Introduction
Cranial nerves are peripheral nerves associated with the brain and, thus, provide sensory, motor, and autonomic innervation to the head and parts of the body. They share many similarities with peripheral nerves that emerge from the spinal cord. Most cranial nerves exit the central nervous system (CNS) at the level of the brainstem (Fig. 14-1). Two cranial nerves (i.e., the olfactory [I] and optic [II]) enter the CNS at the level of the forebrain. Only one cranial nerve, the accessory nerve (cranial nerve [CN] XI), does not emerge from the brain. Instead, it emerges from the spinal cord.
![View of the ventral surface of the brain depicting the sites of entry or exit of most of the cranial nerves. Note that the olfactory nerve (cranial nerve [CN] I) is not shown in this illustration and that the trochlear nerve exits the brainstem on the posterior surface and then passes along the exterior of the lower midbrain to enter the cavernous sinus along with CN VI.](http://what-when-how.com/wp-content/uploads/2012/04/tmp15F28.jpg "View of the ventral surface of the brain depicting the sites of entry or exit of most of the cranial nerves. Note that the olfactory nerve (cranial nerve [CN] I) is not shown in this illustration and that the trochlear nerve exits the brainstem on the posterior surface and then passes along the exterior of the lower midbrain to enter the cavernous sinus along with CN VI.")
FIGURE 14-1 View of the ventral surface of the brain depicting the sites of entry or exit of most of the cranial nerves. Note that the olfactory nerve (cranial nerve [CN] I) is not shown in this illustration and that the trochlear nerve exits the brainstem on the posterior surface and then passes along the exterior of the lower midbrain to enter the cavernous sinus along with CN VI.
Several cranial nerves, such as the hypoglossal nerve (CN XII), are associated only with motor functions. Others are associated with only specific sensory functions, such as the optic nerve (CN II), and still others are associated with a combination of functions. With respect to the latter, such nerves may include both sensory and motor functions (e.g., trigeminal nerve [CN V]); a combination of sensory, motor, and autonomic functions (e.g., glossopharyngeal nerve [CN IX]); or even a combination of somatic motor and autonomic functions (e.g., oculomotor nerve [CN III]). The anatomical and functional properties of each of the cranial nerves (except CN I, II, and VIII to which separate topics are dedicated) will be discussed in this topic.
The overall strategy employed in this topic is to first characterize cranial nerves in terms of the established and accepted classification scheme that has been in use in the literature for many years. This approach should enable the student to begin to learn which nerves have sensory, motor, and/or autonomic components. After analysis of this classification scheme, the anatomy, functions, and clinical correlations associated with each of the cranial nerves is considered separately. Though the rationale underlying the order of presentation of cranial nerves may be viewed as somewhat arbitrary, cranial nerves in this topic are presented beginning with the CN XII and ending with CN I. One reason for this approach is that the important medial-to-lateral columnar anatomical relationships (see "Anatomical Organization of the Cranial Nerves Within the Brainstem," and Figs. 14-2 and 14-3) of sensory, motor, and autonomic components of cranial nerves are best observed at the level of the medulla and pons. Because this relationship can best be demonstrated with CN V to CN X and CN XII, it was accordingly deemed useful to begin with a discussion of these cranial nerves. A second reason is that this approach enables us to "work our way up" the CNS in a manner parallel to the order of presentation of the anatomy of the brainstem and forebrain. It is our belief that the consistency of this approach will further aid in the learning of this complex subject.
Classification of The Cranial Nerves
At this time, it is appropriate to review this classification scheme in more detail and to indicate the categories of classification to which each of the cranial nerves are associated.
Efferent Nerves (From The Central Nervous System)
General Somatic Efferents
A general somatic efferent (GSE) fiber is one whose cell body lies within the brainstem or spinal cord and whose axons innervate skeletal muscle. These neurons innervate skeletal muscle derived from somites. The following cranial nerves, which include GSE components, are derived from the neural tube and are associated with specific somites: oculomotor (CN III), trochlear (CN IV), abdu-cens (CN VI), and hypoglossal (CN XII).
Special Visceral Efferents
A special visceral efferent (SVE) fiber is functionally identical to nerves categorized as GSE. Essentially, this kind of nerve arises from the brainstem and innervates skeletal muscle. The only reason for the change in classification of these cranial nerves is that these neurons innervate skeletal muscle derived from the mesenchyme of the branchial arches. The term "visceral" is based on the fact that, in less developed animals such as fish, the branchial arches are associated with nerves that synapse on postganglionic neurons that then innervate smooth muscles or glands. Cranial nerves that include SVE components are: trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), vagus (CN X), and the spinal accessory (CN XI).
General Visceral Efferents
A general visceral efferent (GVE) fiber is one whose cell body lies within the CNS and whose fibers innervate smooth muscles or glands. The following cranial nerves include GVE components: oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X).
Afferent Fibers (to the Brain)
General Somatic Afferents
A general somatic afferent (GSA) fiber is one whose cell body (usually termed pseudo-unipolar neuron) lies outside the CNS. The peripheral limb of this nerve typically innervates skin, skeletal muscle, tendons, or joints. The receptors associated with such nerves mediate sensations of pain and temperature, conscious (and nonconscious) proprioception, touch, and pressure. The central limb of the nerve innervates specific nuclei within the brainstem. The primary cranial nerve associated with GSA information is the trigeminal (CN V) nerve. Several other cranial nerves also contribute GSA inputs to the brainstem. They include the vagus (CN X), glossopharyngeal (CN IX), and facial (CN VII) nerves, which are all associated with the skin of the ear.
Special Sensory Afferents
Special sensory afferent (SSA) fibers are similar to GSA fibers except that they convey information to the brain from very highly specialized kinds of somatic receptors. The cranial nerves with SSA fibers include the optic (CN II) and auditory-vestibular (CN VIII) nerves.
Special Visceral Afferents
Special visceral afferent (SVA) fibers are similar to SSAs in that they also serve special senses and have specialized receptors. However, SVA nerves are distinguished from SSAs on the basis of the receptor mechanisms. SVA nerves utilize chemoreceptors (i.e., receptors that respond to changes in their chemical environment), whereas receptors associated with SSA nerves use other kinds of mechanisms (such as light or mechanical perturbation) to become activated. Chemoreceptors include those receptors that respond principally to olfactory and taste signals. Olfaction is mediated by the olfactory (CN I) nerve, and taste is mediated by the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves.
General Visceral Afferents
General visceral afferent (GVA) fibers have their receptors located principally in the serous linings or muscle layers of the body viscera. The receptors may include Pacin-ian corpuscles, free nerve endings, or even nerve endings that are poorly encapsulated. The conscious sensations of visceral pain are generally not specific and are poorly localized. The afferent fibers associated with these sensations convey such signals as thirst, hunger, visceral pain, and general unpleasant feelings. Most of the fibers linked with these sensations do not directly involve cranial nerves.
GVA components of cranial nerves convey signals associated primarily with changes in blood pressure. Some authors include chemoreception as well (see discussion below concerning chemoreception and CN IX and X). Cranial nerves associated with GVA inputs into the brain include the glossopharyngeal (CN IX) and vagus (CN X) nerves.
Anatomical Organization of The Cranial Nerves Within The Brainstem
As a brief review, this material is presented here and depicts the anatomical positions of cranial nerve nuclei within the brainstem of the cranial nerves (Fig. 14-2) as well as positions with respect to functional lines (Fig. 14-3). Of significance is the relationship between the locations of cranial nerve nuclei and the cell columns with which they are associated.
![Longitudinal view of the brainstem depicting the position and arrangement of the sensory, motor, and autonomic cell groups that comprise first-order and second-order neurons associated with cranial nerves. Motor nuclei of cranial nerves (CN) III, IV, VI, and XII are classified as general somatic efferent (GSE) and are located near the midline. Motor nuclei of CN V, VII, IX, X (nucleus ambiguus for nerves IX and X), and XI are classified as SVE and are located slightly lateral to GSE neurons. Autonomic nuclei (general visceral efferent) are derived from CN III (Edinger-Westphal nucleus), VII (superior sali-vatory nucleus), IX (inferior salivatory nucleus), and X (dorsal motor nucleus) and are situated slightly more laterally. Sensory neurons lie lateral to motor neurons. General visceral afferent (GVA) neurons include CN IX and X. CN I (not shown in figure), VII, IX, and X include special visceral afferent (SVA) components (involving the nucleus of the solitary tract [solitary nucleus] for each of these nerves except for CN I). General somatic afferent (GSA) and special somatic afferent (SSA) components lie lateral to GVA and SVA components. GSA components are found among CN V (main sensory nucleus of CN V), IX, and X (spinal nucleus of CN V receives inputs from CN IX and X). Cranial nerves that are classified as SSA include CN II (optic) and VIII (auditory-vestibular), which are not shown in this illustration.](http://what-when-how.com/wp-content/uploads/2012/04/tmp15F29.jpg "Longitudinal view of the brainstem depicting the position and arrangement of the sensory, motor, and autonomic cell groups that comprise first-order and second-order neurons associated with cranial nerves. Motor nuclei of cranial nerves (CN) III, IV, VI, and XII are classified as general somatic efferent (GSE) and are located near the midline. Motor nuclei of CN V, VII, IX, X (nucleus ambiguus for nerves IX and X), and XI are classified as SVE and are located slightly lateral to GSE neurons. Autonomic nuclei (general visceral efferent) are derived from CN III (Edinger-Westphal nucleus), VII (superior sali-vatory nucleus), IX (inferior salivatory nucleus), and X (dorsal motor nucleus) and are situated slightly more laterally. Sensory neurons lie lateral to motor neurons. General visceral afferent (GVA) neurons include CN IX and X. CN I (not shown in figure), VII, IX, and X include special visceral afferent (SVA) components (involving the nucleus of the solitary tract [solitary nucleus] for each of these nerves except for CN I). General somatic afferent (GSA) and special somatic afferent (SSA) components lie lateral to GVA and SVA components. GSA components are found among CN V (main sensory nucleus of CN V), IX, and X (spinal nucleus of CN V receives inputs from CN IX and X). Cranial nerves that are classified as SSA include CN II (optic) and VIII (auditory-vestibular), which are not shown in this illustration.")
FIGURE 14-2 Longitudinal view of the brainstem depicting the position and arrangement of the sensory, motor, and autonomic cell groups that comprise first-order and second-order neurons associated with cranial nerves. Motor nuclei of cranial nerves (CN) III, IV, VI, and XII are classified as general somatic efferent (GSE) and are located near the midline. Motor nuclei of CN V, VII, IX, X (nucleus ambiguus for nerves IX and X), and XI are classified as SVE and are located slightly lateral to GSE neurons. Autonomic nuclei (general visceral efferent) are derived from CN III (Edinger-Westphal nucleus), VII (superior sali-vatory nucleus), IX (inferior salivatory nucleus), and X (dorsal motor nucleus) and are situated slightly more laterally. Sensory neurons lie lateral to motor neurons. General visceral afferent (GVA) neurons include CN IX and X. CN I (not shown in figure), VII, IX, and X include special visceral afferent (SVA) components (involving the nucleus of the solitary tract [solitary nucleus] for each of these nerves except for CN I). General somatic afferent (GSA) and special somatic afferent (SSA) components lie lateral to GVA and SVA components. GSA components are found among CN V (main sensory nucleus of CN V), IX, and X (spinal nucleus of CN V receives inputs from CN IX and X). Cranial nerves that are classified as SSA include CN II (optic) and VIII (auditory-vestibular), which are not shown in this illustration.
FIGURE 14-3 Frontal section taken through the middle of the medulla depicting the loci of the different cell columns that comprise the various components of the cranial nerves. Although this illustration is taken through the medulla, this arrangement applies for other parts of the brainstem as well. Note that the motor nuclei of cranial nerves are located medial to those of sensory nuclei. The GSE (general somatic efferent) motor column is located in the most medial position; next to the GSE column lies the special visceral efferent (SVE) and general visceral efferent (GVE) columns. On the lateral side of the sulcus limitans lie the sensory columns, the most medial of which give rise to general visceral afferent (GVA) and special visceral afferent (SVA) neurons. In the far lateral column lie the special somatic afferent (SSA) and general somatic afferent (GSA) columns. Nuc. = nucleus; n. = nerve.
The relationship between cranial nerve nuclei and the sulcus limitans is the key point to note. Recall that the sulcus limitans separates motor from sensory structures. Those structures that lie medial to the sulcus limitans are derived from the basal plate, and those found lateral to the sulcus limitans are derived from the alar plate. This relationship closely resembles that described previously for the spinal cord in which the ventral horn, which is derived from the basal plate, is associated with motor functions, whereas the dorsal horn, which is derived from the alar plate, is associated with sensory functions. The region near the sulcus limitans is associated with auto-nomic functions.
Thus, cranial nerves situated medial to the sulcus limi-tans have motor functions, those located lateral to the sul-cus limitans have sensory functions, and cranial nerve nuclei that lie near the sulcus limitans carry out autonomic functions. As shown in Figure 14-3, the GSE and SVE columns are both located medial to the GVE column and close to the sulcus limitans. Concerning sensory structures, the SSA and GSA cell columns are located in a far lateral position, whereas the SVA and GVA cell columns lie close to the sulcus limitans. This arrangement of sensory, motor, and autonomic nuclei remains relatively constant throughout the brainstem, and knowledge of this organization serves as a useful aid in identification of cranial nerve nuclei.
Cranial Nerves Associated With the Lower Brainstem and Adjoining Regions of the Spinal Cord
Hypoglossal Nerve (Cranial Nerve XII) Component: GSE—Origin, Distribution, and Function. The hypoglossal nerve is mainly a motor nerve (but contains some muscle spindle afferents). Because it innervates muscles derived from somites, it is classified as a GSE nerve.
Origin and Distribution. The hypoglossal nerve originates from the hypoglossal nucleus, which is located within the dorsomedial aspect of the lower medulla (Figs. 14-1 and 14-4). Axons of the hypoglossal nucleus pass ventrolateral^ and exit the brain between the pyramids and the inferior olivary nucleus. The hypoglossal nerve exits the brain through the hypoglossal canal and innervates both extrinsic muscles (the styloglossus, hyoglossus, and genioglossus, which control the shape and position of the tongue) and intrinsic muscles (which control the shape of the tongue).
Function. The general functions of the hypoglossal nerve are to control the shape and position of the tongue by virtue of its innervation of the extrinsic and intrinsic muscles. Contraction of the genioglossus muscle on each side causes the tongue to protrude.
Clinical Disorders. A hypoglossal paralysis can occur if either the cell bodies of the hypoglossal nucleus or axons that form the hypoglossal nerve are damaged. When an individual who has unilateral hypoglossal nerve damage attempts to protrude his tongue, it will deviate to the side of the lesion. This occurs because the genioglossus muscle on the intact side will attempt to pull the half of the tongue, which it innervates forward, whereas the genioglossus muscle on the affected side cannot do so. In other words, the genioglossus muscle on the affected side serves as a pivot with respect to the actions of the genioglossus muscle of the intact side.
![Diagram illustrates the origin and distribution of the hypoglossal nerve (cranial nerve XII [general somatic efferent]) with its innervation of the muscles of the tongue. Also shown in this illustration is the corticobulbar projection to the hypoglossal nucleus, which, for this cranial nerve, is crossed and uncrossed. In some patients, however, lesions of this corticobulbar pathway produce contralateral weakness.](http://what-when-how.com/wp-content/uploads/2012/04/tmp15F31.jpg "Diagram illustrates the origin and distribution of the hypoglossal nerve (cranial nerve XII [general somatic efferent]) with its innervation of the muscles of the tongue. Also shown in this illustration is the corticobulbar projection to the hypoglossal nucleus, which, for this cranial nerve, is crossed and uncrossed. In some patients, however, lesions of this corticobulbar pathway produce contralateral weakness.")
FIGURE 14-4 Diagram illustrates the origin and distribution of the hypoglossal nerve (cranial nerve XII [general somatic efferent]) with its innervation of the muscles of the tongue. Also shown in this illustration is the corticobulbar projection to the hypoglossal nucleus, which, for this cranial nerve, is crossed and uncrossed. In some patients, however, lesions of this corticobulbar pathway produce contralateral weakness.
A paralysis of the tongue, called supranuclear paralysis, may also result from a cortical lesion involving the region of the precentral gyrus, which supplies axons to the hypoglossal nucleus. Because many of the cortical fibers that supply the hypoglossal nucleus are crossed, the con-tralateral hypoglossal nucleus and nerve are affected. Therefore, during attempts to protrude the tongue, it will deviate to the side contralateral to the lesion.
