Anatomy and physiology of the eye

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Introduction0:00–0:29

Our eyes allow us to visualize the world around us. They do this by converting light waves into neural signals so that our brains can process them.
The eye itself is shaped like a sphere that is elongated horizontally, as opposed to being perfectly round, and only the anterior one-sixth of the eye is visible.
The rest of the eye is contained within the orbit, or eye socket, of the skull. Now, the eye consists of three layers: the outermost fibrous layer, the middle vascular layer, and the inner neural layer.

Outer fibrous layer0:29–2:00

The outer fibrous layer contains two main structures: the sclera and the cornea. The sclera makes up the majority of the outer layer and is the white portion of the eye.
It’s like a tough, fibrous covering that protects the more delicate structures within the eye and it also acts as an anchoring point for the extrinsic eye muscles to attach to.
The sclera is like a wall that’s built around the eye, that only has a tiny opening at the back to let the optic nerve through.
As the sclera approaches the anterior portion of the eye it reaches a transition point known as the corneal limbus where it becomes the cornea.
The cornea itself is a transparent, dome shaped clear layer that covers the iris and the pupil. It allows light to enter the eye, and its curved shape helps focus light on the retina in the back of the eye.
At the periphery of the cornea, there are stratified squamous epithelial cells which continually divide and regenerate the cornea, and they help to heal after a corneal injury or abrasion.
The cornea doesn’t contain blood vessels and therefore immune cells can't access the cornea. As a result, it’s one of the few parts of the body that is considered "immune privileged" since it can be transplanted without the fear of an immune response and organ rejection.

Middle vascular layer2:00–4:45

Moving inwardly from the fibrous layer, the next layer of the eye is the middle vascular layer, which is also called the uvea.
Structures within this layer include the iris, pupil, choroid, and ciliary body. The word "iris" derives from a Greek word meaning "rainbow", and that makes sense since the iris is what we think of as the colorful part of the eye.
Eye color is determined by the amount of melanin in the iris. People with a high concentration of melanin have dark brown eyes, those with medium amounts have green eyes, and people with low concentrations of melanin have blue eyes.
The iris sits behind the cornea and it is composed of two distinct groups of muscle: the sphincter pupillae muscle, sometimes referred to as circular muscle, and the dilator pupillae muscle, otherwise known as radial muscle.
These muscles help control the the size of the pupil, which is the central opening at the center of the iris. The sphincter pupillae muscle surround the iris like a tiny circle, and in bright light this muscle tightens around the pupillary opening, reducing the size of the pupil.
When it’s dark, the dilator pupillae muscle pulls the iris radially or outwardly from the pupil which helps increase the diameter of pupillary opening, allowing more light to enter the eye.
After light passes through the cornea and pupillary opening of the iris, it reaches a biconvex transparent structure called the lens which is located in a space called the posterior chamber.
Biconvex means that the lens is curved on both sides. The lens itself can bend, allowing it to become flatter or rounder, and this in turn bends the light entering the eye.
A structure called the ciliary body, controls the degree to which the lens becomes flatter or rounder. The ciliary body includes the ciliary muscle and tiny projections from the ciliary muscle called ciliary processes.
The ciliary processes connect to suspensory ligaments which attach directly to the lens and hold the lens in place behind the iris as well as help it change shape.
When the ciliary muscles relax the ciliary processes pull on the suspensory ligaments like a taut rope, and that makes the lens flatter.
Finally, the last structure of the vascular layer is a membrane called the choroid which is full of blood vessels that provide nutrients to most of the eye.
The choroid is dark brown in color, and that allows it to absorb light. Without the choroid, light would be more likely to reflect and scatter within the eye, preventing the light from being focused on the retina, which is necessary for visual processing.

Inner neural layer4:45–6:57

The third and innermost layer of the eye is the retina. The retina has an inner neural layer that contains ganglion cells which synapse with bipolar cells, which then synapse with the photoreceptors - both rods and cones.
The retina also has an outer pigmented layer consists of a one cell-thick layer of pigmented epithelial cells. So when light enters the eyes, it hits the retina, then goes past the ganglion cells and bipolar cells to hit the rods and cones.
The light that doesn’t hit the photoreceptors makes it to the pigmented layer where the light is absorbed so that it doesn’t scatter and bounce back to the photoreceptors.
The rods are more numerous, about 120 million of them in each eye. Rods are highly sensitive to light and even a single photon can cause them to activate, making them great for seeing in low light conditions, but they only offer black and white vision.
By comparison, there are only about 6 million cones in each eye, and most are located in the macula which is an oval spot in the middle of the posterior retina.
At the center of the macula is the fovea which contains the highest concentration of cones and is the part of the retina that offers the highest visual acuity.
Cones are less sensitive than rods to light, and each cone can detect either a red, green, or blue wavelength of light. So when you see a red apple, only the red cones are activated, whereas when you see a purple flower, both the red and blue cones are activated.
When rods and cones are activated, they depolarize and create an action potential, which triggers depolarization of the bipolar cells, which then triggers the depolarization of the ganglion cells.
The action potential in the ganglion cells travels through their axons to the posterior portion of the retina to form the optic nerve, which leaves the eye through the optic disc medial to the macula.
The optic disc doesn’t have any photoreceptors so it’s known as the blind spot. The optic nerve then carries the visual information to the brain where we process and recognize the visual information.
Finally, let’s zoom out and look at the eye as a whole. With a cross section of the eye, you can see that it’s split up into different chambers: The anterior chamber includes the area from the cornea to the iris, the posterior chamber is this really narrow space between the iris and the lens.

Eye chambers6:57–8:31

And then this larger vitreous chamber includes the space between the lens and the back of the eye. Not to be too confusing, but both the anterior and posterior chambers are located in the anterior section of the eye, while the vitreous chamber is part of the posterior section of the eye.
Typically all of the chambers in the eye are filled with fluid. The chambers in the anterior section are filled with a liquid called aqueous humor, and the posterior section is filled with vitreous humor.
The aqueous humor is a transparent, watery fluid that is secreted by the ciliary epithelium, which in addition to secreting aqueous humor and providing nutrients to the lens and cornea, it provides structural support and helps to keep the shape of the eye.
So that fluid’s secreted into the posterior chamber, and then flows through a narrow space between the front of the lens and the back of the iris through the pupil to the anterior chamber.
From there the fluid flows out of the eye through the trabecular meshwork, which is a spongy tissue that acts like a drain, and this allows the fluid to go down into a circular channel called the canal of Schlemm and finally into aqueous veins that are part of the episcleral venous system—the veins around the sclera of the eye.

Review8:31–9:05

Alright, as a quick recap…..the wall of the eye is made up of three major layers, the fibrous outer layer which contains the cornea and sclera, the middle vascular layer consisting of the iris, pupil, choroid and ciliary body, and finally the neural layer consisting of the retina with its own outer pigmented layer and inner neural layer.
The inner neural layer of the retina is composed of photoreceptor cells that finally convert light into neural signals that travel via the optic nerve to the brain for visual processing.