Vestibular System

Changes in the motion and position of the head are detected by hair cells in the vestibular apparatus of the inner ear (Figure 9-39), a series of fluid-filled membranous tubes that connect with each other and with the cochlear duct. The vestibular apparatus consists of three membranous semicircular ducts and two saclike swellings, the utricle and saccule, all of which lie in tunnels (canals) in the temporal bone on each side of the head. The bony canals of the inner ear in which the vestibular apparatus and cochlea are housed have such a complicated shape that they are sometimes called the labyrinth. The canals that house the semicircular ducts are the semicircular canals (a term often used to denote the semicircular ducts).

The Semicircular Canals

The semicircular ducts detect angular acceleration during rotation of the head along three perpendicular axes. The three axes of the semicircular canals and the ducts within them are those activated while nodding the head up and down as in signifying "yes," shaking the head from side to side as in signifying "no," and tipping the head so the ear touches the shoulder (Figure 9-40).

Semicircular duct

Utricle

Semicircular canal

Cavity in temporal bone

Semicircular duct

Utricle

Semicircular canal

Cavity in temporal bone

Semicircular Ducts

Ampulla Cochlear duct Cochlea

FIGURE 9-39

A tunnel in the temporal bone contains a fluid-filled membranous duct system, part of which can be seen in this cutaway view. The semicircular duct, utricle, and saccule make up the vestibular apparatus. This duct system is connected to the cochlear duct. The purple region on the ducts indicates the locations of the hair (receptor) cells.

Redrawn from Hudspeth. %

Ampulla Cochlear duct Cochlea

FIGURE 9-39

A tunnel in the temporal bone contains a fluid-filled membranous duct system, part of which can be seen in this cutaway view. The semicircular duct, utricle, and saccule make up the vestibular apparatus. This duct system is connected to the cochlear duct. The purple region on the ducts indicates the locations of the hair (receptor) cells.

Redrawn from Hudspeth. %

Semicircular Duct

FIGURE 9-40

Relation of the two sets of semicircular canals.

FIGURE 9-40

Relation of the two sets of semicircular canals.

Receptor cells of the semicircular ducts like those of the organ of Corti, contain hairlike stereocilia. These stereocilia are closely ensheathed by a gelatinous mass, the cupula, which extends across the lumen of each semicircular duct at the ampulla, a slight bulge in the wall of each duct (Figure 9-41). Whenever the head is moved, the bony semicircular canal, its enclosed duct, and the attached bodies of the hair cells all move with it. The fluid filling the duct, however, is not attached to the skull, and because of inertia, tends to retain its original position (that is, to be "left behind"). Thus, the moving ampulla is pushed against the stationary fluid, which causes bending of the stereocilia and alteration in the rate of release of a chemical transmitter from the hair cells. This transmitter activates the nerve terminals synapsing with the hair cells.

The speed and magnitude of rotational head movements determine the direction in which the stere-ocilia are bent and the hair cells stimulated. Neuro-transmitter is released from the hair cells at rest, and the release changes from this resting rate according to the direction in which the hairs are bent. Each hair cell receptor has one direction of maximum neurotrans-mitter release, and when its stereocilia are bent in this direction, the receptor cell depolarizes. When the stereocilia are bent in the opposite direction, the cell hyperpolarizes (Figure 9-42). The frequency of action potentials in the afferent nerve fibers that synapse with the hair cells is related to both the amount of force bending the stereocilia on the receptor cells and to the direction in which this force is applied.

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

II. Biological Control Systems

9. The Sensory Systems

© The McGraw-Hill Companies, 2001

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Semicircular duct

Cupula

Ampulla wall

Cupula

Ampulla wall

Ampulla Vestibular System

At rest

Cupula -Hair cell

Pressure -exerted by stationary fluid

Rotation of head ^^

Ampulla

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Responses

  • Lena
    What contains receptors for rotation in semicircular ducts?
    4 years ago

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