The figure below shows a simplified longitudinal cross section of the human ear. Three parts my be distinguished:
- the external ear, which extends from the pinna to the tympanic membrane via the auditory canal;
- the middle ear, comprising the tympanic membrane, the ossicles (hammer, anvil, stirrup), and the oval window;
- the inner ear or cochlea, which codes the acoustic information into electrical neural impulses when stimulated by the oval window. These impulses are then transmitted to the brain by the auditory nerve.
The external ear receives the airborne sounds and permits the tympanic membrane to be stimulated acoustically according to a pressure operation mode. The Eustachian tube allows the static pressure to be equalized on either side of the tympanic membrane. The external ear also plays a protective role toward the middle and inner ear.
The main function of the middle ear is an impedance matching function between the airborne sound in the auditory canal and fluidborne sounds in the cochlea. The ratio of the areas of the tympanic membrane to the oval window is decisive, but the ossicles also play a role. This system protects, in particular, the inner ear against intense sounds, thanks to its stiffening through the contraction of the muscles: this is referred to as the acoustic reflex, analogous to the behavior of the iris of the eye when in presence of intense light.
The cochlea is a complex hydrodynamic system. It is divided by membranes into 3 liquid-filled canals, known as the scala vestibuli, scala tympani, and scala media, or ductus cochlearis. The first 2 are filled with perilymph and the latter with endolymph. They communicate at the apex through the helicotrema. The oval window permits the scala vestibuli to be stimulated, whereas the round window, a stretched membrane, ends the scala tympani.
The basilar membrane plays a basic role in hearing, for it allows a frequency analysis of the sound to take place, thanks to its behavior as a nonuniform bifilar line. It supports the organ of Corti where the hair cells convert the acoustic information (mechanical motion) into electrochemical signals, which in turn trigger neural impulses in the auditory nerve and auditory cortex.