Hearing

Human ears are a good deal less efficient than those of most animals. We are unable to ‘prick up’ our ears or rotate them towards the source of sound, and we can detect only sounds in the frequency range 20 to 20,000 hertz (cycles per second), whereas a bat can hear sounds of up to 175,000 hertz.

Nevertheless, the human external ear still retains some ability to funnel sound into the external auditory canal, through which the sound waves travel to the eardrum (tympanic membrane). At the eardrum the sound waves – vibrations of air – make the eardrum itself vibrate. These vibrations are carried by the small bones of the middle ear (the malleus, incus and stapes) through the oval window to reach the fluids of the inner ear. Within the spiral cochlea of the inner ear lie the nerve endings where the fluid vibrations are converted into electric nerve impulses, which pass to the brain for interpretation. The mechanism of hearing can conveniently be divided into two parts: conduction and perception. Conduction involves the transmission of sound waves from the external auditory canal, via the eardrum and ossicles, to the oval window and the fluids of the inner ear. It is a purely mechanical process, transferring vibrations from the air to solids (bones), then fluids. Perception of sound begins at the nerve endings in the cochlea, where nerve impulses are generated which pass along the auditory nerve to the hearing centres of the brain. There they are analysed and interpreted.

Conduction in the middle ear.

Sound passes through any medium as a series of pressure waves, like ripples spreading across a pond from a dropped pebble. As the waves get farther from their source they get weaker, not only because they spread out with increasing distance but also because of the resistance of the medium in which they are travelling. This resistance is known as impedance and is significantly different in different media, particularly fluids and solids. The structures of the middle ear act as sound transformers to match the impedances of the outside air and the fluids of the inner ear. The eardrum is a very thin membrane with air on both sides at equal pressures, so little energy loss occurs when sound waves strike it. The handle of the malleus, firmly attached to the tympanic membrane, vibrates also and the waves pass through the incus to the stapes and then to the oval window as through a system of levers, which increase the force of the vibrations by about one-and-a-half times. The eardrum is much larger than the oval window, however, which effectively increases the force of vibrations on the cochlea a further fourteen times.

Conduction in the inner ear

In order to understand how the snail-shaped cochlea works, we must first look at its detailed structure. Its central axis (modiolus), has projecting from it a flange, called the osseous spiral lamina, rather like the thread of a screw. An extension of the flange, the basilar membrane, divides the cochlear tube into two main spirals; the scala vestibuli, and the scala tym-pani. At the top of the spiral the scala vestibuli and the scala tympani are connected to form a coiled, U-shaped tube containing a fluid called perilymph. At the base of the cochlea the scala vestibuli terminates at the oval window and the scala tympani at the round window. Sandwiched between the two scala chambers, lying on the basilar membrane, is a third coiled chamber. It is the scala media, which contains another fluid (endolymph) and is in direct communication with the endolymph of the organs of balance (saccule, utricle and semicircular canals). Between the scala media and the scala tympani is the basilar membrane which bears a strip of specialized cells called the organ of Corti. This consists of more than 20,000 hair cells, each of which has up to 100 hairs that protrude into a gelatinous mass, the tectorial membrane. As the cochlea spirals round, the hairs of the basilar membrane become longer and thicker, like the strings of a piano, and the tectorial membrane becomes longer. It is here that vibrations are transformed into the electric signals of nerve impulses.

Perception

The precise mechanism whereby the cochlea discriminates between the great range of frequencies and intensities of incoming sounds is still something of a puzzle. The simplest explanation, advanced by Hermann von Helmholtz (1821-94) back in 1863, is that different parts of the basilar membrane resonate, or vibrate in harmony, with the different frequencies of vibrations in the fluid, and thus a different set of hair cells is stimulated for each frequency. This is clearly not the whole answer, however. Currently it is suggested that vibrations pass from the perilymph of the scala vestibuli into the endolymph of the scala media, where they vibrate the gelatinous tectorial membrane, which in turn moves the hairs of the hair cells in the basilar membrane. The nerve fibres of the hair cells in the basilar membrane unite to form the cochlear nerve. This joins more nerve fibres from the vestibule and semicircular canals to form the auditory nerve, which passes through the skull bone into the brainstem. Here it divides into separate cochlear and vestibular pathways. After several further relay and connection points, during which some cochlear nerve fibres cross to the opposite side of the brain, these nerves finally end in the cerebral cortex of the temporal lobe. This is where the nerve impulses become perceived as sounds. The acoustic cortex is the area of the brain specialized to interpret auditory nerve impulses. Broadly, it is organized into two ‘maps’ or cell layouts. One is a frequency map, with a strip of cells detecting low notes at one end and high notes at the other (not unlike the cochlea). Overlaid on this is a dimensional map where the brain can detect a difference in nerve impulse frequency equal to only one hundred-thousendth of a second. Sounds from one side of the head arrive a short time later at the ear on the other side. Detecting such minute differences is therefore very important for determining from which direction a sound reaches us. Sound intensity (loudness) is represented by more nerve impulses per fibre and more fibres sending impulses. Integrating these three main constituents – frequency, dimension (space) and intensity – we can hear a jet plane going overhead followed by a whispered lullaby.