There are three distinct sections to the ear structure through which sounds must pass before they reach the auditory nerve centre deep in the brain.
The protective outer ear – the shell, or pinna – channels sound into the ear canal and assists with identification of sound location. Sound waves travel along the ear canal, eventually impacting upon the thin cone-shaped eardrum, causing it to vibrate. The eardrum separates the outer ear from the middle ear.
The middle ear is about the size of a 10 cent coin, though it is only a few millimetres deep. The vibration from the ear drum is passed on to tiny bones: the hammer (malleus), the anvil (incus) and the stirrup (stapes) that amplify and transmit the vibrations on to the inner ear.
The middle ear is filled with air, and the internal and external pressure needs to be maintained at a similar level, or the eardrum becomes stretched and does not vibrate correctly. This pressure is normally maintained by swallowing, as this opens and closes the eustachian tube that connects the middle ear to the back of the throat.
The inner ear – about the size of a large marble – is where the sound energy is changed to mechanical energy then into nerve impulses that travel to the brain.
The inner ear includes the vestibule, semicircular canals and the cochlea. The vestibule and the semicircular canals contain the organs that contribute to balance and to spatial orientation.
The cochlea is the hearing organ. Shaped like a snail shell, it contains 20,000 minute sensory or hair cells that coil around inside the spiral and are moved as a result of sound vibrations. These cells are connected to a number of nerves, which together make up the auditory or “hearing‟ nerve.
The auditory nerve bundle of about 30,000 nerves then passes out of the cochlea and travels to the first auditory centre in the brain, where it branches out and connects with many other nerves to send messages to the higher levels of the brain, i.e. the cortex.
The human ear is now known to be an exceptionally intricate and delicately constructed mechanism. While it is vulnerable to illness or disease, it is nonetheless well protected from direct injury – except for head injury – by the bones of the skull. It is, however, able to be irreversibly damaged by the insidious if invisible exposure to excessive sound, which research indicates is principally a result of working in industrial or noise toxic environments.
Article by Professor Peter Thorne.