The cochlea, a component of the bony labyrinth, is divided into multiple chambers. The vestibule is the space facing the oval window. It is continuous with the scala vestibuli, a spiral-shaped chamber that extends to the apex of the cochlea and merges with the scala tympani at the helicotrema. The scala tympani is another spiral-shaped space that descends back down the cochlea and terminates at the round window. The scala media, enclosed in the membranous labyrinth, separates the scala vestibuli and scala tympani, except at the helicotrema. The scala media, also known as the cochlear duct, is a spiral tube bounded by the basilar membrane, Reissner's membrane, and the stria vascularis. 

The cochlea contains fluid-filled spaces. The fluid in the bony labyrinth, including the scala vestibuli and scala tympani, is called perilymph and closely resembles cerebrospinal fluid. The fluid in the membranous labyrinth, including the scala media, is called endolymph, which differs significantly from perilymph. Endolymph, produced by the stria vascularis, has a high concentration of potassium ions ([K+]) and low concentration of sodium ions ([Na+]), and exhibits a positive potential in relation to the perilymph. As a result, there exists a large potential gradient across the hair cell cilia membranes that extend into the endolymph. 

The organ of Corti, situated within the cochlear duct, is responsible for sound transduction. It is positioned on the basilar membrane and comprises three rows of outer hair cells, one row of inner hair cells, a gelatinous tectorial membrane, and various supporting cells. In humans, the organ of Corti contains 15,000 outer hair cells and 3500 inner hair cells. The rods of Corti provide structural support. Stereocilia, located on the apical surface of the hair cells, make contact with the tectorial membrane.

The organ of Corti receives innervation from nerve fibers of the cochlear division of the vestibulocochlear nerve (CN VIII). In humans, approximately 32,000 auditory afferent fibers originate from sensory ganglion cells in the spiral ganglion. These nerve fibers penetrate the organ of Corti and terminate at the bases of the hair cells. About 90% of the fibers end on inner hair cells, while the remaining fibers terminate on outer hair cells. Therefore, each inner hair cell receives input from approximately 10 afferent fibers, whereas each outer hair cell is supplied by a smaller number of afferent fibers. The inner hair cells primarily convey neural information about acoustic signals that the central nervous system processes for hearing, while the role of the outer hair cells in sensory function is less clear.
Fig. 8.17A ). The bony labyrinth component of the cochlea is subdivided into several chambers. The vestibule is the space facing the oval window (see
Fig. 8.17A ). Continuous with the vestibule is the scala vestibuli, the spiral-shaped chamber that extends to the apex of the cochlea, where it meets and merges with the scala tympani at the helicotrema. The scala tympani is another spiral-shaped space that winds back down the cochlea and ends at the round window (see
Fig. 8.17B ). Separating the two, except at the helicotrema, is the scala media enclosed in the membranous labyrinth.
The scala media, or cochlear duct (see
Fig. 8.17B and ), is a membrane-bound spiral tube that extends along the cochlea, between the scala vestibuli and scala tympani. One wall of the scala media is formed by the basilar membrane, another by Reissner’s membrane, and the third by the stria vascularis (see
Fig. 8.17C
The spaces within the cochlea are filled with fluid. The fluid in the bony labyrinth, including the scala vestibuli and scala tympani, is perilymph, which closely resembles cerebrospinal fluid. The fluid in the membranous labyrinth, including the scala media, is endolymph, which is very different from perilymph. Endolymph, generated by the stria vascularis, contains high [K+] (about 145 mM) and low [Na+] (about 2 mm) and has a high positive potential (about +80 mV) with regard to the perilymph. As a result, a very large potential gradient (about 140 mV) exists across the membranes of the hair cell cilia that extend into the endolymph. (These hair cells, which are the sensory receptors for sound, are discussed in more detail later.)
The neural apparatus responsible for transduction of sound is the organ of Corti (see
Fig. 8.17C ), which is located within the cochlear duct. It lies on the basilar membrane and consists of several components, including three rows of outer hair cells, a single row of inner hair cells, a gelatinous tectorial membrane, and a number of types of supporting cells. The organ of Corti in humans contains 15,000 outer and 3500 inner hair cells. The rods of Corti help provide a rigid scaffold. Located on the apical surface of the hair cells are stereocilia, which can be described as nonmotile cilia that contact the tectorial membrane.
The organ of Corti is innervated by nerve fibers of the cochlear division of the vestibulocochlear nerve (CN VIII). The 32,000 auditory afferent fibers in humans originate in sensory ganglion cells in the spiral ganglion. These nerve fibers penetrate the organ of Corti and terminate at the bases of the hair cells (
Fig. 8.18
Fig. 8.17C ). Approximately 90% of the fibers end on inner hair cells, and the remainder end on outer hair cells. Thus approximately 10 afferent fibers supply each inner hair cell, whereas other afferent fibers diverge to supply about five outer hair cells each. The inner hair cells clearly provide most of the neural information about acoustic signals that the CNS processes for hearing. The sensory function of the outer hair cells is less clear.