CHAPTER 8 The Special Senses 153 branches to each of the vestibular epithelia; centrally, it accompanies the cochlear and facial nerves as they enter the internal auditory meatus of the skull.
Like cochlear hair cells, vestibular hair cells are functionally polarized, and the transduction mechanism is presumed to be similar. When the stereocilia are bent toward the longest cilium (in this case, the kinocilium), conductance of the apical membrane increases for cations and, because of the high K+ concentration of the endolymph, K+ enters, and the vestibular hair cell is depolarized (
Fig. 8.26 ). Conversely, when the cilia are bent away from the kinocilium, the hair cell is hyperpolarized. The hair cell releases an excitatory neurotransmitter (either glutamate or aspartate) tonically, so that the afferent fiber on which it synapses has a resting discharge. When the hair cell is depolarized, more neurotransmitter is released, and the discharge rate of the afferent fiber increases. Conversely, when the hair cell is hyperpolarized, less neurotransmitter is released, and the firing rate of the afferent fiber slows.
Angular accelerations of the head produce minute movement of the endolymph in relation to the head (
Fig. 8.27 ). This happens because the inertia of the endolymph causes it to resist the initial acceleration of the membranous labyrinth. This lag pushes on the cupula, causes the cilia to bend, and consequently changes the discharge rates of the vestibular afferent fibers. All the cilia in a given ampullary crest are oriented in the same direction. In the horizontal canal, the cilia are oriented toward the utricle, and in the other ampullae, they are oriented away from  the utricle.
• Fig. 8.26 Functional Polarization of Vestibular Hair Cells. When the stereocilia are bent toward the kinocilium, the hair cell is depolarized, and the afferent fiber is excited. When the stereocilia are bent away from the kinocilium, the hair cell is hyperpolarized, and the afferent discharge slows or stops. (Redrawn from Kandel ER, Schwartz JH. Principles of Neural Science. New York: Elsevier; 1981.)
Turning motion of head
AmpullaAxis of
Axis of hair cells •Fig. 8.27 Effect of Leftward Head Movement on the Activity of Vestibular Afferent Fibers Supplying Hair Cells in the Horizontal Semicircular Canals. The perspective in the figure is from above the head looking down. In this case, head rotation (acceleration) to the left causes endolymph pressure to the right and the result is increased output from the left canal and decreased output from the right canal. Small arrows intheampullaindicatethefunctionalpolarityofthehaircells.Thelarge curved arrow atthetopindicatesmovementofthehead;smaller curved arrows indicaterelativemovementoftheendolymph.
The way in which angular acceleration of the head affects the discharge of vestibular afferent fibers is exemplified by the activity that originates from the horizontal canals.
Fig.