General Arrangement of Cranial Nerve Nuclei

In the thoracic region of the developing spinal cord, four distinct cell columns can be identified in the gray matter on each side (Figure 17.1A, B). In the basal plate, the general somatic efferent column supplies the striated muscles of the trunk and limbs. The general visceral efferent column contains preganglionic neurons of the autonomic system. In the alar plate, the general visceral afferent column receives afferents from thoracic and abdominal organs. A general somatic afferent column receives afferents from the body wall.

Figure 17.1 Cell columns of the spinal cord and brainstem. (A) Embryonic spinal cord. (B) Adult spinal cord. (C) Embryonic hindbrain. (D) Adult hindbrain. Afferent cell columns: GSA, general somatic afferent; GVA, general visceral afferent; SSA, special somatic afferent; SVA, special visceral afferent. Efferent cell columns: GSE, general somatic efferent; GVE, general visceral efferent; SVE, special visceral efferent.

In the brainstem, these four cell columns can be identified, but they are fragmented, and not all contribute to each cranial nerve. Their connections are as follows.

Three additional cell columns (Figure 17.1C, D) serve branchial arch tissues and the inner ear, as follows.

Figure 17.2 shows the position of the various nuclei in a dorsal view of the brainstem.

Figure 17.2 Posterior view of adult brainstem, showing position of cranial nerve cell columns. L, S, I, M, lateral, superior, inferior, medial vestibular nuclei.

In this chapter, details of the internal anatomy of the brainstem accompany nine representative transverse sections and their captions. Connections (direct or indirect) with the right cerebral hemisphere have been highlighted in accordance with information to be provided.

Background Information

As stated earlier, exteroceptive and conscious proprioceptive information are transferred (by spinothalamic and dorsal column–medial lemniscal pathways, respectively) from left trunk and limbs to right cerebral hemisphere. It was also explained that corticospinal fibers of the pyramidal tract arising from motor areas of the cerebral cortex supply contralateral anterior horn cells and give a small ipsilateral supply of similar nature, and that those arising from the parietal lobe project to the contralateral posterior gray horn.

The same arrangement holds good for the brainstem. The pyramidal tract fibers terminating in the brainstem are corticonuclear. As shown in Figure 17.3, their distribution is predominantly contralateral to somatic and branchial motor nuclei, and entirely contralateral to the somatic sensory nuclei.

Figure 17.3 Posterior view of brainstem, showing distribution of corticonuclear fibers from the right cerebral cortex.

Absent from this figure are the three pairs of motor ocular nuclei. Why? Because these do not receive a direct corticonuclear supply. Instead, their predominantly contralateral supply synapses on adjacent cell groups known as gaze centers having the function of synchronizing conjugate (conjoint parallel) movements of the eyes.

For a basic understanding of neural relationships in the brainstem, it is also essential to appreciate hemisphere linkages to the inferior olivary nucleus and to the cerebellum (Figure 17.4).

Figure 17.4 Anterior view of the four principal motor decussations of the brainstem. Pathways are numbered in accordance with their sequence of activation in voluntary movements: (1) corticopontocerebellar; (2) dentatothalamocortical; (3) corticospinal; (4) olivocerebellar. Also shown is the rubroolivary connection.

The general layout of the reticular formation (Figure 17.5) is borrowed from a figure in Chapter 24 devoted to this topic. It may be consulted when reading under this heading in successive descriptions.

Figure 17.5 Layout of the reticular formation (RF).

Figure 17.6 depicts the main components of the medial longitudinal fasciculus (MLF). This fiber bundle extends the entire length of the brainstem, changing its fiber composition at different levels. This figure, too, may be consulted during study of the brainstem sections to be described, following inspection of C1 segment of the spinal cord.

Figure 17.6 Main fiber composition of the medial longitudinal fasciculus (MLF). PPRF, paramedian pontine reticular formation; RN, reticular nucleus; RST, reticulospinal tract; VN, vestibular nucleus.

Study guide

The presentation departs from the traditional method, which is to describe photographs or diagrams at successive levels in ascending order without highlights. In the present approach:

1 The various nuclei and pathways are highlighted and labeled on the side having primary affiliation with the right cerebral hemisphere.

2 The nuclei and pathways are color-coded by systems, for example red for motor, blue for sensory, green for connections of cerebellum and reticular formation.

3 Highlighting together with color-coding makes it possible to study individual systems in vertical, ‘multiple window’ mode. The descriptive text related to the brainstem sections enables a logical sequence of study whereby afferent pathways can be followed from below upward to thalamic level (commencing with Figure 17.10), and efferent pathways can be followed from above downward (commencing with Figure 17.19). It must be emphasized that, following study in the vertical mode, a horizontal approach must be undertaken, the location of the various systems to be identified at each level. This is because occlusion of a small artery of supply to the brainstem may affect function in a patch that may include several distinct nuclei or pathways.

At each level, miniature replicas of the diagrams in Figure 17.7 are inserted to assist left–right orientation.

Figure 17.7 (A) Anterior and (B) posterior view of brainstem, showing disposition of some major pathways.

Special note: Readers unfamiliar with the internal anatomy of the brainstem may be disconcerted by the amount of new information contained in the series of sections to be described. It may be reassuring to know that all the information will come up again in later chapters. Therefore a sensible approach could be to undertake an initial browse through the sections and to recheck the location of individual items during later reading.

Overview of three pathways in the brainstem

Figure 17.8 shows the posterior column–medial lemniscal and anterolateral pathways already described in Chapter 15. Recall that the latter comprises the lateral spinothalamic tract serving pain and temperature, and the anterior spinothalamic tract serving touch. Within the brainstem, the two are combined as the spinal lemniscus.

Figure 17.8 Posterior column–medial lemniscal and anterolateral pathways. VPN, ventral posterior nucleus of thalamus.

The corticospinal tract, treated in Chapter 16, is shown in Figure 17.9. Also included are corticonuclear projections to the facial and hypoglossal nuclei.

Figure 17.9 Corticospinal tract; two corticonuclear projections.

C1 Segment of Spinal Cord (Figure 17.10)

Blue

The gracile and cuneate fasciculi constitute the posterior column of the spinal cord on each side. Their axons are ipsilateral central processes of posterior root ganglion cells whose peripheral processes receive information from the large tactile nerve endings in the skin, including Meissner’s and Pacinian corpuscles, and from neuromuscular spindles and Golgi tendon organs. The fasciculi terminate in the gracile and cuneate nuclei (Figure 17.12).

Figure 17.10 C1 segment of spinal cord.

(After Noback CR et al. 1996, with permission of Williams & Wilkins.)

The spinothalamic (anterolateral) pathway comprises the anterior and lateral spinothalamic tracts. Unlike those in the posterior column, these two tracts comprise crossed axons. As indicated in Figure 15.10, the second-order neurons traverse the anterior white commissure at all segmental levels before ascending to the thalamus.

The posterolateral tract of Lissauer contains fine first-order sensory fibers that divide and span several cord segments prior to synapsing in the posterior gray horn.

The spinal (descending) tract of the trigeminal nerve contains nociceptive and thermoceptive first-order neurons about to synapse in the posterior gray horn of segments C2 and C3.

Red

The large red area on the left side of the cord represents the (crossed) lateral corticospinal tract. The anterior CST has not yet crossed.

Anterior motor neurons projecting from the anterior gray horn occupy the anterior root of spinal nerve C1 and the uppermost root of the spinal accessory nerve.

The lateral vestibulospinal tract (uncrosssed) descends in the anterior funiculus to activate antigravity muscles on its own side. The medial VST (partly crossed) has emerged from the medial longitudinal fasciculus to activate head-righting reflexes.

Lateral to the anterior gray horn is the autonomic projection from the hypothalamus. Its functions include activation of sacral parasympathetic neurons causing contraction of the bladder and rectum.

Green

The posterior spinocerebellar tract (from the posterior thoracic nucleus) conveys high-speed nonconscious proprioception from the ipsilateral trunk and limbs, notably from muscle stretch receptors.

The pontine reticulospinal tract is descending ipsilaterally to supply motor neurons innervating antigravity muscles. The medullary reticulospinal tract supplies flexor motor neurons.

Spinomedullary Junction (Figure 17.11)

Blue

The gracile and cuneate fasciculi continue to occupy the posterior white column, with the spinal tract and nucleus of the trigeminal nerve alongside. The position of the spinal lemniscus is also unchanged.

Figure 17.11 Spinomedullary junction.

Red

The dominant feature in this diagram is the decussation of the pyramids. Observe the right pyramid: 80% of its fibers cross the midline by decussating with its opposite numbers, to form the left lateral corticospinal tract; 10% enter the ipsilateral anterior corticospinal tract which will cross lower down; and 10% remain ipsilateral among the fibers of the right lateral CST.

Within the lateral tegmentum is the lateral vestibulospinal tract. The red spots in the medial longitudinal fasciculi represent the medial vestibulospinal tract, which descends bilaterally within them.

Green

The posterior spinocerebellar tract is nearing its point of departure into the inferior cerebellar peduncle. The paramedian and lateral reticular formation occupy the tegmentum.

Middle of Medulla Oblongata (Figure 17.12)

Blue

The left posterior column of the spinal cord ascends to mid-medulla before turning ventrally. The gracile fasciculus synapses in the gracile nucleus, the cuneate one in the cuneate nucleus. Second-order neurons give rise to internal arcuate fibers which cross over in the great sensory decussation, then ascend (to thalamus) as the medial lemniscus.

Figure 17.12 Middle of medulla oblongata.

The spinal lemniscus contains the (crossed) anterior and lateral spinothalamic tracts.

The vestibulospinal tract is descending from the vestibular nucleus to the spinal cord.

Red

The pyramid contains the corticospinal tract prior to the pyamidal decussation; the hypoglossal nerve emerges at its lateral edge. Lateral to the XII nucleus is the dorsal nucleus of vagus nerve. The cranial accessory nerve has emerged from the nucleus ambiguus; it will be incorporated into the vagus below the jugular foramen. The dorsal longitudinal fasciculus contains autonomic fibers descending from hypothalamus to spinal cord.

Green

The projections from inferior and accessory olivary nuclei to contralateral cerebellar cortex are shown.

The paramedian and lateral reticular formation and the inferior cerebellar peduncle are seen again now.

Upper Part of Medulla Oblongata (Figure 17.13)

Blue

In the mid-region, the medial lemnisci are continuing their ascent to the thalamus. Laterally, we see the spinal lemniscus, the spinal tract and nucleus of the trigeminal nerve, the solitary tract and nucleus (S.t.n.) and the medial and lateral nuclei of the vestibular nerve. Sensory fibers of the glossopharyngeal nerve synapse in the spinal nucleus of the trigeminal nerve and in the solitary nucleus.

Figure 17.13 Upper medulla oblongata.

Red

The pyramids are in the same position as before. On the anatomical right side, the vagus nerve is emerging anterior to the inferior cerebellar peduncle. On the left side the motor components of the glossopharyngeal nerve derive from the inferior salivatory nucleus and the nucleus ambiguus.

Green

The principal and accessory olivary nuclei are sending fibers across to the contralateral inferior cerebellar peduncle. Dorsal to these are the chemoreceptive area (sensitive to HCO₃ levels in the CSF), the lateral reticular nucleus, pontine reticulospinal tract, and paramedian reticular formation.

Occupying the midline region are the magnus raphe nucleus, and the medial and dorsal longitudinal fasciculi.

Pontomedullary Junction (Figure 17.14)

Blue

Previously seen are the medial and spinal lemnisci, also the spinal tract and nucleus of the trigeminal nerve. Novel are the lateral and trigeminal lemnisci. As explained in Ch. 20, the lateral lemniscus is an auditory fiber bundle ascending to the inferior colliculus, having crossed in the trapezoid body from the superior olivary nucleus. This nucleus is fed by auditory information from the dorsal and ventral cochlear nuclei, where the cochlear nerve terminates.

Figure 17.14 Pontomedullary junction.

Red

Reflex balance pathways here are the medial and lateral vestibulospinal tracts (VST). The medial tract descends to the spinal cord within the medial longitudinal fasciculus (MLF).

Also seen are the corticospinal tract and the emerging abducens and facial nerves.

The dorsal longitudinal fasciculus (DLF) contains autonomic fibers descending to the spinal cord.

Green

At top are the superior cerebellar peduncles, which (Ch. 25) project from the dentate nucleus of the cerebellum to the contralateral thalamus. Below these are the inferior cerebellar peduncles. More centrally are the pontine reticulospinal tract and the paramedian RST.

MID-Pons (Figure 17.15)

Blue

Medial, lateral and trigeminal lemnisci are progressing toward the thalamus. We have reached the upper end of the vestibular, spinal V and solitary nuclei and of the trapezoid body.

Figure 17.15 Mid-pons.

The nervus intermedius contains gustatory fibers for the supply of taste buds in the tongue and palate. They terminate centrally in the solitary nucleus.

Red

The facial nerve is J-shaped, like a bended knee (genu), where it winds around the nucleus of the abducens nerve before piecing the tegmentum, where it is joined by the nervus intermedius. This nerve contains parasympathetic secretomotor fibers that will synapse on autonomic ganglia innervating the lacrimal and submandibular glands (among others).

Green

At top, the arrows indicate imminent intersection of the superior cerebellar peduncles in the roof of the fourth ventricle. At a lower level, the inferior peduncles are entering the cerebellum.

In the basilar pons, millions of corticopontine fibers descend from the cerebral cortex to synapse upon millions of individual neurons collectively called nuclei pontis. These give off transverse fibers which separate the corticospinal tracts into bundles, on their way to create the middle cerebellar peduncles.

Beside the abducens is the pontine gaze center, a node of reticular formation which activates contraction of the lateral rectus muscle in the orbit, thereby causing abduction of the coresponding eyeball.

In the midline, the pontine raphe nucleus sends serotonergic fibers throughout pons and cerebellum.

Upper Pons (Figure 17.16)

Blue

The sensory root of the left trigeminal nerve terminates in the pontine sensory nucleus. From here, axons project across the midline and turn upward as the trigeminal lemniscus. Three lemnisci previously seen are the spinal, lateral, and medial lemnisci.

Figure 17.16 Upper pons.

The mesencephalic tract of the trigeminal nerve (Mes. t. V) contains processes of unipolar neurons in the midbrain, as explained in Ch. 21.

Red

The cerulean nucleus, in the floor of the upper end of the fourth ventricle, is the largest group of noradranergic neurons in the brain. It distributes fine, beaded axons to all parts of the cerebral and cerebellar hemispheres.

The motor nucleus of the trigeminal nerve provides the axons of supply to the masticatory muscles.

Previously seen are the corticospinal fiber bundles and the dorsal longitudinal fasciculus (DLF).

Green

Next to the superior cerebellar peduncle (SCP), the pedunculopontine nucleus is part of the locomotor center (Ch. 24).

Previously seen are the corticopontine and pontocerebellar fibers.

Lower Midbrain (Figure 17.17)

Blue

The medial, spinal, and trigeminal (V) lemnisci are still ascending, whereas the lateral lemniscus is now on target, synapsing in the inferior colliculus – the lower center for hearing.

Figure 17.17 Lower midbrain.

The mesencephalic nucleus of the trigeminal nerve (Mes. n. V) is the only unipolar cell group within the CNS. It serves proprioception within the trigeminal area.

Red

The trochlear nerve is the only cranial nerve to decussate (as shown); also the only cranial nerve to emerge from the dorsal surface of the brainstem.

The DLF contains autonomic fibers traveling to the spinal cord. The crus of the midbrain contains corticonuclear and corticospinal fibers; the former activate motor cranial nerve nuclei. The tectospinal tract is commencing its descent, having crossed from the contralateral superior colliculi. It operates visuospinal refexes whereby the head and trunk are turned in the direction of a source of light.

Green

The fronto- and temporopontine fibers are traveling from the corresponding association areas to reach the ipsilateral nuclei pontis.

Brown/gray

The anterior tegmentum is occupied by compact and reticular elements of the substantia nigra. The compact part, comprising pigmented dopaminergic neurons, is the source of the nigrostriatal pathway to the corpus striatum. The nigrostriatal pathway loses both pigment and cells in those unfortunates bound for Parkinson’s disease (Ch. 33). The reticular part contains GABAergic neurons.

Upper Midbrain (Figure 17.18)

The four occupants of the crus cerebri and the substantia nigra are in the same relative positions. So too are the midbrain raphe nucleus and the ventral tegmental and interpeduncular nuclei.

Figure 17.18 Upper midbrain.

Midbrain–Thalamic Junction (Figure 17.19)

Blue

Ascending to the ventral posterior nucleus of thalamus are 3 lemnisci (medial, spinal, trigeminal). Entering the medial (auditory) geniculate nucleus (MGN) of thalamus is the inferior brachium which arose in the inferior colliculus. The lateral (visual) geniculate nucleus (LGN) receives the superior brachium (not seen here).

Figure 17.19 Midbrain–thalamic junction.

Red

The subthalamic nucleus receives an input from the centromedian nucleus of thalamus and projects to the lentiform nucleus. It may become overactive in Parkinson’s disease (cf. Ch. 33). The red nucleus and the contents of the crus cerebri are unchanged.

Green

In the dorsal tegmentum, the pretectal nucleus belongs to the visual system. It receives an input from the optic tract and projects to both Edinger-Westphal nuclei, giving rise to bilateral pupillary constriction when a light is shone into one eye (Ch. 23).

Near the midline are the centers for upward and downward gaze. Rarely, a pinealoma (pineal gland tumor) may signal its presence by causing paralysis of upward gaze.

The superior cerebellar peduncle is aiming for the ventral lateral nucleus of the thalamus. which lies anterior to the ventral posterior nucleus. From there, a final projection will reach the motor cortex and will coordinate ongoing movements.

Green

The habenular nuclei are connected across the midline and project via the fasciculus retroflexus to the interpeduncular nucleus, which participates in the sleep–wake cycle (Ch. 34).

Orientation of Brainstem Slices in Magnetic Resonance Images (Figure 17.20)

Figure 17.20 shows brainstem slices in magnetic resonance images. Their orientation is the opposite of those in the preceding sections. In photographs and drawings, the convention is to represent anterior structures below. As already mentioned in Chapter 2, in magnetic resonance imaging scans, anterior structures are represented above.

Figure 17.20 Magnetic resonance images of (A) medulla oblongata, (B) pons, and (C) midbrain in the standard radiologic orientation.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California.)

Epilog

Figure 17.21 offers an unlabeled overview of the brainstem sections. Gentle skimmers may opt to photocopy lightly and to crayon pathways.

Figure 17.21 Brainstem review.

Core Information

Cell columns

Cranial nerve cell columns and their representations are as follows.

• General somatic efferent, represented in the medulla by the hypoglossal nucleus, in the pons by the abducens nucleus, and in the midbrain by the oculomotor and trochlear nuclei.

• Special visceral efferent, supplying muscles of branchial arch origin, represented in the medulla by the nucleus ambiguus and in the pons by trigeminal and facial motor nuclei.

• General visceral efferent, represented in the medulla by dorsal nucleus of the vagus and in the medulla and in the pons by the salivatory nuclei.

• General visceral afferent, represented in the medulla by the inferior solitary nucleus.

• Special visceral afferent, represented in the pons by the superior solitary nucleus.

• General somatic afferent, represented by trigeminal sensory nuclei: spinal in the medulla, principal sensory in the pons, and mesencephalic in the midbrain.

• Special sense afferent, represented at the pontomedullary junction by the cochlear and vestibular nuclei.

Ascending pathways

The gracile and cuneate nuclei send internal arcuate fibers across the midline to form the medial lemniscus, which goes through pons and midbrain to reach the thalamus. The spinal trigeminal nucleus sends fibers across the midline to form the trigeminothalamic tract. The posterior spinocerebellar and cuneocerebellar tracts send their fibers into the ipsilateral inferior cerebellar peduncle, where they mingle with olivocerebellar fibers crossing from the inferior and accessory olivary nuclei. The (crossed) anterior spinocerebellar tract enters the superior cerebellar peduncle; its fibers cross a second time within the cerebellar white matter.

• The spinal lemniscus is formed of the anterior and lateral spinothalamic tracts. It is accompanied first by trigeminothalamic fibers, later by the lateral lemniscus and by fibers crossing from the principal trigeminal nucleus completing the trigeminal lemniscus.

• The cochlear nuclei project fibers across the trapezoid body to form the lateral lemniscus, which ascends to the inferior colliculus. Some fibers synapse instead in a superior olivary nuclear relay to the ipsilateral inferior colliculus. Third-order neurons of the inferior colliculus project via inferior brachium to the medial geniculate body. The medial and superior vestibular nuclei send fibers to the oculomotor nucleus to execute the vestibuloocular reflex.

• The upper part of the central tegmental tract contains fibers of the ascending reticular activating system.

• In the ventral tegmentum of the midbrain are the pigmented (compact) substantia nigra giving rise to the nigrostriatal pathway, and the ventral tegmental nucleus giving rise to the mesocortical and mesolimbic pathways. The non-pigmented (reticular) nigral neurons are inhibitory.

Descending pathways other than reticulospinal

Corticonuclear fibers from motor areas of the cerebral cortex are distributed preferentially to contralateral motor cranial nerve nuclei excepting the ocular motor slaves of gaze centers. Corticonuclear fibers from sensory areas synapse in contralateral trigeminal and posterior column nuclei and posterior gray horn of spinal cord.

• Prior to the initiation of a voluntary movement on the left side of the body, the left cerebellar hemisphere is notified by discharges from association areas of the right cerebral cortex, along the corticopontocerebellar pathway. The left cerebellum responds via the dentatothalamocortical pathway, to the right primary motor cortex. Then the right pyramidal tract discharges and on its way down notifies the cerebellum a second time by activating the right red nucleus, which in turn activates the right olivocerebellar tract.

• Corticospinal fibers pass through the middle three-fifths of the cerebral crus and through the basilar pons (where they are segregated into bundles by transverse fibers), finally creating the pyramid of the medulla before four-fifths enter the pyramidal decussation.

• The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract having an antigravity function. The medial and inferior vestibular nuclei give rise to the medial vestibulospinal tract involved in head-righting reflexes. The tectospinal tract belongs to the spinovisual reflex arc. The dorsal longitudinal fasciculus contains ipsilateral central autonomic fibers.

• A sleep-related pathway from the septal area reaches the interpeduncular nucleus by way of the habenular nucleus and fasciculus retroflexus.

Reticular formation

In the uppermost midbrain are the upward and downward gaze centers. (The lateral gaze centers adjoin the abducens nucleus in the pons.) The midbrain also contains an up-going, ‘arousal’ projection from the cuneiform nucleus, a down-going, pain-suppressant projection from the periaqueductal gray matter, and a locomotor generator, the pedunculopontine nucleus.

The pons contains the noradrenergic, cerulean nucleus, also the oral and caudal pontine reticular nuclei, which send ipsilateral pontine reticulospinal tracts to extensor motor neurons. The medullary reticulospinal tract is partly crossed and supplies flexor motor neurons. Three respiratory reticular nuclei also occupy the medulla.

Reference

Literature references to individual brainstem nuclei and pathways are to be found in the relevant chapters.