Traditional Functional Components

Each of the various components of most of the cranial nerves has traditionally been classified as somatic (referring to body wall structures) or visceral (referring to internal organs), afferent (sensory) or efferent (motor), and general or special. The latter pair of terms are used in reference to earlier beliefs concerning the embryological derivation of some of the sensory structures and muscles in the head. Recent new information will allow us to discard these categories later in this article, but their current widespread use necessitates discussing them here.

The first two pairs of classification terms for cranial nerves correspond to the four components of spinal nerves. Sensory nerve components are thus either somatic afferent or visceral afferent, and motor components are likewise either somatic efferent or visceral efferent. Spinal nerve components and some cranial nerve components are additionally classified as general, so they are designated general somatic afferent (GSA), general visceral afferent (GVA), general somatic efferent (GSE), and general visceral efferent (GVE). The sensory GSA components innervate the skin of the face and position sense (proprioception) receptors in head musculature, whereas GVA components innervate the viscera of the thorax and abdomen and a few structures in the head and neck, such as the mucous membranes of the oral cavity. The motor GSE components innervate extraocular eye muscles and the muscles of the tongue, whereas GVE components supply parasympathetic innervation to the thoracic and abdominal viscera and to glands and intraocular muscles in the head.

Two sensory cranial nerve components are categorized as special and thus designated special somatic afferent (SSA) and special visceral afferent (SVA). The SSA category is applied to the auditory and vestibular senses, whereas the SVA category is applied to the sense of taste (gustation). The two most rostral cranial nerves of the traditional 12, for olfaction and vision, are frequently not categorized at all. One cranial nerve motor category is also designated as special—the special visceral efferent (SVE) components of cranial nerves V, VII, IX, and X. Cranial nerve XI is sometimes not categorized but can be included in this traditional SVE category as well. SVE components of cranial nerves innervate muscles of the face, the mandibular and hyoid arches, the throat, and the neck, which all develop embryologically from muscles of the visceral arches that include the mandibular arch for the jaw, the hyoid arch, and a series of branchial arches. The latter give rise to the gill region in fishes and to components of the throat in tetrapods. The variously recognized traditional components of the midbrain and hindbrain cranial nerves are summarized in Table II, whereas the newer, revised classification is presented in Table III.

In the spinal cord, the two sensory afferent and two motor efferent components of the spinal nerves have their central cell groups organized in a dorsal to ventral order of GSA, GVA, GVE, and GSE. These rostro-caudally running functional columns extend into the brain stem so that the central cell groups of the cranial nerves in the hindbrain and midbrain lie in a similar topographic order. Due to the geometry of the brain's ventricular system, these cell columns lie in a lateral to medial order of GSA, GVA, GVE, and GSE. The two additional "special" sensory components of the cranial

Table II

Traditionally Recognized Components of Midbrain and Hindbrain Cranial Nerves"

Table II

Traditionally Recognized Components of Midbrain and Hindbrain Cranial Nerves"

Cranial nerve

Component

Sensory ganglion or motor ganglion and/or nucleus

Oculomotor (III)

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