Human Auditory System

EAR

The human auditory system is one of the most intricate, miraculous, and an ingenious

creation designed to transfer sound waves from environment to brain in a most efficient and

precise manner. The ear can be described as both an analytic microphone and a

microcomputer, sending sound impulses to the brain. Ear is capable of turning the tiniest

disturbances to a form that brain can understand and doing so instantaneously, over an enormous range of pitch and loudness. Being extremely complicated organ, it performs dual function of balancing and perceiving sound.

The auditory system is highly complex and composed of three anatomical compartments, the external, middle and inner ear, which function as an entity. The boundary between the

These projections, known as stereocilia, have

mechanosensitive ion channels (Corey and Hudspeth.1979, Ohmori.1985) and constitute the

hair bundle which is formed of rows of stereocilia that increase in height in one particular

direction across the bundle. Stereocilia are generally arranged in three rows of graded

lengths and a single kinocilium located behind the row of longest stereocilia. In the hair cells of the organ of Corti, the kinocilium is present only during development, but as the cochlea matures it is reduced to remain as a basal body on one side of the stereociliary bundle. The tallest stereocilia of outer hair cells directly contact the tectorial membrane. The tip of each stereocilium is linked to the shaft of its neighbor by thin tip links which are involved in the mechano-transduction process, stereocilia are also attached by transverse (lateral) links, both in the same row and from row to row. There are thought to be at least three different types of lateral links between stereocilia. Ankle links which are absent from the hair cells of the organ of Corti, but present in the hair bundles of mammalian vestibular organs connect stereocilia at their proximal ends. Shaft connectors are present along the mid-region of the stereociliary shaft. Top-connectors link stereocilia laterally just below the level of the

The impulses are passed on to the cochlear branch of the vestibulocochlear (VIII)

nerve and then to the medulla. Within the medulla, most impulses cross to the opposite side

and then travel to the midbrain, to the thalamus, and finally to the auditory area of the

temporal lobe of the cerebral cortex. In the “resting” position of stereocilia the transduction

channels are partially open, leading to a small release of transmitter. This, in turn, generates

a spontaneous activity in the auditory nerve and the ascending auditory pathways, even in the

absence of sound. The cells are thought to recover from the stimulus by pumping out the

potassium through gap junctions (Connexin channels) and voltage gated potassium channels

(Petit.2001).

As clear from the above complexity of hearing process; a large ensemble of proteins

act in concert to orchestrate the function of the sensory cells in the cochlea, through which we

hear, and the vestibular apparatus of the inner ear, the organ that senses gravity and

acceleration. Defects in any one of these proteins results in disturbance of the