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2 7 Video 1 7 Richard Baker Part 2 04 59

In this second part, we’ll look at
the basic function of the inner ear, more specifically at the cochlea, and what
happens if this sense organ is damaged. The inner ear, consists of both
hearing and balance organs. The hearing organ is the cochlea, a snail
shell like, fluid filled spiral tube. As the stapes moves in and out the fluid
in the tube moves backwards and forwards. The cochlea tube is divided down
the middle by a partition, and the sensor in the organ
accordion sits on this membrane. This vascular membrane is narrow and stiff near the stapes at
the base of the cochlea. And gets broader and floppier towards
the tip of the snail shell, or the apex of the cochlea. This change in basilar membrane along
the length is the basis for the cochlea, being able to separate sound
into different frequencies. If we were able to
straighten out the cochlea, we would see that, although the tube
itself gets narrower toward the apex, the movable portion that has
the membrane gets broader and floppier. As the stapes moves rapidly in and out in response to sound, a travelling
wave occurs across the baseline membrane. It is this up and down movement of
the baseline membrane that stimulates the organ of Corti and
causes the nerve fiber to fire. At the base of the cochlea, the basilar
membrane is narrow and stiff and moves best at high frequency sounds. At the apex of the cochlea, the basilar
membrane is wider and floppier and thus moves best to low frequency sounds. This means that the cochlea has
a frequency gradation along its length. What is called a tonotopic organization. If a single frequency sound is played into
the ear, then it produces basilar membrane movement that is restricted to
a particular place in the cochlea. And so, only nerve fibers
attached to that place will fire. If a different frequency sound is played, nerve fibers at a different
place will respond. This is the basic mechanism by which the
cochlea is able to separate sounds into different frequencies. In summary, because of the structure
of the cochlea partition, the cochlea is tonotopic. Different locations along the spiral,
respond best to different frequencies. This is a property of
the structure of the cochlea. That means that the organ of corti,
enhance the auditory nerve fibers nearest the stapes, or base of the spiral respond
best to high frequency sounds, and reaches near the tip of the spiral, the apex,
respond best to low frequency sounds. This means that the cochlea can
separate sounds into different component frequencies. Within the cochlea and sitting on the basilar membrane is a sense
organ of hearing, the organ of corti. A fluid in the tube moves
in response to sound. The basilar membrane and
the organ of corti move up and down. This movement stimulates the sensory
cells within the organ of corti or the hair cells. The organ of corti contains two types
of sensory cells or hair cells. One of these, is the inner hair cells. These detect the movement and stimulate
the auditory nerve to indicate that frequencies corresponding with this place
in the cochlea are present in the sound. In order for these to. The second type of hair cells
are the outer hair-cells. These act like tiny amplifiers that
increase the movement of the basilar membrane at the organ of corti. So that it can be detected
by inner hair-cells. Without these outer hair-cells,
the ear will be much less sensitive and some normal speech sounds
will be inaudible. Two key aspects of the sound
are the intensity of the sound and the frequency content. Sound intensity is a physical aspect of
the sound that relates to what we describe as loudness. The higher the sound intensity,
the more the basilar membrane moves and the more the auditory
nerve fibers are active. For low intensity sounds, the outer hair
cells are needed to amplify the basilar membrane movement, enough for the
inner-hair cells to detect that movement. Without this outer hair
cell amplification, the ear wouldn’t be able to detect low
intensity sounds such as whispers. Sounds such as speech are very complex and
as well as different intensities, they also contain many
different frequency components. As we have seen, the ear is able to separate different
frequencies from each other. The outer hair cells play
a crucial role in this. By amplifying the movement
to the basilar membrane, they make it easier to distinguish
frequencies that are close together. They improve what we call
frequency selectivity, our ability to separate out
different frequency sounds.

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