FIGURE 25.10 • Diagram of two types of sensory hair cells in the sensory areas of the membranous labyrinth. The type I hair cell has a flask-shaped structure with a rounded base. The base is enclosed in a chalicelike afferent nerve ending that has several synaptic boutons for efferent nerve endings. Note the apical surface specializations of this cell, which include a kinocilium and hair bundle. The apical cytoplasm of hair cells contains basal bodies for the attachment of the kinocilium and a terminal web for the attachment of stereocilia. The type II hair cell is cylindrical and possesses several nerve terminals at its base for both afferent and efferent nerve fibers. The apical surface specializations are identical to those of the type I cell. The molecular organization of the stereocilia is depicted in the enlarged rectangle. The top link connects the lateral plasma membrane of the stereocilium shaft (where K transduction channels are located) with the tip of the shorter stereocilium (where the mechanoelectric transduction [MET] channel protein is located). Note that the proximal end of each stereocilium is tapered and its narrow rootlets are anchored within the terminal web (cuticular plate) of the hair cell. Several other fibrillar connectors between neighboring stereocilia are also shown.
 FOLDER 25.3 Clinical Correlation: Vertigo
The sensation 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 neuro-mas 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. Vertigo can also be produced normally in individuals by excessively stimulating the semicircular ducts. Similarly, excessive stimulation of the utricle can produce motion sickness (seasickness, car-sickness, or airsickness) in some individuals. Some diseases of the internal ear affect both hearing and equilibrium. For example, people with Ménière’s syn-drome initially complain of episodes of dizziness and tinni-tus (ringing in the ears) 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).
FIGURE 25.11 • 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.
movement differential between the crista fixed to the wall of the duct and the endolymph. Deflection of the stereocilia in the narrow space between the hair cells and the cupula generates nerve impulses in the associated nerve endings.
The maculae of the saccule and utricle are sensors of gravity and linear acceleration.
The maculae of the saccule and the utricle are innervated sensory thickenings of the epithelium that face the endolymph of the saccule and utricle (see Fig. 25.8c). As in the cristae, each macula consists of type I and type II hair cells, supporting cells, and nerve endings associated with the hair cells. The maculae of the utricle and saccule are oriented at right angles to each another. When a person is standing, the macula of the utricle is in a horizontal plane, and the macula of the saccule is in a vertical plane.