The cochlea spirals around a conical bony axis, the central pillar of cochlea, or modiolus(AC1), which contains the neurons of the spiral ganglion (AB2), the nervefibers originating from them (AB3), and the radix cochlearis(A4) in the center. A double plate of bone, the osseous spirallamina (A – C5), protrudes from the modio-lus far into the cochlear duct (A6, B). It forms a spiral like the cochlea, but it does not reach into the end of the uppermost convolution and terminates in a free, hook-like process, the hamulus of spiral lamina(C7). The bony spiral lamina is mostly hol-low and contains nerve fibers extending to the organ of Corti. Opposite to it at the lateral wall, in the lower half of the basal convolution, lies the secondary spirallamina.
The spiral canal of cochlea contains the membranous cochlear duct (scala media) (A – C8) which is filled with endolymph. Above the duct lies the scalavestibuli (A – C9) and below it the scala tympani (A – C10); both these spaces contain perilymph. The lower wall of the cochlear duct is formed by the basilar membrane (B11) which carries thesensory receptor for hearing, the organ ofCorti(B12). The width of the membrane var-ies in individual convolutions. The fine fibers of the membrane radiate like a fan prior to attaching to the lateral wall of the cochlear canal, forming the spiral ligamentof cochlea (B13), which looks like a sickle incross section. Its part above the basilar membrane forms the lateral wall of the cochlear duct; it is known as vascular stria (B14) because it is rich in endolymph-pro-ducing capillaries. The upper wall of the cochlear duct is a thin membrane of double-layered epithelium, Reissner’s membrane or vestibular wall of the cochlear duct (B15).
The scalavestibuli communicates with the perilymphatic space of the vestibule and turns into the scala tympani at the heli-cotrema(AC16). The scala tympani runstoward the cochlear window which is closed by the secondary tympanicmembrane. The connection between thetwo ducts is made possible by separation of the spiral lamina from the modiolus and formation of the hamulus. In this way, the helicotrema is created medially. Only the scalavestibuli and the cochlear duct ascend to the uppermost tip of the cochlea, the cupula (A17). In contrast to the rest of the cochlea, the cupula thus contains only two membranous spaces.
Frequency analysis in the cochlea.Theoscillations of sound waves are transmitted to the perilymph through the vestibular window via eardrum and auditory ossicles. The resulting movements of the fluid as-cend in the scalavestibuli and descend in the scala tympani to the cochlear window, where the waves of movement are absorbed (C). Movements of the fluid lead to oscilla-tions of the basilar membrane (traveling waves). The site of maximal displacement of the basilar membrane (and hence stimula-tion of the receptor cells in the organ of Corti) depends on the frequency of the traveling wave or stimulating sound. High tonal frequencies cause maximal displace-ment of the basilar membrane in the basal convolutions (where the basilar membrane is narrow), middle frequencies in the middle of the cochlea, and low frequencies in the uppermost convolutions (where the basilar membrane is wide). Hence, different frequencies are registered in different parts of the cochlea, namely, frequencies of 20 000 Hz in the basal convolutions and frequencies of 20 Hz in the uppermost con-volutions. This local arrangement provides the basis of the tonotopic organization of the acoustic system.
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