The basilar membrane and resonance in the cochlea

The basilar membrane is a fibrous membrane that separates the scala media and the scala tympani. It contains 20000-30000 basilar fibres that project from the bony center of the cochlea. The modiolus, toward the outer wall. These fibers are stiff, elastic, reed-like structures that are fixed at their basal ends in the central part of the cochlea (the modiolus) but not fixed at their distal ends except that the distal ends are embedded in the loose basilar mebrane. Because the fibers are stiff and also free at one end, they can vibrate like reeds of a harmonica. The length of the basilar fibers increases progressively as one goes from the base of the cochlea to its apex. The diameters of the fibers, on the other hand, decrease from the base to the helicotrema so that their overall stiffness decreases more than 100-fold. As a result, the stiff, short fibers near the oval window of the oval window of the cochlea will vibrate at a high freaquency, whereas the long, limber fibers near the tip of the cochlea will vibrate at a low freaquency.

Thus, high freaquency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window and low freaquency resonance occurs near the apex mainly because of difference in stiffness of the fibers but also because of increasing “loading” of the basilar membrane with extra amounts of fluid that must vibrate with membrane at the apex.

Transmission of sound waves in the cochlea - the “travelling waves”

If the foot of the stapes moves inward instantaneously, the round window must also bulge outward instantaneously because the cochlea is bounded on the sides by bony walls. Therefore, the initial effect is to cause the basilar membrane at the very base of the cochlea to bulge in the direction of the found window. However, the elastic tension that is built up in the basilar fibers as they bend toward the round window initiates a wave that “travels” along the basilar membrane toward the helicotrema.

Corti organ functions

This organ is the receptor organ that generates nerve impulses in response to vibration of the basilar membrane. Note that it lies on the surface of the basilar fibers and basilar membrane. The actual sensory receptors are 2 types of hair cells, a single row of internal hair cells and 3-4 rows of external hair cells. The bases and sizes of the hair cells synapse with a network of cochlear nerve endings. These lead to the spiral ganglion of Corti, which lies in the modiolus (the center) of the cochlea. The spiral ganglion in turn sends axons into the cochlear nerve and hence into the CNS at the level of the upper medulla.

The auditory pathway

Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nucleus located in the upper part of the medulla. At this point, all the fibers synapse, and second-order neurons pass mainly to the opposite side of the brain stem through the trapezoid to the superior olivary nucleus. However, some second order neurons also pass ipsilaterally to the superior olivary nucleus on the same side. From the superior olivary nucleus the auditory pathway then passes upward through the lateral lemniscus; and some, but not all, of the fibers terminate in the nucleus of the lateral lemniscus. Many bypass this nucleus and pass on the inferior coliculus where either all or almost all of them terminate. From here, the pathway passes to the medial geniculate nucleus, where all the fibers gain synapse. And, finally, the auditory pathway proceeds by way of the auditory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe.

There are 3 crossing of auditory ways:

• at superior oliva level – less fibres part is remained in the limit of hemisphere on the side of which perypheral auditory is located; larger part comes in opposite hemisphere into midbrain; at trapezoid bodies level is partial crossing too; small parts of direct, uncrossing, fibres come from here (trapezoid bodies and superior oliva) to midbrain;

• at midbrain quadrigemina bodies – some fibres are crossed, another part – directly go to the nearest subcortical auditory centers – medial geniculate bodies;

• at cortical level – fibres come here from geniculate bodies.

Humans have binaural interaction or binaural hearing.