A higher magnification of one of the semicircular canals and of the crista ampullaris (CA) within the canal seen in the lower right corner of figure above is provided here. The receptor for movement, the crista ampullaris (note its relationships in figure above), is present in each of the semicircular canals. The epithelial (EP) surface of the crista consists of two cell types, sustentacular (supporting) cells and hair (receptor) cells. (Two types of hair cells are distinguished with the electron microscope.) It is difficult to identify the hair and sustentacular cells on the basis of specific characteristics; they can, however, be distinguished on the basis of location (see inset), as the hair cells (HC) are situated in a more superficial location than the sustentacular cells (SC). A gelatinous mass, the cupula (Cu), surmounts the epithelium of the crista ampullaris. Each receptor cell sends a hair-like projection deep into the substance of the cupula.
The epithelium rests on a loose, cellular connective tissue (CT ) that also contains the nerve fibers associated with the receptor cells. The nerve fibers are difficult to identify because they are not organized into a discrete bundle.
KEY C, cochlea CA, crista ampullaris CN, cochlear nerve CT, connective tissue Cu, cupula DS, duct system (of membranous labyrinth) EP, epithelium FN, facial nerve HC, hair cell M, modiolus OL, oval ligament S, stapes SC, sustentacular cell SG, spiral ganglion V, vestibule black arrow, entry to cochlea white arrow, entry to semicircular canal
PLATE 109 Cochlear Canal and Organ of Corti
The hair cell, a nonneuronal mechanoreceptor, is the common receptor cell of the vestibulocochlear system. Hair cells are epithelial cells that possess numerous stereocilia, modified microvilli also called sensory hairs. They convert mechanical energy to electrical energy that is transmitted via the vestibulocochlear nerve (cranial nerve VIII) to the brain. Hair cells are associated with afferent, as well as efferent, nerve endings. All hair cells have a common basis of receptor cell function that involves bending or flexing of their stereocilia. The specific means by which the stereocilia are bent varies from receptor to receptor, but in each case, stretching of the plasma membrane caused by the bending of the stereocilia generates transmembrane potential changes that are transmitted to the afferent nerve endings associated with each cell. Efferent nerve endings on the hair cells serve to regulate their sensitivity.
Cochlear canal, ear, guinea pig, H&E 65; inset 380.
A section through one of the turns of the cochlea is shown here. The most important functional component of the cochlea is the organ of Corti, enclosed by the rectangle and shown at higher magnification in figure below. Other structures are included in this figure. The spiral ligament (SL) is a948 thickening of the periosteum on the outer part of the tunnel. Two membranes, the basilar membrane (BM ) and the vestibular membrane (VM), join with the spiral ligament and divide the cochlear tunnel into three parallel canals, namely, the scala vestibuli (SV ), the scala tympani (ST ), and the cochlear duct (CD). Both the scala vestibuli and the scala tympani are perilymphatic spaces; these communicate at the apex of the cochlea. The cochlear duct, on the other hand, is the space of the membranous labyrinth and is filled with endolymph. It is thought that the endolymph is formed by the portion of the spiral ligament that faces the cochlear duct, the stria vascularis (StV ). This is highly vascularized and contains specialized “secretory” cells.