Your attention is highly expected for this session on

1. Your attention is highly expected for this session on EAR: hearing mechanism & defects

2. 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)

4. Sound perception depends on the loudness as well as frequency. 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.

5. Sound waves in the air are transmitted as vibrations through a series of structures to the receptor for hearing. 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.

6. Mechanism of Hearing 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.

8. Mechanism of Hearing 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.

9. Malleus is attached to the tymphanic membrane [TM]. 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.

10. Mechanism of Hearing 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.

11. The pressure waves pass to the median canal and vibrate the basilar membrane. The tympanic membrane is connected to a circular membrane called the round window just beneath the oval window.

12. This arrangement allows the pressure waves to transmit through the cochlear fluid.

13. Mechanism of Hearing 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.

15. Cochlea 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.

16. Organ of Corti • Located on the basilar membrane is the organ of corti. • Organ of corti contains auditory receptors • Four rows of hair cells arise from the basilar membrane

17. STIMULATION OF HAIR CELLS: 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.

18. STIMULATION OF HAIR CELLS: 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

19. STIMULATION OF HAIR CELLS: • The site of maximum distortion in the organ of corti is determined by the frequency of sound, • For high pitched sounds the maximum height of waves is near the base of the cochlea and for low pitched near the apex. • The extent of distortion is determined by the loudness of the sound. Interpretations of the impulses is made in the auditory cortex

21. Sound travels in the ear. Vibrates TymphanicMembrane Movements in ear ossicles chain Sound waves 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 Action potential Brain Interprets

22. Defects of the Ear 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). • The hearing loss may be congenital or • due to middle ear fluid, • serious infections, • such as menigitis, • head injury, • listening to very loud music • especially through headphones, repeated exposure to loud sounds, such as machinery.

23. types of hearing loss • There are a few different types of hearing loss: • Conductive • Sensory • Neural and • Mixed (Conductive and sensory combined),

24. This happens when there is a problem with a part of the outer or middle ear. 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

25. 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.

26. Conductive hearing loss 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.

27. This happens when the cochlea is not working correctly because the tiny hair cells are damaged or destroyed. 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

28. Neural 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.

29. Hearing mechanism of Ear

30. Hearing Aid Patients with a conductive defect which does not respond fully to treatment may be helped with a hearing aid.

31. Noise Pollution 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.

32. Noise Pollution • The loud noises [above 130 dB] can cause immediate and permanent • damage to the muscles in the middle ear, • altering the stiffness of ossicles, • damaging the hair cells of the cochlea and • causing the rupture of the ear drum • thereby reducing the hearing efficiency

33. … Noise Pollution 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

34. Write an assignment for … … Balancing function of the Ear

35. References & Acknowledgements end http://alpha.ipfw.edu/histo-embryo/histear.html Encarta Encyclopedia International Text book of Zoology for XII std.