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:

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   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.

   
   
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   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.)

   
   
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   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:

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

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

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

  
  
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  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:

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

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

  
  
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  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.