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Psychology 2115A/B textbook notes 116-131 the ear.docx

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Western University
Psychology 2115A/B

Psych 2115 The Physiology of the Human Ear- Textbook Notes The outer ear (the "pinna") is the fleshy part of the ear that is visible from the outside. The pinna channels sound waves into the ear canal (the "external auditory meatus"). Pinnae also affect sound localization. Sound waves that reach the ear move along the ear canal until they encounter the eardrum ("tympanum"). The ear canal acts as a passive amplifier increasing the amplitude of certain sound frequencies through resonance. Sounds with frequencies between 2000 and 7000 hz benefit from this (frequencies that carry some speech information). The eardrum vibrates in phase with the incoming sound waves moving faster for high frequency sounds and slower for low frequency sounds. The vibrations of the eardrum are transmitted to the transducer mechanism in the cochlea by 3 tiny ossicles (bones) in the middle ear: the malleus ("hammer"), the incus ("anvil"), and the stapes ("stirrup"). The stapes connects to the cochlea via a small membrane covered opening called the oval window. In addition to conducting vibrations to the oval window, the middle ear also amplifies them. Amplification is necessary because the eardrum, to which the malleus is attached, is a large, easy to move flap of skin, whereas the part of the inner ear that must be moved by the stapes, the oval window, is small and difficult to move because it is at the bottom of a long tube filled with fluid. The middle ear increases the pressure applied to the oval window in 3 ways. First, the oval window is 1/15 the size of the ear drum, and when the same force is applied uniformly to two surfaces of different areas, the smaller surface will receive the greater force per unit area. Hence, the difference in vibrating area between the eardrum and oval window causes the pressure exerted by the stapes on the oval window to be about 15 times greater than that of the sound wave on the eardrum. The other pressure increasing mechanisms are more subtle. They depend on the lever action of the malleus and incus and on the way the eardrum buckles as it moves. These two forces increase the force applied by the stapes to the oval window by decreasing its velocity of movement relative to that of the eardrum. In total, the force applied at the stapes is amplified by a factor of about 30 over that of the sound wave on the eardrum by those properties of the middle ear. The middle ear can also decrease the pressure at the oval window relative to that at the eardrum as a protective mechanism. Low to moderate sound pressures can cause the stapes to push directly on the oval window. High sound pressures cause the stapes to move at an angle that greatly reduces the force it applies to the oval window. In addition, high-level sounds of low frequency cause muscles attached to the malleus and the stapes to contract reflexively, thus decreasing the movements of the ossicles and decreasing the force applied to the oval window. These mechanisms reduce the pressure of what otherwise might be damaging sounds. The ossicles of the middle ear are surrounded by air. The pressure of the air in the middle ear is kept approximately equal to that of the surrounding atmosphere by means of the Eustachian tube, which opens into the back of the throat; a pressure differential would cause the eardrum to bulge and stiffen resulting in less responsiveness to the sound striking it. The two Eustachian tubes open briefly every time we swallow, allowing air to flow into the two middle ear cavities from the mouth and lungs. This equalizes air pressure on both sides of the eardrum. Vibrations of the stapes are transmitted to the inner ear via the oval window, which is the boundary between the middle and inner ears. The oval window is at the base of the vestibular canal, one of the three tubes that form the cochlea. The far end of the vestibular canal (called the apex) is connected to another tube called the tympanic canal. This connection is a sort of short section of bent corridor called the helicotrema (this is the center of the swirly part of the cochlea). The tympanic canal has its own membrane-covered opening at its base, which separates it from the airspace in the middle ear, called the round window (round window is at the part which begins the cochlea). These canals are filled with perilymph, a fluid resembling salt water. Because perilymph is not compressible, movements of the stapes can cause the round window to bulge out or flex in from applied pressure. The thi
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