Functional Systems

The vestibular system provides vestibulo-cochlear input to the cerebellum, spinal cord, and oculomotor apparatus to enable the coordination of head, body, and eye movements. It influences extensor muscle tone and reflexes via the lateral vestibulospinal tract (postural motor system). The medial longitudinal fasciculus permits simultaneous, integrated control of neck muscle tone and eye movements. The oculomotor system (p. 86) communicates with the vestibular nuclei, the cerebellum, and the spinal cord via the medial longitudinal fasciculus and pontine projection fibers; thus the control of eye movements is coordinated with that of body movements. Proprioceptive input concerning joint position and muscle tone reaches the vestibular system from the cerebellum (p. 54). Thalamocortical connections permit spatial orientation. Phenomena such as nausea, vomiting, and sweating arise through interaction with the hypothalamus, the medullary "vomiting center," and the vagus nerve, while the emotional component of vestibular sensation (pleasure and discomfort) arises through interaction with the limbic system.

Effect Gravity

Linear acceleration to side

Deflection of hair cells

(effect of gravity)

Head rotation

(above, rotation to right; below, sudden stop)

Linear acceleration to side

Deflection of hair cells

(effect of gravity)

Head rotation

(above, rotation to right; below, sudden stop)

Patients often use the word "dizziness" non-specifically to mean lightheadedness, unsteadiness, reeling, staggering, or a feeling of rotation. Dizziness in this broad sense has many possible causes. Vertigo, or dizziness in the narrow sense, is the unpleasant illusion that one is moving or that the external world is moving (so-called subjective and objective vertigo, respectively). Pathogenesis. Vertigo arises from a mismatch between expected and received sensory input (vestibular, visual, and somatosensory) regarding spatial orientation and movement. Cause. Vertigo occurs as a normal response to certain stimuli (physiological vertigo) or as the result of diseases (pathological vertigo) affecting the labyrinth (peripheral vestibular vertigo), .O central vestibular system (central vestibular u vertigo), or other functional systems (nonvesti-ยง bular vertigo).

^ Symptoms and signs. The manifestations of ver-,2 tigo are the same regardless of etiology. They fall S into the following categories: autonomic (drowsiness, yawning, pallor, sialorrhea, increased sensitivity to smell, nausea, vomiting), mental (decreased drive, lack of concentration, apathy, sense of impending doom), visual (oscillopsia = illusory movement of stationary objects), and motor (tendency to fall, staggering and swaying gait).

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