Vestibular fibers

Secondary vestibular fibers, originating in the vestibular nuclei, join the MLF and supply the motor nuclei of CN III, IV, and VI. These fibers are involved in the production of conjugate eye movements. These compensatory eye movements represent the efferent limb of the vestíbulo-ocular reflex, which enables the eye to remain focused on a stationary target during"movement of the head or neck. Most of our understanding of the vestíbulo-ocular reflex is based on horizontal head turning and a corresponding horizontal movement of the eyes in the direction opposite to that of head turning. For example, when the head turns horizontally to the right, both eyes will move to the left using the following vestíbulo-ocular structures. Head turning to the right stimulates hairs cells in the right semicircular ducts. The right eighth nerve increases its firing rate to the right vestibular nuclei. These nuclei then send axons by way of the MLF to the right oculomotor nucleus and to the left abducens nucleus. The right oculomotor nerve to the right medial rectus adducts the right eye, and the left abducens nerve to the left lateral rectus abducts the left eye. The net effect of stimulating these nuclei is that both eyes will look to the left (Figures IV-5-8 and IV-5-9).

Clinical Correlate

A lesion of the vestibular nuclei or nerve (in this example on the left) produces a vestibular nystagmus with a slow deviation of the eyes toward the lesion (A) and a fast correction back to the right (B).

Lateral rectus muscle

Cerebellar pedunc/es

connus (fast compnn

^suack (slow compp^

Lateral rectus muscle

Cerebellar pedunc/es connus (fast compnn

^suack (slow compp^

4. Both eyes look left

Vestibular nuclei

2. Increases nerve firing rate

3. Stimulates vestibular nuclei

Figure IV-5-8 The Vestibulo-Ocular Reflex

4. Both eyes look left

Medial rectus ''v muscle

1. Endolymph flow stimulates hair cells

Vestibular nuclei

1. Endolymph flow stimulates hair cells

2. Increases nerve firing rate

3. Stimulates vestibular nuclei

Figure IV-5-8 The Vestibulo-Ocular Reflex

First - Slow component (slow tracking)

Second - Fast component (nystagmus)

Second - Fast component (nystagmus)

Figure IV-5-9. Vestibular System Part 2

Clinical Correlate

Vestibular dysfunction may result from either peripheral or central lesions.

Vertigo may result from a lesion of either the peripheral (end organ, nerve) or central (nuclear, brain stem pathways) vestibular structures. Vertigo refers to the perception of rotation, which may involve either the subject or the external space. The vertigo is usually severe in peripheral disease and mild in brain stem disease. Chronic vertigo (i.e., persisting longer than 2-3 weeks) strongly suggests a central lesion.

Vertigo may also be caused by a variety of drugs, including anticonvulsants, aspirin, alcohol, and certain sedatives and antibiotics. Meniere disease is characterized by abrupt recurrent attacks of vertigo lasting minutes to hours accompanied by tinnitus or deafness and usually involving only one ear. Nausea and vomiting and a sensation of fullness or pressure in the ear also are common during the acute episode. The attacks often are severe, and the patient may be unable to stand. The disease usually occurs in middle age and results from distention of the fluid spaces in the cochlear and vestibular parts of the labyrinth.

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