Figure 942

Relationship between position of hairs and action-potential firing in afferent neurons. (a) Resting activity. (b) Movement of hairs in one direction increases the action-potential frequency in the afferent nerve activated by the hair cell. (c) Movement in the opposite direction decreases the rate relative to the resting state. %

Redrawn from Wersall, Gleisner, and Lundquist.

At a constant velocity, the duct fluid begins to move at the same rate as the rest of the head, and the stereocilia slowly return to their resting position. For this reason, the hair cells are stimulated only during changes in the rate of rotation (that is, during acceleration or deceleration) of the head.

The Utricle and Saccule

The utricle and saccule provide information about linear—up and down, back and forth—acceleration and changes in head position relative to the forces of gravity. Here, too, the receptor cells are mechanoreceptors sensitive to the displacement of projecting hairs. The patch of hair cells in the utricle is nearly horizontal in a standing person, and that in the saccule is vertical.

The stereocilia projecting from the hair cells are covered by a gelatinous substance in which tiny stones, or otoliths, are embedded; the otoliths, which are calcium carbonate crystals, make the gelatinous substance heavier than the surrounding fluid. In response to a change in position, the gelatinous otolithic material moves according to the forces of gravity and pulls against the hair cells so that the stereocilia on the hair cells are bent and the receptor cells stimulated.

Vestibular Information and Dysfunction

Information about hair cell stimulation is relayed from the vestibular apparatus to the brainstem via the vestibular branch of cranial nerve VIII (the same cranial nerve that carries acoustic information). It is transmitted via a multineuronal pathway to a system of vestibular centers in the parietal lobe. Vestibular information is integrated with information from the joints, tendons, and skin, leading to the sense of posture and movement.

Vestibular information is used in three ways. One is to control the eye muscles so that, in spite of changes in head position, the eyes can remain fixed on the same point. Nystagmus is a large, jerky, back-and-forth movement of the eyes that can occur in response to unusual vestibular input in normal people but can also be a pathological sign.

The second use of vestibular information is in reflex mechanisms for maintaining upright posture. The vestibular apparatus plays a role in the support of the head during movement, orientation of the head in space, and reflexes accompanying locomotion. Very few postural reflexes, however, depend exclusively on input from the vestibular system despite the fact that the vestibular organs are sometimes called the sense organs of balance.

The third use of vestibular information is in providing conscious awareness of the position and acceleration of the body, perception of the space surrounding the body, and memory of spatial information.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Unexpected inputs from the vestibular system and other sensory systems such as those caused by a stroke, irritation of the labyrinths by infection, or even loose particles of calcium carbonate in the semicircular ducts can induce vertigo, defined as an illusion of move-ment—usually spinning—that is often accompanied by feelings of nausea and lightheadedness. This can also occur when there is a mismatch in information from the various sensory systems as, for example, when one is in a high place looking down to the ground: The visual input indicates that you are floating in space while the vestibular system signals that you are not moving at all. Motion sickness also involves the vestibular system, occurring when unfamiliar patterns of linear and rotational acceleration are experienced and adaptation to them has not occurred.

Meniere's disease involves the vestibular system and is associated with episodes of abrupt and often severe dizziness, ringing in the ears, and bouts of hearing loss. It is due to an increased fluid pressure in the membranous duct system of the inner ear. The dizziness occurs because the inputs from the two ears are not balanced, either because only one ear is affected or because the two are affected to different degrees.

Tongue epithelium

Receptor cell

Tongue epithelium

Receptor cell

Nerve fiber

Outer taste pore

FIGURE 9-43

Structure and innervation of a taste bud. Unlike the taste bud here, each taste bud contains 100 or so receptor cells. The three different types of cells are actually receptor cells in different stages of development. %

Outer taste pore

Nerve fiber

FIGURE 9-43

Structure and innervation of a taste bud. Unlike the taste bud here, each taste bud contains 100 or so receptor cells. The three different types of cells are actually receptor cells in different stages of development. %

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