The Sensory, Motor, and Reflex Functions of the Brain Stem
The Cranial Nerves Are Homologous to the Spinal Nerves
Cranial Nerves Leave the Skull in Groups and Often Are Injured Together
Embryonic Cranial Nerve Nuclei Have a Segmental Organization
The Organization of the Brain Stem and Spinal Cord Differs in Three Important Ways
Cranial Nerve Reflexes Involve Mono- and Polysynaptic Brain Stem Relays
Pattern Generator Neurons Coordinate Stereotypic and Autonomic Behaviors
IN PRIMITIVE VERTEBRATES—REPTILES, amphibians, and fish—the forebrain is only a small part of the brain and is devoted mainly to olfactory processing and to the integration of autonomic and endocrine function with the basic behaviors necessary for survival. These basic behaviors include feeding, drinking, sexual reproduction, sleep, and emergency responses. Although we are accustomed to thinking that human behavior originates mainly in the forebrain, many complex responses, such as feeding—the coordination of chewing, licking, and swallowing—are actually made up of relatively simple, stereotypic motor responses governed by ensembles of neurons in the brain stem.
The importance of this pattern of organization in human behavior is clear from observing infants born without a forebrain (hydrancephaly). Hydrancephalic infants are surprisingly difficult to distinguish from normal babies. They cry, smile, suckle, and move their eyes, face, arms, and legs. As these sad cases illustrate, the brain stem can organize virtually all of the behavior of the newborn.
In this chapter we examine the role of the brain stem in reflex behavior. We also review the cranial nerves, their origin in the brain stem, as well as the ensembles of local circuit neurons that organize the simple behaviors of the face and head.
The brain stem is the rostral continuation of the spinal cord and its motor and sensory components are similar in structure to that of the spinal cord. But the portions of the brain stem that control the cranial nerves are much more complex than the corresponding parts of the spinal cord that control the spinal nerves because cranial nerves mediate more complex behaviors. The core of the brain stem, the reticular formation, is homologous to the intermediate gray matter of the spinal cord but is also more complex. Like the spinal cord, the reticular formation contains ensembles of local-circuit interneurons that generate motor and autonomic patterns and coordinate reflexes and simple behaviors. In addition, it contains clusters of dopaminergic, noradrenergic, and other modulatory neurons that act to optimize the functions of the nervous system. The modulatory actions of these nuclei are described in the next chapter.
The Cranial Nerves Are Homologous to the Spinal Nerves
Because the spinal nerves reach only as high as the first cervical vertebra, the cranial nerves provide the somatic and visceral sensory and motor innervation for the head. Two cranial nerves, the glossopharyngeal and vagus nerves, also supply visceral sensory and motor innervation of the neck, chest, and most of the abdominal organs with the exception of the pelvis. Unlike the spinal nerves, which supply all sensory and motor functions for specific body segments, each cranial nerve is associated with one or more functions and may therefore overlap the physical territory of another cranial nerve.
Assessment of the cranial nerves is an important part of the neurological examination (see Appendix B) because abnormalities of function can pinpoint a site in the brain stem that has been damaged. Therefore, it is important to know the origins of the cranial nerves, their intracranial course, and where they exit from the skull.
The cranial nerves are traditionally numbered I through XII in rostrocaudal sequence. Cranial nerves I and II enter at the base of the forebrain. The other cranial nerves arise from the brain stem at characteristic locations (Figure 45–1). All but one exit from the ventral surface of the brain stem. The exception is the trochlear (IV) nerve, which leaves the midbrain from its dorsal surface just behind the inferior colliculus and wraps around the lateral surface of the brain stem to join the other cranial nerves concerned with eye movements. The cranial nerves with sensory functions (V, VII, VIII, IX, and X) have associated sensory ganglia that operate much as dorsal root ganglia do for spinal nerves. These ganglia are located along the course of individual nerves as they enter the skull.
Figure 45-1 The origins of cranial nerves in the brain stem (ventral and lateral views). The olfactory (I) nerve is not shown because it terminates in the olfactory bulb in the forebrain. All of the cranial nerves except one emerge from the ventral surface of the brain; the trochlear (IV) nerve originates from the dorsal surface of the midbrain.
The olfactory (I) nerve, which is associated with the forebrain, is described in detail in Chapter 32; the optic (II) nerve, which is associated with the diencephalon, is described in Chapters 25 and 26. The spinal accessory (XI) nerve can be considered a cranial nerve anatomically but actually is a spinal nerve originating from the higher cervical motor rootlets. It runs up into the skull before exiting through the jugular foramen to innervate the trapezius and sternocleidomastoid muscles in the neck.
Cranial Nerves Mediate the Sensory and Motor Functions of the Face and Head and the Autonomic Functions of the Body
The ocular motor nerves—the oculomotor (III), trochlear (IV), and abducens (VI) nerves—control movements of the eyes. The abducens nerve has the simplest action; it contracts the lateral rectus muscle to move the globe laterally. The trochlear nerve also innervates a single muscle, the superior oblique, but its action both depresses the eye and rotates it inward, depending on the eye’s position. The oculomotor nerve supplies all of the other muscles of the orbit, including the retractor of the lid. It also provides the parasympathetic innervation responsible for pupillary constriction in response to light and accommodation of the lens for near vision. The ocular motor system is considered in detail in Chapter 39.
The trigeminal (V) nerve is a mixed nerve (containing both sensory and motor axons) that leaves the brain stem in two roots. The motor root innervates the muscles of mastication (the masseter, temporalis, and pterygoids) and a few muscles of the palate (tensor veli palatini), inner ear (tensor tympani), and upper neck (mylohyoid and anterior belly of the digastric muscle).
The sensory fibers arise from neurons in the trigeminal ganglion, located at the floor of the skull in the middle cranial fossa, the central division of the skull, adjacent to the sella turcica, which houses the pituitary gland.
Three branches emerge from the trigeminal ganglion. The ophthalmic division (V1) runs with the ocular motor nerves through the superior orbital fissure (Figure 45–2A) to innervate the orbit, nose, and forehead and scalp back to the vertex of the skull (Figure 45–3). Some fibers from this division also innervate the meninges and blood vessels of the anterior and middle intracranial fossas. The maxillary division (V2) runs through the round foramen of the sphenoid bone to innervate the skin over the cheek and the upper portion of the oral cavity. The mandibular division (V3), which also contains the motor axons of the trigeminal nerve, leaves the skull through the oval foramen of the sphenoid bone. It innervates the skin over the jaw, the area above the ear, and the lower part of the oral cavity, including the tongue.
Figure 45-2 The cranial nerves exit the skull in groups.
A. Cranial nerves II, III, IV, V, and VI exit the skull near the pituitary fossa. The optic (II) nerve enters the optic foramen, but the oculomotor (III), trochlear (IV), and abducens (VI) nerves, and the first division of the trigeminal (V) nerve leave through the superior orbital fissure. The second and third divisions of the trigeminal nerve exit through the round and oval foramina, respectively.
B. In the posterior fossa the facial (VII) and vestibulocochlear (VIII) nerves exit through the internal auditory canal, whereas the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves leave through the jugular foramen. The hypoglossal nerve (XII) has its own foramen.
Figure 45-3 The three sensory divisions of the trigeminal (V) nerve innervate the face and scalp.
Complete trigeminal sensory loss results in numbness of the entire face and the inside of the mouth. One-sided trigeminal motor weakness does not cause much weakness of jaw closure because the muscles of mastication on either side are sufficient to close the jaw. Nevertheless, the jaw tends to deviate toward the side of the lesion when the mouth is opened because the internal pterygoid muscle on the opposite side, when unopposed, pulls the jaw toward the weak side.
The facial (VII) nerve is also a mixed nerve. Its motor root supplies the muscles of facial expression as well as the stapedius muscle in the inner ear, stylohyoid muscle, and posterior belly of the digastric muscle in the upper neck. The sensory root runs as a separate bundle, the nervus intermedius, through the internal auditory canal and arises from neurons in the geniculate ganglion, located near the middle ear. Distal to the geniculate ganglion the sensory fibers diverge from the motor branch. Some innervate skin of the external auditory canal whereas others form the chorda tympani, which joins the lingual nerve and conveys taste sensation from the anterior two-thirds of the tongue. The autonomic component of the facial nerve includes parasympathetic fibers that travel through the motor root to the sphenopalatine and submandibular ganglia, which innervate lacrimal and salivary glands (except the parotid gland) and the cerebral vasculature.
The facial nerve may suffer isolated injury in Bell’s palsy, a common complication of certain viral infections. Early on the patient may complain mainly of the face pulling toward the unaffected side because of the weakness of the muscles on the side of the lesion. Later the ipsilateral corner of the mouth droops, food falls out of the mouth, and the eyelids no longer close on that side. Loss of blinking may result in drying and injury to the cornea. The patient may complain that sound has a booming quality in the ipsilateral ear because the stapedius muscle fails to tense the ossicles in response to a loud sound (the stapedial reflex). Taste may also be lost on the anterior two-thirds of the tongue on the ipsilateral side. If the Bell’s palsy is caused by a herpes zoster infection of the geniculate ganglion, small blisters may form in the outer ear canal, the ganglion’s cutaneous sensory field.
The vestibulocochlear (VIII) nerve contains two main bundles of sensory axons from two ganglia. Fibers from the vestibular ganglion relay sensation of angular and linear acceleration from the semicircular canals, utricle, and saccule in the inner ear. Fibers from the cochlear ganglion relay information from the cochlea concerning sound. A vestibular schwannoma, one of the most common intracranial tumors, may form along the vestibular component of cranial nerve VIII as it runs within the internal auditory meatus. Most patients complain only about hearing loss, as the brain is usually able to adapt to the gradual loss of vestibular input from one side.
The glossopharyngeal (IX) nerve and vagus (X) nerve are mixed but are predominantly autonomic. These closely related nerves transmit sensory information from the pharynx and upper airway as well as taste from the posterior third of the tongue and oral cavity. The glossopharyngeal nerve transmits visceral information from the neck (for example, information on blood oxygen and carbon dioxide from the carotid body, and arterial pressure from the carotid sinus), whereas the vagus nerve transmits visceral information from the thoracic and abdominal organs except for the distal colon and pelvic organs. Both nerves include parasympathetic motor fibers. The glossopharyngeal nerve provides parasympathetic control of the parotid salivary gland, whereas the vagus nerve innervates the rest of the internal organs of the neck, thorax, and abdomen. The glossopharyngeal nerve innervates only one muscle of the palate, the stylopharyngeus, which raises and dilates the pharynx. The remaining striated muscles of the larynx and pharynx are under control of the vagus nerve.
Because many of the functions of nerves IX and X are bilateral and partially overlapping, unilateral injury of nerve IX may be difficult to detect. Patients with unilateral cranial nerve X injury are hoarse, because one vocal cord is paralyzed, and may have some difficulty swallowing. Examination of the oropharynx shows weakness and numbness of the palate on one side.
The spinal accessory (XI) nerve is purely motor and originates from motor neurons in the upper cervical spinal cord. It innervates the trapezius and sternocleidomastoid muscles on the same side of the body. Because the mechanical effect of the sternocleidomastoid is to turn the head toward the opposite side, an injury of the left nerve causes weakness in turning the head to the right. A lesion of the cerebral cortex on the left will cause weakness of muscles on the entire right side of the body except for the sternocleidomastoid; instead, the ipsilateral sternocleidomastoid will be weak (because the left cerebral cortex is concerned with interactions with the right side of the world).
The hypoglossal (XII) nerve is also purely motor, innervating the muscles of the tongue. When the nerve is injured, for example during surgery for head and neck cancer, the tongue atrophies on that side. The muscle fibers exhibit twitches of muscle fascicles (fasciculations), which may be seen clearly through the thin mucosa of the tongue.
Cranial Nerves Leave the Skull in Groups and Often Are Injured Together
In assessing dysfunction of the cranial nerves it is important to determine whether the injury is within the brain or further along the course of the nerve. As cranial nerves leave the skull in groups through specific foramina, damage at these locations can affect several nerves.
The cranial nerves concerned with orbital sensation and movement of the eyes (III, IV, VI, and the ophthalmic division of the trigeminal nerve, V1) are gathered together in the cavernous sinus, along the lateral margins of the sella turcica, and then exit the skull through the superior orbital fissure adjacent to the optic foramen (Figure 45-2A). Tumors in this region, such as those arising from the pituitary gland, often make their presence known first by pressure on these nerves or the adjacent optic chiasm.
Cranial nerves VII and VIII exit the brain stem at the cerebellopontine angle, the lateral corner of the brain stem at the juncture of the pons, medulla, and cerebellum (Figure 45-2B), and then leave the skull through the internal auditory meatus. A common tumor of the cerebellopontine angle is the vestibular schwannoma (sometimes erroneously called an “acoustic neuroma”), which derives from Schwann cells in the vestibular component of nerve VIII. If the tumor is large, it may not only impair the function of nerves VII and VIII but may also press on nerve V near its site of emergence from the middle cerebellar peduncle, causing facial numbness, or compress the cerebellum or its peduncles on the same side, causing ipsilateral clumsiness.
The lower cranial nerves (IX, X, and XI) exit through the jugular foramen (Figure 45-2B) and are vulnerable to compression by tumors at that site. Nerve XII leaves the skull through its own (hypoglossal) foramen and is generally not affected by tumors located in the adjacent jugular foramen, unless the tumor becomes quite large. If nerve XI is spared, the injury is generally within or near the brain stem rather than near the jugular foramen.
Cranial Nerve Nuclei in the Brain Stem Are Organized on the Same Basic Plan As Are Sensory and Motor Regions of the Spinal Cord
Cranial nerve nuclei are organized in rostrocaudal columns that are homologous to the sensory and motor laminae of the spinal cord (see Chapters 22 and 34). This pattern is best understood from the developmental plan of the caudal neural tube that gives rise to the brain stem and spinal cord.
The transverse axis of the embryonic caudal neural tube is subdivided into alar (dorsal) and basal (ventral) plates by the sulcus limitans, a longitudinal groove along the lateral walls of the central canal, fourth ventricle, and cerebral aqueduct (Figure 45–4). The alar plate forms the sensory components of the dorsal horn of the spinal cord, whereas the basal plate forms the motor components of the ventral horn. The intermediate gray matter is made up primarily of the interneurons that coordinate spinal reflexes and motor responses.
Figure 45-4 The developmental plan of the brain stem is the same general plan as that of the spinal cord.
A. The neural tube is divided into a dorsal sensory portion (the alar plate) and a ventral motor portion (the basal plate) by a longitudinal groove, the sulcus limitans.
B–D. During development the sensory and motor cell groups migrate into their adult positions, but largely retain their relative locations. In maturity (D) the sulcus limitans (dashed line) is still recognizable in the walls of the fourth ventricle and the cerebral aqueduct, demarcating the border between dorsal sensory structures (orange) and ventral motor structures (green). The section in D is from the rostral medulla.
The brain stem shares this basic plan. As the central canal of the spinal cord opens into the fourth ventricle, the walls of the neural tube are splayed outward so that the dorsal sensory structures (derived from the alar plate) are displaced laterally whereas the ventral motor structures (derived from the basal plate) remain more medial. The nuclei of the brain stem are divided into general nuclei, which serve functions similar to those of the spinal cord laminae, and special nuclei, which serve functions unique to the head (such as hearing, balance, taste, and control of the branchial musculature).
Adult Cranial Nerve Nuclei Have a Columnar Organization
Overall, the brain stem nuclei on each side are organized in six rostrocaudal columns, three of sensory nuclei and three of motor nuclei (Figure 45–5). These are considered below, in dorsolateral to ventromedial sequence. The columns are discontinuous—the nuclei are not packed solidly along the rostrocaudal axis of the brain stem. Nuclei with similar functions (sensory or motor, somatic or visceral) have similar dorsolateralventromedial positions at each level of the brain stem.
Figure 45-5 Adult cranial nerve nuclei are organized in six functional columns on the rostrocaudal axis of the brain stem.
A. This dorsal view of the human brain stem shows the location of the cranial nerve sensory nuclei (right) and motor nuclei (left).
B. A schematic view of the functional organization of the motor and sensory columns.
C. The medial-lateral arrangement of the cranial nerve nuclei is shown in a cross section at the level of the medulla (compare with Figure 45-4).
Within each motor nucleus, motor neurons for an individual muscle are also arranged in a cigar-shaped longitudinal column. Thus each motor nucleus in cross section forms a mosaic map of the territory that is innervated. For example, in a cross section through the facial nucleus the clusters of neurons that innervate the different facial muscles form a topographic map of the face.
General Somatic Sensory Column
The general somatic sensory column occupies the most lateral region of the alar plate and includes the trigeminal sensory nuclei (N. V). The spinal trigeminal nucleus is a continuation of the dorsal-most laminae of the spinal dorsal horn (Figure 45-5A) and is sometimes called the medullary dorsal horn. Its outer surface is covered by the spinal trigeminal tract, a direct continuation of Lissauer’s tract of the spinal cord (see Chapter 24), thus allowing some cervical sensory fibers to reach the trigeminal nuclei and some trigeminal sensory axons to reach the dorsal horn in upper cervical segments. This arrangement allows dorsal horn sensory neurons to have a range of inputs that are much broader than that of individual spinal or trigeminal segments, and ensures the integration of trigeminal and upper cervical sensory maps.
The spinal trigeminal nucleus receives sensory axons from the trigeminal ganglion (N. V) and from all cranial nerve sensory ganglia concerned with pain and temperature in the head, including geniculate ganglion (N. VII) neurons that relay information from the external auditory meatus, petrosal ganglion (N. IX) cells that convey information from the posterior part of the palate and tonsillar fossa, and nodose ganglion (N. X) axons that relay information from the posterior wall of the pharynx. The spinal trigeminal nucleus thus represents the entire oral cavity as well as the surface of the face.
The somatotopic organization of the afferent fibers is inverted: The forehead is represented ventrally and the oral region dorsally. Axons from the spinal trigeminal nucleus descend on the same side of the brain stem into the upper spinal cord, where they cross the midline in the anterior commissure with spinothalamic axons and join the opposite spinothalamic tract. (For this reason, upper cervical spinal cord injury may cause facial numbness.) The trigeminothalamic axons then ascend back through the brain stem, providing inputs to brain stem nuclei for reflex motor and autonomic responses in addition to carrying pain and temperature information to the thalamus.
The principal sensory trigeminal nucleus lies in the mid pons just lateral to the trigeminal motor nucleus. It receives the axons of neurons in the trigeminal ganglion concerned with position sense and fine touch discrimination, the same types of sensory information carried from the rest of the body by the dorsal columns. The axons from this nucleus are bundled with those from the dorsal column nuclei in the medial lemniscus, through which they ascend to the ventroposterior medial thalamus.
An additional component of the trigeminal sensory system, located at the midbrain level in the lateral surface of the periaqueductal gray matter, is the mesencephalic trigeminal nucleus, which relays mechanosensory information from the muscles of mastication and the periodontal ligaments. The large cells of this nucleus are not central neurons but primary sensory ganglion cells that derive from the neural crest and, unlike their relatives in the trigeminal ganglion, migrate into the brain during development. The central branches of the axons of these pseudo-unipolar cells contact motor neurons in the trigeminal motor nucleus, providing monosynaptic feedback to the jaw musculature, critical for the precise control of chewing movements.
Special Somatic Sensory Column
The special somatic sensory column has inputs from the acoustic and vestibular nerves and develops from the intermediate region of the alar plate. The cochlear nuclei (N. VIII), which lie at the lateral margin of the brain stem at the pontomedullary junction, receive auditory afferents from the spiral ganglion of the cochlea. The output of these nuclei is relayed through the pons to the superior olivary and trapezoid nuclei and bilaterally on to the inferior colliculus (see Chapter 31). The vestibular nuclei (N. VIII) are more complex. They include four distinct cell groups that relay information from the vestibular ganglion to various motor sites in the brain stem, cerebellum, and spinal cord concerned with maintaining balance and coordination of eye and head movements (see Chapter 40).
Visceral Sensory Column
The visceral sensory column is concerned with special visceral information (taste) and general visceral information from the facial (VII), glossopharyngeal (IX), and vagus nerves (X). It is derived from the most medial tier of neurons in the alar plate. All of the afferent axons terminate in the nucleus of the solitary tract. The solitary tract is analogous to the spinal trigeminal tract or Lissauer’s tract, bundling afferents from different cranial nerves as they course rostrocaudally along the length of the nucleus. As a result, visceral sensory information from different afferent nerves produces a unified visceral sensory map of the body in the nucleus.
Special visceral afferents carrying taste information from the anterior two-thirds of the tongue reach the nucleus of the solitary tract through the chorda tympani branch of the facial nerve, whereas those from the posterior parts of the tongue and oral cavity arrive through the glossopharyngeal and vagus nerves. These afferents terminate in roughly somatotopic fashion in the anterior third of the nucleus of the solitary tract. General visceral afferents are relayed through the glossopharyngeal and vagus nerves. Those from the rest of the gastrointestinal tract (down to the transverse colon) terminate in the middle portion of the solitary nucleus in topographic order, whereas those from the cardiovascular and respiratory systems terminate in the caudal and lateral portions.
The solitary nucleus projects directly to parasympathetic and sympathetic preganglionic motor neurons in the medulla and spinal cord that mediate various autonomic reflexes, as well as to parts of the reticular formation that coordinate autonomic and respiratory responses. Most ascending projections from the viscera to the forebrain are relayed through the parabrachial nucleus in the pons, although some reach the forebrain directly from the solitary nucleus. Together the solitary and parabrachial nuclei supply visceral sensory information to the hypothalamus, basal forebrain, amygdala, thalamus, and cerebral cortex.
General Visceral Motor Column
All motor neurons initially develop adjacent to the floor plate, a longitudinal strip of non-neuronal cells at the ventral midline of the neural tube (see Chapter 52). Neurons fated to become the three types of brain stem motor neurons migrate dorsolaterally, settling in three distinct rostrocaudal columns. The neurons that form the general visceral motor column migrate to the most lateral region of the basal plate, just medial to the sulcus limitans.
The Edinger-Westphal nucleus (N. III) lies next to the oculomotor complex just below the floor of the cerebral aqueduct. It contains preganglionic neurons that control pupillary constriction and lens accommodation through the ciliary ganglion.
The superior salivatory nucleus (N. VII) lies just dorsal to the facial motor nucleus and comprises parasympathetic preganglionic neurons that innervate the sublingual and submandibular salivary glands and the lacrimal glands and intracranial circulation through the sphenopalatine and submandibular parasympathetic ganglia.
Parasympathetic preganglionic neurons associated with the gastrointestinal tract form a column at the level of the medulla just dorsal to the hypoglossal nucleus and ventral to the nucleus of the solitary tract. At the most rostral end of this column is the inferior salivatory nucleus (N. IX) comprising the preganglionic neurons that innervate the parotid gland through the otic ganglion. The rest of this column constitutes the dorsal motor vagal nucleus (N. X). Most of the preganglionic neurons in this nucleus innervate the gastrointestinal tract below the diaphragm; a few are cardiomotor neurons.
The nucleus ambiguus (N. X) is a cluster of neurons that runs the rostrocaudal length of the ventrolateral medulla and contains parasympathetic preganglionic neurons that innervate thoracic organs, including the esophagus, heart, and respiratory system, as well as special visceral motor neurons that innervate the striated muscle of the larynx and pharynx, and neurons that generate respiratory motor patterns (see below). The parasympathetic preganglionic neurons are organized in topographic fashion, with the esophagus represented most rostrally and dorsally.
Special Visceral Motor Column
The special visceral motor column includes motor nuclei that innervate muscles derived from the branchial (pharyngeal) arches. Because these arches are homologous to the gills in fish, the muscles are considered special visceral muscles, even though they are striated in mammals. During development these cell groups migrate to an intermediate position in the basal plate and are eventually located ventrolaterally in the tegmentum. The trigeminal motor nucleus (N. V) lies at midpontine levels and innervates the muscles of mastication. Associated with it are the accessory trigeminal nuclei that innervate the tensor tympani, tensor veli palatini, and mylohyoid muscles, and the anterior belly of the digastric muscle.
The facial motor nucleus (N. VII) lies caudal to the trigeminal motor nucleus at the level of the caudal pons and innervates the muscles of facial expression. During development facial motor neurons migrate medially and rostrally around the medial margin of the abducens nucleus before turning laterally, ventrally, and caudally toward their definitive location at the pontomedullary junction (Figure 45–6A). This sinuous course of the axons forms the internal genu of the facial nerve. The adjacent accessory facial motor nuclei innervate the stylohyoid and stapedius muscles and the posterior belly of the digastric muscle.
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