IntheRinne test, avibratingtuningforkisplacedagainstthebonebehindtheperson’sear,andthepersonisaskedtoindicatewhenthesounddiesout.Thetuningforkisthenheldneartheexternalauditorymeatusofthatear.Inpeoplewithnormalhearing,thesoundisagainheardbecausethesoundismoreeffectivelytransmittedtothecochleainair(i.e.,airconduction> boneconduction).Iftheconductionmechanismisdamaged,thesoundisnotheardwhenthetuningforkisheldneartheexternalauditorymeatus.Boneconductioninthiscaseisbetterthanairconduction.Ifthehearinglossissensorineural,thesoundisheardagainwhenthetuningforkisplacedbytheexternalauditorymeatusbecausewithsensorineuralhearingloss,theinnerearandcochlearnervearelessabletotransmitimpulsesregardlessofwhetherthesoundvibrationsreachthecochleaviaairorbone.Thusbecauseairconductionismoreeffectivethanboneconduction,theboneconductionpatternseenwithsensorineuralhearinglossisthesameasinanormalear.
The Vestibular System
The vestibular system detects angular and linear accelerations of the head. Signals from the vestibular system allow the body to make adjustments in posture that maintain balance and trigger head and eye movements to stabilize the visual image on the retina. The following description of the vestibular system emphasizes the sensory aspects of vestibular function, and it introduces the central vestibular pathways. The role of the vestibular apparatus in motor control is discussed in
The Vestibular Apparatus
Structure of the Vestibular Labyrinth
The vestibular apparatus, like the cochlea, consists of a component of the membranous labyrinth located within
CHAPTER 8 The Special Senses the bony labyrinth. The vestibular apparatus on each side is composed of three semicircular canals and two otolith organs (
Fig. 8.23
Fig. 8.17A).
These structures contain endolymph and are surrounded by perilymph. The semicircular canals are named the horizontal, anterior, and posterior canals. The otolith organs are the utricle and the saccule (together indicated as “Vestibule” in
Fig. 8.17A ). Each semicircular canal has a swelling called an ampulla at the point where it joins the utricle. The saccule connects with the cochlea, through which endolymph (produced by the stria vascularis of the cochlea) can reach the vestibular apparatus.
The three semicircular canals on one side are matched with corresponding coplanar semicircular canals on the other side. The horizontal canals on each side of the head correspond, as do the anterior canal on one side and the posterior canal on the other side (see
Fig. 8.23B ). This arrangement allows the sensory epithelia, in corresponding pairs of canals on the two sides, to cooperate in sensing acceleration of the head about three nearly orthogonal axes in space. The horizontal canals are not truly horizontal; rather, they lie in the horizontal plane if the head is tilted down 30 degrees in relation to the horizon.
The ampulla of each of the semicircular canals contains a sensory epithelium called a crista ampullaris, or ampullary crest (
Fig. 8.24 ). An ampullary crest consists of a ridge, transverse to the long axis of the canal, that is covered by epithelium containing vestibular hair cells. These hair cells are innervated by primary afferent fibers of the vestibular nerve, which is a subdivision of the vestibulocochlear nerve (CN VIII).