The Human Ear
2.1 BRIEF DESCRIPTION OF THE EAR
2.1.4 Cochlear Duct or Partition
The cochlear duct (see Figure 2.3), which divides the cochlea into upper and lower galleries, is triangular in cross-section, being bounded on its upper side next to the scala vestibuli by Reissner's membrane, and on its lower side next to the scala tympani by the basilar membrane. On its third side it is bounded by the stria vascularis, which is attached to the outer wall of the cochlea. The cochlear duct is anchored at its apical end to a bony ridge on the inner wall of the cochlear duct formed by the core of the cochlea. The auditory nerve is connected to the central partition through the core of the cochlea.
The closed three sides of the cochlear duct form a partition between the upper and lower galleries and hence the alternative name of cochlear partition. It has been
outer hair cells scala media (endolymph)
Hensen's stripe
tectorial membrane inner
hair cell
basilar
membrane tunnel
of Co rti cochlear
nerve scala tympani
(perilymph) Reissner's membrane stria vascularis
rods of Co rti
spiral sulcus reticular
lamina hair cell
stereocilia
scala vestibuli (perilymph) scala tympani
(perilymph) cochlear duct
(endolymph)
(perilymph)
(a)
(b)
Figure 2.3 (a) Cross-section of the cochlea. (b) Cross-sectional detail of the organ of Corti.
suggested that the potassium rich endolymph of the cochlear duct supplies the nutrients for the cells within the duct, as there are no blood vessels in this organ.
Apparently, the absence of blood vessels avoids the background noise which would be associated with flowing blood, because it is within the cochlear duct, in the organ of Corti, that sound is sensed by the ear.
The organ of Corti, shown in Figure 2.3, rests upon the basilar membrane next to the bony ridge on the inner wall of the cochlea and contains the sound-sensing hair cells. The sound-sensing hair cells in turn are connected to the auditory nerve cells which pass through the bony ridge to the organ of Corti. The supporting basilar membrane attached, under tension, at the outer wall of the cochlea to the spiral ligament (see Figure 2.3) provides a resilient support for the organ of Corti.
The cochlear partition, the basilar membrane and upper and lower galleries form a coupled system much like a flexible walled duct discussed in Section 1.3.4. In this system sound transmitted into the cochlea through the oval window proceeds to travel along the cochlear duct as a travelling wave with an amplitude that depends on the flexibility of the cochlear partition, which varies along its length. Depending on its frequency, this travelling wave will build to a maximum amplitude at a particular location along the cochlear duct as shown in Figure 2.2 and after that location, it will decay quite rapidly. This phenomenon is analysed in detail in Section 2.2.1.
Thus, a tonal sound, incident upon the ear results in excitation along the cochlear partition that gradually increases up to a place of maximum response. The tone is sensed in the narrow region of the cochlear partition where the velocity response is a maximum. The ability of the ear to detect the pitch (see Section 2.4.5) of a sound appears to be dependent (among other things that are discussed in Section 2.1.6 below) upon its ability to localise the region of maximum velocity response in the cochlear partition and possibly to detect the large shift in phase of the partition velocity response from leading to lagging the incident sound pressure in the region of maximum response.
The observations thus far may be summarised by stating that any sound incident upon the ear ultimately results in a disturbance of the cochlear partition, beginning at the stapes (basal) end, up to a length determined by the frequency of the incident sound. It is to be noted that all stimulus components of frequencies lower than the maximum response frequency result in some motion at all locations towards the basal end of the cochlear partition where high frequencies are sensed. For example, a heavy base note drives the cochlear partition over its entire length to be heard at the apical end. The model, as described thus far, provides a plausible explanation for the gross observation that with time and exposure to excessive noise, high-frequency sensitivity of the ear is progressively lost more rapidly than is low-frequency sensitivity (see Section 4.2.3).
As is well known, the extent of the subsequent disturbance induced in the fluid of the inner ear will depend upon the frequency of the incident sound. Very low-frequency sounds, for example 50 Hz, will result in motion of the fluid over nearly the entire length of the cochlea. Note that such motion is generally not through the helicotrema except, perhaps, at very low frequencies well below 50 Hz.
High-frequency sounds, for example 6 kHz and above, will result in motion restricted to about the first quarter of the cochlear duct nearest the oval window. The
situation for an intermediate audio frequency is illustrated in Figure 2.2. An explanation for these observations is proposed in Section 2.2.1