Cranial Nerves: An Overview

Study Aides

Loyola University: Anatomical Sections: Brainstem and Diencephalon

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I. Introduction:

In general, the cranial nerves do for the head and neck what the spinal nerves do for the body in addition to a number of other special functions such as modulation of heart-rate and gastrointestinal motility. The task of discussing the functional anatomy of the 12 cranial nerves (10 of which have associated nuclei within the brainstem) is daunting. Thus, what I plan to do today is provide an overview of the general organizational principles, which will help you master the details of anatomy and function through self-study. Knowledge of these details is not a mere abstraction, but an important tool in the hands of a clinician for locating the site of disease within the brainstem. As you have seen last week, the brainstem is a very busy thoroughfare: Disease when it appears is rarely confined to a single system. The cranial nerves are arbitrarily numbered from I-XII, rostrocaudally. Some of the nerves are purely motor, others sensory and still others are mixed (sensory and motor).

Table 1: The Cranial Nerves and Their Functions Nerve Name Function

Nerve	Name	Function
I	Olfactory	Olfaction
II	Optic	Vision
III	Oculomotor	(1) Motor: Eye movements (extraocular eye muscles, striated muscles of eyelid (2) Autonomic fibers (Edinger-Westphal nucleus): ciliary ganglion (pupillary light reflex).
IV	Trochlear	Eye movements: Superior oblique.
V	Trigeminal	(1) Sensory: cutaneous and proprioceptive sensations from the face, mouth and teeth (2) Motor: muscles of mastication
VI	Abducens	Eye movements: Lateral Rectus
VII	Facial	(1) Sensory: Taste (ant. 2/3 of tongue); cutaneous (skin of external ear). (2) Motor: Muscles of facial expression; lacrimal glands; salivary glands.
VIII	Vestibulocochlear	Hearing; balance.
IX	Glossopharyngeal	(1) Autonomic fibers: Parotid gland (2) Swallowing: visceral sensations from the palate and post 1/3 of the tongue. (3) Innervates: Carotid body (4) Sensory: Taste (post. 1/3 of tongue)
X	Vagus	(1) Autonomic fibers: smooth muscles of the heart, blood vessels, trachea, bronchi, esophagus, stomach and intestine. (2) Motor: Innervates striated muscles: larynx & pharynx (controls speech) (3) Sensory: visceral sensations from the pharynx, larynx, thorax, abdomen Taste buds on epiglottis.
XI	Spinal Accessory	Motor: trapezius, sternocleidomastoid muscles.
XII	Hypoglossal	Motor: intrinsic muscles of the tongue

II. Classification:

For the spinal cord all fibers entering and leaving the CNS may be classified into one of four categories. Two associated with afferent input (sesnory) and two associated with efferent outflow (motor).

Traditionally the motor output is classified into two types:

1. (General) Somatic motor (innervating the skeletal muscles); i.e., alpha and gamma motoneurons of the ventral horn.

2. (General) Visceral (autonomic) motor (innervating the ganglion cells of the autonomic nervous system which in turn innervate the blood vessels, glands and the viscera of the body cavity): Examples include the preganglionic sympathetic neurons of the lateral horns.

There are also two classes of sensory (afferent) fibers:

1. (General) Somatic Afferent (related to the skin, muscles and the joints

2. (General) Visceral (autonomic) Afferent (related to visceral structures such as the walls of the digestive tract.

Within the cranial nerves, all four of these fiber types are present in addition to three other classes:

1. Special Somatic Afferent (related to the so-called special senses: vision, hearing and equilibrium or vestibular).

2. Special Visceral Afferent (related to the special senses of olfaction and taste).

3. Special Visceral Efferent: innervation of the branchiomeric muscles (homologous to the gill or branchial arches of fishes and develop into a variety of structures of the head and neck of higher vertebrates)These include muscles of the larynx, pharynx and face. It should be noted that these muscles are histologically and functionally identical to the somatic musculature of the body.

Table 2: Classifications of Cranial Nerves

Motor	GSE	Skeletal muscles (somites)	III, IV, VI, XII
	GVE	Autonomic control; Parasympathetic	III, VII, IX, X
	SVE	Skeletal muscles (branchiomeric)	V, VII, IX, X
Sensory	GSA	Touch, pain, temperature and proprioception	V, VII, IX, X
	SSA	Vision, hearing and equilibrium	II, VIII
	SVA	Olfaction and Taste	I, VII, IX, X
	GVA	Mechanical, pain temperature and propriocetion.**	V,VII,IX,X

** (Related to internal structures; e.g., mucus membranes of the larynx, viscera etc

III. Developmental Considerations:

A. A key to understanding the internal organization of many of the cranial nerve nuclei within the brainstem is development (ontongeny)

a. The walls of the neural tube has three layers: (i) ventricular layer: next to the lumen all mitosis occurs here, with a few exceptions; in the adult the ependyma is the remnant of the ventricular layer of the neural tube. (ii) Mantle layer: consisting of cells descended from the ventricular layer; becomes the grey matter. (iii) Marginal layer: is the outer layer consisting largely of neurite; becomes the white matter.

b. The alar and basal plate: development of sensory and motor differentiation; sulcus limitans delineates the border between the alar and basal plates.

c. Anterior and posterior horns are, respectively, alar and basal plate derivatives.

d. During development of the brainstem, the expansion of the IV ventricle results in laterally placed sensory nuclei and medial motor nuclei. This is the result of a non-persistent pontine flexure that occurs by the sixth week.

IV. Anatomical Arrangement:

1. From midline, laterally one may find: GSE, SVE, GVE, sulcus limitans, GVA, SVA, GSA, SSA. However, during development GSA (mainly V) migrate ventrolaterally as do the SVE (n. Ambiguus; Mot VII, Mot. V).

V. Conclusions:

A. The cranial nerve nuclei are organized into columns.

1. There are seven longitudinal columns, running roughly parallel throughout the brainstem, although they may not be contiguous (e.g., the general somatic efferents of the extraocular eye muscles and the innervation of the tongue). These seven columns correspond to the seven fiber types discussed above. Their segregation is dependent on the embryological origin of the neuronal cell group (i.e., alar vs. basal plate) and the nature of the target (e.g., somite, branchial arches, special senses etc.) The locations of the columns in medial to lateral plane are dependent on whether or not they are associated with motor or sensory and somatic or visceral innervation. The functional significance is that neurons of similar function are brought together whether or not they are associated with one or more cranial nerves (a good example is taste, and other is somatosensation).

B. Brain stem sensory nuclei receive input from several cranial nerves.

Sensory information of a particular class (somatosensory, taste) is sent to a common nucleus or nuclear complex regardless of which cranial nerve carries the peripheral axons.

C. There is considerable mixing of cranial nerve fiber types in the periphery.

Damage to the cranial nerves may lead to a wide variety of clinical symptoms ranging from sensory to motor to autonomic.

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