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Cranial Nerves and Functional Components
John T. Povlishock, Ph.D.
OBJECTIVES
After studying the material of this lecture, the student should be familiar with:
1. The afferent and/or efferent components of the cranial nerves.
2. The association of functional components with specific cranial nerves and the nuclear
groups with which they are related.
I.
INTRODUCTION
The structure of the spinal cord, in general, is much the same throughout its length, and
the plan of organization of its cell groups and fibers is recognized quite clearly. As the
rostral continuation of the spinal cord, the brainstem shows only a slight resemblance to
the spinal cord, although there are many common principles of organization. In the
medulla and pons, the sulcus limitans is located on the dorsal surface of the brainstem.
Consistent with its functional significance in the development of the spinal cord, in the
brainstem it also indicates the borderline between the zone of efferent (motor) nuclei
located medial to it in the floor plate and the zone of terminal nuclei of the afferent
(sensory) fibers located lateral to it in the alar plate. In embryonic development, the cells
giving origin to efferent and afferent nuclei are arranged in longitudinal columns that
later become partly broken up into distinct cell groups or nuclei. However, most of these
nuclei retain approximately their original place, such that even in the adult those derived
from the different primary columns are found arranged in a longitudinal columnar
manner.
Due to the high degree of differentiation that has occurred in the cranial region over the
course of evolutionary development, the cranial nerves are more complex with regard to
their structure and function than the spinal nerves. However, some of the cranial nerves
contain features that are common to spinal nerves, including the same functional
components as found in spinal nerves. Recall that spinal nerves contain four functional
components: General Somatic Afferent (GSA), General Visceral Afferent (GVA),
General Somatic Efferent (GSE), and General Visceral Efferent (GVE). The somatic
components are related to innervation of derivatives of the somatic mesoderm, viz.
dermatome (skin) and myotome (skeletal muscle), while the visceral components are
related to innervation of the viscera, including smooth and cardiac muscle. The cranial
nerves, when considered as a whole, consist of these same four functional components,
but the development of the special senses and the branchial arches in the cranial part of
the body is accompanied by a corresponding complexity of fiber categories and nuclei.
Consequently, three specialized functional components also are associated with the
cranial nerves: Special Visceral Efferent (SVE), providing innervation to muscles derived
from the embryonic visceral (branchial) arches, Special Somatic Afferent (SSA), related
to vision and audition, and Special Visceral Afferent (SVA), related to olfaction and
gustation (taste). The composition of the different cranial nerves is not identical, some
having only efferent fibers, others only afferent, whereas some are mixed.
II.
GENERAL ORGANIZATION OF THE CRANIAL NERVES AND THEIR
NUCLEI.
* Link to Netter Image 11.64
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The sulcus limitans marks the boundary between the efferent and afferent nuclear zones
in the brainstem. Within these zones, a further differentiation is possible (Figures 1, 2,
and 3). The most medial column of efferent nuclei corresponds closely to the groups of
neurons in the ventral horn of the spinal cord Like the spinal neurons, the efferent fibers
of these somatic efferent cranial nerve nuclei innervate striated muscle, derived
embryologically from the myotomes of the somites. In the head, muscle of myotomic
derivation is represented only by the extraocular muscles and the intrinsic muscles of the
tongue. Accordingly, the GSE nuclei in the brainstem are associated with oculomotor
(III), trochlear (IV), abducens (VI) and hypoglossal (XII) nerves, all of which are located
on the midline immediately ventral to the floor of the 4th ventricle or the cerebral
aqueduct.
Figure 1: General organization and arrangement of cranial nerve nuclei according to their
functional components.
Figure 2: Longitudinal columnar arrangement of cranial nerve nuclei in the
brainstem according to their functional components.
Figure 3
Lateral to the somatic efferent nuclei are found the two categories of visceral efferent
nuclei, forming two separate, interrupted columns. The more medial column of visceral
efferent nuclei is displaced somewhat ventrally, and the axons from these neurons
innervate striated muscle. These muscles, however, are derived from the branchial arches,
and include the masticatory muscles (first branchial arch), the muscles of facial
expression (second branchial arch), and the muscles of pharynx and larynx (third and
fourth branchial arches). These muscles are innervated by neurons in the SVE nuclei in
the brainstem that are associated with the trigeminal (V), facial (VII), glossopharyngeal
(IX), vagus (X), and cranial portion of the accessory (XI) nerves. The SVE nucleus
associated with the glossopharyngeal and vagus nerves is the nucleus ambiguus The more
internal and dorsal column is comprised of the nuclei that belong to the GVE functional
component and consist of the Edinger-Westphal nucleus (III), the superior (VII) and
inferior (IX) salivatory nuclei, and the dorsal motor nucleus of the vagus (X). These
nuclei contain preganglionic parasympathetic neurons, whose axons course to terminate
upon neurons located in various autonomic ganglia that, in turn, innervate smooth muscle
and glands.
The sensory nuclei in the brainstem are located lateral to the sulcus limitans. Immediately
lateral to the sulcus limitans is found a column of neurons that receive visceral afferent
fibers. This consists of only one nucleus, the nucleus of the tractus solitarius, which
extends throughout the length of the medulla. The afferent fibers that terminate in this
nucleus are associated with the nervus intermedius (VII) and the glossopharyngeal (IX)
and vagus (X) nerves. Their cells of origin reside in the ganglia of these nerves, and
before entering the nucleus their axons descend as the tractus solitarius. A distinction is
made between two types of visceral afferent fibers that terminate in corresponding
subdivisions of the nucleus. Fibers related to the GVA functional component convey
general impulses from the viscera and terminate in the caudal portion of the nucleus,
while SVA fibers specifically are related to taste and terminate in the rostral portion of
the nucleus. Although not related to the brainstem, the olfactory (I) nerve also is
associated with the SVA functional component.
The somatic afferent nuclei are found in the most lateral part of the alar plate, and here a
subdivision also can be made. Fibers related to the GSA functional component convey
superficial (and probably also deep) sensation from the face, course through the
trigeminal (V) nerve, and have their cells of origin in the semilunar ganglion or the
mesencephalic nucleus of V. Those from the semilunar ganglion terminate in the
principal sensory nucleus of V or the spinal nucleus of V. Recall that a small GSA
innervation also is provided by the facial (VII), glossopharyngeal (IX), and vagus (X)
nerves, which also terminate in the spinal trigeminal nucleus. The homology of the
cranial GSA nuclei with the spinal GSA nuclei is revealed morphologically in the
continuity of the pars caudalis of the spinal trigeminal nucleus with the substantia
gelatinosa in the dorsal horn of the spinal cord in segments C1 - C2.
The SSA functional component is provided by the vestibulocochlear (VIII) nerve, which
conveys impulses from the sensory apparati (i.e., semicircular canals, otolith organs,
organ of Corti). The vestibular and cochlear nuclei are located in the extreme lateral and
dorsolateral parts of the medulla. The optic (II) nerve also is associated with the SSA
functional component.
Tables 1-5 summarize various aspects of the cranial nerves.
Table 1
Table 2
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Table 3
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Table 4
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Table 5
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* Link to Netter Image 11.63.2
* Link to Netter Image 11.63.3
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* Netter Presenter Image Copyright 2004 Icon Learning Systems. All rights reserved.