The sense of hearing

The ears outer , middle , and inner parts perform the functions of collecting , amplifying and transducing sound energy , respectively .
•The outer ears pinna and ear canal funnel sound waves onto the eardrum , causing it to vibrate .
•The ear – drum vibration is amplified 20- fold through the lever action of the middle ears ossicles (malleus, Incus, stapes) , and differential vibrating surfaces of the eardrum and the inner ears oval window .
•Stapes movement displaces the oval window and subsequently the basilar membrane of the inner ears cochlea , generating frequency- dependent traveling waves in the cochlea .
•The hair cells of the cochlea transduce sound waves into nerve impulse.

Steps in the generation of sensory impulses from the ear
 1. Sound waves enter the external auditory meatus .
 2. Waves of changing pressure causes the tympanic membrane
to vibrations coming from sound wave source  3. Auditory ossicles amplify and transmit vibrations to the and of the stapes .
4. Movement of the stapes at the oval window transmit vibrations to the perilymph in the scala vestibule .
5. Vibrations pass through the vestibular membrane and enter the endolymph of the cochlear duct .
 6. Different frequencies of vibration in endolymph move specific regions of the basilar membrane , thus stimulating
specific sets of receptors cells (hair cells). 
7. As receptor cell becomes depolarized ; its membrane become more permeable to Ca++ ions .
8. In the presence of Ca++ ions , vesicles at the base of receptors cell release neurotransmitter .
 9. Neurotransmitter stimulates the ends of nearby sensory neurons .
 10. sensory impulses are triggered on fibers of the cochlear branch of the vestibulocochlear nerve .
11. Auditory impulses travel into medulla oblongata , midbrain , and thalamus , and are interpreted in the temporal lobes of the cerebrum.

At the end of this lesson, students should learn 

  1. Tympanic membrane and ossicular system
  2. cochlea and its function
  3. hearing pathway
  4. functions of organ of corti
  5. endocochlear potential

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ear.ppt (2.26 MB )