Anatomy of the Ear Pinna: The external ear that we can see,it collects sound from the environment. Tympanic membrane: The eardrum which vibrates when stimulated by sound waves. Ossicles: The bones of the middle ear.They transfer vibrations from mid brain to Cochlea 1. Malleus 2. Incus 3. Stapes Cochlea: The snail-shaped, fluid-filled, bony structure of the inner ear.It contains the basilar membrane and the auditory receptor hair cells. Oval Window: An opening in the bone of the cochlea that reveals a membrane against which the baseplate of the stapes presses, transmitting sound vibrations into the fluid- filled cochlea.
The O rg a n O f Co rti is the receptor organ that generates nerve impulses in response to vibration of the basilar membrane It lies on the surface of the basilar fibers and basilar membrane•It consists of supporting cells and specialized type of nerve cells calledhair cells,which are of two types:-1.A single row of Inner Hair Cells numbering about 3500 and measuringabout 12 micrometers in diameter2.Three or four rows of Outer Hair Cells numbering about 12,000 andmeasuring only about 8 micrometers•Protruding from each hair cell are structures called s te re o c ilia•The tectorial membrane lies above the stereocilia – shearing motionbetween BM and tectorial membrane – causes stereocilia to be displaced
• Its function is to convert BM mechanical movement into neural activity – achieved in the following way :_• The stereocilia are joined by fine links called ‘tip links’.• Deflection of the stereocilia leads to the opening of “transduction channels”.• Flow of potassium ions into the hair cell occurs due to voltage difference between the inside and outside of the hair cell.
• Causes the release of neurotransmitter and the initiation of action potentials in the neurons of the audtiory nerve.• Action potential propagation is in one direction i.e only down the length of the axon• Most of the a ffe re nt neurons make contact with the inner hair cells• Possibly all information about the input sound is conveyed via the inner hair cells
• They have a role in achieving high sensitivity and sharp tuning• Most of the e ffe re nt neurons synapse directly with the outer hair cells• Effe re nt neurons carry information from higher auditory system to cochlea• A re nt neurons carry information from the cochlea to the higher ffe auditory system
• Movement of basilar membrane presses the hair cells against the tectorial membrane• Deflection or bending of stereocilia in one direction depolarizes the hair cells,and bending in the opposite direction hyperpolarizes them.• This in turn excites the auditory nerve fibers synapsing with their bases• Upward movement of the basilar fibers rocks the reticular lamina upward and inward toward the modiolus.• This causes the hairs on the hair cells to shear back and forth against the tectorial membrane,thus the hair cells are excited whenever the basilar membrane vibrates
• About 90% of the auditory nerve fibers are stimulated by the inner hair cells rather than the outer hair cells• It has been proposed that the outer hair cells in some way control the sensitivity of the inner hair cells at different sound pitches,this phenomenon is called “Tuning” of the receptor system• In this phenomenon,a large no of retrograde nerve fibers pass from the brain stem to the vicinity of the outer hair cells• Stimulation of these nerve fibers can actually cause shortening of the outer hair cells and change their degree of stiffness• These effects suggest a retrograde nervous mechanism for control of the ear’s sensitivity to different sound pitches,activated through the outer hair cells