________________
Human Body
51
Both are filled with fluid. Both consists of three parts : a vestibule, a cochlea and three semicircular canals. The receptors for hearing are found within a complex structure which lies within the cochlear duct. More than 20,000 stiff hair-like fibres run across the floor (Basilar membrane) of the duct. Their lengths increase progressively from about 0.04 m.m. to 0.5 m.m. They can vibrate like the reeds of a harmonica ; shorter ones at high frequency and longer ones at low frequency. The transmitted sound waves set different portions or the basilar fibres vibrating, stimulating different cells of the receptors which relay their impulses along the auditory nerve to the brain. The Physiology of Hearing
Sound waves enter the external auditory canal and strike tbe eardrum setting it vibrating. The vibrations are transmitted to the three tiny ossicles-the hammer, the anvil and the stirrup-in sequence. The foot plate of the stirrup presses against the covering membrane of the oval window, setting up pressure waves in the fluid which winds through the spiral coils of the snail. The pressure produces a wave that travels along the floor i.e., the basilar membrane. This looks something like a long xylophone which gets wider as it stretches out along the coil of snail. At the end, it is 12 times as wide as at the base near the oval window. Each sound sets up sympathetic (resonant) vibrations in a particular place. These, in turn, stimulate the hair cells generating a receptor potential, which activates neurons of the cochlear nerve. The impulses transmitted to the brain carry data on the place of the basilar membrane to enable the cerebral cortex to determine the frequency of the sound. Nervous Pathway for Hearing
Like the messages of the other sense organs, the sounds detected by the ears are not meaningful, until they are analysed and interpreted in the brain. The main nervous pathway for hearing go upward to the cerebral cortex in the upper part of the temporal lobe. Each ear sends impulses to both sides of the brain and even a total destruction of the hearing centre in one hemisphere would not interfere with hearing.
Jain Education International
For Private & Personal Use Only
www.jainelibrary.org
