Welcome to this presentation on the visual pathway. 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. This is necessary to focus the light on the fovea centralis,
the point at which we have the greatest visual acuity. The first structure that light will pass
through is that of the cornea, which you see labelled here. Next, the light will pass through
the aqueous humor and then through an aperture of the iris that we call the pupil.
You see that opening labeled here. The lens is the next structure through which light will pass
along its travel. Then its greatest journey is going to be through the vitreous humor
and then the light will hit the retina. By striking the retina, photoreceptor cells will
become activated. This slide depicts the major layers of the retina that will allow the
visual scenes to be delivered through the optic nerve. First, the pigment epithelium
is shown here. This is the external most component. The purpose of the pigment epithelium
is to absorb any scattered light which helps with visual acuity. The photoreceptors are shown here.
We have cones and rods in through here. We’ll speak to those in detail shortly.
From here, photoreceptors will activate, stimulate bipolar cells. Then the bipolar cells will
communicate and activate ganglion cells. Then the axons from these ganglion cells will then form
the optic nerve. The optic nerve is forming right here on the right upper portion of this image.
Now, let’s take a look in greater detail about the photoreceptors. First, we have the rods.
Here’s a nice illustration demonstrating a rod. Then the cones are shown here, at least one cone.
You can see this cone-shaped appearance here to the outer segment of this photoreceptor.
Now, let’s describe the functions of the cones and the rods. The cones are located specifically
within the fovea centralis exclusively. There are no rods at this place. As you move outside
of the fovea centralis, you’re within the macula lutea, the cone density will start to decrease
and then rods will start to increase in density. Then once you get to the periphery of the macula
lutea, then you have exclusively rods in the peripheral aspects of the retina.
Cones are necessary to operate in bright light. They are responsible for conferring color vision.
In order to do so, there’s a population of red, green, and blue cones. They are responsible for
high visual acuity. If we have an excessive loss of cones, this is a cause of legal blindness.
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. As a result, they have poor
visual acuity. They are responsible for achromatic vision, so we see scales or shades of gray
and not color. Excessive loss of rods impairs our ability to see in low light or during night time.
Thus, this can be a cause of night blindness. The retina in certain situations can become detached.
When that happens, there’s a separation of the neural retina which would be the boundary
between the photoreceptors and the pigment epithelium. As a result of this separation of
the neural retina from the pigment epithelium, blood supply to the photoreceptor cells
is disrupted. The blood supply to this area is from blood vessels that reside in the choroid,
which will be down in this area here. 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. Retinal detachment can cause a shower of pepper or
floaters as visual sensations. The shower of pepper or the floaters that are seen visually
are extravasated red blood cells due to a disruption of the blood supply.