The Semicircular Canals

The relative position of the kinocilium establishes the functional polarity of the hair cell—namely, which direction of movement excites the cell and which inhibits the cell. Directional sensitivity of each of the individual vestibular organs is determined by the specific orientation of hair cells within the organ and the overall shape of the organ. In the semicircular canals, hair cells are clustered in a thickened zone of epithelium, the ampullary crest. A gelatinous, diaphragm-like mass, the cupula covers the kinocilia and stereocilia of the hair cells and stretches to the roof of the ampulla. When the head is rotated, the inertia of the fluid in the semicircular canals generates a force against the stereocilia, causing them to bend.

The three pairs of semicircular canals are orthogonally oriented with respect to one another, and matched pairs on either side of the head respond to similar angles of movements. However, the response of each member of the pair is equal and opposite. The functional polarity of the hair cells in the pair of lateral or horizontal canals is shown in Fig. 3. In this example, the head turns to the left but the endolymph lags behind because of inertia, causing the fluid to be shifted or displaced to the right. This moves the stereocilia in the left canal in an excitatory direction and stereocilia in the right canal in an inhibitory direction. The opposite occurs when the head is turned to the right.

The anterior semicircular canal on one side lies approximately in the same plane as the posterior canal on the opposite side, and they act in concert as a functional pair similar to the pair of horizontal canals. The sum effect is that when the head tilts or turns in any of the three ordinal directions, the brain receives two reports: an increase in the firing rate of vestibular nerve fibers from one side and a decrease from the opposite side.

By comparing the activity of all three pairs of canals, the brain can perform a vector analysis and compute the precise head position during any turning movement. The semicircular canals signal changes in angular or rotational acceleration and do not transmit information about steady-state head position.


The utricle and saccule each contains a region of specialized epithelium, called the macula, which is analogous to the ampullary crests of the semicircular canal. Cilia of hair cells within this region project into the otolithic membrane, an overlying gelatinous matrix studded with small accretions of calcium called otoliths. The macula of the utricle lies roughly in the horizontal plane when the head is held erect, so that otoliths rest directly upon it. If the head is tilted or undergoes linear acceleration in the horizontal plane (e.g., when accelerating in a car), the otoliths deform the gelatinous mass, which in turn bends the cilia of the hair cells. Similar mechanisms operate in the saccule; however, the macula of the saccule is oriented vertically when the head is in its normal position and thus selectively responds to vertically directed linear force (e.g., when riding in an elevator). Because of the constant gravitational pull on the otoliths, hair cells from both of these organs transmit tonic information about the position of the head in space as well as

A. Vestibular hair cell

B. no movement C. movement toward kinocilium D. movement away from kinocilium

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