Chapter 6: Somatic Sensation: Trigeminal and Viscerosensory Systems

In neuroanatomy, the study of sensation and motor control of cranial structures has traditionally been separate from that of the limbs and trunk. This is because cranial nerves innervate the head, and spinal nerves innervate the limbs and trunk. We can see similarities, however, in the functional organization of the cranial and spinal nerves and of the parts of the central nervous system with which they directly connect. For example, sensory axons in cranial nerves synapse in sensory cranial nerve nuclei in the brain stem. Similarly, sensory axons in spinal nerves synapse on neurons of the dorsal horn of the spinal cord and the dorsal column nuclei. The motor cranial nerve nuclei in the brain stem, like the motor nuclei of the ventral horn, contain the motor neurons whose axons project to the periphery.

This chapter examines the trigeminal system, which mediates somatic sensations—mechanosensations and the protective senses, pain, temperature, and itch—from the face and head. This system is analogous to the dorsal column–medial lemniscal and anterolateral systems of the spinal cord (see Chapters 4 and 5). The chapter also considers the brain stem neural system that processes sensory information from the body's internal organs. Both the peripheral territories innervated and the central nervous system processing centers of the viscerosensory system are closely aligned with those of the trigeminal system. Since the cranial nerves innervate the face and head, we will begin with an overview of the general organization of the cranial nerves and a characteristic feature of the cranial nerve nuclei, their columnar organization. Knowledge of the columnar organization helps explain the functional organization of the cranial nerves and nuclei because the location of the column provides important information about function. Knowledge of the cranial nerves is an essential part of the neurological exam.

Among the 12 pairs of cranial nerves (Figure 6–2; Table 6–1), the first two—olfactory (I) and optic (II)—are purely sensory. The olfactory nerve, which mediates the sense of smell, directly enters the cerebral hemisphere, and the optic nerve, for vision, enters the thalamus. The other 10 cranial nerves enter and leave the brain stem. The oculomotor (III) and trochlear (IV) nerves, which are motor nerves, exit from the midbrain. They innervate muscles that move the eyes. The trochlear nerve is further distinguished as the only cranial nerve found on the dorsal brain stem surface.

The pons contains four cranial nerves. The trigeminal (V) nerve is located at the middle of the pons. It is a mixed nerve; it has both sensory and motor functions, and it consists of separate sensory and motor roots. This separation is reminiscent of the segregation of function in the dorsal and ventral spinal roots. The sensory root provides the somatic sensory innervation of the facial skin and mucous membranes of parts of the oral and nasal cavities and the teeth. The motor root contains axons that innervate jaw muscles.

The remaining pontine nerves are found at the pontomedullary junction. The abducens (VI) nerve is a motor nerve that, like the oculomotor and trochlear nerves, innervates eye muscles. The facial (VII) nerve is a mixed nerve and has separate motor and sensory roots. The motor root innervates the facial muscles that determine our expressions, whereas the sensory root primarily innervates taste buds and mediates taste. The facial sensory root is sometimes called the intermediate nerve. (The intermediate nerve also contains axons that innervate various cranial autonomic ganglia [Chapter 11].) The vestibulocochlear (VIII) nerve is a sensory nerve and has two separate components. The vestibular component innervates the semicircular canals, saccule, and utricle and mediates balance, whereas the cochlear component innervates the organ of Corti and serves hearing.

The medulla has four cranial nerves, each of which contains numerous roots that leave from different rostrocaudal locations. Although the glossopharyngeal (IX) nerve is a mixed nerve, its major functions are to provide the sensory innervation of the pharynx and to innervate taste buds of the posterior one third of the tongue. The motor function of the glossopharyngeal nerve is to innervate a single pharyngeal muscle and peripheral autonomic ganglion (see Table 6–1). The vagus (X) nerve, a mixed nerve, has myriad sensory and motor functions that include somatic and visceral sensation, innervation of pharyngeal muscles, and much of the visceral autonomic innervation. The spinal accessory (XI) and hypoglossal (XII) nerves subserve motor function, innervating neck and tongue muscles, respectively (see Table 6–1).

Important Differences Exist Between the Sensory and Motor Innervation of Cranial Structures and Those of the Limbs and Trunk

The peripheral organization of sensory (afferent) fibers in cranial nerves is similar to that of spinal nerves. The organization of the primary sensory neurons that innervate the skin and mucous membranes of the head—mediating the somatic senses—is virtually identical to that of the sensory innervation of the limbs and trunk (Figure 6–3). In both cases, the distal portion of the axon of pseudounipolar primary sensory neurons is sensitive to stimulus energy, and the cell bodies of these primary sensory neurons are located in peripheral ganglia. The proximal portion of the axon projects into the central nervous system to synapse on neurons in the medulla and pons. The peripheral sensory ganglia, which contain the cell bodies of the primary sensory neurons of the different cranial nerves, are listed in Table 6–1.

Despite these similarities, three important differences are evident in the anatomical organization of primary sensory neurons in spinal and cranial nerves:

1. For the senses of taste, vision, hearing, and balance, a separate receptor cell transduces stimulus energy (Figure 6–3). The receptor activates synaptically the primary sensory neuron, which transmits information—encoded in the form of action potentials—to the central nervous system. For the spinal and trigeminal somatic sensations, the distal ending of the primary sensory neuron is the sensory receptor for all but one receptor (see Chapter 4; Merkel's receptor). Thus, the primary sensory neuron mediates both stimulus transduction and information transmission.

2. Primary sensory neurons in cranial nerves have either a pseudounipolar or a bipolar morphology (Figure 6–3). (As is discussed in Chapter 7, a retinal projection neuron is analogous to the primary sensory neurons because it transmits sensory information to the thalamus.)

3. Stretch receptors in jaw muscles, which signal jaw muscle length and thus mediate jaw proprioception (or temporal-mandibular joint angle detection), are pseudounipolar primary sensory neurons, but their cell bodies are located within the central nervous system, not in peripheral ganglia. Most primary sensory neurons derive from the neural crest cells, a group of cells that emerge from the dorsal region of the neural tube. Most neural crest cells migrate peripherally and give rise to the neurons whose cell bodies lie outside of the central nervous system. These neurons include most of the primary sensory neurons that innervate body tissues and the peripheral components of the autonomic nervous system (see Chapters 4 and 15). The primary sensory neurons that mediate jaw proprioception derive from a special group of neural crest cells that do not migrate from the central nervous system to the periphery.

The structures innervated by the motor fibers of cranial nerves, similar to motor fibers in spinal nerves, include striated muscle and autonomic postganglionic neurons. In contrast to striated muscle of the limbs and trunk, which develop from body somites, cranial striated muscle develops from either the cranial somites or the branchial arches. The branchial arches correspond to gills that are present early in human development, representing the evolutionary derivatives of aquatic vertebrates. The extraocular and tongue muscles originate from somites, whereas jaw, facial, laryngeal, palatal, and certain neck muscles are of branchiomeric origin.

There Are Seven Functional Categories of Cranial Nerves

Seven functional categories of cranial nerve enter and exit the brain stem. (See Box 6–1.) Four of these categories are similar to those of the spinal nerves:

• 1. Somatic sensory fibers in cranial nerves subserve touch, pain, itch, and temperature senses, as well as jaw and limb proprioception.

• 2. Viscerosensory fibers mediate visceral sensations and chemoreception from body organs and help regulate blood pressure and other bodily functions.

• 3. Somatic skeletal motor fibers are the axons of motor neurons that innervate striated muscle that develops from the somites.

• 4. Visceral (autonomic) motor fibers are the axons of autonomic preganglionic neurons.

Because cranial nerves innervate structures that are more complex than those innervated by spinal nerves—the highly specialized sensory organs of the eye, ear, and tongue, as well as the muscles that develop from the branchial arches—there are three additional categories of cranial nerves:

• 5. Axons that innervate the eye subserve vision, and those that innervate the inner ear mediate hearing and balance.

• 6. Fibers that innervate taste buds and the olfactory mucosa mediate taste and smell, respectively.

• 7. Branchiomeric skeletal motor fibers are the axons of motor neurons that innervate striated muscle that develops from the branchial arches.

Cranial Nerve Nuclei Are Organized into Distinctive Columns

As we learned in Chapter 11, the spinal cord has a segmental organization that emerges early in development. Each spinal segment provides the sensory and motor innervation to a corresponding body segment, or somite (see Figure 4-5). The developing caudal brain stem, the pons and medulla, also is segmented. Segmentation may be a mechanism for establishing a basic plan of organization, or "building block," for the various parts of the spinal cord and brain stem. This segmental plan is maintained into maturity for the spinal cord. In the mature brain stem, however, segmentation is obscured by later elaboration of neural interconnections. The developing pons and medulla have eight segments, termed rhombomeres (Figure 6–4A), that provide the sensory and motor innervation for most of the head through the cranial nerve peripheral projections. The midbrain and region of midbrain-pons junction may also have an early rhombomeric segmental organization. In contrast to the spinal cord, where each segment contains a pair of dorsal and ventral roots, each rhombomere is not associated with a single pair of sensory and motor cranial nerve roots.

The cranial nerve sensory and motor nuclei are analogous to the dorsal and ventral horns, respectively. Cranial nerve sensory nuclei contain neurons that receive sensory information directly from cranial structures via cranial sensory nerves. Cranial nerve motor nuclei contain the cell bodies of motor neurons, whose axons course through cranial motor nerves to innervate their peripheral targets. Whereas this organization is similar to that of the spinal sensory and motor regions, three important differences exist between the developmental plans of the spinal cord and the brain stem.

First, the sensory and motor nuclear columns in the medulla and pons are aligned roughly from the lateral surface to the midline rather than being oriented in the dorsoventral axis, as in the spinal cord. This is because during development the cavity in the neural tube of the hindbrain expands dorsally ("opens up") to form the fourth ventricle (Figure 6–4B). Compare the dorsoventral organization in Figure 6–4B1, which is before the neural tube expands and therefore is organized like that of the spinal cord, with Figure 6–4B3, where the nuclei are obliquely lateral-to-medial.

Second, in brain stem development, immature neurons migrate more extensively from the ventricular floor to distant sites than in the spinal cord. Cranial nerve nuclei have relatively simple roles in processing afferent information or transmitting motor control signals. However, most other brain stem nuclei have more complex integrative functions. Whereas brain stem integrative nuclei also derive from developing neurons of the sensory and motor plates in the ventricular floor, the immature neurons that give rise to these structures migrate to their destinations in more dorsal or ventral regions (Figure 6–4B3). Most neurons migrate radially (ie, at right angle to the neuraxis) along local paths established by special astrocytes, which are a class of glial cells.

Third, as a consequence of the greater diversity of cranial sensory and motor structures, there is further differentiation of the cranial nerve nuclei. Because there are seven functional categories of cranial nerves, there are also seven categories of cranial nerve nuclei. Nuclei of each of these categories form discontinuous columns that extend rostrocaudally through the brain stem (Figures 6–5A). The seven functional categories are distributed through only six discrete columns, however, because two of the sensory categories synapse on neurons in a single column but at separate rostrocaudal locations. The sensory columns are lateral to the motor columns (Figure 6–5A, B). The somatic sensory, hearing, and balance columns tend to be lateral to the viscerosensory and taste columns. The somatic skeletal motor column is medial to the autonomic motor column. The branchiomeric motor column contains neurons that are located in the region of the reticular formation. The sulcus limitans, a shallow grove that separates the sensory and motor columns during development, remains as a landmark on the floor of the fourth ventricle in the adult brain. We will further examine the distinct locations of motor neurons innervating muscles of somatic or branchiomeric origins in Chapter 11.

Because cranial nerve nuclei that serve similar functions are aligned in the same rostrocaudal columns, knowledge of the locations of these columns aids in understanding their functions. Figure 6–6 shows the longitudinal organization of the cell columns forming the cranial nerve nuclei in the mature brain stem.

Somatic sensation of the head, including the oral cavity, is carried by four cranial nerves. The trigeminal nerve innervates most of the head and oral cavity and is the most important of the four nerves. The facial, glossopharyngeal, and vagus nerves innervate small areas of the skin around the external ear and the mucous membranes and organs of the body. The facial, glossopharyngeal, and vagus nerves also contain sensory fibers that mediate taste (see Chapter 9).

The sensory fibers that innervate surface skin and oral mucosa project into central trigeminal nuclei, whereas the sensory fibers that innervate the mucous membranes of the pharynx and larynx and other internal (visceral) structures project to the caudal portion of the solitary nucleus (Figure 6–7). An important exception exists: A small number of sensory fibers that innervate the pharyngeal and laryngeal mucosa project information to the trigeminal nuclei. Sensory information transmitted to the trigeminal nuclei is thought to contribute to our conscious awareness of cranial sensations. By contrast, information transmitted to the caudal solitary nucleus may not necessarily reach consciousness. Although we are aware of visceral pain, we become conscious of other visceral stimuli only under special circumstances, such as when we become nauseated after eating a certain food or when we feel full after eating a large meal. Some internal stimuli are never perceived. For example, a change in intra-arterial pressure, even a hypertensive episode, can occur unnoticed.

Three trigeminal sensory nuclei serve cranial somatic sensations (eg, from the skin and jaw muscles) from the nerves just described (Figure 6–7). These sensory fibers terminate in two of the trigeminal sensory nuclei, the main (or principal) trigeminal sensory nucleus and the spinal trigeminal nucleus. The third sensory nucleus, the mesencephalic trigeminal nucleus, is not a site of termination of primary sensory fibers. Rather, it is equivalent to a peripheral sensory ganglion because it contains the cell bodies of certain trigeminal primary sensory fibers (see below).

The sensory axons of the trigeminal nerve enter the ventral pons (Figure 6–2). Sensory axons from the facial, glossopharyngeal, and vagus nerves enter the brain stem more caudally. Just as in spinal nerves, functional differences distinguish individual sensory axons in these nerves. Large-diameter fibers, which mediate mechanical sensations, terminate mostly in the dorsal pons, in the main trigeminal sensory nucleus. Small-diameter fibers—which mediate pain, temperature sensations, and itch—mostly travel in the spinal trigeminal tract and terminate in the spinal trigeminal nucleus. (Some large-diameter mechanoreceptive fibers in the spinal trigeminal tract and nucleus play a role in cranial reflexes; see below.) These differences set the stage for two anatomically and functionally distinct trigeminal ascending sensory systems (Figure 6–8A, B). One system is primarily for cranial touch and dental mechanical senses and is analogous to the dorsal column–medial lemniscal system. The other system is for cranial pain, temperature senses, and itch, and is analogous to the anterolateral system.

The main trigeminal sensory nucleus mediates facial mechanical sensations

Most neurons in the main trigeminal sensory nucleus receive mechanoreceptive information. Projection neurons in this nucleus give rise to axons that decussate in the pons and ascend dorsomedially to fibers from the dorsal column nuclei in the medial lemniscus. The ascending second-order trigeminal fibers—collectively termed the trigeminal lemniscus—synapse in the thalamus, in its ventral posterior medial nucleus (Figure 6–8A). (Recall that the ventral posterior lateral nucleus is the thalamic spinal somatic sensory relay nucleus.) From here, the axons of thalamic neurons project, via the posterior limb of the internal capsule, to the lateral part of the primary somatic sensory cortex, in the postcentral gyrus. The secondary somatic sensory cortex and the posterior parietal cortex also process cranial mechanosensory information (see Chapter 11). These higher-order somatic sensory areas receive their major input from the primary somatic sensory cortex. Because of similarities in connections, the main trigeminal sensory nucleus is anatomically and functionally similar to the dorsal column nuclei (which comprise the gracile and cuneate nuclei).

A much smaller pathway originates from the dorsal portion of the main trigeminal sensory nucleus (Figure 6–7A), sometimes termed the dorsal trigeminothalamic tract. This pathway ascends ipsilaterally to the ventral posterior medial nucleus and processes mechanical stimuli from the teeth and soft tissues of the oral cavity.

The pathway for jaw proprioception, the conscious awareness of how wide we open our mouth, begins with stretch receptors that encode jaw angle (see Figure 6–14). The cell bodies for these mechanoreceptors are in the trigeminal mesencephalic nucleus, and the receptors project to the main trigeminal sensory nucleus and to more rostral portions of the spinal trigeminal nucleus. The projection to the spinal nucleus is analogous to the projection of limb muscle receptor to the deep layers of the dorsal horn. The trigeminal brain stem neurons project to the ventral posterior medial nucleus and then to area 3a of the primary somatic sensory cortex. This is the pathway for conscious awareness of temporal-mandibular joint angle. Jaw proprioceptive information also is transmitted to the cerebellum for jaw muscle control (see Chapter 13).

The spinal trigeminal nucleus mediates cranial pain sensation

The spinal trigeminal nucleus has a rostrocaudal anatomical and functional organization with three components (Figures 6–7 and 6–8B): the oral nucleus, the interpolar nucleus, and the caudal nucleus. The functions of the spinal trigeminal nucleus are similar to those of the dorsal horn of the spinal cord, with which it is continuous. Similar to the limb and trunk functions of the dorsal horn, the spinal trigeminal nucleus plays an essential role in facial and dental pain, temperature sensation, and itch and a much lesser role in facial mechanical sensations. In addition, the interpolar and oral nuclei participate in trigeminal reflexes and in transmitting sensory information to jaw motor control structures, such as the cerebellum.

The major ascending trigeminal pathway from the spinal trigeminal nucleus terminates in the contralateral thalamus (Figure 6–8B). The organization of this path, termed the trigeminothalamic tract, is similar to that of the spinothalamic tract, and it also ascends along with fibers of the anterolateral system. Trigeminothalamic axons terminate in three principal locations in the thalamus: the ventral posterior medial nucleus, the ventromedial posterior nucleus, and the medial dorsal nucleus. As discussed in Chapter 5, these thalamic sites have different cortical projections and mediate different aspects of pain and temperature senses. The ventral posterior medial nucleus projects to the primary somatic sensory cortex in the lateral part of the postcentral gyrus, and the ventromedial posterior nucleus projects to the insular cortex. These projections are important in perception of pain, temperature, and itch. The medial dorsal nucleus projects to the anterior cingulate gyrus. Both the insular cortex and the anterior cingulate gyrus are thought to participate in the affective and motivational aspects of facial pain, itch, and temperature senses. Like the spinal pain systems, the ascending trigeminal pain system also engages the parabrachial nucleus, which contributes to the affective aspects of pain through projections to the amygdala and hypothalamus.

The central branches of glossopharyngeal and vagal axons innervate: the pharynx, the larynx, the esophagus, other portions of thoracic and abdominal viscera, and peripheral blood pressure receptive organs. After entering the brain stem, the axons collect into the solitary tract of the dorsal medulla and terminate in the surrounding caudal solitary nucleus. The solitary nucleus is divided into two functionally distinct parts (Figure 6–7): a rostral portion for taste (considered in Chapter 9) and a caudal portion that serves viscerosensory functions. The caudal solitary nucleus projects information to various brain structures for a diversity of functions. For conscious awareness of viscerosensory information, such as a sense of fullness or nausea, there is an ascending projection (Figure 6–9), via the parabrachial nucleus of the pons, to a portion of the ventral posterior nucleus of the thalamus. The viscerosensory thalamic neurons, which are distinct from the ones that process mechanical information and those that process taste (see Chapter 11), project to the insular cortex. Other projections of the caudal solitary and parabrachial nuclei participate in a variety of visceral reflex and autonomic functions, such as regulation of blood pressure or gastrointestinal motility.

Separate Sensory Roots Innervate Different Parts of the Face and Mucous Membranes of the Head

The trigeminal nerve consists of three sensory roots that innervate the skin and mucous membranes of separate regions of the head: the ophthalmic division, the maxillary division, and the mandibular division (Figure 6–10A). The maxillary and mandibular divisions also innervate the oral cavity. As in the spinal somatic sensory systems, the cell bodies of the trigeminal sensory fibers that mediate cranial touch, pain, temperature, and itch are found in the trigeminal or semilunar ganglion, a peripheral sensory ganglion (see Table 6–1). By contrast, the cell bodies of stretch receptors found in jaw muscles are located in the central nervous system, in the mesencephalic trigeminal nucleus (Figure 6–7). The trigeminal nerve also innervates stretch receptors in the extraocular muscles, but the cell bodies of these fibers are located in the semilunar ganglion, and their axons course within the ophthalmic division of the trigeminal nerve.

Unlike dorsal roots of adjacent spinal cord segments, where the dermatomes overlap extensively, the trigeminal dermatomes (ie, the area of skin innervated by a single trigeminal sensory nerve division) overlap very little. Thus, a peripheral anesthetic region is more likely to occur after damage to one trigeminal division than after damage to a single dorsal root. Trigeminal neuralgia is an extraordinarily painful neurological condition, often described as a fiery pain that radiates near the border of the ophthalmic and maxillary roots or at the border of the maxillary and mandibular roots.

In addition to the trigeminal nerve, the intermediate (a branch of the facial nerve), glossopharyngeal, and vagus nerves innervate portions of the skin of the head. The external ear is innervated by the intermediate and glossopharyngeal nerves, and the external auditory meatus is innervated by the intermediate and vagus nerves (Figure 6–10A). Both the trigeminal and vagus nerves innervate the dura. The cell bodies of the sensory fibers in the facial nerve are located in the geniculate ganglion, and those of the glossopharyngeal and vagus nerves are located in the superior ganglion of each nerve (see Table 6–1 for nomenclature).

Although the glossopharyngeal and vagus nerves innervate small patches of surface skin, they have a more extensive innervation of the mucous membranes and body organs. The glossopharyngeal nerve innervates the posterior one third of the tongue, the pharynx, portions of the nasal cavity and sinuses, and the eustachian tube. The vagus nerve innervates the hypopharynx, the larynx, the esophagus, and the thoracic and abdominal viscera. The innervation of the pharynx and larynx by the glossopharyngeal and vagus nerves is essential for normal swallowing and for keeping the airway clear of saliva and other liquids during swallowing (see Chapter 11). Branches of the glossopharyngeal and vagus nerves also innervate arterial blood pressure receptors in the carotid sinus and aortic arch, respectively. These branches are part of the baroreceptor reflex, for blood pressure regulation. For example, they mediate the pressor response to standing. The vagus nerve alone also innervates respiratory structures and the portion of the gut rostral to the splenic flexure. Pelvic visceral organs are innervated by primary sensory fibers that project to the sacral spinal cord. The spinal pathway for pelvic visceral sensation is not well understood but is thought to parallel the organization of the spinal somatic sensory pathways. The pathway for pelvic pain was described in Chapter 5.

After entering the pons, the fibers of each nerve division travel into discrete portions of the spinal trigeminal and solitary tracts en route to the trigeminal and solitary nuclei, where they terminate. The spinal trigeminal tract (Figure 6–10B) is organized like an inverted face: the roots of the intermediate, glossopharyngeal, and vagus nerves as well as the mandibular division of the trigeminal nerve are located dorsal; the ophthalmic division of the trigeminal nerve is located ventral; and the maxillary division of the trigeminal nerve is in between. Axons in the spinal trigeminal tract, in turn, synapse on neurons in the spinal trigeminal nucleus (Figures 6–11A, B and 6–12). The caudal solitary nucleus and tract are located in the caudal medulla (Figure 6–11A); its location is inferred from staining methods that reveal neuronal cell bodies.

The Key Components of the Trigeminal System Are Present at All Levels of the Brain Stem

The three trigeminal nuclei have distinct sensory functions. The spinal trigeminal nucleus is primarily important in facial pain, temperature senses, and itch. Despite its name, it is located principally in the medulla and the caudal pons. The main trigeminal sensory nucleus mediates facial touch sense and oral mechanosensation and is located in the pons. The mesencephalic trigeminal nucleus contains the cell bodies of stretch receptors that signal jaw muscle length, which is the key sensory signal for jaw proprioception. Despite its name, it is located both in the rostral pons and in the midbrain.

The spinal trigeminal nucleus is the rostral extension of the spinal cord dorsal horn

The dorsal horn extends rostrally into the medulla as the spinal trigeminal nucleus. Three nuclear subdivisions comprise the spinal trigeminal nucleus, from caudal to rostral: the caudal nucleus, the interpolar nucleus, and the oral nucleus. The caudal nucleus and the dorsal horn of the spinal cord are similar structurally and functionally. The laminar terminations of afferent fibers and the origins of trigeminothalamic, trigeminoreticular, and trigeminomesencephalic neurons are like those of the dorsal horn (see Figure 5–3). In fact, the caudal nucleus is sometimes called the medullary dorsal horn because it is so similar to that of the spinal cord dorsal horn. As the nuclear components of the dorsal horn have counterparts in the trigeminal system, so too does the spinal sensory tract. Lissauer's tract extends into the medulla as the spinal trigeminal tract. The spinal trigeminal tract is lightly stained (Figure 6–11) because it contains thinly myelinated and unmyelinated axons; this is similar to Lissauer's tract (see Figure 5-4).

The spinal trigeminal nucleus and tract have a rostrocaudal organization

Important insights into the functions of the spinal trigeminal nucleus and tract have been gained from a neurosurgical procedure to relieve intractable facial pain. This operation transects the spinal trigeminal tract and produces selective disruption of such pain and temperature senses with little effect on touch. (Spinal trigeminal tractotomy is rarely done today, however, because analgesic drugs have proved a more effective and consistent therapy.) If the tract is transected rostrally, near the border between the caudal and interpolar nuclei, facial pain and temperature senses over the entire face are disrupted. Transection of the tract between its rostral and caudal borders spares pain and temperature senses over the perioral region and nose.

This clinical finding shows that the spinal trigeminal tract has a rostrocaudal somatotopic organization in addition to a mediolateral organization (Figure 6–10B). Trigeminal fibers that innervate the portion of the head adjacent to the cervical spinal cord representation (Figure 6–10A) project more caudally in the spinal trigeminal tract and terminate in more caudal regions of the caudal nucleus than those that innervate the oral cavity, perioral face, and nose.

The spinal trigeminal nucleus also has a rostrocaudal organization. Proceeding rostrally from the cervical spinal cord, neurons of the spinal dorsal horn process somatic sensory information (predominantly pain, itch, and temperature senses) from the arm, neck, and occiput (ie, region d in Figure 6–10B). Neurons of the trigeminal caudal nucleus, located near the spinal cord–medulla border, process somatic sensory information from the posterior face and ear (ie, regions b and c in Figure 6–10B). These neurons receive input not only from the mandibular trigeminal division but also from the intermediate, glossopharyngeal, and vagus nerves. Farther rostrally, the neurons process information from the perioral region and nose (ie, region a in Figure 6–10B). Finally, neurons in the most rostral part of the trigeminal caudal nucleus, as well as farther rostrally in the trigeminal spinal nucleus (not shown in Figure 6–10B), process pain and temperature information from the oral cavity, particularly from the teeth. This organization is termed "onion skin" because of the concentric ring configuration of the peripheral fields processed at a given level by the medullary dorsal horn.

The caudal nucleus extends from approximately the first or second cervical segment of the spinal cord to the medullary level at which point the central canal "opens" to form the fourth ventricle (Figures 6–7 and 6–11). The interpolar nucleus extends from the rostral boundary of the caudal nucleus to the rostral medulla (Figures 6–7 and 6–12). Finally, the oral nucleus extends from the rostral boundary of the interpolar nucleus to the level at which the trigeminal nerve enters the pons (see Figures 6–7 and AII–9).

The posterior inferior cerebellar artery (PICA) provides the arterial supply to the dorsolateral portion of the medulla (Figure 6–12; see Chapter 3). The PICA is an end-artery with little collateral flow from other vessels into the territory it serves. As a consequence, the dorsolateral region of the medulla becomes infarcted when the artery is occluded (Figure 6–12). The medial region of the medulla is spared with such an occlusion because of the collateral blood supply to this area from the contralateral vertebral artery and the anterior spinal artery. Occlusion of the PICA produces a complex set of sensory and motor deficits, termed the lateral medullary, or Wallenberg, syndrome. This syndrome produces a distinctive pattern of somatic sensory signs, which are examined in the case at the beginning of the chapter.

The main trigeminal sensory nucleus is the trigeminal equivalent of the dorsal column nuclei

Rostral to the spinal trigeminal nucleus is the main trigeminal sensory nucleus (Figure 6–13C). This part of the trigeminal nuclear complex mediates touch sensation of the face and head and mechanosensation from the teeth. Most of the neurons in this nucleus give rise to axons that decussate and ascend to the ventral posterior medial nucleus of the thalamus. Their axons are located in the trigeminal lemniscus, dorsomedial to axons of the medial lemniscus. This is another example of segregation of functions. The main trigeminal sensory nucleus is the trigeminal equivalent of the dorsal column nuclei because both nuclei project to the contralateral ventral posterior nucleus (but separately to the medial and lateral subdivisions) and both structures subserve touch sensation (but from different body regions). The trigeminal lemniscus also contains a small number of axons from neurons in the spinal trigeminal nucleus.

A portion of the main trigeminal sensory nucleus receives mechanoreceptive signals from the soft tissues of the oral cavity and the teeth and gives rise to an ipsilateral pathway that terminates in the ventral posterior medial nucleus of the thalamus. Apart from transmitting mechanical information from the oral cavity, the particular function of this ipsilateral path, in relation to the contralateral trigeminal lemniscus, is not known.

The mesencephalic trigeminal nucleus and tract contain the cell bodies and axons of jaw muscle stretch receptors

The mesencephalic trigeminal nucleus, located in the lateral portion of the periventricular and periaqueductal gray matter (Figure 6–13A, B), contains the cell bodies of muscle spindle sensory receptors that innervate jaw muscles. Therefore, this nucleus is equivalent to a peripheral sensory ganglion. The peripheral branch of the primary sensory neuron (Figure 6–3), carrying sensory information to the central nervous system, ascends to the mesencephalic trigeminal nucleus in the mesencephalic trigeminal tract (note its myelinated axons lateral to the nucleus in Figure 6–13A). The central branch also projects through the mesencephalic trigeminal tract, to terminate in various brain stem sites important for jaw muscle control and jaw proprioception. For example, a monosynaptic projection to the trigeminal motor nucleus mediates the jaw-jerk (or closure) reflex (Figure 6–14), which is analogous to the knee-jerk reflex. Jaw muscle afferents terminate in the main trigeminal and rostral spinal trigeminal nuclei (Figures 6–13A and 6–14). Together these regions play a role in jaw proprioception. In the midbrain, the medial lemniscus and trigeminal lemniscus have migrated laterally (Figure 6–13A). The trigeminal lemniscus terminates in the medial division of the ventral posterior nucleus.

The Caudal Solitary and Parabrachial Nuclei Are Key Brain Stem Viscerosensory Integrative Centers

The caudal solitary nucleus (Figures 6–7 and 6–11A) receives input from visceral receptors—chemoreceptors (such as receptors sensitive to blood carbon dioxide), mechanoreceptors (such as mechanoreceptors beneath the mucous membrane of the larynx and arterial pressure receptors), vascular pressure receptors (such as baroreceptors in the carotid body), and nociceptors. Neurons in this nucleus have diverse ascending projections. There are also local projections in the medulla and pons that play important roles in controlling blood pressure and respiration rate and in regulating gastrointestinal motility and secretions. Sensory information processed here, especially mechanical and noxious stimuli from the larynx and pharynx, is important for initiating protective reflexes, such as the laryngeal closure reflex to prevent fluid aspiration into the lungs. The caudal solitary nucleus has descending projections to the spinal cord for directly controlling portions of the autonomic nervous system.

The ascending projections of the caudal solitary nucleus are focused on the parabrachial nucleus (Figure 6–13B; see Figure AII–12), which, in turn, has diverse forebrain projections. One ascending projection of the parabrachial nucleus is to the parvocellular (small celled) division of the ventral posterior medial nucleus of the thalamus, which is the thalamic viscerosensory relay (see next section). The parabrachial nucleus also transmits viscerosensory information rostrally to the hypothalamus and the amygdala (see Figure 6–9), two brain structures thought to participate in a variety of autonomic and endocrine functions, for example, feeding and reproductive behaviors (see Chapter 15). As discussed in Chapter 5, the parabrachial nucleus is also involved in transmitting information about somatic pain to areas of cortex that are important in emotions. Viscerosensory control of bodily functions is considered further in Chapter 15, which covers the hypothalamus and autonomic nervous system.

Somatic and Visceral Sensation Are Processed by Separate Thalamic Nuclei

The ventral posterior medial division of the ventral posterior nucleus, often simply termed ventral posterior medial nucleus (Figure 6–15A), processes mechanosensory information from the head. This information, in turn, is projected to the lateral postcentral gyrus, forming the face and head cortical sensory representations (Figure 6–16). The ventral posterior medial nucleus is the trigeminal complement to the spinal mechanosensory nucleus, the ventral posterior lateral nucleus. Similar to the overrepresentation of the hands (see Chapter 4), the representations of the tongue and perioral region in the primary somatic sensory cortex are larger than the cortical representations of other body parts because they are used more extensively during speech and chewing, for example. In many species of rodents and carnivores, the face representation is more extensive than that of the fingers or tongue and perioral regions, because the large whiskers of these species are their principal tactile discriminative and exploratory organs. The primary somatic sensory cortex, in turn, projects to higher-order somatic sensory areas in the parietal and insular cortex for the further elaboration of the sensory message (see Figure 4–12). The MRI (Figure 6–15B) is through a similar part of the thalamus as in part A, showing the approximate location of the ventral posterior nucleus.

Similar to the systems for spinal somatic sensory information processing, cranial pain, itch, and temperature are transmitted to the ventral posterior medial and ventromedial posterior nuclei. These thalamic nuclei project to the postcentral gyrus and the insular cortex, respectively. As discussed in Chapter 5, these cortical regions play a role in perception of pain, itch, and temperature senses. In addition, the insular representation may be important in memories of painful experiences and the somatic and autonomic behaviors that pain evokes. The medial dorsal nucleus (Figure 6–15) is the third thalamic area to receive information about pain, temperature, and itch. It receives trigeminal thalamic and spinothalamic inputs and projects to the anterior cingulate cortex, which is important in the emotional aspects of pain, itch, and temperature perception.

Viscerosensory information from the pharynx, larynx, esophagus, and other internal organs is processed by a thalamic region that is located posterior to the ventral posterior medial nucleus and this area projects to the cortex within the lateral sulcus, where the viscera are represented (Figure 6–16B). This nucleus, which is located within an ill-defined posterior thalamic region, may take on one of several names, including ventral posterior medial parvocellular nucleus (which also processes taste; Chapter 9) and the posterior nucleus.

The Cranial Nerves

Of the 12 cranial nerves (Table 6–1 and Figure 6–2), the first two, the olfactory (I) and optic (II) nerves, are sensory and enter the telencephalon and diencephalon directly. The 3rd through 12th cranial nerves enter and exit from the brain stem directly. Two cranial nerves, the oculomotor (III) and the trochlear (IV), are motor nerves located in the midbrain. The pons contains the trigeminal (V), a mixed nerve; the abducens (VI), a motor nerve; the facial (VII), a mixed nerve; and the vestibulocochlear (VIII), a sensory nerve. The medulla also contains four cranial nerves: the glossopharyngeal (IX) and vagus (X) are mixed nerves, whereas the spinal accessory (XI) and hypoglossal (XII) are motor nerves.

Cranial Nerve Nuclei Columns

Separate columns of cranial nerve nuclei course through the brain stem along its rostrocaudal axis (Figures 6–4, 6–5, 6–6, 6–7). Each column has a separate sensory (afferent) or motor function, and the nomenclature conforms to that for the cranial nerves: (1) skeletal somatic motor, (2) branchiomeric skeletal motor, (3) visceral (autonomic) motor, (4) viscerosensory and taste, (5) somatic sensory, and (6) hearing and balance (Table 6–1). Each column has its own mediolateral location (Figures 6–4 and 6–6). The sulcus limitans separates the sensory from the motor nuclear columns (Figure 6–4 and 6–5).

Trigeminal Sensory System

Somatic sensation of cranial structures is mediated predominantly by the trigeminal nerve, which has three sensory divisions (Figure 6–10A): ophthalmic, maxillary, and mandibular. The cell bodies of the primary sensory neurons that innervate the skin and mucous membranes of the head are located in the semilunar (or trigeminal) ganglion. Three other cranial nerves also innervate portions of the head: (1) the intermediate (VII) nerve (a branch of the facial nerve) innervates the skin of the ear; (2) the glossopharyngeal (IX) nerve innervates the posterior tongue and portions of the oral cavity, nasal cavity, pharynx, and middle ear (Figure 6–10A); and (3) the vagus (X) nerve innervates the skin of the ear and mucous membranes of the larynx. The cell bodies of the primary afferent fibers in the facial nerve are located in the geniculate ganglion, and those of the glossopharyngeal and vagus nerves are located in the superior ganglion of each nerve. Afferent fibers in these four cranial nerves enter the brain stem and ascend and descend in the spinal trigeminal tract (Figures 6–8 and 6–11). The fibers of the trigeminal nerve, whose cell bodies lie in the semilunar ganglion, terminate in two of the three major components of the trigeminal nuclear complex: the main (or principal) trigeminal sensory nucleus (Figures 6–8A and 6–13C) and the spinal trigeminal nucleus (Figures 6–7, 6–8B, 6–10, and 6–11). The spinal trigeminal nucleus has three subdivisions: the oral nucleus, the interpolar nucleus, and the caudal nucleus (Figure 6–7). The afferent fibers of the facial, glossopharyngeal, and vagus nerves terminate in the spinal trigeminal nucleus.

Mechanoreceptive afferent fibers of the trigeminal nerve terminate predominantly in the main trigeminal sensory nucleus. The majority of the ascending projection neurons of this nucleus send their axons to the contralateral ventral posterior medial nucleus of the thalamus (Figure 6–15). From here, thalamic neurons project, via the posterior limb of the internal capsule, to the face representation of the primary somatic sensory cortex, which is located in the lateral portion of the postcentral gyrus (Figure 6–16). Projections from the primary cortex engage the secondary somatic sensory cortex and the posterior parietal lobe. A small pathway ascends ipsilaterally to the thalamus and cortex, conveying mechanical information from the mouth, especially the teeth (Figure 6–8A).

Pain, temperature, and itch afferents from cranial structures enter and descend in the spinal trigeminal tract. The ascending pathway for pain and temperature sensations originates from the spinal trigeminal nucleus, primarily from the caudal and interpolar nuclei (Figures 6–7 and 6–10). The axons of most ascending projection neurons in these nuclei decussate. The ascending fibers course with the anterolateral system in the lateral medulla and pons en route to the rostral brain stem and thalamus. The thalamic nuclei in which the fibers terminate are the ventral posterior medial nucleus, the ventromedial posterior nucleus, and the medial dorsal nucleus (Figure 6–15).

Afferent fibers carrying proprioceptive information from the jaw muscles form the mesencephalic trigeminal tract (Figure 6–14). Their cell bodies are located in the mesencephalic trigeminal nucleus and are unique because they are the only primary sensory neurons with cell bodies located in the central nervous system (Figure 6–3). These afferents project to the main trigeminal nucleus and rostral parts of the spinal trigeminal nucleus, which give rise to a pathway to the ventral posterior medial nucleus and primary somatic sensory cortex.

Viscerosensory System

Viscerosensory receptors are innervated by the glossopharyngeal (IX) and vagus (X) nerves, which project into the caudal solitary nucleus (Figures 6–7 and 6–9). Axons ascend ipsilaterally from the solitary nucleus to the parabrachial nucleus in the rostral pons (Figure 6–13B). The third-order neurons project to the hypothalamus and limbic systems, structures for regulating behavior and autonomic responses. Other third-order neurons project to the medial part of the ventral posterior nucleus and then to the visceral representation in the insular cortex (Figures 6–9 and 6–16). Pelvic visceral organs are innervated by primary sensory fibers that project to the sacral spinal cord. Apart from the dorsal column (Chapter 5), the brain circuits for pelvic visceral sensation are not well understood.