Physiology of Vision (Nursing)

00:01 So again, here we have the cornea as the sort of outermost layer.

00:05 The space between the iris is the pupil.

00:08 And then, we have this lens, which is sort of biconvex, and it's flexible, it's not fixed in shape, so it can be altered.

00:19 And it can be altered by the ciliary muscle, pulling on those ligaments and changing [unintelligible] so slightly the shape of the lens.

00:29 Something called accommodation.

00:33 And ultimately, all of these things are helping light go straight back to that special dark area with the higher concentration of photoreceptor cells called the fovea or the fovea centralis.

00:47 And so essentially, this light that's getting passed back there is where the image is being projected onto the retina.

00:55 Now, it's actually being inverted in that process.

00:59 So it's hitting the retina and sort of an upside down way.

01:01 But fortunately, our brain will compensate for that.

01:06 Now, the shape of the eye and the lens can affect how these images are perceived by the retina.

01:14 And those are problems and what we call refraction, how the light is bent by the cornea, lens, and shape of the eyeball itself.

01:22 So in a typical eye, there's really no distortion of this image as it passes through the structures to reach the retina.

01:33 In a myopic eye, it can result in what's called nearsightedness, which means things they've be very near to be seen clearly, and things that are further away have a hard time being seen clearly because of the way it's being refracted.

01:50 In order to compensate for that, a concave lens can be used to help direct the image so that it hits the retina at the ideal spot.

02:01 So that's where optometry comes in.

02:05 The opposite will be a hyperopic eye.

02:07 And that will be far sighted to where it's basically the opposite.

02:11 And you have a harder time seeing things that are close to the eyeball.

02:16 And to correct that, you would add a convex lens.

02:20 So again, light can hit the retina at just the right spot.

02:27 The visual pathways are pretty complicated.

02:30 And as I mentioned right away, just in passing that images get inverted onto the retina.

02:35 And so the brain has to do a lot of processing to get the visual sensation basically processed properly.

02:45 And it's even more confusing when we look at how these visual pathways are related to the brain itself before it can even do that processing.

02:55 So it's important to know how these pathways work.

02:59 So you can actually help pinpoint where deficits might be based on the areas that patients can't see in the visual field.

03:07 And so here, when we talk about the visual field, we're talking about the actual area that you're looking at.

03:14 So for color coding purposes, the left visual field is drawn in this red, and the right visual field is colored here in blue.

03:24 And we can see that each eye is going to see portions of both the right and left visual fields.

03:30 Course the right, we'll see more of the right.

03:32 The left we'll see more of the left.

03:34 But either way, both of those are going to go back into the optic nerve of the respective eye.

03:43 Then it's going to have something weird.

03:46 It's going to cross. So some of these nerves are going to cross over to the opposite side at something called the optic chiasm.

03:53 That's what chiasm means. It means like a crossing.

03:57 And then, it will become something called the optic tract.

04:01 And then, it will reach the visual areas of the thalamus and eventually the visual cortex in the occipital lobe.

04:08 And so a lots going on before it even reaches the occipital lobe to be processed.

04:14 That also means a lot of things can go wrong before it reaches the occipital lobe.

04:19 So for example, in this image, we have the two circles representing the left and right essentially input there, with the left and right eyes seen.

04:29 And so if you have blockage of the left optic nerve, you'll essentially see nothing coming from the left eye. Right.

04:38 So that's why that circle is all in black there.

04:41 A little more complicated, but important to know, is if there's damage at the optic chiasm because that is actually something that's pretty common.

04:52 And it might not be obvious just by looking at this diagram, but the optic chiasm sits right by the sella turcica where the pituitary is.

05:04 Why is that important? Pituitary tumors can then press directly on the optic chiasm.

05:11 And why that's important? Is they can damage stuff that's trying to pass through the optic chiasm.

05:19 And if you look carefully at this diagram and how it's color coordinated, you'll see that the, for example, let's just look at the left eye, the right visual field is going all the way back to the - staying on the left side of the brain.

05:40 But that left visual field in the red there is actually crossing over at the optic chiasm.

05:48 And the opposite is true for the right eye.

05:51 So if you have some sort of loss of the lateral visual fields, so in other words, the left part of the left eye and the right part of the right eye, that's a very particular form of visual deficit.

06:04 Where if you look at this diagram, you can say, you know, the only way you're really going to get this visual deficit is if there's something pushing on the optic chiasm and that's going to trigger you to think about, oh, wow, there might be something wrong with the pituitary gland.

06:19 And so it's a little quirk if you will about these visual pathways that is pretty useful to help you detect some potentially serious pathologies.

06:29 Moving further back and somewhat less common.

06:32 If you get into these optic tracks, you're going to get, again, a very characteristic type of visual deficit.

06:38 So if it's the left optic track, it's past the optic chiasm.

06:43 And so at this point, the only thing it's carrying is right visual field information.

06:50 And the opposite will be true if we had it on the other side.

06:54 But that's where we're going to lose the right visual field, just in both eyes.

06:59 So that particular type of visual field defect would point you to something in this area of the visual pathway.

07:08 Getting even more specific, moving further back, you can get very finely detailed sort of like, right upper outer, we're getting into very fine details.

07:18 And we know we're moving further back into the brain and so on and so forth.

07:22 And so that the idea is when you get a certain picture for which defects there are in the visual field, you can trace them back to certain parts of the visual pathway.

07:34 And especially in the case of pituitary tumors, it might alert you to something beyond just a visual field defect, but maybe something else going on.