Academic Reading — Test 33

Reading passage

Hearing begins with movement. When a sound is produced, it travels through the air as waves of pressure, alternately compressing and rarefying the molecules around us. These pressure waves are gathered by the outer ear and funnelled towards the eardrum, a thin membrane that vibrates in sympathy with the incoming sound. Yet vibration of the eardrum is only the beginning of a remarkable chain of events. The true work of hearing — the conversion of mechanical movement into the electrical language of the nervous system — takes place deep within the skull, in a small, spiral-shaped structure known as the cochlea. The cochlea is roughly the size of a pea and is coiled like the shell of a snail, completing about two and a half turns. Despite its modest dimensions, its internal architecture is extraordinarily precise. It is filled with fluid and divided along its length into separate chambers by delicate membranes. Before sound can reach this fluid, however, its vibrations must cross the middle ear. Here, three tiny bones, collectively called the ossicles, form a bridge between the eardrum and the cochlea. The ossicles act as a system of levers that amplify the force of the vibrations. This amplification is essential, because the cochlea is filled with liquid, and liquid is far harder to set in motion than air. Without the mechanical advantage provided by the ossicles, much of the sound energy would simply be reflected away and lost. The last of the three bones presses against a small flexible region called the oval window. As it pushes in and out, it creates pressure waves that travel through the fluid inside the cochlea. Running along the length of the spiral is the basilar membrane, a structure whose physical properties change gradually from one end to the other. Near the oval window the basilar membrane is narrow and stiff, while at the far end, towards the tip of the spiral, it is wider and more flexible. Because of this variation, different parts of the membrane respond best to different frequencies of sound. High-pitched sounds cause the greatest movement near the base, whereas low-pitched sounds produce their strongest effect near the apex. In this way, the cochlea effectively sorts a complex sound into its component frequencies, rather like a prism separating white light into colours. Resting upon the basilar membrane is the organ of Corti, which contains the cells that perform the actual conversion of motion into nerve signals. These are the hair cells, named for the tiny bundles of hair-like projections, called stereocilia, that protrude from their upper surface. When the basilar membrane moves up and down in response to the travelling pressure waves, the stereocilia are bent against an overlying structure. This bending is the critical mechanical event. Tiny channels in the tips of the stereocilia are pulled open, allowing charged particles to flow into the hair cell and altering its electrical state. The cell responds by releasing a chemical messenger at its base, which in turn excites the nerve fibres connected to it. Thus a purely physical bending is translated into a chemical and then an electrical message. Humans possess two functional categories of hair cell, and their roles are not identical. The inner hair cells, arranged in a single row, are the principal sensors; they are responsible for sending the great majority of auditory information to the brain. The outer hair cells, arranged in three rows, behave quite differently. Rather than simply reporting sound, they actively change their length in response to it, sharpening and amplifying the movement of the basilar membrane. This biological amplifier greatly improves the ear's sensitivity and its ability to distinguish between sounds of similar pitch. Damage to the outer hair cells is a common cause of hearing loss, and because these cells cannot regenerate in mammals, such loss is generally permanent. The signals generated by the hair cells are carried away from the cochlea by the auditory nerve, a thick bundle of fibres that conveys the coded information to the brainstem and, ultimately, to the regions of the brain devoted to hearing. The information is encoded in two complementary ways. The position along the basilar membrane from which a signal originates tells the brain about the frequency of the sound, while the rate and pattern of nerve impulses convey its timing and intensity. Only when these streams of information reach and are interpreted by the brain does the listener experience what we recognise as sound. The cochlea, then, does not hear in any conscious sense; it is a sophisticated translator, turning the trembling of a membrane into a language the brain can read.