Lecture A – Eye and Vision

this chapter is going to cover the
special senses specifically we’re gonna start with the vision and there’s also
taste smell and in the air not only is their hearing but there’s also
equilibrium so there’s going to be five special senses that are covered in this
chapter and what’s important about this is their special sensory receptors so
there’s not going to be the same general receptors that you learned about in the
peripheral nervous system these are going to be more special receptors that
are much more complex so for example in vision we’re gonna be specifically
referring to the photoreceptors and the photoreceptors are going to be the rods
and also the cone so the eye itself is going to have three layers to it going
to have a lens and also some liquid areas called the humours there’s also
some accessory structures which we’re going to talk about next there’s the
eyebrows the eyelids the conjunctiva the lacrimal apparatus and also the
extrinsic muscles so the lacrimal apparatus is shown here on this slide
and specifically it begins with the lacrimal gland which is found on the
lateral side of the eye and this lacrimal gland is very important it’s
going to produce important fluid the tears which contains mucus important
antibody and also lysozyme so it’s very important for protection of our eye every
time that we blink this fliud is going to be released through the excretory ducts
of the lacrimal glands and it’s going to lubricate the eye it’s gonna wash from the
lateral side to the medial side so this fluid is very important it’s going to
cleanse the eye it’s going to protect the eye surface and it’s also going to be
important for moistening and lubricating the eye so you can see the path we’ve
shown here the excretory ducts crossing the line medially and turn going into the lacrimal punctum the openings
and then finally draining into the nasal lacrimal duct as you can see here and
then eventually it’s going to flow outward through the nose so this
explains when we have sniffles or a congested nose that there’s really no
way no where for the fluid to actually leave the eye so that explains
the watery eyes and makes sense one of the other important accessory
structures would be the extrinsic eye muscles and there are six of them and
four out of six of them kind of makes sense based on their words their the
rectus muscles and they are located above below or to the side of the eye
and so you need to know their action as well as the cranial nerve that controls
them and the cranial nerve number three the oculomotor is gonna control four out of six of them the two
exceptions would be the lateral rectus muscle which you see here and it’s
controlled by cranial nerve number six the abducens nerve and the superior oblique
is going to be another one of these exceptions controlled by the trochlear
nerve and one of the reasons is because it kind of acts like a pulley system if
we look at the superior oblique muscle is located on top of the eye and it’s
going to depress the eye and also turn it laterally one of the reasons that
there’s more muscles that help to turn it laterally is because it balances out
the ones that turn immediately so need to know to function as well as the
cranial nerves when some of these are working correctly we have a risk of
having double vision which is called diplopia and there’s also another
disorder called strabismus and there is external strabismus and also internal
strabismus and that turns the eye either laterally or medially so looking
at the three layers of the eye we are gonna first start with the outermost layer of
the eye and this outermost layer of the eye is a very tough layer it’s actually a type of dense avascular
so without blood vessels and it’s a type of connective tissue and has two parts
the sclera is actually going to be continuous with the outer layer of the
brain called the dura mater and in the front we can see that the sclera
actually becomes the cornea as well the cornea is really interesting tissue because it’s
the only tissue in the human body that can actually be transplanted without any
fear of rejection the middle layer is the blood vessel layer and it is called
the choroid and it’s the middle coat of the eyeball and it has the blood
vessels these blood vessels are very important for absorbing the light and
preventing the light from spreading so it doesn’t it keeps the light and the
image focused on the back of the retina where the image can actually form the choroid is going to be made it’s going to become the ciliary body and then there’s
also what’s called a ciliary process and the ciliary process is going to secrete
fluid into the anterior segment and this is the anterior segment is made up of
aqueous humor and what we see here is the two examples of what I automatically
happens to the iris the iris is not just for color its most important job is that
it’s made up of two smooth muscle smooth muscles that are going to respond to the
parasympathetic nervous system and also the sympathetic nervous system so the
parasympathetic nervous system is going to be specifically responsive to close
division and also very bright light then if you think about the opposite what
happens when you go into a very dark room your sympathetic division is
going to be activated and your pupils are going to dilate so this is not only
for distant vision but it’s also for very dim light as well so that’s the middle layer of the eye
and then probably the most important layer of the eye because this is where photo
transduction actually occurs this is going to be the sensory layer or the retina and what happens here is there’s millions of photoreceptors and those
photo receptors are going to be the rods as well as the cones and you need to
know the pathway of light what happens is once the late it goes right through
the people and it’s focused hopefully right on the fovea centralis the
back of the eye and we can see the pathway of light going from left to right
on your screen and the photoreceptors are first can be activated the rods and
cones then there is a graded potential between the photoreceptor and the
bipolar cell another graded potential between the bipolar cell and the
ganglion cell and it’s the ganglion cell which is going to produce the most
important part which is the action potential which is going to continue
along the optic nerve cranial nerve number 2 the occipital lobe of the
brain for interpretation so looking at the back of that I we see
the blood supply shown here on this slide the neural retina has two
different supplies the outer third is the choroid which remember recall is the
middle layer and the inner two-thirds is from the central artery and vein and we
can see that shown right here and the most important part of this is the
macula lutea so when patients get macular degeneration that’s going to be
a degeneration of the very important macula so if we’re thinking the macula
lutea kind of like a dartboard the very center of the dartboard would be the
fovea centralis and the fovea centralis is going to have the highest amount of
cone so it’s kinda like the best possible vision that we have the next
slide showing the anterior segment and also the posterior segment first of all the
posterior segment is containing the vitreous humor and it is a clear gel
which most interestingly is actually present from when we are born up until we die so never actually
changes the aqueous humor however is going to be in the anterior segment and
it’s very important because it nourishes the lens nourishes the cornea and also
removes waste and if there’s problems with aqueous humor for example clouding
of the lens that could lead to a cataract which is what we see in this
picture right here so cataracts are very common they’re easily removed and we see
the pathway of production of the fluid in aqueous humor on this chart we had
the aqueous humor producing the fluid is produced by filtration number one here
and then it flows into the aqueous humor and then it’s going to be drained by
this vein also called the Canal of Schlemm but also called the scleral venous sinus
so when the pressure increases in this area it can actually lead to glaucoma so
glaucoma is a homeostatic imbalance that is associated with increased intraocular
pressure in the aqueous humor

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