Mechanism of hearing by rohit allbi gs 1st year mbbs .pptx

mechanism of hearing

1. Mechanism of hearing

2. Sound waves Sound waves are alternate compression and rarefraction of molecules that strike the tympanic membrane to produce the sensation of sound. They travel through an elastic medium like air or water. It travels in the air at a speed of 340m/s at 20 degree Celsius at sea level The audible range of frequency for human ear is 20-20,000 HZ but more sensitive to sounds with frequency of 1,000-4,000 HZ . The subjective sensation produced by the frequency of sound is known as pitch . Male pitch – 120HZ , Female pitch – 250 HZ . Pitch discrimination is best between 1,000 – 3,000 HZ . The distance between the two adjacent peaks is known as the wavelength of sound . The amplitude of sound waves determines the loundness or intensity of the sound . Number of decibles = 10 log Intensity of the sound heard / Intensity of standard sound. The human audibility curve ranges from 0db to 100db, and the human ear is more sensitive to intensity of about 60 db at a frequency of 1,000 – 4,000 HZ

3. Mechanism of masking The hearing threshold for a given sound increases in the presence of background sound or noise . This phenomenon is knowing as masking . Sensitivity of the auditory apparatus to a given tone decreases in the presence of other sounds . The degree to which a given sound can be masked depends on the frequency of the masking sound . A low frequency of sounds masks high frequency sounds more effectively . In Conduction deafness , sound is better heard in the defective ear .

4. Transmission of sound waves Ossicular conduction : sound waves transmitted primarily through tympanic membrane and the ossicular chain causing vibration of the footplate of stapes . This is known as ossicular conduction . Air conduction : sound waves travel through air in the middle ear cavity and vibrate the secondary tympanic membrane to reach perilymph of the scala tympani . Bone conduction : sound waves get mechanically transmitted through the bones and reach the inner ear .

5. Transmission through inner ear Sound waves are transmitted from footplate of stapes to the fluid of the inner ear . When the stapes moves inward , the pressure in the perilymph of scala vestibuli increases . The pressure wave is transmitted to the perilymph of the scala tympani and finally produces outward movement of the round window into the middle ear . The up and down movements of the basilar membrane produce upward and downward movements of the organ of corti .

6. Transduction of sound waves Genesis of receptor potential in hair cells : Process of depolarisation- A deflection of stereocilia of as little as 0.5nm generates a response in the hair cell . When the shorter stereocilia bend towards the taller stereocilia the cation channels open , allowing entry of k+ and ca+2 into the hair cells . The resulting depolarisation causes opening of voltage gated ca+2 channels at the base of the hair cells. This results in release of glutamate into the synaptic cleft producing action potentials in the afferent nerve fibers . The number of action potentials increases with the increased release of neurotransmitter . The resting membrane potential is restored by diffusion of k+ along the electrochemical gradient through the leaky k+ channels at the basolateral membrane

7. Process of repolarisation : Role of cochlea: 1.The cochlea encodes the frequency and amplitude of sound waves that is further processed in the auditory pathway higher up in the cortex . 2 .cochlea acts as a fine transducer as well as a frequency analyser . Special features of basilar membrane: 1.The basilar membrane is narrow at the base and wider towards the apex , also stiff at the base and elastic towards the apex . 2. These mechanical characteristic enable the membrane to respond differently to the wide range of sound frequencies along its length . 3. The basilar membrane is the first for pitch discrimination , representation of tone is maintained in the cochlear neurons as well as in the neurons of the auditory pathway and this is called tonotopic organisation . 4. The basilar membrane also responds to different intensities of sound waves

8. Travelling wave : The organ of corti including the basilar membrane vibrates in accordance with undulation created in the perilymph , distorts in the form of a travelling wave . High frequency sound cause maximal displacement of the membrane near its base and low frequency sounds generate waves that attain maximum amplitude . The perception of different tones by the brain depends upon the type of wave generated in the choclear fluid and the basilar membrane . This phenomenon is known as travelling wave hypothesis .

9. Shearing force bends cilia : The basilar membrane and the tectorial membrane are joined by the cilia of the outer hair cells . When the stapes moves , the pressure gradient tends to deform both the membranes in the same direction . The pressure gradient creates the lateral shearing force that causes bending of the cilia . When the basilar membrane moves up , the stereocilia bend away from the modiolus , and the hair cells depolarise . When the basilar membrane moves down , the stereocilia bend toward the modiolus , and the hair cells repolarise .

10. Thank you By ROHIT ALLBI G S