Brainstem, Cerebellum, and Cranial Nerves

Overview

Remembering the segmental anatomy of the brainstem is easier if sites of exit of the cranial nerves (CN) are thoroughly understood. Fig. 2.4.1 shows a ventral view of the brainstem with sites of exit of each cranial nerve, and Fig. 2.4.2 shows the vascular supply of the brainstem.

The oculomotor nerve exits the midbrain on the ventral surface between the cerebral peduncles. The trochlear nerve exits the midbrain near the inferior colliculus and wraps around the cerebral peduncle. The optic nerve only projects a small proportion of its fibers to the midbrain, but the optic tracts sit just above the oculomotor and trochlear nerves as they travel toward the orbit.

Immediately below the midbrain is the pons, which on ventral view appears to wrap around the brainstem and attach to the cerebellar hemispheres on either side. The trigeminal nerve exits the lateral aspect of the pons.

The abducens, facial, and vestibulocochlear nerves exit at the pontomedullary junction. The most medial is the abducens nerve, which has a long course past the pons to project to the orbit. The facial nerve exits laterally to the abducens and just adjacent to the vestibulocochlear nerve. The glossopharyngeal, vagus, and hypoglossal nerves exit the medulla, and the accessory nerve has fibers that exit both at the level of the medulla and at the upper cervical cord.

Localization of lesions affecting the brainstem is easier if a systematic approach is used. When faced with a difficult diagnostic problem, the neurologist should try to rely on this method rather than trying to fit the symptoms into one of the many syndrome descriptions. Important questions to answer include the following:

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  Is the lesion intraaxial or extraaxial?

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  What is the rostrocaudal location of the lesion?

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  Is the intraaxial lesion unilateral or bilateral?

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  What clinical conditions can produce damage in this localization?

Isolated single cranial nerve palsies are usually extraaxial, that is, not in the substance of the brainstem. Multiple cranial nerve palsies can still be due to extraaxial disease. An intraaxial lesion is suggested by the combination of cranial nerve palsies and signs of damage to ascending and descending tracts. For example, an isolated CN VI palsy, which causes impaired abduction of the ipsilateral eye, is relatively common and can result from microvascular disease, trauma, and increased intracranial pressure; it is often idiopathic. In contrast, a CN VI palsy combined with damage to the medial longitudinal fasciculus (MLF), which causes internuclear ophthalmoplegia (INO), is due to a lesion in the pons.

Rostrocaudal localization is most easily identified by carefully determining which cranial nerves are affected. For example, an oculomotor lesion with signs of intraaxial damage suggests a midbrain lesion, whereas glossopharyngeal and vagus damage suggest a medullary lesion.

The determination of whether the lesion is unilateral or bilateral requires an understanding of which tracts are crossed and at which level. The descending corticospinal tracts cross in the medulla. Dorsal column axons, which serve touch, vibration, and position sense, ascend the cord uncrossed, synapse in the nucleus gracilis and nucleus cuneatus in the medulla, and cross as they ascend through the medulla, forming the medial lemniscus. The spinothalamic tract axons, which convey pain and temperature sensations, cross at the level of the spinal cord segment and ascend crossed in the cord. The spinothalamic tract stays lateral in the brainstem and ascends to the midbrain where it joins with the medial lemniscus to enter the thalamus.

When the lesion has been determined to be intraaxial and the rostrocaudal and unilateral or bilateral localization has been made, individual syndromes can be considered. Specific etiologies are suggested by associated symptoms; for instance, acute onset of a deficit suggests a stroke, sudden onset of headache and vomiting suggests a hemorrhage, and a slowly progressive deficit suggests an expanding mass lesion.

Midbrain

Fig. 2.4.3 shows a cross section of the midbrain between the superior and inferior colliculi. Important landmarks are the cerebral peduncles, red nucleus, substantia nigra, aqueduct and surrounding periaqueductal gray matter, medial lemniscus, MLF, and midbrain tectum.

The cerebral peduncles carry descending information from the cerebral hemispheres to the brainstem and spinal cord. The red nucleus receives outflow from the contralateral cerebellum and sends efferents to the thalamus. The medial lemniscus carries ascending fibers from the nucleus gracilis and nucleus cuneatus in the medulla to the thalamus. The substantia nigra has extensive interconnections with other nuclei, but the most important are its reciprocal connections with the striatum.

The oculomotor nuclear complex is adjacent to the midline and ventral to the aqueduct. The trochlear nucleus is in a similar position, but it is caudal to the oculomotor nucleus although still within the midbrain. The MLF carries outflow from the contralateral paramedial pontine reticular formation, the center that governs lateral gaze, and projects to the oculomotor nuclear complex.

Blood supply to the midbrain is largely through the basilar artery, which lies on the ventral surface. The basilar artery sends several penetrating branches into the midbrain before bifurcating into the posterior cerebral arteries (PCAs), which wrap laterally around the midbrain. The PCAs also give off penetrating branches.

Pons

Fig. 2.4.4 shows a cross section of the pons. There are rostrocaudal differences in organization, so this figure is slightly stylized to show important structures. Important landmarks include the corticospinal tract, pontine nuclei, medial lemniscus, spinothalamic tract, lateral lemniscus, nuclei and intraaxial portions of the abducens, trigeminal and facial nerves, vestibular nucleus, paramedian pontine reticular formation (PPRF), and MLF.

The corticospinal tract lies in the ventral aspect of the pons, and it is still uncrossed at this level. The medial lemniscus, carrying fibers from the nuclei gracilis and cuneatus, is already crossed. The PPRF, which is important for lateral gaze, sends output to the abducens nucleus and through the MLF to the midbrain. The MLF is not dedicated solely to the PPRF; however, it also carries descending axons of the medial reticulospinal and vestibulospinal tracts and the tectospinal tract. The motor and sensory nuclei of the trigeminal nerves lie rostral to the abducens and facial nuclei, which lie in the caudal half of the pons. The facial nerve exits the facial nucleus and moves dorsally to loop around the abducens nucleus before moving ventrally and medially to exit the brainstem at the pontomedullary junction.

The vertebral arteries join to form the basilar artery near the pontomedullary junction, and the basilar artery ascends to the midbrain. There are major and minor circumferential branches as well as direct penetrating paramedial branches of the basilar artery. Major circumferential branches include the anterior inferior cerebellar artery (AICA) and superior cerebellar arteries. Transverse arteries have a variable projection bilaterally and send penetrating arteries into the pons.

Medulla

Fig. 2.4.5 shows a cross section of the medulla. There are rostrocaudal differences in organization, so the figure is slightly stylized. Important landmarks include the corticospinal tracts, olivary nucleus, medial lemniscus, MLF, vestibular nucleus, nucleus ambiguus, hypoglossal nucleus and nerve, nucleus of the tractus solitarius, descending sympathetic tract, spinothalamic tract, and inferior cerebellar peduncle.

The blood supply to the medulla is from branches of the vertebral arteries because the basilar artery has not formed yet at this location. The major vessels are the posterior inferior cerebellar artery (PICA) and medial branches of the vertebral artery, which join to form the anterior spinal artery.

The corticospinal tract travels in the medullary pyramids. The corticospinal tract decussates in the medulla, with the arm axons crossing slightly rostral to the crossing of the leg axons. This arrangement allows a very localized lesion to produce corticospinal dysfunction affecting the ipsilateral arm and contralateral leg; although in practice, this combination is uncommon.

Underlying the olives are the olivary nuclei, which project somatosensory information to the contralateral cerebellum. Nucleus gracilis and nucleus cuneatus are in the most caudal section of the medulla, on the dorsal surface, in continuity with the dorsal columns. Outflow crosses the midline and forms the medial lemnisci. The spinothalamic tract is a continuation of the same tract in the spinal cord and stays lateral through its course in the medulla and into the pons. The nucleus ambiguus provides axons that control muscles of the pharynx and larynx via the glossopharyngeal, and vagus nerves. The solitary tract nucleus receives taste and visceral sensation input from the facial, glossopharyngeal, and vagus nerves. The descending sympathetic tract projects into the cervical cord. The MLF contains the medial vestibulospinal and reticulospinal tracts and the tectospinal tract. The restiform body forms the inferior cerebellar peduncle.

Anatomy

The cerebellum sits dorsal to the brainstem in the posterior fossa. All of its inputs and outputs pass through the brainstem. The cerebellum can be divided into several phylogenetic and physiologic divisions; in clinical practice, the most important distinction is between disorders affecting the vermis and those affecting the cerebellar hemispheres.

The cerebellum is divided into the anterior lobe, posterior lobe, and flocculonodular lobe. The anterior lobe is the section rostral to the primary fissure of the cerebellum. This lobe receives prominent input from the spinocerebellar pathways and is involved in gait and truncal coordination. The posterior lobe receives extensive input from the cerebral hemispheres and is concerned with coordination of individual limb movements. The flocculonodular lobe receives input from the vestibular nuclei and is concerned with proximal coordination.

Inputs to the cerebellum are largely through the middle and inferior cerebellar peduncles, although the ventral spinocerebellar tract and a few other minor afferent tracts pass through the superior cerebellar peduncle. Outflow from the cerebellum is through the superior cerebellar peduncle. Axons from the cerebellar cortex project to the deep nuclei, which in turn project axons through the superior cerebellar peduncle to the contralateral red nucleus, contralateral ventrolateral nucleus of the thalamus, reticular formation, and vestibular nuclei.

The cerebellum is involved in control of movement. The flocculonodular lobe, anterior lobe, and especially the vermis are involved in control of gait and truncal movement ( Fig. 2.4.6 ). The posterior lobe, especially the hemispheres, is involved in coordinating movements of individual ipsilateral limbs.

Function

The cerebellum can be considered to have three major roles, which are:

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  Maintenance of balance and posture,

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  Moment-to-moment coordination of the trunk and limbs, and

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  Higher order cognitive functions.

Each of these regions has distinct inputs and outputs that allow their respective regions to serve these functions. These are divided functionally into the:

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  Vestibulocerebellum (responsible for balance and posture),

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  Spinocerebellum (responsible for moment-to-moment coordination), and

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  Neocerebellum (responsible for higher order cognitive function).

With one exception (the vestibulocerebellum), cerebellar zones send output to the brainstem via the four (on each side) cerebellar nuclei. Each hemisphere of the cerebellum controls the ipsilateral side of the body.

In addition, three fiber bundles (the cerebellar peduncles) carry information in and/or out of the cerebellum. They are termed the superior, middle, and inferior cerebellar peduncles.