NEUROSCIENCE: AUDITORY SYSTEM -- Sound Transmission: Sequence

SOUND TRANSMISSION

A. Sequence for Sound Wave Transmission

1. Collection of sound waves in the external ear canal (external auditory meatus); concentrates sound waves

2.    Vibration of the tympanic membrane (ear drum)

a. large area (relatively), so helps to amplify sound

b. transmits sound vibrations to middle ear ossicles

3.    Conduction by ossicles through middle ear

a.    ossicular function: sound transmission from tympanic membrane to oval window of the cochlea

b. middle ear muscles (tensor tympani and stapedius, innervated by C.N.s V and VII respectively): muscle contraction decreases efficiency of ossicular sound transmission

function: protect against cochlear damage due to excess sound energy; tympanic reflex

loud sound muscle contraction

Note: cannot protect against very loud sound or sound of sudden onset because of response latency (0.25 sec) and limited attenuation (10db)

Note: decreased hearing sensitivity after exposure to loud sound; initially sensitivity recovered with time but with repeated exposure may become permanent

c. Eustachian tube: equalizes pressure between middle ear and atmosphere to prevent stress on tympanic membrane by venting middle ear to nasopharynx

Note: Eustachian tube normally closed, but opened by swallowing, chewing, yawning, etc.; can be blocked by middle ear infection (otitis media: middle ear inflammation)

4. Vibration of the oval window

5. Transmission through the cochlear fluid

6. Vibration of the basilar membrane and excitation of the hair cells of the Organ of Corti