Human Physiology, 14th edition (2016)

Sensory Physiology 281

(a) Head upright

(b) Head bent forward

Sensory

axons

Hair cell

Supporting cell

Otoliths

Otolithic

membrane

Stereocilia of hair

cell bands stimulating

neurons

Otolithic membrane

sags

Gravitational force

Figure 10.16 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.

Supporting

cells

Hair

cells

Cupula

Endolymph in

semicircular canal

(a)

Movement of

endolymph

bends cupula

Sensory

nerve fibers

(b)

processes of these cells are embedded in a gelatinous mem-

brane, the cupula ( fig. 10.16 ), which has a higher density than

that of the surrounding endolymph. Like a sail in the wind, the

cupula can be pushed in one direction or the other by move-

ments of the endolymph.

The endolymph of the semicircular canals serves a func-

tion analogous to that of the otolithic membrane—it provides

inertia so that the sensory processes will be bent in a direction

opposite to that of the angular acceleration. As the head rotates

to the right, for example, the endolymph causes the cupula to

be bent toward the left, thereby stimulating the hair cells. Hair

cells in the anterior semicircular canal are stimulated when

doing a somersault, those in the posterior semicircular canal

are stimulated when performing a cartwheel, and those in the

lateral semicircular canal are stimulated when spinning around

the long axis of the body.

Neural Pathways

Stimulation of hair cells in the vestibular apparatus activates

sensory neurons of the vestibulocochlear nerve (VIII). These

fibers transmit impulses to the cerebellum and to the vestibu-

lar 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.  10.17 ). Neurons in the oculomo-

tor center control eye movements, and neurons in the spinal

Figure 10.15 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.