A word in your shell-like

5th December 2003 at 00:00
Jonathan Ashmore explains the inner workings of the ear.

The cochlea is the organ of hearing. This picture shows its soft tissue. This would normally be hidden within the skull; here the bone has been removed to reveal the tube itself. It is like showing the spiral stairs in an underground station by removing all the earth and station except for the staircase itself. Our coiled cochlea (which means "snail" in Latin) would pack neatly into a small pea, but if unwrapped it would stretch 35 millimetres.

When sound enters the ear canal, the eardrum vibrates. The movement passes through the ossicular bones of the middle ear into the fluid-filled cochlea. The middle ear converts sound travelling in air to sound travelling in fluid. When you have a cold, the middle ear often fills with mucus and stifles the sound. The middle ear is believed to have evolved about 250 million years ago. It allowed animals to hear sound travelling through air, rather than through water or from the ground through their legs to early inner ears. Keen nightclubbers know that sounds, except treble tones, travel through the ground. When you hear bass sounds, you may be using your vestibular (balance-sensing) organs in your inner ear. It is unlikely you are using your cochlea.

Although the basic mechanisms of hearing have been known for a long time, recent discoveries have rewritten some of the textbooks. For example, we now know that the cochlea contains what is effectively a biological hearing aid. The coils you see in the picture are partly formed by a flexible collagenous membrane, the basilar membrane. All the sensory cells which detect sound and inform the brain are aligned in rows along this basilar membrane, which runs the length of the cochlear spiral.

With increased magnification it becomes apparent that the sensory cells have small filaments projecting from one end. For obvious reasons these are called hair cells, but there is otherwise no connection with the hair on your head. If tipped backwards and forwards slightly by sound, the filaments produce an electrical signal in the cell, which is sent on to the brain via the auditory nerve.

About 3,500 hair cells in a single row are connected to the auditory nerve, but if you were to count the total number of hair cells in each cochlea you would find about 15,000. The additional cells amplify sound.

Sound entering the ear makes the basilar membrane vibrate. The movements are very small and no more than a few nanometres in amplitude (one nanometre is one-billionth of a metre). These are displacements no more than the width of a molecule, yet the hair cells can detect them. However, even this finding puzzled scientists as the physics alone suggested that the movements in the cochlea should be even smaller: there is just not enough energy in a sound wave to deflect the basilar membrane that much.

Since we now know that the normal cochlea has a "biological amplifier" built into it, many types of hearing loss can be traced to defects in the proper functioning of the hair cells forming part of this amplifier. In this delicate piece of machinery, mutations in any one of 30 identified genes associated with the hair cells and the cochlea are known to lead to deafness.

You can hear different frequencies because, although sound normally travels quickly in liquids, sound in the cochlea produces a slow wave-like swell which travels along the basilar membrane at about 15 metres a second until it reaches a place corresponding to the initiating sound frequency.

Biologically important sounds, such as speech, contain many different frequency components. It can be helpful to think of the cochlea as being like a piano in water, with the basilar membrane being the strings. Only those strings tuned to the incoming frequencies will respond to vibrations.

It has been found quite recently that some of the sensory hair cells in the cochlea also act like fast muscles, undoing the damping-down effects of the fluid and allowing resonant vibrations to build up quickly.

These hair cells can sense small movements in the cochlea and at the same time feed back forces to increase vibrations produced by incoming sounds.

Thus the tiny cochlea sends the brain huge amounts of information.

Because a normal cochlea contains feedback between biological "microphones" and "amplifiers", it emits sounds if appropriately stimulated: when a small click is delivered to the ear of a person with normal hearing, it is possible to measure a faint re-emitted sound. This feature can be used to screen newborn babies for deafness many months before conventional hearing tests can be given.

Jonathan Ashmore is Bernard Katz professor of biophysics at University College London.

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