Eyeball and Retina

00:01 Welcome to this presentation on the visual pathway.

00:08 The first topic that I'm going to guide you through on this journey along the pathway is the passage of light through the various components of the eyeball.

00:21 And this is necessary to focus the light on the phobia centralis the point at which we have the greatest visual acuity.

00:29 The first structure that light will pass through is that of the cornea, which you see labeled here.

00:38 Next, the light will pass through the aqueous humor.

00:43 And then through an aperture of the iris that we call the pupil.

00:48 And you see that opening labeled here.

00:51 The lens is the next structure through which light will pass along it's travel.

00:56 And then its greatest journey is going to be through the vitreous humor.

01:01 And then the light will hit the retina.

01:05 And by striking the retina photoreceptor cells will become activated.

01:13 At this slide depicts the major layers of the retina that will allow the visual scenes to be delivered through the optic nerve.

01:25 First, the pigment epithelium is shown here.

01:30 This is the external most component.

01:34 The purpose of the pigment epithelium is to absorb any scattered light, which helps with visual acuity.

01:42 The photoreceptors are shown here.

01:45 And we have cones and rods in through here.

01:49 And we'll speak to those in detail shortly.

01:54 From here, photoreceptors will activate stimulate bipolar cells.

02:03 And then the bipolar cells will communicate and activate ganglion cells.

02:10 And then the axons from these ganglion cells will then form the optic nerve.

02:16 And the optic nerve is forming right here on the right upper portion of this image.

02:23 Now, let's take a look in greater detail about the photoreceptors.

02:30 First, we have the rods.

02:33 And here's a nice illustration demonstrating the rod.

02:42 And then the cones are shown here at least one cone And you can see this cone shaped appearance here to the outer segment of this photo receptor.

02:56 Now, let's describe the functions of the cones and the rods.

03:04 The cones are located specifically within the fovea centralis exclusively, there are no rods at this place.

03:12 As you move outside of the fovea centralis you're within the macula lutea.

03:17 The cone density will start to decrease and then rods will start to increase in density.

03:23 And then once you get to the periphery of the macula lutea then you have exclusively rods in the peripheral aspects of the retina.

03:35 Cones are necessary to operate in bright light.

03:41 They are responsible for conferring color vision.

03:45 And in order to do so there's a population of red, green, and blue cones.

03:51 They are responsible for high visual acuity.

03:56 And if we have an excessive loss of cones, this is a cause of legal blindness.

04:07 Rods which start to show up outside of the fovea centralis and then we see them exclusively outside of the macula are responsible for low light vision.

04:23 As a result, they have poor visual acuity and they are responsible, for a chromatic vision.

04:31 So we see scales or shades of gray and not color.

04:36 And excessive loss of rods impairs our ability to see in low light or during nighttime does this can be a cause of night blindness.

04:52 The retina in certain situations can become detached And when that happens there is a separation of the neuro retina, which would be the boundary between the photoreceptors and the pigment epithelium.

05:14 As a result to this separation, the neural retina from the pigment epithelium blood supply to the photoreceptor cells is disrupted.

05:23 The blood supply to this area is from blood vessels that reside in the choroid which will be down in this area here.

05:32 And so you are dependent then on the diffusion of the components from the blood to maintain the viability of the retina that becomes lost when we have detachment.

05:45 Possible symptoms are a shower of pepper or floaters as visual sensations.

05:51 This is do extravasated red blood cells that came from a disruption of the blood supply.

05:56 You may also see flashing lights, or a shadow, or curtain over the field of vision.

06:02 The retinal detachment can result from different causes.

06:06 The most common one is rhegmatogenous retinal detachment, which is a structural collapse of the vitreous humor that pulls the retina away from the retinal pigmented epithelium.

06:17 Then there's a tractional retinal detachment where scar tissue exerts tractional force and is pulling the retina away from the retinal pigmented epithelium.

06:28 This may be seen in diabetes for example.

06:31 Another cause is exudative retinal detachment.

06:35 In this case, fluid leaks from blood vessels and separates the retina.

06:40 It may be caused by inflammation or abnormally leaky blood vessels.