The hearing system is divided into three parts each with a different function. The Outer Ear which consists of the external ear or pinna, the ear canal and the eardrum or tympanic membrane, captures and guides sounds. The Middle Ear transmits the movements of the eardrum to Inner Ear through three ossicles; the Hammer, Anvil and Stirrup. The Eustachian tube also empties into the middle ear and communicates with the respiratory system. Finally, the Inner Ear serves both for balance and hearing and is divided into the semi-circular canals and the cochlea. The cochlea is filled with fluid. Sound from the middle ear pushes the fluid in the cochlea that moves the 15000 hair cells. The hair cells each analyze frequencies and as the hair cells move, a neural influx results. The neural influx is lead by the auditory nerve (a group of neural fibers) towards the hearing area of the cerebral cortex which that interprets the message to deliver an auditory sense.

Hearing or perception of sounds is an essential sense in human communication. Hearing serves for two functions, communication (speech and language) and awareness (perception of our environment). Hearing take place through the auditory system as previously described. Sounds move as vibrations or waves though the air. The ear can differentiate two parameters: frequency, the number of vibrations per second (in Hertz), which allows hearing low or high pitched sounds; and amplitude or intensity (in decibels) which allows to recognize loud and soft sounds. There are many types of sounds, for instance, pure sounds that have one frequency and musical sounds, which have one frequency plus harmonics and noise that has no specific frequency.

Perception of these frequencies is different according to species. The human ear can distinguish frequencies between 20 Hz and 20 000 Hz, with an optimal perception between 1000 and 3000 Hz. Moles are capable of hearing infrasounds (low pitched sounds) below 20 Hz. Ultrasounds (high pitched sounds) above 20 000 Hz are heard by dogs up to 40 000 Hz and by bats up to 160 000 Hz. A loud or intense sound can cause harm and destruction to the hair cells of the Inner Ear, resulting in hearing loss.

Hearing is a fragile sense. There are only 15 000 non-renewable hearing cells. Destruction of these cells can cause irreversible hearing losses. There are no medical treatments that can cure hearing losses due to hair cell damage. Assistance is available with hearing aids and cochlear implants

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There are many causes of hearing losses. Natural loss due to aging (presbycusis) can begin at age 55-60. Noise is also a cause for hearing loss, for instance professional noise, loud music (MP3, concert, club), urban noise (cars, transportation) and occupational noise (loud tools or machinery). There are also medical causes to hearing losses, for instance infectious diseases such as ear infections and meningitis. Many drugs such as antibiotics, anti-inflammatory, diuretics, oestrogen, vitamin A, quinine, can also have an influence on hearing losses. Ostospongiosis is a disease where the ossicles of the middle ear are surrounded by a spongy mass, blocking the transmission of sounds. A tumour on the auditory nerve (acoustic neuroma) can also alter hearing. Some hearing impairments can also have a genetic origin, and many individuals are born with varying degrees of hearing loss. Finally, tobacco can also be a cause for hearing loss.