The Healthy Hearing System
The outer part of the ear (called the pinna ) channels sound travelling in the air into the ear canal. Some higher pitch sounds are enhanced by the shape of the ear canal as they reach the ear drum. Sound is transformed into mechanical vibration at the ear drum .
The eardrum is attached to a chain of three hearing bones that act as a lever enhancing sounds while transferring the signal through the middle ear to the inner ear. The last of these middle ear bones (the stapes ), is attached to the oval window. a thin tissue covering and entry point to the fluid-filled inner ear known as the cochlea .
The cochlea is a snail-shaped tube within the skull that contains sensory hearing cells. These sensory cells are situated on a flexible membrane tissue – the basilar membrane. When the oval window vibrates the basilar membrane and sensory hearing cells are displaced.
Two of the important sensory hearing cells are the outer hair cells and the inner hair cells. Outer hair cells act like a biological amplifier/attenuator, boosting soft sounds and dampening loud sounds. Inner hair cells transfer sound information to the auditory nerve.
The auditory nerve transfers sound information to various brainstem and auditory cortex regions in the brain so that information can be processed and meaning interpreted.
Common Problems with the Hearing System
Conductive Hearing Loss
An excessive build-up of ear wax in the ear canal can reduce sound transmission and hearing in an ear. An Ear wax blockage is best treated by an Ear
Specialist, Ear Nurse Specialist or trained Audiologist.
The presence of fluid behind the ear drum (otitis media or ‘glue ear’) can cause hearing loss by reducing movement of the ear drum and middle ear hearing bones. Although this condition usually resolves within ten weeks without treatment, advice should be a General Practice physician.
Otosclerosis is the abnormal growth of the hearing bones in the middle ear. Usually the stapes becomes fixed to the oval window and interferes with sound transmission into the inner ear. Otosclerosis is sometimes treated by surgery or through the use of hearing aids.
Sensory Hearing Loss
Inner ear sensory hearing cells may be malformed or become damaged.
Outer hair cells deteriorate over time so that only about 70% are intact by 70 years of age. This can result in reduced hearing of high pitch sounds. Hearing aids are usually very beneficial in compensating for this type of hearing loss.
Exposure to excessive noise can accelerate damage to outer hair cells causing a greater magnitude of hearing loss. Appropriate hearing protection should be used when in the presence of excessively loud sound to reduce damage to sensory hearing cells. Hearing aids are usually very beneficial in compensating for this type of hearing loss.
Damage to inner hair cells can occur due to excessive noise, ear disease or degenerative conditions.
Hearing aids are most effective at compensating for reduced function of outer hair cells. which is the most common type of permanent hearing loss. If the function of inner hair cells is very poor then a cochlear implant may provide better hearing ability.