Human hearing is the process by which vibrations in air are transduced (converted) into nerve impulses in the auditory nerve. This great biological feat is accomplished through the interaction of various subsystems in the ear. In this article, we will explore the process of hearing from the moment a sound wave reaches the ear to the point where a nerve signal is generated.
First of all, the incident sound waves encounter the pinna - the outer portion of the ear. This dish-shaped structure serves much the same purpose as a satellite dish: it collects sound waves and directs them into the ear. The pinna is formed primarily of cartilage, which explains its relative rigidity. There is a degree of genetic variation in the shape of the pinna, which explains the different shapes and sizes of ears that we encounter.
The sound waves collected by the pinna are next directed into the auditory canal - a tubular structure. The walls of the auditory canal are lined with earwax, which prevents small objects - such as insects - from adhering. At the end of the auditory canal lies the tympanic membrane (or eardrum) which absorbs the incoming sound wave. These waves cause the tympanic membrane to vibrate at the frequency of the sound, effectively converting sound waves into mechanical vibration.
The above structures - the pinna, auditory canal, and tympanic membrane - form what is known as the outer ear. Past the tympanic membrane lies the middle ear, which is not normally exposed to the external environment. In this inner ear are three small bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These three bones serve to conduct the mechanical vibrations of the tympanic membrane to the inner ear, which we will discuss shortly. The tympanic membrane vibrates, inducing vibrations in the malleus, which then in turn causes the incus and stapes to vibrate. Finally, the stapes vibrates against the oval window of the cochlea - part of the inner ear - which then transduces the mechanical vibrations into nerve impulses.
Finally, the vibrations are transferred into the cochlea of the inner ear. The inner ear is the fluid-filled component of the ear, and is responsible for balance (in sensing acceleration) and hearing. The cochlea is named after the snail, as this structure is coiled much like a snail's shell. The cochlea contains two canals (or scala): the vestibular canal and the tympanic canal. Vibrations of the oval window cause the fluid in the vestibular canal to vibrate. These vibrations reach the end of the tympanic canal - the 'apex' - and then travel down the vestibular canal. Part of the wall of the vesitbular canal - the basilar mambrane - vibrates as the fluid does. Attached to this membrane are hair cells. Each hair cell has many hairs, one of which is embedded in the adjacent, stationary tectorial membrane. Thus, when the basilar membrane vibrates, so do the hair cells, causing the hair embedded in the tectorial membrane to bend. The hairs of the hair cell are chained together, causing the displacement of all hairs. This stimulates ion channels in the hair cells to open, causing an action potential. This action potential is then carried through the auditory nerve to the temporal lobe of the brain, where the sound is interpreted.