For humans, speech, music, and other environmental sounds form the basic elements of language, social relations, and adaptive response to environmental stimuli. Our auditory system can detect rapid changes of sound intensity (measured in decibels, dB) and frequency (measured in hertz, Hz). In humans, sound intensity and frequency roughly correspond to loudness and pitch.
The task of the auditory system is to convert changes in air pressure in the environment into the neural activity that permits our brain to perceive and attach meaning to the sounds that we hear.
Each part of the ear performs a specific function in hearing. The external ear captures, focuses, and filters sound. The middle ear concentrates sound energies. Finally, the inner ear transduces mechanical energy into neural activity.
Sound waves are first collected by the outer ear, which consists of the external ear, or pinna, and a canal that leads to the eardrum. The configuration of the external ear amplifies sound, particularly at the frequency ranges of 2,000 to 5,000 Hz. Not coincidentally, most human speech sounds are distributed in the frequency range around 3,000 Hz.
From the auditory canal, the sound waves vibrate the eardrum, which in turn vibrates three tiny bones in the middle ear—the malleus, incus, and stapes. The stapes vibrates a small membrane at the base of the cochlea, the oval window, which transmits amplified vibrational energy to the fluids of the cochlea. The round window serves to dampen the movement of fluid within the chochlea.
The complex structures of the inner ear convert sound into neural activity. In mammals the auditory portion of the inner ear is a coiled structure called the cochlea. The region nearest the oval-window membrane is the base of the spiral; the other end, or top, is referred to as the apex.
Along the length of the cochlea are three parallel canals: the tympanic canal, the vestibular canal, and the cochlear canal. The principal elements for converting sounds into neural activity are found on the basilar membrane, a flexible structure that separates the tympanic canal from the cochlear canal.
Let's take a closer look at the basilar membrane by "unrolling" the cochlea and peering inside. The basilar membrane is about five times wider at the apex of the cochlea than at the base, even though the cochlea itself narrows toward its apex. It vibrates in response to sound transmitted to the fluid-filled cochlea by deflections of the oval window initiated by the bones of the middle ear.
High-frequency sounds displace the narrow, stiff base of the basilar membrane more than they displace the wider, more flexible apex. Mid-frequency sounds maximally displace the middle of the basilar membrane. Finally, lower frequency sounds maximally displace the apex.
Within the cochlear canal and atop the basilar membrane is the organ of Corti—the collective term for all the elements involved in the transduction of sounds. The organ of Corti includes three main structures: the sensory cells (or hair cells), an elaborate framework of supporting cells, and the terminations of the cochlear nerve fibers.
The heights of the stereocilia increase progressively across the hair cell, so the tops approximate an inclined plane. Atop the organ of Corti is the tectorial membrane. The stereocilia of the outer hair cells extend into the indentations in the bottom of the tectorial membrane.
The movements of fluid in the cochlea produce vibrations of the basilar membrane. These vibrations bend the stereocilia inserted into the tectorial membrane. Depending on the direction of the bend, the hair cells will either increase or decrease the firing rate of cochlear nerve fibers.
In this tutorial we have seen how the ear captures sound energy and transduces this mechanical energy into nerve impulses. The external ear captures, focuses, and filters sound. The bones (ossicles) of the middle ear focus the sound energy onto the oval window, generating pressure waves that sweep along the basilar membrane of the cochlea. The structure of the basilar membrane allows it to respond to vibrations of different frequencies. The organ of Corti contains the sensory hair cells (about 15,000 in humans) that convert the mechanical energy into action potentials that are transmitted to the cerebral cortex.