Human Auditory System:

The three basic elements of hearing are sound waves, auditory perception and cognition. A sound wave travels in all directions and the properties of the sound can be affected by temperature, humidity, composition and density of the air. Also, if indoors, the sound can be affected by the size, shape, surface texture and composition of walls, floors and ceilings. This complex sound wave eventually reaches our two ears which are separated  by less than 20 centimetres.

The irregular shape of our pinnae (outer ears) allows us to distinguish slightly sounds coming from different directions. Sound waves travel through the air, past the pinna, and into the outer ear canal (meatus), where they strike the timpanic membrane (eardrum), which vibrates sympathetically. A chain of three small bones (middle ear) attached to the inner surface of the timpanic membrane transmits the vibrations to another membrane stretched across one of the openings of the inner ear, or cochlea.

It is known that the primary structure of the cochlea, the basilar membrane, carries out a frequency analysis of the incoming sound wave, such that each frequency within the auditory spectrum causes a maximum displacement to occur at a particular point on the membrane. Sensory receptors, known as hair cells, reside on the basilar membrane. The hair cells are arranged into a single row of approximately 3,500 inner hair cells and three rows of approximately 20,000 outer hair cells. The transduction of membrane displacement to electrical energy takes place in the inner hair cells. Bending of the inner hair cell cilia due to basilar membrane displacement, produces a change in the overall resistance of the cell, thus modulating current flow through the hair cell. Modulation of the current flow is produced by bending of the cilia in one direction only. This modulation is directly proportional to the degree of bending of the cilia.

The outer hair cells are not sensory cells, rather they effect an automatic gain control loop which modulates the mechanical motion of basilar membrane. The outer hair cells are the target of descending fibres from high centres of the auditory pathway. These cells are normally inhibited, but when the inhibition is reduced, they act to push in the direction of motion of the basilar membrane. The outer hair cells have the role of a muscle which can change the characteristics of the membrane adaptively, thus providing the amplification effect for low-level stimuli.

Communication of hair cell response to the nervous system is achieved via the auditory nerve. The fibres of the auditory nerve arise from bipolar neurons which reside in clusters of cells known as the spiral ganglion. The spiral ganglion, located in the cochlea, contains a relatively small number of neurons. Approximately 30,000 fibres emanate from the ganglion to enervate the hair cells in a specific pattern. Enervating fibres form the afferent nerve, while the fibres, from the spiral ganglion cells which project to higher auditory structures, are called the eighth nerve. 

Since each afferent fibre enervates a single inner hair cell, and each inner hair cell is sensitive to motion in a specific section of the basilar membrane (corresponding to a particular frequency in the stimulus), the fibres themselves are labelled by frequency. Thus, the fibres of the eighth nerve fire optimally at a characteristic frequency and these fibres are organised to preserve the enervation pattern on the basilar membrane. The distribution of firing eighth nerve fibres is reported to be a duplicate of the sound spectrum, and the characteristic frequencies progress logarithmically from the basal end to the apical end of the membrane. 

The eventual awareness of sound as music occurs on the highest level of processing in the nervous system in the brain.

TOM SCARFF
1 Martello Court
Portmarnock
Dublin
Ireland.

Email: tscarff@eircom.net