How human hearing works
The human ear is responsible for converting variations in
air pressure – from speech, music, or other sources – into the neural activity that
our brains can perceive and interpret. The ear can be divided into three
sections: the outer ear, the middle ear and the inner ear. Each of these parts
performs a specific function in processing sound information.
Sound waves are first collected by the outer ear, which is
made up of the external ear (also called the pinna) and a canal that leads to
the eardrum. The external ear amplifies sound, particularly at the frequency
ranges of 2,000 to 5,000 Hz – a range that is important for speech perception.
The shape of the external ear is also important for sound localisation –
picking up where the sound is coming from.
From the ear canal, the sound waves vibrate the eardrum,
which in turn vibrates three tiny bones in the middle ear. These three tiny
bones are called the malleus, incus and stapes. The stapes vibrates a small
membrane at the base of the cochlea (which is called the oval window) which
transmits amplified vibrational energy to cochlea, which is full of fluid. The
round window separates the tympanic canal from the middle ear.
The inner ear converts sound into neural activity. The
auditory portion of the inner ear is a coiled structure called the cochlea. The
region nearest the oval-window membrane is the base of the spiral; the other
end, or top, is referred to as the apex.
Inside the length of the cochlea are three parallel canals;
the tympanic canal, the vestibular canal, and the middle canal. The main
elements for converting sounds into neural activity are found on the basilar
membrane, a flexible structure that separates the tympanic canal from the
middle canal.
This diagram shows the cochlea ‘unrolled’ so that we can see
the basilar membrane more clearly.
The basilar membrane is about five times wider at the apex (top)
of the cochlea than at the base, even though the cochlea itself gets narrower towards
its apex. It vibrates in response to sound transmitted to the cochlea from the
middle ear.
High frequency sounds displace the narrow, stiff base of the
basilar membrane more than they displace the wider, more flexible apex.
Mid-frequency sounds maximally displace the middle of the basilar membrane.
Lower frequency sounds maximally displace the apex.
Within the middle canal and on top of the basilar membrane
is the organ of Corti. The organ of Corti is the collective term for all the
elements involved in the transduction of sounds. It includes three main
structures: the sensory cells (hair cells), a complicated framework of
supporting cells, and the end of the auditory nerve fibres.
On the top of the organ of Corti is the tectorial membrane.
The stereocilia of the outer hair cells extend into indentations in the bottom
of the tectorial membrane.
The movement of fluid in the cochlea produces vibrations of
the basilar membrane. These vibrations bend the stereocilia inserted into the
tectorial membrane. Depending on the direction of the bend, the hair cells will
either increase or decrease the firing rate of auditory nerve fibres.
References:
The human ear - http://bcs.whfreeman.com/thelifewire/content/chp45/4502001.html
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