The auditory system may be considered to consist of three components (Evans & Christensen, 1979): the outer, middle, and inner ears. The outer ear, consisting of the pinna and ear canal extending up to the tympanic membrane, serves to direct sound waves toward the receptor organ. Considerable variation exists in the conformation of the pinna between species and breeds, but the attached muscles enable orientation of the external ear toward sound sources (King, 1993). Coordinated movements of both pinnae still occurs in animals with unilateral deafness when alerting to auditory stimuli. Analyses of the human ear canal have shown that its shape and dimensions optimize the transmission of the sound frequencies important in speech communication, but similar analyses have not been done for domestic species. Apocrine glands in the skin of the canal produce cerumen, a sebaceous product of cellular breakdown that serves to cleanse the canal (Johnson & Hawke, 1988). Great variation in the rate of cerumen production occurs, with long-haired dog breeds being more prolific cerumen producers and hence requiring greater grooming attention. Chronic infections may result in stenosis or occlusion of the canal and blockage of sound transmission.
The air-filled middle ear includes the tympanic membrane, the ossicles (malleus, incus, stapes), their associated muscles and ligaments, and the opening of the auditory tube, which provides communication with the pharynx as well as a route for infection. Sound vibrations in the ear canal are transmitted to the tympanic membrane, and in turn are transmitted through the articulations of the ossicles to the attachment of the foot plate of the stapes on the membrane of the oval window. The ossicles amplify the vibrations of sound and in turn pass them on to the fluid-filled inner ear. The ossicular muscles, the stapedius and the tensor tympani, enable reflex damping of sound transmission in response to abrupt noises and in anticipation of loud vocalization by reducing ossicle movement. Innervation of the stapedius and tensor tympani muscles is by the trigeminal and facial nerves, respectively.
The cochlea (Lat.: snail shell) and the semicircular canals constitute the inner ear. The cochlea, a coiled structure enclosing three fluid-filled chambers (Figure 1), is encased in the temporal bone with two membranous surfaces exposed at its base: the oval window and the round window. The foot plate of the stapes adheres to the oval window, transmitting sound vibrations into the cochlea. Two of the three cochlear chambers are contiguous at the apex. Inward deflections of the oval window caused by the foot plate of the stapes compress the fluid in the scala vestibuli; this compression wave travels along the coils of the cochlea in the scala vestibuli to the apex, then travels back down the coils in the scala tympani. The round window serves as a pressure-relief vent, bulging outward with inward deflections of the oval window. The third cochlear chamber, the scala media or cochlear duct, is positioned between the scala vestibuli and scala tympani. Pressure waves from sound traveling up the scala vestibuli and back down the scala tympani produce a shearing force on the hair cells of the organ of Corti in the cochlear duct. The hairs (cilia) of the hair cells are imbedded in the gelatinous tectorial membrane (Figure 2), which has a relatively high inertial resistance to movement, so that sound-induced shearing forces bend the hairs. This bending produces mechanical opening of ionic channels (Fettiplace, 1990), depolarizing the hair cells due to K+ influx. Within the cochlea, hair cell sensitivity to frequencies progresses from high frequencies at the base to low frequencies at the apex. The cells in the single row of inner hair cells passively respond to deflections of sound-induced pressure waves. Cells in the rows of outer hair cells can elongate or shorten in response to motion of the basilar membrane to actively produce amplification or attenuation of the response of the inner hair cells (Møller, 1993). Efferent innervation by fibers from the olivary nucleus caudally and the dorsal nucleus of the trapezoid body rostrally also provide regulation of the sensitivity of the inner and outer hair cells (Liberman, 1991). The scala vestibuli and scala tympani are filled with perilymph, similar in composition to extracellular fluid, while the cochlear duct is filled with endolymph, similar in composition to intracellular fluid. The hair cells synapse on processes of neurons of the spiral ganglia, initiating signal transmission into the central nervous system via the eighth cranial nerve. On the lateral wall of the cochlear duct is the stria vascularis, a three-cell-layer thick vascularized epithelium not bounded by a basal lamina (Santi, 1988). The tissue is rich in Na+-K+-ATPase, responsible for secretion of high levels of K+ into the endolymph of the cochlear duct. Also present in the stria vascularis are melanocytes, which appear critical to the maintenance of the stria (see below) but whose function at this site is unknown.
Fig. 1. Schematic diagram of a cross-section of the cochlea, demonstrating the scala vestibuli, scala tympani and scala media or cochlear duct. The organ of Corti rests on the basilar membrane, with the hair cell cilia imbedded in the gelatinous tectorial membrane. The outer margin of the cochlear duct contains the stria vascularis. (From Bloom, W. & Fawcett, D.W. (1975). A Textbook of Histology, 10th edn. Philadelphia: W.B. Saunders. Reproduced with permission.)
Fig. 2. Drawing of the organ of Corti, demonstrating the inner and outer hair cells and the spiral ganglion cells that become the cochlear nerve. (From Pansky, B. & Allen, D.J. (1980), Review of Neuroscience, New York: Macmillan. Reproduced with permission.)