Ch 09 sec 9-6 9-7


So before we move onto the ear and the special
senses of equilibrium and hearing, we should discuss
photoreception and how our receptors in the eye actually
respond to light stimulation. So photoreceptors respond
to photons and physics where your photons are really,
really small packages of energy. Radiant energy to be more
specific and they travel upon wavelengths. The distance
between the peaks of the waves is determined by the
color of the light so for example, photons of red light
have the longest wavelength and the least energy and photons
of violet light have the shortest wavelength and most energy.
So rods respond to the presence or the absence of photons
regardless of wavelength. So they’re very sensitive and
very effective in dim light. But they don’t provide us with
any information about the color. That’s where the cones come
in. So we have three different types of cones. We have blue
cones, green cones, and red cones. These cones contain
pigment sensitive to blue, green, or red wavelengths of light.
So since cones are less sensitive, they function in only
very bright light. Color blindness can occur when one or more
types of a cone is not functioning or if it’s missing.
The most common is a red/green color blindness where the red
cones are missing. This is a great picture of a photoreceptor.
So the pigment epithelium is the layer of the
retina that borders the posterior cavity. It’s the very
top of this figure. The outer segment of the photoreceptor contains
hundreds of thousands of flattened discs and these
contain visual pigments that absorb photons and initiate
photo reception. The pigments are made of a compound rodopsin
that contains opsin and retinal. Retinal is derived
from vitamin A. And retinal is the same in rods and cones
but opsin is different. The inner segment contains organelles
and synapses with bipolar cells. Photoreception begins when a photon strikes
rhodopsin. So if you look at the very top of in the middle
of this top middle of this figure, you can see a photon
as that yellow wiggly arrow. It comes in and it strikes rhodopsin
which is respresented by this big purple blob with
the red bent key in it. Okay, so photoreception begins when
a photon strikes rhodopsin and the retinal and the opsin are
going to break apart. And this is referred to as bleaching.
This alters the rate of neurotransmitter release into
a synapse with a bipolar cell. For a rod or a cone to be
able to respond to light again, the opsin and the retinal must
recombine. So you can see that the retinal, the bent red
key, once it is hit with a photon it changes shape and it
no longer is bound to that rhodopsin. In fact, now you have just
a retinal and an opsin. And it requires ATP to restore that
retinal shape so that it looks like a bent key again
and to get it to bind to opsin again to form our rhodopsin.
So it’s kinda nice that this rhodopsin is rechargeable and recyclable
and that we can keep forming images otherwise this
would be a one time event. The visual information received
by the bipolar cell is then passed along to the ganglion
cell and the ganglion’s axons form the optic nerves or cranial nerve
number 2. These are gonna convert at the optic chiasm
and about half of the fibers cross the chiasm to the
opposite side of the optic tract. The other half do not cross.
The optic tracts then relay the visual information to the thalamic
nuclei which act as switching and processing centers
and send the sensation to the reflex centers of the superior
nuclei of the midbrain which controls our reflexes and/or
the visual cortex of the cerebrum. So that is the end
of the eye and the sense of vision section of the textbook.
The next section looks at the sense of equilibrium and hearing
in the ear. So the special senses of equilibrium and hearing
are provided by the ear and the ear is divided into three
anatomical divisions: the external ear, the middle ear, and the
internal ear. The external ear is the visual portion of
the ear that collects sound waves and funnels them towards the middle
ear. The middle ear is a chamber with structures
that amplify sound waves and then transfer those waves
to the internal ear which contains sensory organs for hearing
as well as sensory organs for equilibrium. So let’s take a look
at that. The external ear is composed of the auricle or the pinna
I think your lab manual calls it and this is a fleshy cup
that directs sound waves into the ear and directly into the external
acoustic meatus or the auditory canal. This is going to contain
many glands that are specific for secreting earwax. Those sound
waves are going to strike the tympanic membrane or the ear drum.
The ear drum is a thin sheet that vibrates when the sound
waves strike it. In the middle ear, which is also called the
tympanic cavity. This is an air-filled chamber and we have an auditory
tube. This is also called a pharyngotympanic or a eustachian
tube. But anyway, this auditory tube leads to the pharynx which
allows for pressure equalization on either side of the ear drum.
The middle ear also has three small bones that connect the
tympanic membrane to the internal ear. So let’s take a closer look
at that middle ear and those three small bones. The first
bone, the first auditory ossicle which is attached to the
ear drum is the malleus and the malleus vibrates on the incus which is
attached to the malleus and is the innermost middle bone. Sometimes it’s
called anvil and that is attached to the stapes. So the stapes
has a base that nearly fills the oval window into the internal ear.
So let’s talk about that internal ear. The internal ear has sensory
structures that are protected by a bony labyrinth. Inside
of this bone labyrinth is a membranous labyrinth and it’s around this
membranous labyrinth you have perilymph. So perilymph is a fluid
that flows between the bony and the membranous labyrinth and
these tubes follow the contures of the bony labyrinth
and on the inside of that membranous labyrinth you have endolymph.
Okay, so you have perilymph around the membranous
labyrinth and endolymph on the inside of it. So you can
see that the oval window is covered with a thin membrane that separates
the perilymph in the cochlea from the air in the middle
ear and just below that oval window you can see the round window
is labeled and this is opening in the bone which is covered
with a membrane of the cochlea. So there are three parts of the bony labyrinth:
the vestibule contains a membranous saccule and utricle
with receptors with gravity and linear acceleration. The
semicircular canal contain membranous semicircular ducts with
receptors for rotational acceleration and these two components,
the vestibule and semicircular canal make up the
vestibular complex. So this is going to provide us with a sense
of balance. The third portion of the bony labyrinth is
the cochlea and this contains the membranous cochlear duct and
the sensory receptors for hearing. So in the ear, we have hair cells which are
the sensory receptors. Hair cells are surrounded by supporting cells
and hair cells are going to synapse with dendrites of sensory
neurons. The free surface of the hair cells is covered
with stereocilia which is where they got their name hair cell.
So it’s the movement of this stereocilia that alters a neurotransmitter
release and so bending the stereocilia in one direction
triggers depolarization. Bending the stereocilia in the other direction
leads to hyperpolarization. Now that you have an idea of the structures
of the ear, let’s discuss how it’s used for the special senses
of hearing and equilibrium. So there’s two aspects of equilibrium:
dynamic equilibrium and static equilibrium. Dynamic
equilibrium is responsible for maintaining balance while
in motion and this is monitored by your semicircular ducts.
Static equilibrium is maintaining balance and posture while you’re
motionless, while you’re holding still. So this is monitored
by the saccule and utricle. The sensory receptors in the
semicircular ducts are going to respond to rotational movements of the head and so this is a figure that shows
the three ducts. The anterior, posterior, and lateral
ducts and these are organized into three planes. Transverse,
frontal, and saggital. Notice how each one contains an
ampulla. Inside each ampulla we have sensory receptors.
The crista ampullaris contains hair cells that
are embedded into latin structure called the cupula. When
the head rotates endolymph pushes against a crista and activates
those hair cells. In the vestibule pictured here
in this black box the saccule receptors respond with gravity
and linear acceleration so the utricle receptors respond
to horizontal acceleration. So hair cells of the saccule
and utricles are clustered in an oval maculae. The hair cells in the macula project into
a gelatinous membrane. The surface of the membrane is covered with
dense, heavy calcium carbonate crystals called otoliths.
So gravity pulls the otoliths, pulling on the hair cells. Hair cells of the vestibule and the semicircular
ducts synapse with neurons of the vestibular branch of cranial
nerve number 8. Which then synapse with neurons in the vestibular
nuclei of the pons and medulla oblongata. And information
is then relayed to the cerebrum, the cerebral cortex,
the cerebellum, or motor nuclei in the brain and the spinal
cord. The hair cell receptors in the cochlear duct
are similar to those of the vestibular complex. They provide
us with the sense of hearing. They are responsible
for conveying vibrations from the tympanic membrane to the
oval window. The auditory ossicles are going to convert
sound energy in the air to a pressure pulse in the perilymph
of the cochlea. This is gonna stimulate hair cells along the
cochlear spiral. The pitch or the frequency is determined by
which part of the cochlear duct is stimulated whereas volume
or intensity is determined by how many hair cells are activated
at that site. So this is a sectional view of the cochlear
duct or the scala media which shows two chambers. The
scala vestibuli or the vestibular duct which is pictured on
the top portion and the scala tympani or the tympanic duct. The
scala vestibuli and the scala tympani are both filled with
perilymph and they’re a continuous chamber. This is a closer look at the spiral organ
of corti which is located in the cochlear duct on the basilar membrane.
Hair cell stereocilia project onto the tectorial membrane
which is attached to the wall of the cochlear duct.
So as waves strike the basilar membrane moving it up and down,
the hair cells are pushed against the tectorial membrane
bending the stereocilia. So the 6 steps to hearing are
involved in this figure. In step number one, sound waves are striking
the tympanic membrane causing it to vibrate and in step
two you can see that vibrations in the tympanic membrane are
gonna vibrate the auditory ossicles. These ossicle vibrations
are going to lead to vibrations as pressure on the perilymph
through the oval window in step three. Step four, the
pressure pulses are going to distort the basilar membrane.
In step five, movement of that basilar membrane distorts the hair
cells against the tectorial membrane. This leads to a neurotransmitter
release and then step six that neurotransmitter release
that action potential is going to travel to the central
nervous system through the vestibular cochlear nerve which
is cranial nerve number 8. So the cochlear branch of the vestibular cochlear
nerve, cranial nerve 8, those axons arrive from the spiral
ganglion to the cochlear nuclei of the medulla oblongata to
the inferior colliculi at the midbrain to the nucleus of
the thalamus to the auditory cortex of the temporal lobe. And that concludes the hearing and equilibrium
section of chapter 9. I have a link to hearing video
that if you would like to watch it, you may. All you have to do is
open up the powerpoint file which is in Blackboard in
learning plans.

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