Cranial Nerve Motor Nuclei and Brain Stem Motor Functions

Clinical Case

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CLINICAL CASE | Hemiparesis and Lower Facial Droop

A 69-year-old man, with a history of hypertension and cigarette smoking, suddenly developed difficulty walking as he was returning home from shopping. Upon reaching his apartment, he was unable to raise a cup of coffee with his right hand. He called his daughter for assistance, who later noted that his speech was slurred. She brought him to the emergency room.

On neurological examination, somatic sensations on his limbs and trunk were normal. His cranial nerve functions were also found to be normal except for a flattening of the right nasolabial fold. The patient understood verbal commands, and speech was intact but slurred (disarthric). He was able to extend his tongue fully at the midline. On further testing, the patient’s right arm and leg strength were found to be 3 out of 5. (Note, strength is qualitatively assessed according to a 0 to 5 scale, where 0 is complete paralysis and 5 is normal. In between, 1 is the presence of a small muscle contraction but no movement; 2, movement, but not against gravity; and 3, movement against gravity but not against resistance.) Left arm and leg strength were normal (ie, 5/5). His gait required support. Reflex testing revealed a stronger knee jerk and other tendon reflexes on the right side compared with the left.

Figure 11–1A is a MRI that shows brain structure well. The image in part B, at the same level as A, shows more clearly an intense signal in the ventral pons, on the patient’s left side. This corresponds to the site of an infarction. Note that the bright signals in the temporal poles are artifacts. Part C shows the level of the MRIs in relation to brain stem vasculature. The site of infarction is represented on the ventral pontine surface.

Unfortunately, the patient died several years later, due to complications related to the stroke he suffered. Figure 11–1D shows a myelin-stained cervical spinal section after supraspinal stroke. Two prominent regions of demyelination, and accompanying axon degeneration, are noted (arrows); one on the right side (contralateral to infarction) in the dorsolateral white matter and the other in the left (ipsilateral) ventromedial white matter.

Answer the following questions based on your reading of this and the previous chapter.

1. Occlusion of what artery likely produced the infarction?

2. Why are the only somatic motor signs a flattening of the contralateral nasolabial fold and contralateral limb muscle weakness?

3. Why is the knee jerk reflex stronger (hyperreflexia) on the paretic (weakened) side?

Key neurological signs and corresponding damaged brain structures Selective flattening of nasolabial fold

The nasolabial fold is produced by tone in facial musculature; flattening signifies a loss of tone, and associated weakness or paralysis of facial musculature. There is no loss of capacity to contract upper facial muscles. In this patient’s case, where the lesion is in the descending cortical fibers in the pons, sparing of upper facial control is likely due to control by both the contralateral and ipsilateral cortical motor areas. Since the lesion is limited to the contralateral pathway, spared ipsilateral descending fibers could mediate control. There is no loss of other cranial motor functions; these, like the upper face, are under more bilateral cortical control. Thus, unilateral lesion will not seriously weaken or paralyze muscle groups under bilateral control (see Figure 11–5). Nevertheless, there can be marked control impairments.

Contralateral limb muscle weakness

Limb muscles, as well as lower facial muscles, receive a predominant contralateral control by the corticospinal and corticobulbar systems. Upper face (and other cranial muscles) and trunk muscles receive predominant bilateral control. Unilateral lesion of these systems will therefore disrupt contralateral limb and lower facial muscle control.

Hyperreflexia concurrent with muscle weakness

Hyperreflexia is a characteristic of lesion of the corticospinal, as well as brain stem, descending motor pathways. The precise mechanism is not well understood, but likely involves maladaptive plasticity in the spinal cord after the lesion (see Box 10–1). The hyperreflexia after the lesion is typically paralleled by progressively increasing muscle tone.

Disproportionate complex motor control impairment

The lesion produced mild facial muscle weakness; tongue protrusion at the midline was intact indicating significant spared control. Despite relatively modest cranial motor signs, the patient’s speech is slurred. This reflects disproportionate impairment in the complex coordination of perioral muscles needed for clear speech. Similarly, with such a lesion, limb muscles are weak and there is also disproportionate incoordination and slowing of movements. This is common with corticospinal and corticobulbar lesions. Spared brain stem pathways—such as the rubrospinal, vestibulospinal, and reticulospinal pathways—may help the patient to regain strength and balance, but the cortical pathways are essential for fine control.

Reference

Brust JCM. The Practice of Neural Science. New York, NY: McGraw-Hill; 2000.

FIGURE 11–1

Hemiparesis after unilateral ventral pontine stroke. A. MRI through the pons showing a lesion in the left ventral pons. This is a FLAIR image. Arrow points to the infarcted region. B. MRI through the same level showing more clearly the infarction (arrow). Note that the bright signals in the temporal poles are artifacts. C. Ventral brain stem showing site of infarction (ellipse) and approximate plane of section of MRIs. D. Myelin-stained section from deceased stroke patient showing demyelination (and accompanying axon degeneration) in the left lateral and right ventral columns (arrows). (MR images in are courtesy of Dr. Blair Ford, Dept. of Neurology, Columbia University College of Physicians and Surgeons.)

Striking parallels exist between the functional and anatomical organization of the spinal and cranial somatic sensory systems. In fact, the principles governing the organization of one are nearly identical to those of the other. A similar comparison can be made between motor control of cranial structures and that of the limbs and trunk: Cranial muscles are innervated by motor neurons found in the cranial nerve motor nuclei, whereas limb and axial muscles are innervated by motor neurons in the motor nuclei of the ventral horn. A similar parallel exists with the control of body organs. Control of the glands and smooth muscle of the head, as well as the pupil, is mediated by parasympathetic preganglionic neurons located in cranial nerve autonomic nuclei. Abdominal visceral organs are controlled by parasympathetic neurons in the sacral cord.

This chapter examines in detail the cranial nerve motor nuclei innervating facial, jaw, and tongue muscles, as well as the muscles for swallowing. It also examines the cortical control of these nuclei, which is accomplished by the corticobulbar tract. This pathway is the cranial equivalent of the corticospinal tract, and the two pathways share numerous organizational principles. Knowing the patterns of corticobulbar connections with the cranial motor nuclei has important diagnostic value because it helps clinicians to understand the cranial motor signs produced by brain stem damage. This knowledge also helps clinicians to plan the proper therapy for the patient to avert potentially life-threatening sequelae. The autonomic and extraocular motor nuclei of the brain stem are also examined to achieve greater knowledge of regional anatomy. Such knowledge is essential for localizing central nervous system damage after trauma.

Organization of Cranial Motor Nuclei

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There Are Three Columns of Cranial Nerve Motor Nuclei

As we saw in Chapter 6, the cranial nerve sensory and motor nuclei are organized into columns that course rostrocaudally throughout the brain stem (Figure 11–2). The sensory columns are located laterally and the motor columns, medially. The sensory nuclei derive from a portion of the developing neural ectoderm, the alar plate, of the brain stem and the motor nuclei, from the basal plate (Figure 11–3). The two collections of developing neurons undergo migration and further subdivision to give rise to the various columns of sensory and motor nuclei.

FIGURE 11–2

Dorsal view of the brain stem (without the cerebellum), showing the locations of cranial nerve nuclei. The top left inset, which is a lateral view of the diencephalon and basal ganglia, shows the various cranial nerves, the spinal accessory nerve, and a ventral root. The bottom inset is a schematic cross section through the medulla, showing the location of cranial nerve nuclear columns. (Adapted from Nieuwenhuys R, Voogd J, van Huijzen C. The Human Central Nervous System: A Synopsis and Atlas. 4th ed. London: Springer-Verlag; 2007.)

FIGURE 11–3

Development of the cranial nuclei. A–D. Schematic section through the hind brain at three developmental time points (A–C) and maturity (D). The space within the sections is the fourth ventricle. During development the fourth ventricle, initially flattened dorsoventrally just like the spinal cord, expands dorsally. This has the effect of transforming the dorsoventral sensory-motor nuclear organization characteristic of the spinal cord, into the lateromedial organization of sensory and motor nuclei in the caudal brain stem (the future medulla and pons). Developing neurons in the alar plate will become sensory cranial nuclei near the ventricular floor and, in the basal plate, cranial motor nuclei. Additionally, neurons from the plates migrate to more distant locations to serve more integrative functions.

The cranial nerve motor nuclei are organized into three columns (Figure 11–2): somatic skeletal, branchiomeric, and autonomic. Nuclei in the somatic skeletal motor column contain motor neurons that innervate striated muscle derived from the occipital somites (see Figure 6-4): the extraocular and tongue muscles. This column is close to the midline. Nuclei of the branchiomeric motor column contain motor neurons innervating striated muscle derived from the branchial arches (ie, branchiomeric or visceral, opposed to somatic, origin): facial, jaw, palatal, pharyngeal, and laryngeal muscles. This column is lateral to the somatic skeletal motor column (Figure 11–2) and is displaced ventrally from the ventricular floor (Figure 11–2, bottom inset). Nuclei of the autonomic motor column contain the parasympathetic preganglionic neurons that regulate the functions of cranial exocrine glands, smooth muscle, and many body organs. The autonomic motor column is lateral to the somatic skeletal motor column (Figure 11–2). Sometimes these three columns are termed general somatic motor, special visceral motor, and general visceral motor columns, respectively (see Box 6–1).

Neurons in the Somatic Skeletal Motor Column Innervate Tongue and Extraocular Muscles

Four nuclei comprise the somatic skeletal motor column (Figure 11–2). Three of these nuclei contain motor neurons that innervate the extraocular muscles: the oculomotor nucleus, the trochlear nucleus, and the abducens nucleus. The oculomotor nucleus is located in the rostral midbrain and innervates the medial rectus, inferior rectus, superior rectus, and inferior oblique muscles, which move the eyes (see Figure 12–4), as well as the levator palpebrae superioris muscle, an eyelid elevator. The motor axons course within the oculomotor (III) nerve. Motor neurons in the trochlear nucleus course in the trochlear (IV) nerve and innervate the superior oblique muscle. The abducens nucleus contains the motor neurons that project their axons to the periphery through the abducens (VI) nerve and innervate the lateral rectus muscle. The neuroanatomy of eye muscle control is the focus of Chapter 12. The hypoglossal nucleus is the fourth member of the somatic skeletal motor column (Figure 11–2). The axons of motor neurons in the hypoglossal nucleus course in the hypoglossal (XII) nerve and innervate intrinsic tongue muscles, including the genioglossus, hypoglossus, and styloglossus.

The Branchiomeric Motor Column Innervates Skeletal Muscles That Develop From the Branchial Arches

Three cranial nerve nuclei constitute this nuclear column: the facial motor nucleus, the trigeminal motor nucleus, and the nucleus ambiguus. The facial motor nucleus contains the motor neurons that innervate the muscles of facial expression. These axons course in the facial (VII) nerve. The axons of motor neurons of the trigeminal motor nucleus course in the trigeminal (V) nerve and innervate principally the muscles of mastication: masseter, temporalis, and external and internal pterygoid muscles. The nucleus ambiguus contains motor neurons that innervate striated muscles of the pharynx and larynx. This nucleus and its efferent projections through cranial nerves are organized rostrocaudally. A small number of motor neurons in the most rostral portion of the nucleus ambiguus course in the glossopharyngeal (IX) nerve and innervate one pharyngeal muscle, the stylopharyngeus. Most motor neurons in the nucleus send their axons through the vagus (X) nerve to innervate the pharynx and larynx. Because the pharyngeal muscles are innervated by the vagus nerve, a lesion of the nucleus ambiguus produces difficulty in swallowing. The vagus nerve is the efferent component of the gag reflex. In this reflex, mechanical stimulation of the pharynx, using a cotton swab for example, produces reflex contraction of the pharyngeal muscles. The glossopharyngeal nerve contains the afferent fibers that innervate mechanoreceptors of the pharynx that comprise the afferent limb of the gag reflex (see Chapter 6). The most caudal portion of the nucleus ambiguus contains laryngeal motor neurons whose axons course in a portion of the accessory (XI) nerve. This cranial nerve consists of distinct cranial and spinal roots, and only axons in the cranial root have their cell bodies in the nucleus ambiguus. These axons are probably displaced vagal fibers. They join the vagus nerve as they exit from the cranium and innervate the same structures as the vagus; accordingly, they are sometimes considered to be part of the vagus nerve.

Cell bodies of axons in the spinal root of the spinal accessory nerve are located in the spinal accessory nucleus (Figure 11–2). This nucleus is a part of the ventral horn of the upper cervical spinal cord—from the pyramidal decussation to about the fourth or fifth cervical segments—not the branchiomeric motor column. Axons in the spinal root of the spinal accessory nerve innervate the sternocleidomastoid muscle and the upper part of the trapezius muscle, which develop from the somites and not the branchial arches.

The Autonomic Motor Column Contains Parasympathetic Preganglionic Neurons

The autonomic motor column contains neurons that regulate the function of various body organs, smooth muscles, and exocrine glands. These neurons are part of the parasympathetic nervous system, a division of the autonomic nervous system (see Chapters 1 and 15). In contrast to the innervation of skeletal muscle, which is mediated by a single motor neuron (Figure 11–4A), the innervation of smooth muscle and glands is accomplished by two separate neurons: preganglionic and postganglionic neurons (Figure 11–4B). Parasympathetic preganglionic neurons are located in the various nuclei that comprise the autonomic motor column; these neurons are also found in the sacral spinal cord (see Chapter 15). Parasympathetic postganglionic neurons are located in peripheral autonomic ganglia.

FIGURE 11–4

A. Somatic motor neurons have their cell body located in the central nervous system. Their axon projects directly to their peripheral targets, which are striated muscles. B. Parasympathetic preganglionic neurons are located in nuclei within the central nervous system, whereas postganglionic neurons are located in peripheral ganglia. B1–B3 show examples of three parasympathetic functions: pupillary constriction (B1), secretions (B2), and visceral functions (B3).

The autonomic motor column, which is lateral to the somatic skeletal motor column (Figure 11–2), contains four nuclei. The Edinger-Westphal nucleus is located in the midbrain and in the pretectal region, dorsal to the oculomotor nucleus (Figure 11–4B1). It participates in pupillary constriction and lens accommodation. The parasympathetic neurons in the nucleus send their axons into the oculomotor (III) nerve to synapse on postganglionic neurons in the ciliary ganglion. These neurons innervate the ciliary muscle and the constrictor muscles of the iris.

Parasympathetic preganglionic neurons are also located in nuclei of the caudal pons and medulla (Figure 11–4B2). Neurons of the superior and inferior salivatory nuclei are located in the pons and medulla. They are somewhat dispersed, not forming a discrete cell column. The axons of neurons of the superior salivatory nucleus course in the intermediate nerve. They synapse in two peripheral ganglia: (1) the pterygopalatine ganglion, where postganglionic neurons innervate the lacrimal glands and glands of the nasal mucosa, and (2) the submandibular ganglion, from which postganglionic parasympathetic neurons innervate the submandibular and sublingual salivary glands. The intermediate nerve is sometimes considered to be the sensory branch of the facial nerve because it contains afferent fibers, which are the axons of pseudounipolar neurons of the geniculate ganglion (see Chapters 6 and 9). The inferior salivatory nucleus contains parasympathetic preganglionic neurons whose axons course in the glossopharyngeal nerve and synapse on postganglionic neurons in the otic ganglion (Figure 11–4B2). Parasympathetic postganglionic neurons in the otic ganglion innervate the parotid gland, which secretes saliva.

The dorsal motor nucleus of the vagus nerve forms a column of parasympathetic preganglionic neurons beneath the floor of the fourth ventricle in the medulla (Figure 11–4B3). These neurons synapse in extracranial parasympathetic ganglia, called terminal ganglia (Figure 11–4B3). These ganglia are located in the viscera of the thoracic and abdominal cavities, including the gastrointestinal tract proximal to the splenic flexure of the colon. The functions of the vagal parasympathetic neurons include regulating heart rate (ie, slowing), gastric motility (ie, increasing), and bronchial muscle control (ie, contracting to constrict airway). (The colon distal to the splenic flexure is innervated by parasympathetic preganglionic neurons of the sacral spinal cord [see Chapter 15].)

The remaining sections of this chapter will focus on the cortical control of cranial motor nuclei and their regional anatomy in the pons and medulla.