When a person first begins to spin, the inertia of endolymph within the semicircular ducts causes the cupula to bend in the opposite direction. As the spin continues, however, the inertia of the endolymph is overcome and the cupula straightens. At this time, the endolymph and the cupula are moving in the same direction and at the same speed. If movement is suddenly stopped, the greater inertia of the endolymph causes it to continue moving in the previous direction of spin and to bend the cupula in that direction.
Bending of the cupula affects muscular control of the eyes and body through the neural pathways previously discussed. During a spin, this produces smooth movements of the eyes in a direction opposite to that of the head movement so that a stable visual fixation point can be maintained. When the spin is abruptly stopped, the eyes continue to move smoothly in a direction opposite to that of the spin (because of the continued bending of the cupula) and then are jerked rapidly back to the midline position. This produces involuntary oscillations of the eyes called vestibular nystagmus. People
(control of eye movements)
■ Figure 10.16 Neural pathways involved in the maintenance of equilibrium and balance. Sensory input enters the vestibular nuclei and the cerebellum, which coordinate motor responses.
experiencing this effect may feel that they, or the room, are spinning. The loss of equilibrium that results is called vertigo.
Vertigo as a result of spinning is a natural response of the vestibular apparatus. Pathologically, vertigo may be caused by anything that alters the firing rate of one of the vestibulocochlear nerves (right or left) compared to the other. This is usually due to a viral infection causing vestibular neuritis. Severe vertigo is often accompanied by dizziness, pallor, sweating, nausea, and
Joint, tendon, muscle, and cutaneous receptors vomiting due to involvement of the autonomic nervous system, which is activated by vestibular input to the brain stem.
Vestibular nystagmus is one of the symptoms of an inner-ear disease called Meniere's disease. The early symptom of this disease is often "ringing in the ears," or tinnitus. Since the endolymph of the cochlea and the endolymph of the vestibular apparatus are continuous through a tiny canal, the duct of Hensen, vestibular symptoms of vertigo and nystagmus often accompany hearing problems in this disease.
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