal, laryngeal, diaphragmatic, intercostal and ab- Efferent axons from the pontine nuclei constitute the transverse pontine
dominal muscles. (pontocerebellar) fibres, which decussate and continue as the major


Brainstem
317.e1
12
retPaHC
The medial accessory olivary nucleus is a curved grey lamina, concave
laterally, between the medial lemniscus and pyramid and the ventro-
medial aspect of the principal olivary nucleus. The dorsal accessory
olivary nucleus is a similar lamina, dorsomedial to the principal olivary
nucleus. Both accessory nuclei are connected with the cerebellum. These
nuclei are phylogenetically older than the principal olivary nucleus, and
are connected with the paleocerebellum. In all connections, cerebral,
spinal and cerebellar, the olivary nuclei display specific topographical
organization (Ch. 22).
Termination of special visceral afferents (taste) within the nucleus
shows a viscerotopic pattern, predominantly in the rostral region (the
gustatory nucleus). 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, predominately 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 affer-
ents; this is the cardiorespiratory nucleus. Ventral and interstitial sub-
nuclei probably receive tracheal, laryngeal and pulmonary afferents and
play an important role in respiratory control and possibly rhythm gen-
eration. 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 ambiguus contains several cellular subgroups, and some
topographical representation of the muscles innervated has been estab-
lished. 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.


Brainstem
318
3
nOitCes
Fourth ventricle
Juxtarestiform body
Superior vestibular nucleus
Genu of facial nerve
Lateral vestibular nucleus
Restiform body
Abducens nucleus
Spinal nucleus and
tract of trigeminal nerve
Vestibulocerebellar
Facial nerve fibres
Middle
Central tegmental
cerebellar
tract
peduncle
Medial Facial nucleus
longitudinal
fasciculus Trapezoid nucleus
Abducens
nerve Trapezoid body
Vestibular
nerve
Medial lemniscus
Descending tracts
(corticospinal, etc.)
Transverse fibres of pons Pontine nuclei
Fig. 21.11 A transverse section through the pons at the level of the facial colliculus.
Superior medullary velum
Fourth ventricle
Superior cerebellar
Mesencephalic tract of peduncle
trigeminal nerve
Principal sensory
Medial longitudinal nucleus of trigeminal nerve
fasciculus
Motor nucleus
of trigeminal nerve
Central tegmental tract
Middle cerebellar peduncle
Corticospinal and
corticopontine fibres
Lateral lemniscus
Trapezoid nucleus
Medial lemniscus and
trapezoid body
Transverse pontine fibres
Fig. 21.12 A transverse section of the pons at the level of the principal sensory and motor nuclei of the trigeminal nerve.
constituent of the contralateral middle cerebellar peduncle. All pon- pons. Functionally related subcortical and cerebrocortical afferents con-
tocerebellar fibres end as mossy fibres in the cerebellar cortex, and a verge, e.g. those from the somatosensory cortex and dorsal column
degree of somatotopy is maintained in these connections. nuclei. There is also non-specific input from the reticular formation,
Corticopontine fibres, from the frontal, temporal, parietal and occip- raphe nuclei, locus coeruleus and periaqueductal grey matter.
ital cortices, end on neurones in the pontine nuclei. Frontopontine
fibres end rostral to the level of the trigeminal nerve, and connect with
Pontine tegmentum


pontine neurones that 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, pos- The pontine tegmentum varies in cytoarchitecture at different levels
terior parietal, extrastriate visual and cingulate neocortices. Projections (Haines 2015, Nieuwenhuys et al 2008, Olszewski and Baxter 1954).
from prefrontal, temporal and striate cortices are sparse. The terminal However, a consistent feature is that all cranial nerves associated with
fields, although divergent, form topographically segmented patterns the pons have their nuclei (motor or sensory) within the tegmentum,
resembling overlapping columns, slabs or lamellae within the pons. although their axons may traverse the basilar pons. The pedunculopon-
Subcortical projections to the pontine nuclei include those from the tine nucleus, as its name implies, lies in the caudal part of the tegmen-
superior colliculus to the dorsolateral pons, and the medial mammil- tum of the midbrain and extends into the rostral part of the dorsolateral
lary nucleus to the rostromedial pons and pretectal nuclei. The lateral tegmentum of the pons; it is described below.
geniculate nucleus, dorsal column nuclei, trigeminal nuclei, hypotha- A transverse section through the lower pontine tegmentum transects
lamus and cerebellar nuclei also project to restricted neurones of the the facial colliculus in the floor of the fourth ventricle (see Fig. 21.11;


Pons
319
12
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Medial longitudinal fasciculus Facial colliculus The medial lemniscus (see Figs 21.11, 21.12) retains its position in
the ventral pontine tegmentum, where it lies a little lateral to the
Abducens nucleus median raphe, and is joined medially by fibres from the principal
sensory nucleus of the trigeminal nerve. The trigeminal lemniscus, ante-
rolateral system, and the lateral lemniscus and its nucleus all lie
dorsolaterally.
Cochlear nuclei
Fibres of the cochlear division of the vestibulocochlear nerve partially
encircle the restiform body laterally and end in the dorsal and ventral
cochlear nuclei. The dorsal (posterior) cochlear nucleus forms a bulge,
the auditory tubercle, on the posterior surface of the restiform body,
and is continuous medially with the vestibular area in the rhomboid
Facial fossa. The ventral (anterior) cochlear nucleus is ventrolateral to the
nerve dorsal cochlear nucleus, and lies between the cochlear and vestibular
Facial fibres of the vestibulocochlear nerve.
nucleus Cochlear nerve fibres, which are derived from neuronal somata in
Descending cortical fibres Abducens nerve the spiral ganglion, bifurcate on entering the brainstem and terminate
in both dorsal and ventral cochlear nuclei (see Fig. 38.20). The ventral
Pons Pyramid of medulla oblongata
cochlear nucleus has a complex cytoarchitecture.
Fig. 21.13 A transverse section of the pons, viewed from the rostral It contains many neuronal types with distinct dendritic field charac-
aspect, showing the central course of fibres of the facial nerve.
teristics. Marked topographical order has been demonstrated in coch-
lear nerve terminals within the nucleus. Different parts of the spiral
ganglion and differing stimulation frequencies are related to neurones
Fig. 21.13). This colliculus contains the nucleus of the abducens nerve that are serially arrayed anteroinferiorly in the ventral nucleus. All coch-
and the fibres of the internal genu of the facial nerve. More deeply lear nerve fibres enter the nucleus. There are approximately 25,000


placed are the facial nuclei, the nearby vestibular and cochlear nuclei, axons in the human cochlear nerve, and they project on to a much larger
and other isolated neuronal groups. The vestibular nuclei are laterally number of neurones in the cochlear nucleus. The number of cochlear
placed in the rhomboid fossa of the fourth ventricle, subjacent to the fibres in the lateral lemniscus greatly exceeds that in the cochlear nerve.
vestibular area, which spans the rostral medulla and caudal pons (see A minor fraction of the cochlear neurones receive terminals from the
Figs 21.3, 21.5). nerve, though each fibre may connect with several neurones. Terminals
The striae medullares of the fourth ventricle (see Fig. 21.10) are an are limited to the anteroinferior region of the ventral nucleus, where
aberrant cerebropontocerebellar connection, in which the arcuatocere- the neurones are probably mostly local interneurones.
bellar fibres are involved. Axons from arcuate nuclei spread round the The dorsal (posterior) cochlear nucleus is generally continuous with
medulla, above and below the olivary eminence, where they are super- the ventral nucleus, from which it is separated only by a thin stratum
ficially visible as the circumolivary fasciculus. All these fibres, collec- of nerve cell bodies and fibres. Giant cells predominate, and their den-
tively known as the external arcuate fibres, enter the restiform body (see dritic fields are aligned with the incoming auditory fibres.
Fig. 21.9). Some fibres from arcuate nuclei pass dorsally through the Though the cellular origins are not precisely known, axons of most
medulla near its midline (as arcuatocerebellar fibres), decussate near neuronal types in the cochlear nuclei leave to end at pontine levels in
the floor of the fourth ventricle, then turn laterally under the ependyma the superior olivary, trapezoid and lateral lemniscal nuclei (see Fig.
and enter the cerebellum through the inferior peduncle via the resti- 21.12). They leave the cochlear nuclei by three routes. The largest group


form body. of axons lies ventrally and decussates as the trapezoid body, level with
In addition to the tracts already noted at lower levels, the lower the pontomedullary junction (see Figs 21.11, 21.12). Most of these
pontine tegmentum contains the trapezoid body, lateral lemniscus and axons ascend slightly, decussate and relay in the contralateral nuclei. A
emerging fibres of the abducens and facial nerves. The medial lemnis- few do not cross, and synapse in the ipsilateral superior olivary nuclei.
cus is ventral, its transverse outline now a flat oval. It extends laterally From both nuclei, the next-order axons ascend in the corresponding
from the median raphe (see Figs 21.11, 21.12) and is laterally related to lateral lemniscus. Occasional decussating fibres traverse the contralat-
the anterolateral system and trigeminal lemniscus. The fibres of the eral superior olive and enter the lateral lemniscus to relay in lemniscal
latter originate from neurones of the contralateral spinal trigeminal nuclei.
nucleus, serving pain and thermal sensibility in facial, nasal and oral Some axons from ventral cochlear neurones pass dorsally, superficial
cavities and the conjunctiva. Here these fibres form a transverse band to descending trigeminal spinal fibres, cerebellar fibres in the restiform
that is composed, in lateral order from the midline, of the medial lem- body and axons of the dorsal cochlear nucleus. This bundle of ventral
niscus, trigeminal lemnisci, the anterolateral system and the lateral cochlear fibres is smaller than that of the trapezoid decussation. It
lemniscus. swerves ventromedially across the midline, ventral to the medial longi-
The trapezoid body contains cochlear fibres, mainly from the ventral tudinal fasciculus, as the intermediate acoustic striae. Its further path
cochlear and trapezoid nuclei. They ascend transversely in the ventral is uncertain, but it most likely ascends in the contralateral lateral
tegmentum, pass either through or ventral to the medial lemniscal lemniscus.


fibres, and decussate with the contralateral fibres in the median raphe. The most dorsally placed axons issue from the dorsal cochlear
Below the emerging facial axons, the trapezoid fibres turn rostral to nucleus. They curve dorsomedially round the restiform body towards
enter the lateral lemniscus. As the lateral lemniscus ascends, it lies near the midline as the dorsal acoustic striae, ventral to the striae medullares.
the dorsolateral surface of the brainstem. Rostrally, its fibres enter the They incline ventromedially and cross the midline to ascend in the
inferior colliculus and medial geniculate body. The ascending auditory contralateral lateral lemniscus, probably relaying in its nuclei.
pathway is described in detail on page 655. The nuclei of the lateral lemniscus consist of small groups of neu-
The medial longitudinal fasciculus is paramedian, ventral to the rones (counts of 18,000–24,000 have been recorded in humans) that
fourth ventricle, near the midline and adjacent to motor nuclei of lie among the fibres of the lateral lemniscus. Dorsal, ventral and inter-
cranial nerves III, IV, VI and XII. It is the main intersegmental tract in mediate groups probably receive afferent axons from both cochlear
the brainstem, particularly for interactions between nuclei of cranial nuclei. Their efferents enter the midbrain along the lateral lemniscus
nerves innervating the extraocular muscles and the vestibular system. and terminate in the inferior colliculi.
In the lower pons it receives fibres from vestibular, and perhaps Some efferent axons travel in the cochlear nerves to the spiral organ;
dorsal trapezoid, nuclei. Its greater part is formed by vestibular these comprise the olivocochlear tract (bundle of Rasmussen). Though
contributions. few in number, they are involved in hearing, perhaps by modulating
A transverse section at an upper pontine tegmental level contains sensory transduction through reflexes via cochlear nuclei. The neurones
trigeminal elements (see Fig. 21.12), but otherwise shows little notable of origin are located at the hilus and along the lateral border of the


alteration from a section through a lower pontine tegmental level. Its lateral superior olivary nucleus and lateral edge of the ventral trapezoid
dorsolateral parts are invaded by the superior cerebellar peduncles. The nucleus. Fibres from both sides proceed to both cochleae.
small nucleus of the lateral lemniscus, a relay in the auditory pathway,
Vestibular nuclei
lies medial to its tract in the upper pons and receives some lemniscal
terminals. Some of its efferent fibres enter the medial longitudinal The vestibular complex consists of the medial (Schwalbe), lateral
fasciculus, while others return to the lemniscus. (Deiters’ nucleus), superior (Bechterew) and inferior vestibular nuclei


Brainstem
319.e1
12
retPaHC
The superior olivary complex is sited in the tegmentum of the caudal
pons, lateral in the reticular formation at the level of the pontomedul-
lary junction. This complex includes several named nuclei and nameless
smaller groups. In humans, the lateral superior olivary nucleus is made
up of some six small cellular clusters. The medial (accessory) superior
olivary nucleus is large and compact. Cells in the immediate vicinity of
these nuclei give rise to the olivocochlear tract. The trapezoid nucleus
is medial and the retro-olivary group, also a potential source of cochlear
efferents, is dorsal. Some internuclear connections have been described.
The medial superior olivary nucleus receives impulses from both spiral
organs, and may be involved in sound source localization. The superior
olivary complexes and the trapezoid nuclei are relay stations in the
ascending auditory projection. These intricate connections have not
been definitively established in humans.
The medial nuclei of the trapezoid body are small in humans. They
have a ventral component, which consists of large neurones scattered
among the trapezoid fascicles, and a more compact dorsal nucleus,
medial to the superior olivary complex. This nucleus lies at the level of
the exiting abducens nerve roots, anterior to the central tegmental tract.
It is likely that the human trapezoid nuclei function in the auditory
relay pathway. Some trapezoid axons may enter the medial longitudinal
fasciculus, and ascend to end in trigeminal, facial, oculomotor, troch-
lear and abducens nuclei, where they mediate reflexes involving tensor
tympani, stapedius and oculogyric muscles, respectively.


Brainstem
320
3
nOitCes
(Haines 2013). The medial vestibular nucleus is the largest. It broad- the tectobulbar tract (from the deep layers of the superior colliculus);
ens, and then narrows as it ascends from the upper olivary level into the paramedian pontine reticular formation (which lies rostral and
the lower pons, where it is located lateral and superior to visceromotor caudal to the nucleus); the nucleus prepositus hypoglossi; and the
nuclei of cranial nerves IX and X. It is crossed by the striae medullares, contralateral medullary reticular formation.
running nearer the floor, and is continuous with the nucleus intercala-
Facial nucleus
tus below. The inferior vestibular nucleus (which is the smallest) lies
between the medial vestibular nucleus and restiform body, from the The facial (motor) nucleus lies in the caudal pontine reticular forma-
level of the upper end of the nucleus gracilis to the pontomedullary tion, posterior to the dorsal trapezoid nucleus and ventromedial to
junction. It is traversed by descending fibres of the vestibular nerve, the spinal tract and nucleus of the trigeminal nerve (Haines 2013,
from the cerebellum, and the vestibulospinal tract. The lateral vestibu- Nieuwenhuys et al 2008). Groups of facial neurones form columns,
lar nucleus lies just above the inferior nucleus and is ventrolateral to which innervate individual muscles or which correspond to branches
the upper part of the medial nucleus. It ascends almost to the level of of the facial nerve. Neurones innervating muscles in the scalp and upper
the abducens nucleus and its rostral end is continuous with the caudal face are dorsal, and those supplying the lower facial musculature are
end of the superior nucleus. The lateral vestibular nucleus is composed ventral.
of large multipolar neurones, which are the main source of lateral ves- Efferent fibres of the large motor neurones of the facial nucleus form
tibulospinal tract fibres. The superior vestibular nucleus is small and the motor root of the facial nerve. The motor nucleus is part of the


lies above the medial and lateral nuclei. It extends higher into the pons pharyngeal efferent column, but it lies much more deeply in the pons
than other subdivisions, and occupies the upper part of the vestibular than might be expected and its axons have an unusual course (see Fig.
area. 21.13). At first they incline dorsomedially towards the fourth ventricle,
Through its connections, the vestibular system influences move- caudal to the abducens nucleus, and ascend medial to it, near the
ments of the eyes and head, and of the muscles of the trunk and limbs, medial longitudinal fasciculus. They then curve round the rostral pole
in order to maintain equilibrium (Nieuwenhuys et al 2008). The ves- of the abducens nucleus and descend ventrolaterally through the reticu-
tibular nuclei receive fibres from the vestibular part of the vestibuloco- lar formation. Finally, they pass between their own nucleus medially
chlear nerve, cerebellum, brainstem reticular formation and spinal and the spinal nucleus of the trigeminal nerve laterally. They emerge
cord. They send axons to the cerebellum, medial longitudinal fascicu- between the olive and restiform body at the cerebellopontine angle.
lus, spinal cord and lateral lemniscus. Evidence suggests that the ves- The facial nucleus receives corticonuclear fibres for volitional control.
tibular apparatus is spatially represented in the nuclei. Vestibular fibres Neurones that innervate muscles in the scalp and upper face receive
of the vestibulocochlear nerve enter the medulla between the restiform bilateral corticonuclear fibres, while those supplying lower facial mus-
body and the spinal tract of the trigeminal nerve, and approach the culature receive a predominately contralateral innervation. Clinically,
vestibular area, where they bifurcate into descending and ascending upper and lower motor neurone lesions of the facial nerve can be dif-
branches. The former descend medial to the restiform body and end in ferentiated because the former results in paralysis that is confined to
medial, lateral and inferior vestibular nuclei; the latter enter the supe- the contralateral lower face (supranuclear facial palsy), whilst the latter


rior and medial nuclei. A few vestibular fibres enter the cerebellum results in a complete ipsilateral paralysis (Bell’s palsy).
directly through the juxtarestiform body (the medial part of the inferior The facial nucleus also receives ipsilateral afferents from the nucleus
cerebellar peduncle), and end in the fastigial nucleus, flocculonodular solitarius and from the sensory nucleus of the trigeminal nerve, estab-
lobe and uvula. lishing important reflex connections.
The vestibular nuclei project extensively to the cerebellum and also Some efferent fibres of the facial nerve originate from visceromotor
receive axons from the cerebellar cortex and the fastigial nuclei. Vestibu- neurones in the superior salivatory nucleus, which is in the reticular
locerebellar fibres (primary and secondary) travel via the juxtarestiform formation dorsolateral to the facial nucleus. These preganglionic para-
body mainly to the flocculus and nodule. Cerebellovestibular fibres pass sympathetic neurones belong to the general visceral efferent column.
to the nuclei, also via the juxtarestiform body. They arise mainly in the They send fibres into the sensory root of the facial nerve, a part of
flocculus and nodule (posterior lobe), but some fibres are derived from the intermediate nerve. These fibres travel via the chorda tympani to the
the anterior lobe and fastigial nucleus. The vestibular nuclear complex submandibular ganglion and via the greater petrosal nerve and the
projects to the pontine reticular nuclei and to motor nuclei of the ocu- nerve of the pterygoid canal to reach the pterygopalatine ganglion.
logyric muscles in the medial longitudinal fasciculus. The complex also
projects to the spinal cord via medial and lateral vestibulospinal tracts, Corneal and blink reflexes
the former one part of the medial longitudinal fasciculus (see Fig. Touching the cornea of one eye elicits reflex closure of both eyes via the
20.18). corneal reflex. The afferent limb of the pathway involves mainly naso-
Information from the vestibular nuclei also reaches the cerebral ciliary branches of the ophthalmic division of the trigeminal nerve, with


cortex by way of the thalamus (probably via posterior parts of the ven- cell bodies in the trigeminal ganglion and central processes synapsing
troposterior complex and the medial pulvinar). The primary vestibular on second-order neurones in the ipsilateral spinal tract and chief
cortical area is located in the parietal lobe at the junction between the sensory nuclei of the trigeminal nerve. These neurones drive a polysy-
intraparietal and the postcentral sulci, which is adjacent to that portion naptic chain of interneurones in the lateral reticular formation in the
of the postcentral gyrus where the head is represented. There may be pons and medulla, which, in turn, activate neurones innervating the
an additional representation of the vestibular system in the superior palpebral components of orbicularis oculi in both ipsilateral and con-
temporal gyrus near the auditory cortex. tralateral facial motor nuclei; the stimulated muscles contract to produce
a bilateral blink. The sweep of the eyelids carries lacrimal secretions
Abducens nucleus
across the eye, helping to remove any irritating particles.
The abducens nucleus is a group of approximately 6500 neurones, Shining a bright light into one eye elicits a similar reflex closure of
occupying a paramedian position in the central grey matter, in line with both eyes via the blink reflex; the afferent limb of this reflex involves
the trochlear, oculomotor and hypoglossal nuclei, with which it forms stimulation of the retina and optic pathway.
a somatic motor column (see Fig. 21.3). It lies laterally adjacent to the
Trigeminal sensory nucleus
medial longitudinal fasciculus, which is the means by which vestibular,
cochlear and other cranial nerve nuclei, especially the oculomotor, On entering the pons, the fibres of the sensory root of the trigeminal
connect with the abducens. The abducens nucleus contains large motor nerve run dorsomedially towards the principal sensory nucleus (see
neurones and small multipolar interneurones, which are intermixed, Fig. 21.12). About 50% of the fibres divide into ascending and descend-
although the latter are most heavily concentrated in its lateral and ing branches; the others ascend or descend without division. The


ventral aspects. Axons of abducens interneurones cross the midline at descending fibres form the spinal tract of the trigeminal nerve, which
the level of the nucleus and ascend in the medial longitudinal fasciculus terminates in the medially adjacent spinal nucleus of the trigeminal
to all three medial rectus subnuclei of the oculomotor nucleus. nerve.
Efferent abducens motor axons pass ventrally through the reticular Some ascending trigeminal fibres, many of them heavily myelinated,
formation, trapezoid body and medial lemniscus, and traverse the synapse around the small neurones in the principal sensory nucleus
ventral pons to emerge at its inferior border (see Fig. 21.13); they inner- (see Fig. 21.12), which lies lateral to the motor nucleus and medial to
vate the lateral rectus muscle. the middle cerebellar peduncle, and is continuous inferiorly with the
The abducens nucleus receives afferent connections from corticonu- spinal nucleus of the trigeminal nerve. The principal nucleus is consid-
clear fibres (which are principally contralateral, some of the fibres being ered to be mainly concerned with tactile stimuli.
aberrant corticospinal fibres that descend from the midbrain to this Other ascending fibres enter the mesencephalic nucleus (Fig. 21.14).
level in the medial lemniscus); the medial longitudinal fasciculus (by This column of cells contains the only primary sensory unipolar neu-
which it is connected to oculomotor, trochlear and vestibular nuclei); rones with somata in the CNS rather than in a sensory ganglion. The


midbrain
321
12
retPaHC
Cerebral cortex (lower postcentral gyrus) Jaw jerk reflex
Third ventricle Rapid stretching of the muscles that close the jaw (masseter, temporalis,
medial pterygoid) activates muscle spindle afferents, which travel via
Thalamus
the mandibular division of the trigeminal nerve to the brain stem. The
cell bodies of these primary afferent neurones are located in the mes-
Sensory area of
encephalic trigeminal nucleus. Collaterals project mono synaptically to
face, orbit, nose
and mouth the motor nucleus of the trigeminal nerve in the pons. Motor axons
Forebrain arising in the nucleus travel via the mandibular nerve to innervate the
muscles that act on the temporomandibular joint and close the jaw
(Chapter 32).
Most fibres that arise in the trigeminal sensory nuclei cross the
Lentiform complex
midline and ascend in the trigeminal lemniscus. They end in the con-
Internal capsule tralateral ventral posteromedial thalamic nucleus, from which third-
Ventral posteromedial order neurones project to the postcentral gyrus (areas 3, 1, 2). Some
nucleus of thalamus
trigeminal nuclear efferents ascend to the ventral posteromedial nucleus
of the thalamus.
Midbrain
Trigeminal motor nucleus
The motor nucleus of the trigeminal nerve is ovoid in outline and lies
in the upper pontine tegmentum, under the lateral part of the floor of
the fourth ventricle (see Fig. 21.3). It lies medial to the principal sensory
Trigeminothalamic tract
nucleus, and is separated from it by fibres of the trigeminal nerve. It
forms the rostral part of the pharyngeal (special visceral) efferent
column.
The motor nucleus contains characteristic large multipolar neu-
rones interspersed with smaller multipolar cells. The neurones are
Midbrain organized into a number of relatively discrete subnuclei, the axons
from which innervate individual muscles. It receives fibres from both
corticonuclear tracts. These fibres leave the tracts at the nuclear level or
Motor nucleus of


higher in the pons (aberrant corticospinal fibres), and may end on
Mesencephalic nucleus trigeminal nerve motor neurones or interneurones. The motor nucleus receives afferents
of trigeminal nerve
from the sensory nuclei of the trigeminal nerve, possibly including
Principal sensory
nucleus of some from the mesencephalic nucleus, which form monosynaptic
trigeminal nerve reflex arcs for proprioceptive control of the masticatory muscles. It also
Pons receives afferents from the reticular formation, red nucleus and tectum,
Spinal tract
the medial longitudinal fasciculus and possibly from the locus coeru-
of trigeminal nerve
leus. Collectively, these represent pathways by which salivary secretion
Trigeminal nerve
and mastication may be coordinated.
Axons of second-
tensor tympani and stapedius reflex
order neurones
crossing in pons Loud sound elicits reflex contraction of tensor tympani and stapedius,
(secondary pain and which attenuates movement of the tympanic membrane and middle
temperature fibres of ear ossicles. Afferent impulses travel in the cochlear nerve to the coch-
Vi mandibular division lear nuclei in the brainstem. Efferent fibres to tensor tympani arise in
of trigeminal nerve)
the motor nucleus of the trigeminal nerve and travel in the mandibular
Motor root
Vii division of the nerve. Efferent fibres to stapedius originate in the facial
Viii Spinal nucleus of nucleus and travel in the facial nerve.
Medulla trigeminal nerve
Salivatory nuclei
Axons of second-order
neurones crossing The salivatory nuclei are near the upper pole of the vagal nucleus, just
in upper medulla above the pontomedullary junction and near the inferior pole of the
(secondary pain facial nucleus. They are divided into superior and inferior salivatory
and temperature fibres nuclei, which send preganglionic parasympathetic fibres into the facial
of maxillary division
and glossopharyngeal nerves, respectively, for the control of the salivary
of trigeminal nerve)
and lacrimal glands.
Spinal tract of trigeminal nerve Spinal tract of
Spinal nucleus of trigeminal nerve trigeminal nerve MIDBRAIN
Axons of second-order


Medulla neurones crossing in EXTERNAL FEATURES AND RELATIONS
lower medulla and
upper cervical cord
The midbrain traverses the tentorial notch and connects the pons and
(secondary pain and
cerebellum with the diencephalon. It is the shortest brainstem segment,
temperature fibres
of ophthalmic division not more than 2 cm in length, and much of it lies in the posterior
of trigeminal nerve) cranial fossa (Haines 2013, Nieuwenhuys et al 2008). Lateral to it are
the parahippocampal gyri, which hide its sides when the inferior surface
Dorsolateral fasciculus of the brain is examined. For descriptive purposes, the midbrain may
(tract of Lissauer) be divided into a tectum containing the pretectal area and corpora
quadrigemina, and a basis pedunculi consisting of the substantia nigra
Fig. 21.14 The trigeminal nerve and its central connections.
and the crus cerebri (see Fig. 21.2). The former contains pigmented cells
and the latter clinically important descending tracts. The midbrain teg-
peripheral processes of these neurones may convey proprioceptive im- mentum is the central midbrain area between the tectum and the
pulses from the masticatory muscles, and possibly also from the teeth substantia nigra; the cerebral aqueduct is located at the interface of
and the facial and oculogyric muscles. The nucleus is the relay for the tectum and tegmentum within the periaqueductal grey (Haines 2013).
‘jaw jerk’, the only significant supraspinal monosynaptic reflex, elicited The cerebral peduncle may be regarded as the midbrain located below
by lightly tapping the relaxed open jaw in a downward direction. Nerve the tectum (the tegmentum plus basis pedunculi).
fibres that ascend to the mesencephalic nucleus may give collaterals to The crura cerebri are superficially corrugated and emerge from
the motor nucleus of the trigeminal nerve and to the cerebellum. the cerebral hemispheres. They converge as they descend, forming the


Brainstem
322
3
nOitCes
Nucleus of inferior
colliculus
Cerebral aqueduct
Mesencephalic trigeminal
tract and nucleus
Periaqueductal grey
matter
Lateral
lemniscus
Trochlear nucleus
Central tegmental
tract Medial longitudinal
fasciculus
Medial lemniscus
Reticular formation
Parieto-, occipito- and Superior cerebellar
temporopontine fibres peduncle
Decussation of superior
cerebellar peduncles
Corticospinal and
corticonuclear fibres
Substantia nigra
Interpeduncular fossa
Frontopontine fibres
Posterior perforated
substance
Crus cerebri
Fig. 21.15 A transverse section of the midbrain at the level of the inferior colliculi.
caudolateral boundaries of the interpeduncular fossa, and eventually INTERNAL ORGANIZATION
enter the basilar pons (Figs 21.15–21.16). At the level of the tentorial
incisure, the basilar artery divides in the interpeduncular fossa into the Transverse sections of the midbrain
right and left P segments of the posterior cerebral arteries. The superior
1
cerebellar arteries branch from the basilar artery immediately before
In transverse section, the cerebral peduncles are seen to be composed
this bifurcation. The posterior cerebral and superior cerebellar arteries
of dorsal and ventral regions separated by the substantia nigra (see Figs
both run laterally around the ventral surface of the crus, the former
21.15–21.16). On each side, the dorsal region is the tegmentum and
passing above the tentorium cerebelli, the latter passing below. The
the ventral part is the crus cerebri. The tegmenti are continuous across
oculomotor and trochlear nerves lie between these two arteries. The
the midline but the crura are separated.
roots of the oculomotor nerve emerge from the medial aspect of
the crus via the oculomotor sulcus. The posterior communicating artery
Crus cerebri
joins the posterior cerebral artery on the medial surface of the peduncle
in the interpeduncular fossa. The numerous small openings in the


interpeduncular fossa constitute the posterior perforated substance, The crus cerebri is approximately semilunar in section, being convex
which is pierced by central branches of the posterior cerebral arteries. externally and concave internally. It contains corticonuclear, corticospi-
The optic tract is located at the point where the crus emerges from the nal and corticopontine fibres. Corticonuclear fibres terminate at various
cerebral hemispheres. brainstem levels, many in association with the cranial nerve nuclei.
The superior and inferior colliculi are two paired eminences on the Corticospinal fibres pass uninterrupted through the brainstem to enter
dorsal surface of the midbrain (see Fig. 21.5). They lie rostral to the the medullary pyramid and, thereafter, form the corticospinal tracts of
superior medullary velum, inferior to the pineal gland and caudal to the spinal cord (see Fig. 20.17). Corticonuclear and corticospinal fibres
the posterior commissure. Below the splenium of the corpus callosum, occupy about the middle two-thirds of the crus cerebri. Corticopontine
they are inferior and medial to the pulvinar of the thalamus. The supe- fibres arise in the cerebral cortex and form two groups, both of which
rior and inferior colliculi are separated by a cruciform sulcus. The upper end in the pontine nuclei. Frontopontine fibres from the frontal lobe,
limit of the sulcus expands into a depression for the pineal gland, and principally areas 6 and 4, traverse the internal capsule and then occupy
a median frenulum veli is prolonged from its caudal end down over the the medial fourth of the ipsilateral crus cerebri. Temporopontine fibres,
superior medullary velum. The trochlear nerve emerges lateral to the which originate largely from the posterior region of the temporal lobe,
frenulum and caudal to the inferior colliculus, passes ventrally over traverse the internal capsule and then run in the lateral part of the
the lateral aspects of the cerebral peduncles and traverses the interpe- ipsilateral crus. Parietopontine and occipitopontine fibres are also


duncular cistern to the petrosal end of the cavernous sinus. The superior described in the crus, lying medial to the temporopontine fibres; col-
colliculi, larger than the inferior, are centres for visual reflex responses. lectively, these fibres occupy about the lateral third of the crus. There
The inferior colliculi are a part of the ascending auditory pathway. are few fibres from the primary sensory cortex in corticopontine
A brachium passes ventrolaterally from the lateral aspect of each projections.
colliculus (see Fig. 21.5). The brachium of the superior colliculus (supe-
Substantia nigra
rior quadrigeminal brachium) passes below the pulvinar, partly over-
lapping the medial geniculate body, and conveys fibres from the retina
to the superior colliculus. The brachium of the inferior colliculus (infe- The substantia nigra is a large nucleus that extends through the entire
rior quadrigeminal brachium) ascends ventrally and conveys fibres length of the midbrain, located between the crus cerebri ventrolaterally
from the lateral lemniscus and inferior colliculus to the medial genicu- and ascending lemniscal fibres dorsomedially (see Figs 21.15–21.16).
late body. Its medial part is traversed by oculomotor axons passing to their point


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Superior colliculus
Cerebral aqueduct
Periaqueductal grey
matter
Reticular formation
Central tegmental tract
Medial geniculate
nucleus
Parieto-, occipito- and Oculomotor nucleus
temporopontine fibres
Medial lemniscus
Corticospinal Medial longitudinal
and cortico- fasciculus
nuclear fibres Red nucleus
Frontopontine
fibres Substantia nigra
Posterior perforated
Oculomotor nerve substance
Interpeduncular fossa
Crus cerebri
Fig. 21.16 A transverse section of the midbrain at the level of the superior colliculi.
Cerebral aqueduct Superior colliculus meso limbic dopamine system, which supplies the ventral striatum,
neighbouring parts of the dorsal striatum, and the prefrontal and ante-
Periaqueductal grey rior cingulate cortices. The dopaminergic neurones of the pars compacta
Dorsal raphe (A9) and adjacent ventral tegmental area (A10) also contain cholecysto-
Red nucleus
5-HT group B7 kinin (CCK) or somatostatin.
Raphe 5-HT Pars lateralis The pars reticularis extends rostrally as far as the subthalamic region.
group B8 Pars compacta Substantia It consists of clusters of large multipolar neurones, many of which
nigra intermingle with fibres of the crus cerebri. It is considered to be a
Pars reticulata
homologue of the internal (medial) segment of the globus pallidus,
which it resembles in terms of cell type and connectivity. It contains
neurones that are very similar to those of the globus pallidus; their
disc-like dendritic trees, like those of the pallidum, are orientated at
right angles to afferents from the striatum, probably making en passant
contacts. Most pars reticularis neurones are GABAergic and also contain
high levels of iron. They project rostrally to the ventral anterior and
Dopamine
dorsomedial thalamic nuclei and the deep (polysensory) layers of the
group A9 Crus cerebri


superior colliculus, and caudally to the pedunculopontine nucleus and
Dopamine group A10 brainstem reticular formation, from where impulses are relayed to
(paranigral nucleus)
spinal ventral horn neurones. The connection from the striatum to the
superior colliculus, via the pars reticularis, is thought to function in the
Dopaminergic neurones control of gaze in a manner analogous to the pathway that initiates
general body movement via the pallidum, thalamus and supplementary
Serotoninergic neurones motor cortex. The uncontrolled or fixed-gaze disturbances of advanced
GABAergic neurones Parkinson’s disease, progressive supranuclear palsy and Huntington’s
disease tend to support this.
Fig. 21.17 A transverse section through the midbrain to show the
There are reciprocal connections between the substantia nigra and
arrangement of dopaminergic cell groups A9 and A10 in the substantia
the basal ganglia, and these are discussed in Chapter 24. Briefly, topo-
nigra (left) and serotoninergic cell groups B7 and B8 in the raphe.
graphically organized striatonigral axons, some of which may be col-
laterals of striatopallidal axons, originate from the caudate nucleus and
putamen and project to the pars reticularis. They utilize GABA and
of exit in the interpeduncular fossa. The substantia nigra is divided into substance P or encephalin, and are distributed differentially in the pars
a larger part consisting of a dorsal pars compacta and a ventral pars reticularis in that enkephalinergic axons terminate in the medial part,
reticularis and a smaller pars lateralis. whereas substance P axons terminate throughout. The head of the
The pars compacta consists of darkly pigmented neurones, which caudate nucleus projects to the rostral third of the substantia nigra,
contain neuromelanin granules. Their arrangement is irregular and they while the putamen projects to all parts. The fibres end in axodendritic
partially penetrate the subjacent pars reticularis. Pigmentation increases synapses. A small number of GABAergic pallidonigral fibres from the


with age, is most abundant in primates, maximal in humans, and external (lateral) segment of the globus pallidus end mostly in the pars
present even in albinos. The pigmented neurones synthesize dopamine reticularis. The subthalamic nucleus sends an important glutamatergic
as their neurotransmitter and correspond to the dopaminergic cell projection to the pars reticularis and to the globus pallidus. Some cor-
group A9 (Fig. 21.17). They project predominately to the caudate ticonigral fibres appear to exist, passing from precentral and postcentral
nucleus and putamen in a topographically organized fashion, but also gyri to neurones in the pars reticularis: this is predominantly a gluta-
project to the globus pallidus and subthalamic nucleus. Together with matergic projection.
the retrorubral nucleus (A8), they comprise most of the dopaminergic The pars lateralis projects to the ipsilateral superior colliculus, and
neurone population of the midbrain and are the source of the meso- may be involved in the control of saccadic eye movements.
striatal dopamine system that projects to the striatum. The pars com- Two other dopaminergic cell groups are found in the ventral tegmen-
pacta of each side is continuous with its contralateral counterpart tum: cell group A10 in the rostromedial region, which corresponds to
through the ventral tegmental dopamine group A10, which is some- the ventral tegmental area (of Tsai), and cell group A8 in the dorsola-
times known as the paranigral nucleus. This is the source of the teral reticular area, which forms the nucleus parabrachialis pigmentosus


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(Nieuwenhuys 1985). The whole ventral tegmental system of dopamin- appears only in fresh material and is caused by a ferric iron pigment in
ergic neurones acts as an integrative centre for adaptive behaviour. It its multipolar neurones. The latter are of varying size, and their relative
projects via a number of pathways, mainly through ascending fibres in proportions and arrangement vary between species. In primates the
the ipsilateral medial forebrain bundle. These pathways are a mesodi- more caudal magnocellular part is decreased, and there is a reciprocal
encephalic system, which terminates in thalamic and hypothalamic increase in the size of the more rostral parvocellular part. Small multipo-
nuclei; a mesostriatal projection to the caudate nucleus and putamen; lar neurones occur in all parts of the nucleus. In humans, the larger
a mesolimbic pathway to the nucleus accumbens, olfactory tubercle, neurones of the magnocellular part have been estimated to be as few
lateral septum, interstitial nucleus of the stria terminalis, amygdala and as 200 in number. The magnocellular element is considered phyloge-
entorhinal cortex; and mesocortical fibres that project to most cortical netically old, which accords with the parvocellular predominance in
areas, particularly the prefrontal, orbitofrontal and cingulate cortices. primates. Rostrally, the red nucleus is poorly demarcated, and it blends
into the reticular formation and caudal pole of the interstitial nucleus.
Mesencephalic tegmentum
It is traversed and surrounded by fascicles of nerve fibres, including
many from the oculomotor nucleus.
The mesencephalic tegmentum (see Fig. 21.2) is continuous caudally The principal afferent connections of the red nucleus are corticorub-
with the pontine tegmentum. At inferior collicular levels, grey matter is ral and cerebellorubral fibres. Uncrossed corticorubral fibres originate
restricted to collections of neurones forming the reticular formation from primary somatomotor and somatosensory areas. In animals, the


and the central (periaqueductal) grey surrounding the cerebral aque- red nucleus receives fibres from the contralateral nucleus interpositus
duct. The trochlear nucleus lies in the latter region, near the midline, (which corresponds to the human globose and emboliform nuclei) and
in a position corresponding to the abducens and hypoglossal nuclei at dentate nucleus, via the superior cerebellar peduncle. It has bilateral,
more caudal levels. It extends through the lower half of the midbrain, and probably reciprocal, connections with the superior colliculi. In
just caudal to the oculomotor nucleus and immediately dorsal to the humans, the rubrospinal tract is small and originates from the magno-
medial longitudinal fasciculus. cellular part of the red nucleus. These fibres decussate in the ventral
The mesencephalic nucleus of the trigeminal nerve occupies a lateral tegmental decussation, ventral to the tectospinal decussation, and ven-
position in the central grey matter. It ascends from the principal sensory tromedial to the red nucleus. On reaching the grey matter ventral to the
nucleus of the trigeminal nerve in the pons to the level of the superior inferior cerebellar peduncle, the tract turns caudally to descend adjacent
colliculus in the midbrain. Its large ovoid neurones are unipolar, like to the lateral lemniscus. It continues descending ventral to the spinal
those in peripheral sensory ganglia. They are arranged in many small tract of the trigeminal nerve and nucleus throughout the medulla, and
groups that extend as curved laminae on the lateral margins of the enters the upper part of the cervical cord intermingled with fibres of the
periaqueductal grey matter; neurones are most numerous caudally. lateral corticospinal tract (Nathan and Smith 1982). Some efferent
Apart from these nuclei, the mesencephalic tegmentum contains axons form a rubrobulbar tract to motor nuclei of the trigeminal, facial,
many other scattered neurones, most of which are included in the oculomotor, trochlear and abducens nerves. Few fibres reach the cervi-
reticular formation. cal cord.


The white matter of the mesencephalic tegmentum contains the The largest group of efferents from the red nucleus in humans is a
majority of the tracts present in the pontine tegmentum. The decussa- descending component of the ipsilateral, central tegmental tract. Ini-
tion of the superior cerebellar peduncles is particularly prominent. tially, this tract lies ventrolateral to the medial longitudinal fasciculus
Numerous fibres that enter the mesencephalic tegmentum arise from and dorsolateral to both the red nucleus and the decussation of the
the cerebellar nuclei, pass ventromedially, and decussate at the level of superior cerebellar peduncles (see Figs 21.10–21.12, 21.15). Most of its
the inferior colliculus (see Fig. 21.15). Thereafter, these ascending fibres fibres arise from the parvocellular part of the red nucleus and pass to
laterally encapsulate and penetrate the red nucleus, in which some the ipsilateral inferior olivary complex in the medulla. Some tract fibres
fibres, or their collaterals, terminate (see Fig. 21.16). However, the terminate in the brainstem reticular nuclei. Ascending and descending
majority of these fibres ascend to terminate in the nucleus ventralis axons from the brainstem reticular formation also run in the central
lateralis of the thalamus. A relatively small number of uncrossed fibres tegmental tract. These axons include dorsal and ventral ascending
are believed to end in the periaqueductal grey matter and reticular noradrenergic bundles, a ventral ascending serotoninergic bundle, and
formation, the interstitial nucleus and the nucleus of the posterior com- some fibres of dorsal and ventral ascending cholinergic bundles.
missure. Descending crossed fibres from the decussation of the superior As mentioned elsewhere, lesions of the corticospinal system in
cerebellar peduncle also end in the pontine nuclei, the medullary reticu- humans result in permanent paresis or paralysis. In monkeys, although
lar formation, the nuclei of the inferior olivary complex and, possibly, initially complete, the paralysis disappears and good recovery ensues.
some cranial nerve motor nuclei. The explanation for this interprimate variability in recovery from corti-


The medial longitudinal fasciculus adjoins the somatic efferent cell cospinal lesions appears to lie in the differential capacity of the rubro-
column, dorsal to the decussating superior cerebellar peduncles (see spinal system to compensate for loss of corticospinal drive. Monkeys
Fig. 21.15). The medial, trigeminal, lateral and spinal lemnisci (antero- never fully recover from combined lesions of both the corticospinal and
lateral system) form a curved band dorsomedial to the substantia rubrospinal tracts, which suggests that the two systems are functionally
nigra. Fibres in the medial, trigeminal and spinal lemnisci continue a interrelated in the control of movement. Both encode force, velocity
rostral course to synapse with neurones in the lateral (VPL) and medial and direction parameters, but the rubrospinal system primarily directs
(VPM) parts of the ventral posterior thalamic nucleus. Some fibres of activity both during the terminal phase of a movement and preceding
the lateral lemniscus end in the nucleus of the inferior colliculus, or a movement. There is, thus, overlap of activity in the two systems for
send collaterals to it. The remaining lateral lemniscal fibres join with all parameters during movements of limbs and even of individual
fibres originating in the inferior colliculus and enter the brachium of digits. The corticospinal system is most active during the learning of
the inferior colliculus, which carries them to the medial geniculate new movements, whereas the rubrospinal system is most active during
body. the execution of learned automated movements.
At the level of the superior colliculus, the tegmentum contains the The rubro-olivary projection, which travels in the central tegmental
red nucleus, which extends rostrally into the subthalamic region. The tract, connects the red nucleus indirectly to the contralateral cerebellum
ventromedial central grey matter around the cerebral aqueduct also via olivocerebellar fibres. The cerebellum is thought to play a role in
contains the elongated oculomotor nucleus, which is adjacent to the motor learning, and so the rubro-olivary system could switch the


medial longitudinal fasciculus, and caudally reaches the trochlear control of movements from the corticospinal to the rubrospinal system
nucleus. The oculomotor nucleus is divisible into neuronal groups that for programmed automation. The relative absence of a rubrospinal
are correlated with the motor distribution of the oculomotor nerve. A system in humans could explain the poor recovery of motor function
group of parasympathetic neurones that control the activity of smooth after stroke.
muscle within the eyeball, the Edinger–Westphal preganglionic nucleus,
Oculomotor nucleus
lies dorsal to the oculomotor nucleus. The Edinger–Westphal complex,
based on its connections, consists of an Edinger–Westphal pregangli- The nuclear complex from which the oculomotor nerve arises consists
onic nucleus projecting only to the ciliary ganglion, and an Edinger– of several groups of large motor neurones (collectively, the oculomo-
Westphal centrally projecting nucleus that has a variety of targets in the tor nucleus; see Fig. 21.16) and smaller preganglionic parasympa-
central nervous system but does not target the ciliary ganglion (Kozicz thetic neurones (the Edinger–Westphal preganglionic nucleus). The
et al 2011). motor neurone groups of the oculomotor nucleus innervate, in dorso-
ventral order, the ipsilateral inferior rectus, inferior oblique and
Red nucleus medial rectus. A medially placed column of cells innervates the con-
The red nucleus is a pink, ovoid mass, approximately 5 mm in diameter tralateral superior rectus; the axons from this subnucleus decussate in
and lying dorsomedial to the substantia nigra (see Fig. 21.16). The tint its caudal part.


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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 supe-
rior 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 skel-
etal muscles.


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The Edinger–Westphal preganglionic nucleus, composed of small, has dorsomedial and ventrolateral zones, which are covered by a dorsal
multipolar, parasympathetic neurones, lies dorsal to the main oculo- cortex. In humans, the cortex has four cytoarchitectonic layers: layer I
motor nucleus. Its neurones give rise to axons that travel in the oculo- contains small neurones with flattened radial dendritic fields; layer II,
motor nerve and synapse with postganglionic neurones in the ciliary medium-sized neurones with ovoid dendritic fields aligned parallel
ganglion (Kozicz et al 2011). with the collicular surface; layer III, medium-sized neurones with spher-
Fascicles of axons from these subnuclei course forwards in the mid- ical dendritic fields; and layer IV, large neurones with variably shaped
brain and emerge as the oculomotor nerve in the interpeduncular fossa. dendritic fields.
The fascicles are most probably arranged from medial to lateral subserv- Most efferent fibres from the inferior colliculus travel via the inferior
ing the pupil, inferior rectus, medial rectus, levator palpebrae superioris brachium to the ipsilateral medial geniculate nucleus. Lemniscal fibres
and superior rectus, and inferior oblique. The human oculomotor nerve relay only in the central nucleus, and some pass without relay to
contains approximately 15,000 axons. the medial geniculate nucleus. In humans, the ventral division of the
Afferent inputs to the oculomotor nuclear complex include fibres medial geniculate nucleus receives a topographic projection from the
from the rostral interstitial nucleus of the medial longitudinal fasciculus central nucleus and the dorsal division receives a similar projection
and the interstitial nucleus of Cajal, both of which are involved in the from the dorsal cortex. Some colliculogeniculate fibres do not relay in
control of vertical and torsional gaze. Other inputs come, either directly the geniculate nucleus but continue, with those that do, via the auditory
or indirectly, from the nuclei of the posterior commissure, the intersti- radiation to the auditory cortex. A descending projection from the audi-


tial nucleus of Cajal, the frontal eye fields, the superior colliculus, the tory cortex reaches the inferior colliculus via the medial geniculate
dentate nucleus and other cortical areas. The medial longitudinal fas- nucleus. Some fibres may traverse this projection without relay. This
ciculus carries connections from the trochlear, abducens and vestibular descending path may produce effects at levels from the medial genicu-
nuclei; the medial and lateral vestibular nuclei project to the medial late nucleus downwards, and it probably links with efferent cochlear
rectus subnucleus. fibres, through the superior olivary and cochlear nuclei.
Afferent inputs to the Edinger–Westphal preganglionic nucleus
Superior colliculus
come primarily from the pretectal nuclei bilaterally, mediating the
pupillary light reflex, and from the visual cortex, mediating The superior colliculus is an obviously laminated structure. At succes-
accommodation. sive depths from the external surface, it may be divided into seven layers
termed zonal, superficial grey, optic, intermediate grey, deep grey, deep
Trochlear nucleus white and periventricular strata, composed alternately of neuronal
The trochlear nucleus lies in the ventromedial periaqueductal grey somata or their processes. The zonal layer consists chiefly of myelinated
immediately adjacent to the medial longitudinal fasciculus at the level and non-myelinated fibres from the occipital cortex (areas 17, 18 and
of the inferior colliculus (see Figs 21.3, 21.15). It is in line with the 19), which arrive as the external corticotectal tract. It also contains a
ventromedial part of the oculomotor nucleus, in the position of the few small neurones, which are horizontally arrayed. The superficial grey
somatic efferent column. The trochlear nucleus is caudal to the oculo- layer (stratum cinereum) forms a crescentic lamina over the deeper
motor nucleus and distinguished by the smaller size of its neurones. layers and contains many small multipolar interneurones, on which
The afferent inputs to the trochlear nucleus are similar to those described cortical fibres synapse. The optic layer consists partly of fibres from the


for the oculomotor nucleus. Trochlear efferent fibres pass laterodorsally optic tract. As they terminate, they permeate the entire anterior–
round the central grey matter, descending caudally medial to the mes- posterior extent of the superficial layers with numerous collateral
encephalic nucleus as they do so, to reach the upper end of the superior branches. This arrangement provides a retinotopic map of the contra-
medullary velum, where they decussate and emerge lateral to the frenu- lateral visual field, in which the fovea is represented anterolaterally.
lum and caudal to the inferior colliculus. A few fibres may remain Retinal axons terminate in clusters from specific retinotectal neurones
ipsilateral. and as collaterals of retinogeniculate fibres. The layer also contains
some large multipolar neurones. Efferent fibres to the retina are said to
Medial longitudinal fasciculus start in this layer.
The medial longitudinal fasciculus (see Figs 21.12, 21.15) is a heavily The intermediate grey and white layers collectively constitute the
myelinated composite tract, lying adjacent to the midline, ventral to the main receptive zone. The main afferent input is the medial corticotectal
periaqueductal and periventricular grey matter. It ascends to the inter- path from layer V neurones of the ipsilateral occipital cortex (area 18),
stitial nucleus of Cajal, which lies in the lateral wall of the third ventri- and from other neocortical areas that are concerned with ocular follow-
cle, just above the cerebral aqueduct. The fasciculus retains its position ing movements. Afferent fibres are also received from the contralateral
relative to the central grey matter through the midbrain, pons and upper spinal cord (via spinotectal fibres in the anterolateral system), the infe-
medulla, but is displaced ventrally by the motor (pyramidal) decussa- rior colliculus, and the locus coeruleus and raphe nuclei (from noradren-
tion containing corticospinal fibres. ergic and serotoninergic neurones). The deep grey and deep white layers
The medial longitudinal fasciculus interconnects the oculomotor, adjacent to the periaqueductal grey matter are collectively called the


trochlear, abducens, Edinger–Westphal preganglionic, vestibular, retic- parabigeminal nucleus. They contain neurones whose dendrites extend
ular and spinal accessory nuclei, coordinating conjugate eye move- into the optic layer, and whose axons form many of the collicular
ments and associated movements of the head and neck. Lesions cause efferents.
internuclear ophthalmoplegia. All four vestibular nuclei contribute The superior colliculus receives afferents from many sources includ-
ascending fibres. Those from the superior nucleus remain uncrossed, ing the retina, spinal cord, inferior colliculus and occipital and tempo-
while the others are partly crossed. Some fibres reach the interstitial ral cortices. The first three of these pathways convey visual, tactile and
and posterior commissural nuclei, and some decussate to the contra- probably thermal, pain and auditory impulses. Collicular efferents
lateral nuclei. Descending axons, from the medial vestibular nuclei pass to the retina, lateral geniculate nucleus, pretectum, parabigeminal
and perhaps the lateral and inferior nuclei, partially decussate and nucleus, the inferior, medial and lateral pulvinar, and to numerous
descend in the fasciculus to form the medial vestibulospinal tract, sites in the brainstem and spinal cord. Fibres passing from the pulvi-
which travels in the medial longitudinal fasciculus into the ventral nar are relayed to primary and secondary visual cortices and form an
funiculus of the spinal cord (see Ch. 20 and Fig. 20.18). Fibres join the extrageniculate retinocortical pathway for visual orientation and
fasciculus from the dorsal trapezoid, lateral lemniscal and posterior attention.
commissural nuclei, which means that both the cochlear and vestibu- The tectospinal and tectobulbar tracts start from neurones in the
lar components of the vestibulocochlear nerve may influence move- superior colliculi. They sweep ventrally around the central grey matter
ments of the eyes and head via the medial longitudinal fasciculus. to decussate ventral to the oculomotor nuclei and medial longitudinal
Some vestibular fibres may ascend in the medial longitudinal fascicu- fasciculi as part of the dorsal tegmental decussation. The tectospinal


lus as far as the thalamus. tract descends ventral to the medial longitudinal fasciculus throughout
the brainstem, diverges ventrolaterally at the level of the motor decus-
sation, and moves medially into the ventral funiculus in association
Tectum
with the medial longitudinal fasciculus. Tectospinal fibres descend to
cervical segments. The tectobulbar tract, mainly crossed, descends near
Inferior colliculus the tectospinal tract, and ends in the pontine nuclei and motor nuclei
The inferior colliculus (see Fig. 21.15) is part of the ascending auditory of the cranial nerves, particularly those innervating the oculogyric
pathway and is the principal site of termination of the lateral lemniscus. muscles. It subserves reflex ocular movements. Other tectotegmental
It has a central, ovoid, main nucleus, which is lateral to the periaque- fibres reach various tegmental reticular nuclei in the ipsilateral mesen-
ductal grey matter. It is surrounded by a lamina of nerve fibres, many cephalic and contralateral pontomedullary reticular formation (gigan-
from the lateral lemniscus, which terminate in it. The central nucleus tocellular reticular, caudal pontine reticular, oral pontine reticular


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The central nucleus contains indistinct laminae. Bands of cells with
disc-shaped or stellate dendritic fields orthogonally span the fibre layers
in which the terminals of lateral lemniscal fibres ramify. The neurones
are sharply tuned to frequency, and the laminae may represent the
structural basis of tonal discrimination. Experimental studies have
found cells driven by low frequencies in the dorsal laminae, and others
driven by high frequencies in the ventral laminae. Neurones are broadly
frequency-tuned in the dorsal cortex and lateral nucleus.
Inferior collicular projections to the brainstem and spinal cord
appear to traverse the superior colliculi before they descend. In this way
they connect with the origins of the tectospinal and tectotegmental
tracts. These projections are relatively small and probably mediate reflex
turning of the head and eyes in response to sounds.
In experimental animals, lesions of either the inferior colliculus or
its brachium produce defects in tonal discrimination, sound localiza-
tion and auditory reflexes. The effects of such lesions are poorly docu-
mented in humans.


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nuclei), and the substantia nigra and red nucleus. Tectopontine fibres, Reticular neurones have long dendrites that spread along the long
which probably descend with the tectospinal tract, terminate in dorso- axis of the brainstem. The dendrites may spread into 50% of the cross-
lateral pontine nuclei, with a relay to the cerebellum. sectional area of their half of the brainstem, and they are intersected
by, and may synapse with, a complex of ascending and descending
Pretectal nucleus
fibres. Many axons of reticular neurones ascend or descend, or bifurcate
The pretectal nuclei, of which the olivary pretectal nucleus is one, is a in both directions. They often travel far, perhaps through the whole
poorly defined mass of neurones at the junction of the mesencephalon brainstem and beyond.
and diencephalon. It extends from a position dorsolateral to the pos- Multitudes of afferent fibres converging on individual neurones, and
terior commissure caudally towards the superior colliculus, with which the myriad destinations of efferent fibres provide the structural basis for
it is partly continuous. It receives fibres from the visual cortex via the considering the reticular formation to be ‘polymodal’, ‘diffuse’ or ‘non-
superior quadrigeminal brachium, from the retina via the lateral root specific’ in function.
of the optic tract, and from the superior colliculus. Its efferent fibres A contrasting dendritic form is also found, in which dendrites are
reach the Edinger–Westphal preganglionic nuclei bilaterally. Those that short, sinuous or curved, and branch profusely at the perimeter of a
decussate pass ventral to the aqueduct or through the posterior com- nuclear group, defining a boundary between it and its environs. In dif-
missure. In this way, the sphincter pupillae contract in both eyes in ferent zones, the proportion of different sizes of neuronal somata varies.
response to impulses from either eye. This bilateral light reflex may not Some regions contain only small to intermediate multipolar cells (‘par-