Sensory Hair Cells of the Vestibular Apparatus

The utricle and saccule provide information about linear acceleration—changes in velocity when traveling horizontally or vertically. We therefore have a sense of acceleration and deceleration when riding in a car or when skipping rope. A sense of rotational, or angular, acceleration is provided by the semicircular canals, which are oriented in three planes like the faces of a cube. This helps us maintain balance when turning the head, spinning, or tumbling.

The receptors for equilibrium are modified epithelial cells. They are known as hair cells because each cell contains twenty to fifty hairlike extensions. All but one of these hairlike extensions are stereocilia—processes containing filaments of protein surrounded by part of the cell membrane. One larger extension has the structure of a true cilium (chapter 3), and it is known as a kinocilium (fig. 10.13). When the stereocilia are bent in the direction of the kinocilium, the cell membrane is depressed and becomes depolarized. This causes the hair cell to release a synaptic transmitter that stimulates the dendrites of sensory neurons that are part of the vestibulocochlear nerve (VIII). When the stereocilia are bent in the opposite direction, the membrane of the hair cell becomes hyperpolarized (fig. 10.13) and, as a result, releases less synaptic transmitter. In this way, the frequency of action potentials in the sensory neurons that innervate the hair cells carries information about movements that cause the hair cell processes to bend.

Semicircular canals:

Anterior Posterior Lateral

Semicircular ducts of the membranous labyrinth

Semicircular ducts of the membranous labyrinth

Membranous ampullae:

Anterior Lateral Posterior

Cochlear nerve

Cochlea

Cochlear duct

Connection to cochlear duct

Apex of cochlea

Membranous ampullae:

Anterior Lateral Posterior

Cochlear nerve

Cochlea

Cochlear duct

Connection to cochlear duct

Apex of cochlea

■ Figure 10.12 The labyrinths of the inner ear. The membranous labyrinth (darker blue) is contained within the bony labyrinth.

Kinocilium

Stereocilia

Cell membrane

Kinocilium

Stereocilia

Cell membrane

At rest

Stimulated

Stimulated

J_I_I_I_I_I_I_L

Inhibited

■ Figure 10.13 Sensory hair cells within the vestibular apparatus. (a) A scanning electron photograph of a kinocilium and stereocilia. (b) Each sensory hair cell contains a single kinocilium and several stereocilia. (c) When stereocilia are displaced toward the kinocilium (arrow), the cell membrane is depressed and the sensory neuron innervating the hair cell is stimulated. (d) When the stereocilia are bent in the opposite direction, away from the kinocilium, the sensory neuron is inhibited.

Sensory Physiology

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