Understanding Photoreceptors, Eye Anatomy, and Common Vision Problems
Photoreceptors and the Sense of Vision
Humans see in 3 dimensions and color. The eyes are the organs where light-sensitive photoreceptors are located, allowing us to see.
Anatomy of the Eye
The eye is formed by organs and appendages attached to it, including:
- Eyelids
- Eyelashes
- Eyebrows
- Lacrimal glands (produce tears, lubricate, moisten, and clean the eyeball; tears also have bactericidal properties)
Eyeball Structure
- Sclera: A thick protective sheath, also called the white of the eye. The anterior portion of the sclera is transparent, allowing light rays to pass through and form the cornea.
- Choroid: A highly vascularized layer with many blood vessels. Its function is to provide nutrients to the retina.
- Iris: The pigmented portion located in front of the lens. The central hole is the pupil, which appears black.
- Aqueous humor: A clear liquid filling the cavity between the cornea and the lens.
- Lens: The eye lens, located under the cornea.
- Vitreous humor: A jelly-like liquid located between the lens and the retina.
Common Vision Problems
Blindness
Loss of vision in one or both eyes.
Cataracts
Reduced transparency of the cornea, causing impaired vision.
Glaucoma
Occurs when intraocular pressure is too high.
Retinal Detachment
Separation between the retina and the choroid.
Colorblindness
An inherited condition characterized by the lack of one or more types of cones, which are specialized for capturing specific colors.
Conjunctivitis
Inflammation of the conjunctiva caused by allergy, irritation, or infection (bacterial or viral).
Retina and Photoreceptors
The retina is the innermost layer of the eye and contains light-sensitive cells:
- Rods
- Cones
These cells have a pigment in their inner membranes capable of being excited by light; rhodopsin is the most important.
Rods
Cylinder-shaped. They are very sensitive to light and allow for night vision in black and white.
Cones
Cone-shaped. They need higher intensity light (daylight), provide visual acuity to settle details, define objects, and, most importantly, enable color vision.
Fovea
Located in the back of the retina, it is a very thinned area without rods and with numerous densely crowded cones: the point of maximal visual acuity.
Optic Nerve
Runs from the retina of the eye through an area where there are no cones or rods: the blind spot.
How Vision Works
When light rays reach the eye through the cornea, lens, and aqueous and vitreous humors, they stimulate the cones and rods of the retina. Like a camera, an image is formed on the retina. The images formed on the retina are inverted from top to bottom and right to left. Our brain is responsible for interpreting them. The excitation of the pigments contained in the cones and rods generates an electrical current that is transmitted via the optic nerve to the brain.
Refractive Errors
Hyperopia (Farsightedness)
Occurs when the eyeball is shorter than normal, and the image is formed further back than the retina (objects up close are not seen clearly).
Myopia (Nearsightedness)
The opposite of farsightedness.
Astigmatism
The curvature of the cornea is not the same in all points, making it impossible to focus everything in the same plane.