Lecture D – Hearing & Balance

there are three major areas of the ear
and those three main areas include first of all the outer or the external ear the
middle ear and then also the inner ear and we’re going to look at each of them
individually the inner ear is where all the sensory reception happens kinda like
the retina where the neurons are located it’s going to be where the
transduction of the stimulus is converted into a nerve impulse so the
inner ear actually has two of those special senses that were mentioned
previously and the first one is going to be equilibrium and this happens in an
area called the vestibule and the other place is in the semicircular canals and
we’ll be talking about them coming up the other major area of the inner ear is
of course hearing and the hearing sensory receptor is located in the
cochlea so when we look at the anatomy of the outer ear first of all the
outermost portion that we physically see that everybody can see which is visible
is going to be an area of elastic cartilage and this is the elastic
cartilage that helps to concentrate the sound waves and direct the sound waves
or funnel the sound waves into the external auditory meatus so when we look
at this diagram we can see imagine the sound waves coming in this direction and
they’re traveling in through the external auditory meatus or the external
auditory canal both terms mean the same thing now lined external auditory canal
are some specialized glands and these are specialed sweat glands called
Ceruminous glands and they produce earwax the function of the ear wax is to
trap foreign bodies and also to prevent insects from getting into the ear
pretty kind of a pretty nasty thought so the sound waves as they come in they hit
the tympanic membrane the tympanic membrane is most commonly referred to as
the eardrum and it’s basically a membrane and it’s going to transfer
the sound wave to the middle ear canal so the middle ear canal is located in
the tympanic membrane or I’m sorry tympanic cavity and in the tympanic
cavity tube itself that we can see here is the pharyngotympanic tube its commonly
referred to as the eustachian tube also the auditory tube they don’t
usually use the term eustachian anymore in a lot of text books but it’s the same
thing so the eustachian tube is usually gonna be flattened and it’s usually
gonna be collapsed except when it’s performing the function to equalize
pressure between middle ear and the external air pressure so when you yawn
or swallow this tube is going to open up but normally it would be collapsed so
we look at the bones in the middle ear these are very very small bones and it’s
really interesting that between the bones there is actually many synovial
joints so synovial joints as you know now are those freely movable joints located
between the ossicles and there’s important muscles that are located which are going
to prevent damage to the ear so these muscles are going to contract when the
sound is very very high so they’re gonna contract when the sound is low so when the sound
is very very loud not low and it’s to protect the hearing receptors so if you
go to a really really loud concert these muscles are gonna contract for example
Stapedius muscle is what is connected to the stapes the third ossicle the order of
contraction of these ossicles are the malleus first then the incus
and then finally the stapes the stapes is going to be covering up another
membrane called the oval window and that’s kind of the beginning the
separation between the middle ear and the inner ear so the next slide is
showing the vestibule and the vestibule is for static equilibrium so it is one
of the two parts that are specifically involved with balance or equilibrium so
what we’re seeing first of all here within the ear there are what’s called
a bony labyrinth and then also membranous labyrinth and the utricle and
the saccule are within the vestibule and they are essentially
membranous sacks and these sacks are very important because they’re going to
contain import receptors called the maculae and these receptors are going to
be where the receptors are located to transduce or convert a nerve impulse so
the inner ear is made up of two parts for balance or equilibrium the bony
labyrinth is what we’re seeing here and then there’s another part of the called the
semicircular canals which we will look at on the next slide and contained
within the membranous labyrinth are series of sacks and ducts and there’s a
couple different fluids that are in this area there is what’s called perilymph
and the perilymph is going to be similar to cerebrospinal fluid and it’s
actually continues with the cerebrospinal fluid and then there’s another
type that will see coming up called edolymph and the endolymph is gonna be
located in the hearing receptor in the cochlea specifically where the organ of
Corti so that’s the hearing receptor and the endolymph is going to be rich in
potassium so it’s very similar to what’s inside of the cells the intra- cellular
fluid so the next part of equilibrium is the semicircular canals and this is
associated with dynamic equilibrium and there are canals in three different
directions three different planes and you can see
that one is anterior one is posterior and one is lateral so these this flood is
going to basically be moving in different directions when our head
rotates and an angular movement so it requires the rate of change over time maintains the dynamic equilibrium so
balanced when we’re moving so imagine Dancing with the Stars the type of
rotational movement that occurs there that’s what’s going to activate the
semicircular canals and the receptors that are found within it and the
specific receptors equilibrium receptors that you should know associated with the
semicircular canals are called the Cristae ampullaris just like in
the previous slide those equilibrium receptors for the vestibule were
referred to as the maculae so maculae for vestibule and also Cristae
ampullaris for semicircular canals so we’re now going to look at the hearing
receptor and the hearing receptor is what’s located in the cochlea so the
cochlea is divided into three chambers imagine what would happen if we took
this snail shape structure we kind of rolled it out as a very long tube and we
were to cut that tube in a transverse section you would actually see these
three individual sections and these three sections are very important the
top section is the scala vestibule and it contains perilymph and the perilymph is remember similar
to the cerebrospinal fluid the middle area is called the scala media and
that’s where the cochlear duct is so that has endolymph in it which is
inside the cell the intracellular fluid so recall that its potassium rich
and then finally at the bottom there is the scala tympani and that’s going to
contain perilymph as well which is also the cerebrospinal fluid but the
most important part of this is the spiral organ of Corti and that’s where
the individual receptors are located we see down in this lower right hand corner
of the slide here these are where the receptors are going to be activated
which send a nerve impulse into the brain so just to summarize the
transmission of sound into the inner ear what we see here first is that the outer
ear sends the signal into the middle ear which is where the ossicles are
located so essentially we have sound energy that is converted into mechanical
energy these are mechanoreceptors in the middle ear and then finally we have the
movement of fluid in the inner ear the displacement of these fluids that perilymph and the endolymph and what ends up happening is the receptors in
the organ of Corti are going to be stimulated so the bottom layer the
bottom membrane of organ of Corti is called the basilar membrane and when
those those receptors move that the hairs are displaced that activates the
receptors and a nerve impulse goes into the brain for interpretation so this is kind of the summary of what’s
happening in the inner ear the outer ear the pinna or the oracle both terms
apply the external auditory canal or the external auditory meatus tympanic
membrane the ear drum and once the ear drum is stimulated that transfers the energy into the ossicles
the malleus the incus and the stapes to the oval window and the oval window is going
to be covered up by the stapes so when the stapes is the third ossicle to vibrate the oval window moves and that transfers the energy to the inner ear
and these fluids then are going to move so eventually for hearing the organ of
Corti is going to be stimulated

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