It’s probably best for all of us to think of
the eye like a camera when we first start.
This gives us some working knowledge of what things
we’re gonna be able to change whether eye
to get the best image possible to the brain.
And it is very important
to get that best image possible,
both in terms if you were trying to forge
for food or runaway from a predator
or even have a good social interaction
with a neighbor or colleague.
So if you think of the eye being like a camera,
what are the couple components that the camera
that we can now relate to the eye and the eye structure?
One of the big one is the aperture.
Aperture is the whole in which the light travels through.
And you can alter that to get more light
into a camera or to get more light into the eye.
The other interesting or the important thing
is you can adjust the focus of the lens
so that you can get an optimal clarity of the image.
Those are two important items
and your eye does the very same thing,
by altering the pupil with is the same as the aperture
and altering the lens is the same as adjusting the focus.
In addition, to the lens, you also have
some sort of film or way to record the image.
So in the olden days when we use
photography film to put the image on,
that is, think of that like the retina,
its collecting the information that’s being sent
through a cornea, the various humors, into the lens
and then back to back to the eye.
That photography or photographic film
is the same as photoreceptors.
So, going through these humors and lenses.
We always need to go through what is the path of the
light has travelling to get back to the retina.
The aqueous humor is that first humor that we have for fluid
that will eventually be drained into a canal.
That will then be collected up by the lymphatic system.
And remember we talked about before you need
to have that flow from the posterior chamber
through the pupil into the
anterior chamber before its drained.
Finally, to think about is that
various humors will reflect the light.
What is light refraction mean?
It means that the light is bent slightly.
So for every object you travel through, you’ll need
to account for how much that light is being bent.
So air versus the cornea, versus the aqueous humor,
versus the lens, versus the vitreous humor.
All will just the light bend.
that needs to be accounted for when you were
showing an image to the back of the retina.
What kind of process that occurs?
The less light bending the better.
And that is why you don’t have
structures that are emplaced
that allow the light to travel
through that absorb that light.
Cos you don’t want any, what we call
chromophores which absorbs the light
because you want that light to pass
through with the least bending as possible.
Now the lens itself also refracts
light but it can accommodate.
What we mean by accommodate is change
with and how much it is stretch
so that you can adjust the focal length
to the back of the retina.
This allows you to see both near things, readjust
see far things, readjust see near things again.
It is how you’re moving your focus in and out
because you can’t focus on both
near and far things at the same time.
Just like with your camera, you have
to adjust the lens to look at near things.
You have to adjust the lens to look at far things.
How that adjustment looks?
Is all in terms of muscle contractions on the lens itself.
So you have ciliary muscles
that will either contract or relax.
If they are contracting, they are relaxing the lens.
If they are relaxed, the lens tenses up.
That may seemed counter in two direct first.
But remember, you have these
little fibers called zonule fibers.
and these are polling on the lens at all times.
So actually, by contracting the ciliary muscles you were living
that tension being polled on with these zonule fibers.
How do you adjust pupil size?
Pupil size is the aperture how much
light you’re getting through the pupil
is gonna be a very important
for how bright your image is.
If you have high light levels,
the sphincter muscles are going
to constrict to get smaller
and that decreases the pupil diameter
and how much light can move in.
If light levels are too low, the opposite happens.
You have muscles are called radial muscles.
Those will constrict which dilate the pupil.
Why is this sympathetic
parasympathetic component important?
Well sympathetic nervous system
responds is to cause a pupil dilation
and a parasympathetic nerve response
is to cause the pupils constriction.
These are always balance in each other.
So in fact, they’re always have one,
both of these play at the same time.
You can adjust it one direction or another
if you do something like a pupillary light test.
You take a small flash light
and just move it towards the eye.
You can get the eye to change
size in terms of the pupil.
The physician will often times utilize this
principle of having both of these in play at the same time.
Because you can remove one or
the other to get the opposite response.
So for example, a parasympathetic nervous system
response to cause a sphincter muscle constriction
is using acetylcholine.
Therefore, you can give a blocker of acetylcholine
like atropine and get dilation of the eye.
So you can block the parasympathetic
to get pupil dilation.
Cos this are always going on at the same time.
These remember are just the aperture
or how much light gets into the eye.
Therefore, if you have your pupils
dilated by something like atropine
you might need to wear your some sort
of sun protection as you go outside
because your pupils will be dilated.