Cochlea
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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