Your attention is highly expected for this session on . EAR: hearing mechanism & defects. EAR: hearing mechanism & defects. Ears provides the two vital but very different senses: Hearing & Balance.
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EAR: hearing mechanism & defects
Ears provides the two vital but very different senses: Hearing & Balance
Audible frequencies to humans is in the range of 20-20,000 cycles per second.(cps) [20-20000 Hz].
Only the young people are able to hear this full range.
Loudness of the sound is correlated with the amplitude of sound wave and pitch with frequency ( No. of waves / unit time)
The human ear is most sensitive to 50 – 5000 Hz range.
But most sounds we normally hear fall within 500-5000 Hz.
The decibel scale is used to measure the intensity of sound.
The physical vibrations are detected and converted into electric signals.
Nerves carry the electrical signals to the brain where they are interpreted.
The interpreted messages allow us to perceive the subtleties of sounds such as speech and music.
Functions of External Ear
Sound waves enter the external auditory meatus,
pass along the external auditory canal and
fall on the tympanic membrane.
. . this causes tymphanic membrane to vibrate.
Functions of Middle Ear
The middle ear is an air filled cavity in the temporal bone which opens via the auditory tube into the nasophaynx.
The three auditory ossicles[Malleus, Incus and Stapes]are localised in the middle ear.
Stapes is attached to the membranous oval window.
Incus articulates with these 2 bones
Thus the vibrations of the TM are transmitted to the oval window.
TM has an area of 90 mm2 and the footplate of the stapes 3.2 mm2 and the lever system by the ossicles multiples the force 1.3 times.
The vibrations of the oval window generate pressure waves in the fluid filling the vestibular canal.
The tympanic membrane is connected to a circular membrane called the round window just beneath the oval window.
Functions of Inner Ear and Cochlea
The inner ear (labyrinth) is made up of Cochlea and the vestibule.
cochlear portion of the labyrinth is a tubule coiled 2.75 times.
the cochlea is divided into 3 compartments by the basilar and the Reissner’s membranes.
The middle compartment contains Endolymph and the other two (scala vestibuli & scala tymphani) contains Peri lymph.
Movements of the foot plate of the stapes set up a series of waves in the perilymph of the scala vestibuli.
This in turn causes the vibrations of the vestibular membrane and hence of endolymph in the scala media.
These waves deflect the Reissner’s membrane and this in turn produces disturbances in the basilar membrane which bends the hair cells in the organ of corti
This leads to the development of action potentials in the related nerve fibers which are transmitted along the auditory nerve
Movements in ear ossicles chain
Vibration of oval window
Basilar membrane disturbed
Deflection in Reissner’s membrane
Waves in the Endolymph
Waves in Perilymph
Bending of hair cells
Transmission by auditory nerve
Several defects of the ear leads to hearing loss or even deafness. Such loss or impairment happens when there is a problem with one or more parts of the ear(s).
Most kids with conductive hearing loss have a mild hearing loss and it is usually temporary because in most cases medical treatment can help.
Conductive hearing loss
Commonest causes of conductive hearing loss is blockage of the external auditory meatus with wax secreted from ceruminous glands in the skin lining the meatus.
In some people wax accumulates in the meatus and hardens, sometimes pressing against the eardrum.
Normal hearing is usually restored after the hardened wax is removed with a special syringe.
Injury to the head can also cause the ossicles of the middle earto become disconnected from one another, thus breaking the conductive path to the cochlea.
Another cause is the perforated ear drum. This Perforation can be caused by infection in the middle ear or by mechanical injury resulting from a near by explosion or a sudden blow to the head.
Depending on the loss, a person may be able to hear most sounds (although they would be muffled); only some sounds; or no sounds at all.
Sensory hearing impairment is almost always permanent and the ability to talk normally may be affected.
Sensory hearing loss
This happens when there is a problem with the connection from the cochlea to the brain.
Malfunction of the cochlea and acoustic nerve can be the cause of hearing loss, eventhough vibrations are conducted perfectly into the inner ear.
Such hearing loss is called sensori-neural [perceptive] hearing loss.
Acquired forms of this condition can result from infection, head injury, blast from explosions or exposure to excessive noise.
Patients with a conductive defect which does not respond fully to treatment may be helped with a hearing aid.
Any unpleasant and unwanted sound is considered as Noise.
The best measurement unit of the sound is decibel.
The permissible noise levels is 81 -120 dB.
There are various sources of noises and the sources are broadly classified in to industrial and non-industrial sources.
Prolonged exposure to the unpleasant Noises
Partially responsible for the increased consumption of alcohol, drugs, tranquilizers and sleeping pills.
Sever mental disturbances and violent behaviour,
Aggravation of Asthma and Ulcers
… Balancing function of the Ear
Encarta Encyclopedia International
Text book of Zoology for XII std.