Video Transcript
In this video, we will learn about the structure and function of the components of the eye. Then we’ll learn about common eye defects and how they can be treated. So let’s get started and see what the eye has to teach us.
The eye is a sensory organ, meaning that it takes in information from our surroundings. Specifically, the eyes are responsible for taking in visual information. And they do this by detecting light. However, our eyes can’t make sense of this information on their own. The eyes are connected by nerves to the brain, where the light patterns they sense are decoded.
So what do we mean by decoded? We tend to think of vision as automatic, but actually it’s a skill we learn over time. We know this from instances where vision is restored to a person who has been blind since birth or early childhood. Learning to distinguish objects based on their appearance and other points of visual acuity can take these patients weeks, months, or much longer to master. The eye is a small, if important, part of our vast and complex nervous system. Next, let’s take a closer look at the parts of the eye and how they work together to allow us to see.
Here, we have an external view of the eye shown from the front and from the side. We can see the sclera or the white part of the eye. The dark circle in the middle of the eye is an opening called the pupil. The iris is the ring between the sclera and the pupil that we refer to as our eye color. And the iris controls the size of the pupil. The tissues inside of the eye are sensitive and easily damaged. So when the eye is exposed to a source of bright light, the pupil constricts and gets smaller, which allows less light to enter. In contrast, when lighting is low or dim, the pupil dilates and becomes larger, which lets more light enter the eye so we can see better.
The iris is able to change shape because it’s opened and closed by small muscles. And this automatic response to lighting conditions is known as the pupillary reflex. Surrounding the eye, we notice the eyebrows, which help to direct sweat away from the eye, and the eyelashes, which are supersensitive hairs that trigger the blinking reflex keeping foreign particles away from the eye. And the surface of the eye is kept moist by secretions from glands found within the eyelids.
Next, let’s take a look at the inner workings of the eye. To do this, we’ll look at a cross-sectional diagram of the view from the side. This cross-sectional diagram allows us to see the internal structures of the eye. And the parts we’ve already identified have been labeled. The eye is filled with clear gel-like liquid called vitreous humor. And the back of the eye is coated with special tissue called the retina. The retina is covered with special photoreceptor cells. Photo- is a word part that means light, so these are cells that detect light.
There are two types of photoreceptor cells called rods and cones. Rod cells can tell light from dark but are unable to detect colors. And cone cells are responsible for our color vision. Our retina contains many more rod cells than there are cone cells. And the cone cells need much more light than the rod cells in order to work, which is why it’s very hard to tell colors apart in dim lighting situations.
In the rear of the eye is the optic nerve, which carries signals from the eye to the the brain. In the front of the eye is a thick, clear outer layer called the cornea. And within the eye, we have another clear structure called the lens. Surrounding the lens are suspensory ligaments. And the suspensory ligaments connect the lens to the ciliary muscles, which allow the lens to change shape. But why does the lens change shape? Well, we’re able to see objects because they reflect light into the eye, which is detected by the photoreceptor cells on the retina. And that light has to be focused just right for the retina to get a clear image.
The cornea does most of the work, focusing the light from our surroundings so that it can be detected by the retina. But depending on the distance between the eye and the object being viewed, the light needs to refract, or bend, to slightly different degrees in order to focus the image clearly. When we’re viewing an object that’s close to the eye, the light it reflects needs to refract a little more in order to properly focus the image on the retina. So when an object is close to the eye, the ciliary muscles contract, drawing them inwards. The suspensory ligaments slack in and the lens assumes a shape closer to that of a sphere, meaning it gets shorter and fatter, which causes the light to refract more so that the retina gets a clear image.
In contrast, when we’re viewing an object that’s far from the eye, the light needs to refract a little less. In this case, the ciliary muscles relax, which draws them outward. The suspensory ligaments pull on the lens, drawing it into a longer and thinner shape. This causes the light to refract less and to focus clearly on the retina. This process, in which the lens of the eye changes shape in order to fine-tune focus, is called accommodation.
So when all the parts of the eye work together properly, the cornea focuses the light, the iris lets just enough light through the pupil. The ciliary muscles stretch the lens to just the right thickness to fine-tune the image. And the photoreceptor cells on the retina send signals to the brain through the optic nerve. And we think to ourselves, “Oh wow, a blue square!” But what if something goes wrong? Someone looking at a blue square may find themselves thinking, “What is that blurry blue object?” And this may indicate that they have a vision problem known as a refractive error.
Refractive errors occur when a defect within the eye prevents light from focusing on the retina properly. There are three main types of refractive errors: far or long sightedness, near or short sightedness, and astigmatism.
If the blue square is clear when it’s far from the eye but blurry when it’s brought closer, you may be farsighted. Farsightedness can be caused by either the lens of the eye being a little too thin or the eye itself being a little too short. In either case, light from nearby objects ends up being focused slightly behind the retina, so images are not completely clear. Farsightedness is corrected with the use of convex or converging lenses, which compensate by shifting images slightly forwards so that the retina can detect them clearly.
If the blue square is blurry when it’s far away but becomes clear as it’s brought closer, you may be nearsighted. Nearsightedness occurs when the lens is a little too thick or the eye itself is too long. In either case, the light from faraway objects is focused slightly in front of the retina, so images are not completely clear. Nearsightedness is corrected with the use of concave or diverging lenses, which compensate by shifting images slightly backward so that the retina can detect them clearly.
If the blue square is blurry at any distance or changes focus as it moves from left to right or top to bottom, you may have astigmatism. Astigmatism is a refractive error usually caused by an irregularly shaped cornea, which, instead of having a normal spherical profile, has a more oblong shape, something like the shape of a rugby ball or an American football. Astigmatism is corrected by using lenses designed specifically for the shape of the patient’s cornea.
Most refractive errors can be easily treated using glasses or contact lenses. However, some people choose a surgical solution known as laser eye surgery to correct these issues. Laser eye surgery uses a laser to change the shape of the surface of the cornea to compensate for refractive errors. It’s a popular choice among individuals for whom glasses or contact lenses may interfere with their profession, such as pilots.
First, the protective layer of the cornea is sliced from the eye using a laser or a thin blade. This creates a flap that’s peeled back like the cover of a book. Then a laser reshapes the exposed surface of the patient’s cornea to compensate for their refractive issue. For correction of nearsightedness, the final shape of the cornea is more flat. And when correcting for farsightedness, the cornea is given a more curved profile. Finally, the flap is reattached to the cornea and allowed to heal.
Now that we’ve learned about refractive errors, let’s look at two more common eye issues and their corrective measures.
Returning to our blue square vision test, what if you’re seeing the square, but you’re not sure what color it is? Then you may have a condition known as color blindness. Recall that on the retina, there are two types of photoreceptor cells, rods and cones. Color blindness occurs when the cone cells on the retina do not function properly, making it harder to distinguish between certain colors. Glasses with color filters can be used to assist some people with certain types of color blindness, but most individuals learn to compensate on their own. Interestingly, the allele for color blindness is recessive and located on the X chromosome. So this condition is much more common in males than in females.
Finally, if you can see the blue square, but it’s cloudy and dim, you may be suffering from cataracts. Cataracts are a clouding of the lens of the eye caused by a buildup of proteins. Cataracts are most commonly the result of aging but can be present from birth or caused by disease or environmental factors. Cataracts are treated by cataract surgery, in which the lens of the eye is usually removed and a plastic replacement is inserted. Cataracts can be caused by too much sun exposure, which is a good reason, besides looking fashionable, to make sure you remember your sunglasses on bright days.
Let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned about the structure and the function of the major components of the eye. We also learned about common vision problems and their corrective measures, including refractive errors, such as far- or long-sightedness and near- or shortsightedness, color blindness, and cataracts.
