Figure 2410

Diagram of the crista ampullaris within a semicircular duct. The cellular organization of the crista ampullaris of a semicircular duct is shown in the large diagram and the enlarged rectangle. The crista ampullaris is composed of both type I and type II sensory hair cells and supporting cells. The stereocilia and kinocilium of each hair are embedded in the cupula that projects toward the nonsensory wall of the ampulla.

son is standing, the macula of the utricle is in a horizontal plane, and the macula of the saccule is in a vertical plane.

The gelatinous polysaccharide material that overlies the maculae is called the otolithic membrane (Fig. 24.11). Its outer surface contains 3- to 5-/xm crystalline bodies of calcium carbonate and a protein (Fig. 24.12). Otoliths are heavier than the endolymph. The outer surface of the otolithic membrane lies opposite the surface in which the stereocilia of the hair cells are embedded. The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista. Stereocilia of the hair cells are bent by gravity in the stationary individual when the otolithic membrane and its otoliths pull on the stereocilia. They are also bent during linear movement when the individual is moving in a straight line and the otolithic membrane drags on the stereocilia because of in ertia. In both cases, movement of the otolithic membrane triggers an action potential.

The spiral organ of Corti is the sensor of sound vibrations

The cochlear duct divides the cochlear canal into three parallel compartments or scalae:

• Scala media, the middle compartment in the cochlear canal

• Scala vestibuli

• Scala tympani

The cochlear duct itself is the scala media (Figs. 24.13 and 24.14). The scala vestibuli and scala tympani are the spaces above and below, respectively, the scala media. The scala media is an endolymph-containing space that is continuous with the lumen of the saccule and contains the spiral organ of Corti, which rests on its lower wall (see Fig. 24.14).

The scala vestibuli and the scala tympani are perilymph-containing spaces that communicate with each other at the apex of the cochlea through a small channel called the be-licotrema (see Fig. 24.13). The scala vestibuli begins at the

The sense of rotation without equilibrium (dizziness, vertigo) signifies dysfunction of the vestibular system. Causes of vertigo include viral infections, certain drugs, and tumors such as acoustic neuroma. Acoustic neuromas develop in or near the internal acoustic meatus and exert pressure on the vestibular division of cranial nerve VIII or branches of the labyrinthine artery. Also, vertigo can be produced normally in individuals by excessive stimulation of the semicircular ducts. Similarly, excessive stimulation of the utricle can produce motion sickness (seasickness, carsickness, or airsickness) in some individuals.

Some diseases of the internal ear affect both hearing and equilibrium. For example, people with Ménière's syndrome initially complain of episodes of dizziness and tinnitus (ringing) and later develop a low-frequency hearing loss. The causes of Ménière's syndrome are related to blockage of the cochlear aqueduct, which drains excess endolymph from the membranous labyrinth. Blockage of this duct causes an Increase in endolymphatic pressure and distension of the membranous labyrinth (endolymphatic hydrops).

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