The main features of cochlear structure are seen in Figures 20.1 and 20.2. The cochlea is pictured as though it were upright, but in life it lies on its side, as shown earlier in Figure 19.1. The central bony pillar of the cochlea (the modiolus) is in the axis of the internal acoustic meatus. Projecting from the modiolus, like the flange of a screw, is the osseous spiral lamina. The basilar membrane is attached to the tip of this lamina; it reaches across the cavity of the bony cochlea to become attached to the spiral ligament on the outer wall. The osseous spiral lamina and spiral ligament become progressively smaller as one ascends the two and one half turns of the cochlea, and the fibers of the basilar membrane become progressively longer.

Figure 20.1 The cochlea in section.

Figure 20.2 Organ of Corti at three levels of magnification. Arrows indicate directions of impulse traffic. OCB, fibers of olivocochlear bundle.

The basilar membrane and its attachments divide the cochlear chamber into upper and lower compartments. These are the scala vestibuli and the scala tympani, respectively, and they are filled with perilymph. They communicate at the apex of the cochlea, through the helicotrema. A third compartment, the scala media (cochlear duct), lies above the basilar membrane and is filled with endolymph. It is separated from the scala vestibuli by the delicate vestibular membrane.

Sitting on the basilar membrane is the spiral organ (organ of Corti). The principal sensory receptor epithelium consists of a single row of inner hair cells, each one having up to 20 large afferent nerve endings applied to it. The hair cells rest upon supporting cells, and there are ancillary cells too. The organ of Corti contains a central tunnel, filled with perilymph diffusing through the basilar membrane. On the outer side of the tunnel are several rows of outer hair cells, attended by supporting and ancillary cells.

All of the hair cells are surmounted by stereocilia. Unlike the vestibular hair cells, they have no kinocilium in the adult state. The stereocilia of the outer hair cells are embedded in the overlying tectorial membrane. Those of the inner hair cells lie immediately below the membrane.

The outer hair cells are contractile (in tissue culture), and they have substantial efferent nerve endings (Figure 20.2). In theory, at least, oscillatory movements of outer hair cells could influence the sensitivity of the inner hair cells through effects on the tectorial or basilar membrane.

Sound transduction

The vibrations of the tympanic membrane in response to sound waves are transmitted along the ossicular chain. The footplate of the stapes fits snugly into the oval window, and vibrations of the stapes are converted to pressure waves in the scala vestibuli. The pressure waves are transmitted through the vestibular membrane to reach the basilar membrane. High-frequency pressure waves, created by high-pitched sounds, cause the short fibers of the basilar membrane in the basal turn of the cochlea to resonate and absorb their energy. Low-frequency waves produce resonance in the apical turn where the fibers are longest. The basilar membrane is therefore tonotopic in its fiber sequence. Not surprisingly, the inner hair cells have a similar tonotopic sequence. In response to local resonance, the cells become depolarized and liberate excitatory transmitter substance from synaptic ribbons (Figure 20.2).

The nerve fibers supplying the hair cells are the peripheral processes of the bipolar spiral ganglion cells lodged in the base of the osseous spiral lamina.

Cochlear nerve

The bulk of the cochlear nerve consists of the myelinated central processes of some 30,000 large bipolar neurons of the spiral ganglion. Unmyelinated fibers come from small ganglion cells supplying dendrites to the outer hair cells. (Motor fibers do not travel in the cochlear nerve trunk.) The cochlear nerve traverses the subarachnoid space in company with the vestibular and facial nerves, and it enters the brainstem at the pontomedullary junction.

Central auditory pathways

The general plan of the central auditory pathway from the left cochlear nerve to the cerebral cortex is shown in Figure 20.3. The first cell station is the cochlear nuclei, where all cochlear nerve fibers terminate upon entry to the brainstem. From here, some second-order fibers project all the way to the opposite inferior colliculus by way of the trapezoid body and lateral lemniscus. The inferior brachium links the inferior colliculus to the medial geniculate body, which projects to the primary auditory cortex in the temporal lobe.

Figure 20.3 Dorsal view of brainstem showing basic plan of central auditory pathways. The strand linking the two inferior colliculi is the collicular commissure.

A small but important purely ipsilateral relay passes from the superior olivary nucleus to the higher auditory centers.

Functional anatomy (Figure 20.4)

(Reproduced from Kretschmann, H-J. and Weinrich, W. Neurofunctional Systems: 3D Reconstructions with Correlated Neuroimaging: Text and CD-ROM. New York: Thieme 1998, with kind permission of the authors and the publisher.)

Primary auditory cortex

The upper surface of the temporal lobe shows two or more transverse temporal gyri. The anterior one (the gyrus of Heschl) contains the primary auditory cortex (Figure 20.6). Tonotopic arrangement is preserved in Heschl’s gyrus, its posterior part being responsive to high tones and its anterior part to low tones. The auditory cortex responds to auditory stimuli within the contralateral sound field. In cats, destruction of a patch of primary cortex on one side produces a sigoma or ‘deaf spot’ in the contralateral sound field. In humans, ablation of the superior temporal gyrus (in the course of tumor removal) does not cause deafness, but it significantly reduces ability to judge the direction and distance of a source of sound.