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displacement of kinocillia hair cell membrane potential depolarization hyperpolarization tonic release of neurotransmitter decreased release increased release action potentials in afferent neurons more frequent less frequent time

A. Horizontal section of the skull showing the semicircular canals foramen magnum . temopral bone left semicircular canals posterior horizontal anterior cribriform plate (nose)

B. Right horizontal canal

head rotating to the left (counterclockwise)

direction of endolymph displacement head rotating to the left (counterclockwise)

C. Anterior view of the semicircular canals

B. Right horizontal canal direction of endolymph displacement

head stationary

right ear right nerve decreased activity right ear

right nerve decreased activity left ear left ear

left nerve increased activity head rotating to the left head rotating to the left

FIGURE 3 Paired responses of semicircular canals. (A) Three pairs of semicircular canals as they are positioned in the head; the two horizontal canals (dark blue bars) lie in the same plane and form a functional pair. The same is true for right anterior and left posterior canals (light blue bars). (B) As the head rotates to the left, the endolymph lags behind, dragging the cupula and hair cell tips to the right. (C) Rotation of the head to the left excites hair cells in the left horizontal canal and inhibits hair cells in the right horizontal canal.

FIGURE 3 Paired responses of semicircular canals. (A) Three pairs of semicircular canals as they are positioned in the head; the two horizontal canals (dark blue bars) lie in the same plane and form a functional pair. The same is true for right anterior and left posterior canals (light blue bars). (B) As the head rotates to the left, the endolymph lags behind, dragging the cupula and hair cell tips to the right. (C) Rotation of the head to the left excites hair cells in the left horizontal canal and inhibits hair cells in the right horizontal canal.

linear acceleration due to changes in gravity, acceleration, or deceleration of movement or tilting of the head.

Unlike the simple parallel arrangement of hair cells in the semicircular canals, the organization of hair cell clusters in both the utricle and the saccule is more complex (Fig. 4). A curved border called the striola crosses the surface of the macule in both the utricle and the saccule. The functional axes of all hair cells are oriented at right angles to the striola, producing an overall fan-shaped arrangement. Thus, a tilt of the head in any direction will activate the subpopulation of hair cells that has a corresponding axis of polarity, inhibit those with opposite polarity, and have no effect on those orthogonally aligned. These sets of signals from the utricle and saccule allow the brain to continuously monitor the position of the head in space.

FIGURE 2 Structure and function of vestibular hair cells. (A) Hair cells are modified epithelial cells with the tips of the "hairs," or stereocilia, immersed in the unique fluid endolymph, which has a potassium concentration higher than intracellular concentrations. When the cilia are deflected toward the large kinocilium, tiny elastic fibers open potassium channels in the tips of the stereocilia, allowing potassium to move in and depolarizing the cell. (B) to (D). Movement changes the membrane potential, regulates the release of the neurotransmitter (glutamate), and changes the frequency of action potentials in the afferent neurons.

Vestibular Nuclei and Central Vestibular Pathways

B. Macula in on the floor of the utricle

A. Vestibular system semicircular canals anterior posterior horizontal

perilymph endolymph utricle with macula saccule with macula

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