Lecture E – Sound to the Internal Ear

so just to summarize first of all there
is sound which are essentially vibrations in the air and those
vibrations first are directed into the external auditory canal and that is
going to cause vibrations and the tympanic membrane and the tympanic
membrane remember is simply the eardrum so once this membrane vibrates then
that’s going to transfer information to the ossicles and those ossicles then
are going to vibrate in order so there’s the malleus the incus and the stapes
and the stapes is what’s going to be covering up the oval window once that
oval window vibrates then there’s gonna be movement of the fluid which is found
in the inner ear and that’s what we’re gonna take a little bit more closer look
at in this short mini lecture so once this fluid is activated its going to
activate the sensory receptors or the sensory neurons so this is going to be
the site of the transduction just like we learned a lot about in some of the
other previous special senses as well so this slide is showing you a little more
detail for the cochlea so we see the route of sound waves to go through the
ear first of all there’s the sound waves
they vibrate on the tympanic membrane what we see at number one shown
right here and so the greater the intensity the sound so greater intensity
that’s simply going to correlate to greater movement of the eardrum so the
more that it’s going to vibrate and you can and that’s going to increase the
pressure on the smaller and smaller membranes so greater movement of the
eardrum it’s almost like the analogy of if you think about greater surface area increasing the
force or smaller surface area increasing the force so since this oval window is
smaller than the tympanic membrane that’s going to increase the force here
kind of like the idea that a sharp spike heel for a woman shoe is gonna great
create a lot more force on the floor than with the heel of a mens shoe for
example so the movement of the fluid is going to be amplified because of that
increase in pressure the perilymph is what we find in the scala tympani
here and that fluid is going to move back and forth in the higher the
frequency the sound the greater chance that it’s going to actually bypass this
area and go directly through to the cochlear duct and the basilar membrane
is the membrane on the bottom of the cochlear duct and that’s going to
vibrate so this slide is showing a little more detail of the basilar
membrane and what we see here shown on the left ear is higher frequency sounds
so the higher frequency sounds shown as 20,000 hurts and at the end of the right
hand side of the slide we see lower frequency sounds and what that’s going
to translate to is that the basilar membrane is going to mechanically
process these sound signals before the signals actually reach the receptors and
the end result is that there are neurotransmitters that are going to be
released just like in the retina for example and that is going to end up
leading to action potentials and these action potentials would be in the
cochlear nerve which is simply going to be a branch of the vestibule of cochlear
nerve which is cranial nerves number eight the other component of the
vestibule cochlear nerve is from the vestibule which is static
equilibrium the slide is showing the auditory pathway to the brain now so
we’re gonna look at is the difference between the first order neuron the second
order neuron and third order neuron as you’ve learned about before the first
order neuron is what goes from the spiral organ of Corti to the medulla
and at the medulla some of the nerves are going to cross over some of them are
not gonna cross over and when they cross over the term for that is decussate so not
all of them will cross over just like we learned about in the visual pathway as well
the second order neuron is going from the medulla up into the thalamus the relay
center of the brain and this happens to be a specific nucleus called the medial
geniculate nucleus remember the lateral geniculate nucleus was involved with
vision and then finally the destination for the third order neuron where
interpretation occurs is gonna be the primary auditory cortex so the
mechanisms that actually happened for equilibrium in orientation our first
shown here for static equilibrium and static equilibrium it’s important that
you know that that is going to be important for controlling posture for
example so it’s the movement left and right and kind of in a forward backwards
motion so when are you utricle and saccule respond to linear
acceleration so imagine you know putting the brakes on the car or accelerating
forward and backward motion so what happens simply put is that we see the
stereocilia gonna move in different directions back and forth based on the movement of
the head we see tilting the head this way is
going to cause depolarization the opposite way is going to cause
hyperpolarization but the end result is that there is going to be action
potentials going on to go through the vestibule nerve eventually to the brain
and remember that the receptors are called the maculae and the two sacks
with in the vestibule would be the utricle and the saccule this slide is showing rotational or angular equilibrium and this would be the type
of movement that occurs in the semicircular canals so at the base of
the semicircular canals we have receptors they’re called the
crista ampullaris and those are going to be activated much like the maculae
were activated in the utricle and saccule but they’re gonna be due to
rotational equilibrium so great example of this is this ice skater spinning and
that would be activating the base of her semicircular canals all this information
has to go into the brain from different receptors we’ve kind of learned about some of this before remember
that there has to be constant input coming from all the receptors in the
body from the skin the muscles and the joints recall that these are called
proprioceptors so there was a specific track that you learned about in
the central nervous system called the spinocerebellar tract and
all that information is going to the cerebellum to be processed there’s
information that’s coming from vision from the visual receptors and also for
balance from the vestibule receptor so there’s lots of information that has to
be processed by the brain and all that information then is going to be
integrated and finally there’s certain eye movements that happened there
certain neck movement that happen but all of this has to be
done in order to ensure that we make the next appropriate movement otherwise if
we had to think about all this we would already have been on the floor falling before that happens all this
stuff all the input is automatically integrated

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