Like cochlear hair cells, each vestibular hair cell contains a set of stereocilia on its apical surface. However, unlike cochlear hair cells, vestibular hair cells also contain a large single kinocilium. The cilia on ampullary hair cells are embedded in a gelatinous structure called the cupula. The cupula and the crista occlude the lumen of the ampulla completely. Movement of endolymph, produced by angular acceleration of the head about an axis perpendicular to the plane of the canal, deflects the cupula and consequently bends the cilia on the hair cells. The cupula has the same specific gravity as endolymph, and thus it is unaffected by linear acceleratory forces, such as gravity.
The sensory epithelia of the otolith organs are called the macula utriculi and the macula sacculi (
Fig. 8.25 ). The utricle is oriented nearly horizontally; the saccule is oriented vertically. Their hair cells are embedded in the epithelium that overlies each macula. As in the ampullary crests, the stereocilia and kinocilia of the macula project into a gelatinous mass. However, the gelatinous mass in the macula contains numerous otoliths (“ear stones”) composed of calcium carbonate crystals. Together, the gelatinous mass and its otoliths are known as an otolithic membrane. The otoliths increase the specific gravity of the otolithic membrane to about twice that of the endolymph. Hence, the otolithic membrane tends to move when subjected to acceleration, whether linear (such as that produced by gravity) or angular, particularly when the center of rotation is outside the head.
• Fig. 8.23 A, Lateral view of the right semicircular canals of a rhesus monkey that were dissected after being filled with plastic. Note the ampullae associated with each canal. Scale is in millimeters. B,Overheadviewofthebaseoftheskullshowingtheorientationofstructuresoftheinnerear.Coplanar pairsofsemicircularcanalsincludethehorizontalcanals,aswellastheanteriorandcontralateralposterior canals.(A,CourtesyofDr.JohnSimpson,New YorkUniversity SchoolofMedicine.B,Redrawnfrom Haines DE [ed]. Fundamental Neuroscience for Basic and Clinical Applications. 3rd ed. Philadelphia: ChurchillLivingstone;2006.) •Fig. 8.24 Drawing of an Ampullary Crest Inside an Ampulla.Thestereocilia andthekinocilium of eachhaircellextendintothecupula,whichextendsacrosstheentirecross-sectionoftheampulla.Head movement(acceleration)totherightwouldresultinendolymphpressuretotheleftanddeflectionofthe cupulatotheleft.
Innervation of Sensory Epithelia of the  Vestibular Apparatus
The cell bodies of the primary afferent fibers of the vestibular nerve are located in the Scarpa ganglion. The neurons are bipolar, and their cell bodies, as well as axons, are myelinated. Peripherally, the vestibular nerve gives off separate • Fig. 8.25 Structure of one of the otolith organs, the saccule. Note the orderly variation in kinocilium orientation, as well as their mirror symmetry with regard to the striola. (Redrawn from Lindeman HH. Adv Otorhinolaryngol 1973;20:405.)