The retina is made of nervous tissue. It contains layers of neurons, intricately hooked up to one another with numerous synapes. Much is known about the neuronal connectivity among the neurons in the retina, but that information is too detailed for our course.
There are two sets of cells in the retina that you need to know by name: photoreceptors and retinal ganglion cells. The rest of the types of neurons you do not need to know by name… but you should keep in mind that the retina has layers of cells. Light enters the eye through the pupil and the photons have to hit the visual sensory receptor cells to cause any sort of visual stimulus. The sensory receptors are called photoreceptor cells because they receive photons. What you might not expect is that the photons have to travel through the entire retina in order to hit the photoreceptors at the proximal end (as shown below).
This may seem counterintuitive, but you should remember that this photoreceptor side of the retina is adjacent to the choroid coat. So, this means that any light photons that pass by the photoreceptors will immediately run into the choroid coat, and not be able to overstimulate the photoreceptors.
There are two types of photoreceptors– rods and cones. The rods are used for our black & white vision, mainly in our peripheral vision. The cones are used for color vision, mainly right in the center of our vision.
The retinal ganglion cells are the output cells of the retina. They receive all the information from the other neurons in the retina, and send their axons out from the eye to form the optic nerve. Their axons run along the surface of the retina facing the vitreous humor and join together at the optic disk to form the optic nerve. Retinal ganglion cells are the only neurons of the retina to leave the retina.
A bit about photoreceptors… and rods versus cones:
Rods are photoreceptors that see only the presence or absence of light, but not color. So these are black and white viewers. The beauty of these rods is that they are extremely sensitive to light, so that only a teeny bit of light is enough to get them working. The color photoreceptors, cones, need more light. You have probably noticed that if you are in a room with very dim lighting, you can see where things are, but not what color they are. That is because the rods will work in this dim light, but the cones will not– so no colors are detectable.
Another thing about rods is that they are found in abundance in our peripheral vision, but not right in the center of our field of view (not in the fovea). Another way to realize just how remarkably sensitive rods are to light is to go outside (on a warmer night) and look at the stars. If you focus your eyes on one particular star, you will be able to see other stars in your peripheral vision. Find one of those stars in your peripheral vision that is not very bright. Now, move your focus to try to see that star. It may no longer be visible! Since your rods are not in your fovea, when you look directly at that dim star, you can only view it with your less sensitive cones, and they may not allow you to see it. Try it out!
The rounded area in the middle of each is where its nucleus is found; this area is called the inner segment of the photoreceptor. The top areas, pointy and short on the cones while longer (although they don’t look longer in this image) and more rectangular on the rods, are where phototransduction occurs. That is, those top areas are where the photons interact to cause an electrical signal. These areas are called the outer segments. I know that your book does not give the terms inner and outer segments, but it is easier for me to use these to refer to the specific region.
Cones are for color vision. Cones are sensitive to either blue, green, or red light, but not all three. The way we can see purple, for example, is when our red cones and our blue cones are both excited… then they feed back onto other retinal cells and combine their inputs. Our brain interprets that as purple.
Don’t think that cones are only good for color vision! Activation of cones also tends to lead to a sharper visual image; this is because of the neural connections that cones make within the retina.
How do photoreceptors transduce light photons to an electrical signal?
Within the outer segment of the photoreceptors are stacks and stacks of internal membranes, called disks (visible in your figure 12.41 in your textbook). Remember, membranes are made up of lipid and protein. In photoreceptors, the membranes of the disks are loaded with pigment molecules (proteins).
In rods, the pigment molecule that is abundant in the disks is called rhodopsin. This molecule is affected by light. When light hits it, it breaks into two molecules:
With time, in the absence of light, the reaction can go the other way so that rhodopsin levels are replenished.
Meanwhile, the opsin that is released when rhodopsin is broken down is an enzyme. This enzyme causes a chemical cascade that ultimately affects the conductance of a sodium channel. It actually does it in a rather backward way… normally the sodium channel that is in the rod is open a bit, so the rod is constantly depolarized a bit (not enough to kill it!). Opsin leads to the closure of this sodium channel, so that the final effect of light on the rod is to hyperpolarize it.
So, light causes a hyperpolarization of the rod by triggering a chemical cascade of events that starts by the breakdown of rhodopsin. Whew!
When you stare at a bright light for a bit, and then look away or close your eyes, you still see a bright light. We discussed this briefly in class today. You still see the light even with your eyes closed because the rhodopsin was broken down and hasn’t had a chance to recover yet. With a bit of time, the rhodopsin is restored and the glowing light in your visual field disappears.
Cones work in a similar fashion, but they do not have rhodopsin in them– they use other pigments. The pigment molecules in cones for blue light only break in the presence of light waves travelling at the appropriate frequency for the color blue. And cones for red and green each have specific pigments that only break when red or green light, respectively, hit them.
How can a hyperpolarization cause us to see something? Well, when our photoreceptors hyperpolarize, they release less neurotransmitter onto their postsynaptic neurons within the retina. Therefore, the way our visual neurons signal each other is different in the presence of light than in the absence of light. It should seem possible to you that all we really need is achange in signalling to indicate the presence of light… not necessarily a depolarization, but just a change. That really is all we need.
The retina contains a map of our visual world, with our center of vision upon the fovea.
When we open our eyes and look anywhere, everything that we see is what we can call our visual field. This visual field is visible because light from everywhere within it is hitting our retina. The point where we focus our visual attention is the point that is hitting the center of our retina, called the fovea centralis. This fovea contains only cones. The fovea lies within a larger ring of the retina called the macula, which contains cones as well as rods.
Where is our visual attention focused? You can tell by looking for what portion of the visual field is on the fovea centralis. In this case, then, we are looking right in the middle of the leaves.
The macula also has many, many cones, so we are seeing the leaves in the middle of the tree as green. But the closer we get to the perimeter of the retina, the fewer the cones until there are only rods.
Notice also that the optic disk takes up a portion of the space of the retina. The right eye is thus not able to see the leaves at that spot on the tree that is reflecting light onto the optic disk.