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| Mechanism of Hearing |
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| The sound waves are directed towards the ear canal by the pinna. |
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| The waves that enter the canal are concentrated and made to strike against the tympanum. |
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| The vibrations are picked up by the malleus on the other side. |
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| These vibrations are transmitted to the fenestra ovalis via the incus and the stapes. |
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| The vibrations that strike the oval window are amplified 22 times more than those that struck the tympanum. |
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| These vibrations travel along the vestibular canal to the end of the cochlea and then to the tympanic canal. The vibrations are also transmitted via the Reisnner's membrane to the basilar membrane and then to the tympanic canal. |
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| Note that the vibrations travel along the vestibular and tympanic canals in the opposite directions. |
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| From the basilar membrane the vibrations are picked up by the sensory hair cells of the organ of corti and transmitted as action potentials to the neurons of the auditory nerve fibres. |
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| The exact mechanism of transformation of the sound waves into the action potentials is not known. |
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| The action potentials are then transmitted as nerve impulses to the auditory cortex of the brain through the auditory nerve. |
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| (b) |
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| (a) Axonal membrane showing distribution of ions and ion channels |
| (b) Recording of nerve action potential |
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| Nature of sound |
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| Sound is a form of energy. It is produced by the vibration of particles and it travels in a wave-like motion. The number of waves passing a point per second is called the frequency and its unit is Hertz (Hz). The pitch of the sound (high or low sounding) is measured by the frequency. Man can hear between 16 and 20,000 Hz. Beyond this the frequency is referred to as ultrasonic. Other animals hear and produce ultrasonic sounds. For e.g., dogs can hear up to 40,000 Hz and bats up to 100,000 Hz. The pitch is perceived by the specific regions of the auditory cortex in the brain. |
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| The loudness of the sound is determined by the number of sensory cells stimulated at the same frequency. Low sound will stimulate fewer number of cells than loud sound. Thus the loudness is determined by the rate of nerve impulses reaching the auditory cortex. |
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