To better understand the mechanism and the varied causes of hearing loss, one needs to have a notion about the structure of the hearing organ. Human auditory analyzer is the most complicated system comprising three parts — outer ear, middle ear и inner ear.
The function of the outer ear is to locate sounds, amplify them and convey to the eardrum (tympanic membrane). The outer ear consists of auricle, ear canal and eardrum. Tympanic glands, situated in the ear canal, secrete earwax that accumulates in the outer ear. The earwax serves to prevent dust and dirt from getting into the ear. Another function of the ear canal is maintaining the temperature and the humidity of the eardrum at a fixed level.
The middle ear is represented by a cavity behind the eardrum. The cavity contains a chain of tiny bones which are called the hammer, the anvil and the stirrup bone. The middle ear cavity is connected to the nasopharynx by means of the auditory (eustachian) tube. The tube is generally closed, and it opens only during swallowing or sucking movements. The elements of the middle ear serve to conduct the sounds detected by the ear.
The inner ear, separated from the middle ear by the oval window, consists of the cochlea and the semicircular canals. The semicircular canals form the peripheral part of vestibular apparatus and, strictly speaking, are not related to the function of the auditory analyzer. The cochlea is a complex system of tubes filled with lymph. Inside the cochlea there is the membranous labyrinth with the receptory apparatus on its wall. The membranous labyrinth is also filled with lymph.
How does the information about the acoustic environment travel to the human brain? First of all, the sound waves come through the auricle into the ear canal, causing the eardrum to vibrate. Those vibrations are then communicated to the system of the middle ear bones. The hammer is connected to the eardrum and the stirrup bone is connected to the cochlea, thus together they make up a flexible lever system, amplifying the vibrations of the eardrum many times. Passing through this system into the internal ear, the vibration wave causes the fluid inside the cochlea to move. The hair cells which cover the walls of the cochlea receptory apparatus detect those vibrations. Every hair cell can detect only the vibrations of a certain frequency. Hair cells transform the energy of mechanical vibrations into electric impulses which are then conducted to the brain via the auditory nerve. Each one of the finest fibers comprising the auditory nerve is connected to a specific part of the cochlea surface, that’s why it can be stimulated only by the sound of a certain frequency. When nerve impulses come to the brain via the auditory nerve fibers, they are transformed back into auditory images. Human brain relies on its experience to decipher nerve impulses correctly. The knowledge of the acoustic image of the world that we accumulate during our whole life helps us not just to detect sounds, but to interpret the received signals accurately and without delay, already in the process of sound perception.