Mechanism of Hearing, Animation

sounds are produced by vibrating objects
the vibrations of a sound source cause the surrounding air molecules to move
back and forth creating a series of alternating regions of high and low
pressures a sound wave is basically a pressure wave it propagates in the form
of fluctuations in air pressures the loudness of a sound is determined by the
amplitude of sound waves which represents the strength of vibrations
produced by the sound source the stronger the vibrations the higher
the amplitude of sound waves the louder the sound the pitch of a sound is
related to the frequency of sound waves which indicates how fast the sound
source vibrates the higher the frequency the higher the pitch frequency is measured in Hertz a young
human ear can detect sounds in the range of 20 to 20,000 Hertz some animal
species can hear frequencies well beyond this range hearing is the process by
which the ear transforms sound vibrations into nerve impulses that can
be interpreted by the brain as sounds the human ear has three distinct regions
called the outer middle and inner ear the outer ear funnels sound waves
through the auditory canal to the tympanic membrane also called eardrum
which separates the outer ear from the middle ear the eardrum is attached to a
chain of three small bones in the middle ear called the ossicles the malleus
incus and stapes sound waves caused the tympanic membrane to vibrate and the
vibrations are transmitted through the three bones to the oval window where the
inner ear begins since the eardrum is much larger in area
than the oval window the sound pressure that arrives at the oval window is much
greater than the original pressure received by the eardrum this
amplification is essential for the stapes to push against the higher
resistance of the fluid in the inner ear the organ of hearing in the inner ear is
the cochlea essentially a long tube that is coiled up in a spiral to save space
the cochlea is composed of three fluid- filled chambers the central chamber
known as the cochlear duct is where mechanical vibrations are transformed
into nerve impulses there are four rows of hair cells within the cochlear duct
supported on the basilar membrane the movements back and forth of the stapes push on the fluid in the cochlear duct causing the basilar membrane and
the hair cells to move up and down these movements bend the cilia of hair cells
opening the mechanically gated potassium channels on their surface influx of
potassium depolarizes the cells stimulating them to send nerve impulses
to the cochlear nerve and on to the brain our ability to differentiate
sounds of different loudness and pitch depends on the ability of the cochlea to
respond differently to different amplitudes and sound frequencies louder
sounds caused more hair cells to move and generate greater nerve signals to
the brain different frequencies stimulate different parts of the basilar
membrane which acts like a set of piano strings the basilar membrane is
narrowest and stiffest at the base near the oval window and widest and most
flexible at the far end high frequency sounds with more energy can move the
stiffer part of the membrane while low-frequency sounds can only move the
more flexible part thus high pitch sounds excite nerve fibers that are closer
to the oval window while low pitch sounds sends signals through the fibers
at the far end


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