Study guidelines
- 1.
Describe the mechanism of how vibrations created by sound waves result in activation of the organ of Corti.
- 2.
Be able to identify the scala vestibuli, scala tympani, and scala media and components of the spiral organ.
- 3.
Reproduce the central auditory pathway from the spiral ganglion to primary auditory cortex.
- 4.
Describe the role of the olivocochlear bundle.
Auditory system
The auditory system comprises the cochlea, the cochlear nerve, and the central auditory pathway travelling from the cochlear nuclei in the brainstem to the cortex of the temporal lobe. The central auditory pathway is more elaborate (provides for more processing of the signal) than the somatosensory or visual pathways because the ‘same’ sounds are detected by both ears. To signal the location of a sound as well as perform further processing prior to transmission to the thalamus and cortex, a very complex neuronal network is in place. Numerous connections (mainly inhibitory) between the two central pathways exist to accomplish this task as well as to magnify minute differences in intensity and timing of sounds that exist during normal, binaural hearing.
The cochlea
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 fibres of the basilar membrane become progressively longer.
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 (Reissner 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 endolymph 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 ). It has been suggested that the oscillatory movements of outer hair cells influence the sensitivity of the inner hair cells through effects on the tectorial or basilar membrane.
Sound transduction
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 fibres 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 fibres are the longest. The basilar membrane is therefore tonotopically organised in its fibre sequence. Not surprisingly, the inner hair cells have a similar tonotopic sequence. In response to local resonance the cells depolarise and liberate excitatory transmitter substance from synaptic ribbons ( Figure 20.2 ).
The nerve fibres 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 myelinated central processes of some 30,000 large bipolar neurons of the spiral ganglion. Unmyelinated fibres come from small ganglion cells supplying dendrites to the outer hair cells. (Motor fibres 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 . All entering cochlear nerve axons (first-order auditory neurons) terminate ipsilaterally on the cells of the cochlear nuclei located at the level of the entrance of the nerve, commonly described as at the pontomedullary junction. From here, some second-order neurons ascend through the brainstem to reach the medial geniculate body, but most do not. Most of the second-order auditory neurons synapse along the way on one or more of several brainstem nuclei that form the ‘auditory way stations’ (see below). These provide further processing of the auditory information before it reaches the level of the thalamus in ways that are still under active investigation. Students sometimes question whether these ‘long neurons’ should be properly referred to as ‘third-order’, ‘fourth-order’ (and so on) sensory neurons in the auditory pathway because it is hard to call such long neurons ‘interneurons’. It is probably best to say that second-order neurons in the auditory pathway may be comprised of two or more distinct neurons that collectively serve the ‘function’ of the second-order neurons in projecting (and processing) auditory information from the termination point of the peripheral nerve to the dorsal thalamus.
