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.
Somatotopic organization of the trigeminal system. A. Peripheral innervation territories of the three divisions of the trigeminal nerve and the intermediate and vagus nerves. B. The organization of the spinal trigeminal tract for the portion of the medulla that includes the caudal nucleus. The "onion skin" pattern of representation of trigeminal afferents in the caudal nucleus corresponds to a, b, and c; d corresponds to the rostral spinal cord representation. Regions marked a (located rostrally), b, and c (located caudally) correspond to the concentric zones on the face indicated in A. The intraoral representation is located rostral to region a in in the medulla (B, right); cervical representation is located caudally (ie, region d). (Adapted from Brodal A. Neurological Anatomy. 3rd ed. New York, NY: Oxford University Press, 1981.)
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 organization of the spinal cord–medulla junction and the caudal medulla. Myelin-stained transverse sections through the spinal cord–medulla junction (B) and the caudal nucleus—rostral to the pyramidal decussation (A). At both levels, the spinal trigeminal tract is located dorsolateral to the nucleus. The inset shows the approximate planes of section.
The arterial supply of the medulla. A. The vertebral-basilar arterial system on a portion of the ventral and lateral brain stem. Occlusion of the posterior inferior cerebellar artery (PICA) can lead to infarction of the circled territory. The spinal trigeminal tract (light blue) and nucleus (dark blue) are shown in relation to the infarcted region. PICA occlusion will lead to damage of both the nucleus locally, and the descending trigeminal axons from this level caudally. B. Myelin-stained section through the mid-medulla (plane shown in A). PICA occlusion (yellow) will: (1) interrupt descending trigeminal fibers and damage the trigeminal nucleus (causing ipsilateral loss of facial pain and temperature senses); and (2) interrupt ascending anterolateral system fibers (causing contralateral loss of pain and temperature senses on limbs and trunk). Other damage caused by PICA occlusion will be considered in later chapters.
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.
Myelin-stained sections through the pons at the level of the midbrain (A), the parabrachial nucleus (B), and the main sensory nucleus of the pons (C).
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.
Jaw proprioception and jaw jerk reflex. The mesencephalic trigeminal nucleus, which contains cell bodies of primary sensory neurons innervating stretch receptors in jaw muscles, and the trigeminal motor nucleus, containing jaw muscle motor neurons, are part of the jaw-jerk reflex circuit. Ascending branch trigeminal sensory nucleus is important in jaw proprioception.
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.
Thalamus. A. Myelin-stained section through ventral posterior nucleus. B. MRI. Myelin-stained coronal section through the ventral posterior nucleus of the thalamus. The magnocellular and parvocellular portions of the medial division (ventral posterior medial nucleus) are the trigeminal and taste relay nuclei, whereas the lateral division (ventral posterior lateral nucleus) is the relay nucleus for the medial lemniscus (ie, spinal sensory input).
A. Lateral view of the cerebral hemisphere, showing the locations of the face area laterally and the limbs and trunk area medially. B. A coronal slice through the postcentral gyrus showing the somatotopic organization, as represented by the homunculus. (B, Adapted from Penfield W, Rasmussen T. The Cerebral Cortex of Man: a Clinical Study of Localization of Function. New York, NY: Macmillan; 1950.)
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.