Neural Pathways

Stimulation of hair cells in the vestibular apparatus activates sensory neurons of the vestibulocochlear nerve (VIII). These

(a) Head upright

Otoliths

Otoliths

Otolithic Membrane

Otolithic membrane (gelatinous material)

Sensory nerve fibers Hair cells Supporting cells

Otolithic membrane (gelatinous material)

Sensory nerve fibers Hair cells Supporting cells

■ Figure 10.14 The otolith organ. (a) When the head is in an upright position, the weight of the otoliths applies direct pressure to the sensitive cytoplasmic extensions of the hair cells. (b) As the head is tilted forward, the extensions of the hair cells bend in response to gravitational force and cause the sensory nerve fibers to be stimulated.

(a) Head upright

Vestibular Reflex Head Bent Forward
(b) Head bent forward

© The McGraw-Hill Companies, 2003

Chapter Ten

Vestibular Response Head Turn

Supporting cells

Supporting cells

Endolymph Cupula
Movement of endolymph bends cupula

■ Figure 10.15 The cupula and hair cells within the semicircular canals. (a) Shown here, the structures are at rest or at a constant velocity. (b) Here, movement of the endolymph during rotation causes the cupula to bend, thus stimulating the hair cells.

fibers transmit impulses to the cerebellum and to the vestibular nuclei of the medulla oblongata. The vestibular nuclei, in turn, send fibers to the oculomotor center of the brain stem and to the spinal cord (fig. 10.16). Neurons in the oculomotor center control eye movements, and neurons in the spinal cord stimulate movements of the head, neck, and limbs. Movements of the eyes and body produced by these pathways serve to maintain balance and "track" the visual field during rotation.

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