Human Eye Anatomy and Physiology: A Comprehensive Look
**Cornea**
The cornea has no blood vessels. It is transparent due to the layer of cells above the eye. It is formed preferentially by epithelial carcinoma (there are more, but this is the most abundant). To the front is a layer that protects the related tear, and to the back, we find the aqueous humor. It is the first lens of the optical system. It transmits light and is responsible for refraction (the deviation of light when it passes from one medium to another). It is also a physical barrier against harmful substances and microorganisms. It has a great regenerative capacity. The sclera and cornea form the sclerocorneal angle, which has to do with the drainage of the aqueous humor.
**Iris**
The iris is a muscular structure located between the cornea and the lens, with a central hole, the pupil, through which we see the back of the eye. This structure has a pigment that gives color to the eyes (melanin). If there is more pigment, the eyes will be brown or dark; however, if there is less pigment, the eyes will be green or blue. The iris has two types of fibers:
- Circular fibers: When they contract, they produce a decrease in the pupil’s size. This contraction is called miosis (constriction).
- Radial fibers: Their contraction causes dilation, and this is called mydriasis.
An example of contraction, or rather the reflex of contraction, would be the photomotor reflex to a stimulus; that is, when light is shone into the eye, the pupil contracts (miosis). Thus, we can say that the iris serves as a diaphragm, regulating the entry of light into the eye.
**Lens**
The lens is a biconvex lens located behind the iris, formed by concentric layers of crystalline proteins. It is avascular; therefore, it does not receive blood irrigation. It is surrounded by a transparent capsule, together with the ciliary muscle and the zonules of Zinn. Its refractive index is very high, so it can divert a lot of light to focus an image on the retina. That is why its role is in accommodation (explained below).
Accommodation
Accommodation is an involuntary process. It is transmitted via the optic nerve to the brain, and the brain sends a response via the oculomotor nerve (cranial nerve III), which innervates the ciliary muscle. When an object approaches, the motor nerve sends a signal to the ciliary muscle. This stretches the muscle, and thus, the pupil becomes smaller.
Chambers of the Eye
The eye is divided into two chambers:
- Vitreous chamber: Contains vitreous humor.
- Aqueous chamber: Contains aqueous humor and consists of two parts:
- Anterior chamber: Located between the cornea and the lens.
- Posterior chamber: The space between the zonules.
Aqueous humor maintains intraocular pressure. It is produced in the ciliary body, then flows to the posterior chamber, and through the pupil, it passes to the anterior chamber. At the iridocorneal angle, it is drained by being absorbed through the canal of Schlemm. From the canal, it goes into the veins.
**Retina**
The retina is the inner layer filled with photoreceptors. There is an area that is completely blind to the retina (anterior retina), and another area that is photosensitive (posterior retina). Retinal detachment can occur in the posterior retina. The retina is vascularized. We find two distinct structures:
- Optic disc: Also known as the blind spot, where the optic nerve and the central retinal artery and vein enter.
- Macula lutea: An area located at the midpoint of the posterior retina; it is the point of maximum vision (above the central fovea).
In terms of its histology, the retinal pigmented layer is just below the neovascularization. The neural or nerve layer has three types of neurons:
- Photoreceptors (neurons): There are two types: cones, which are cylindrical and responsible for daytime vision and color vision, and rods, which are responsible for night vision.
- Bipolar cells
- Ganglion cells
Physiology of Vision
Regarding the physiology of vision, we have two processes:
Image Formation in the Retina
The mechanism responsible for refraction is the deviation of light by the lens. The other mechanism is due to accommodation, achieved through the process of constriction of the pupil (letting in more or less light). The lens does this by varying its curvature. The entry of light is reduced by reducing the curvature, and the image is formed.
Transduction of the Stimulus into a Nerve Impulse
Light waves reach the retina and stimulate the receptor cells, which are responsible for color vision: cones (three, one for each primary color) and rods, which specialize in night vision. Rods have a number of visual pigments, such as rhodopsin or photopigments, and iodopsin. The light stimulus reaches the retina and causes a change in the structure of the pigment, which produces a nerve impulse. This is transmitted to bipolar cells, which are directed to ganglion cells. All these cells form the optic nerve.
Optic Pathway
The optic nerve conducts information to the optic chiasm. From there, it goes to the optic chiasm, the area where the fibers of the two eyes cross. From there, it goes to the thalamus, and from the thalamus to the cerebral cortex. From the cerebral cortex, it specifically addresses the occipital cortex (the area responsible for vision), where there is an interaction between different areas, interacting with image memory, reading, writing, etc.