EYE (HUMAN)
Parts of the Eye:
- Sclera: The outermost layer of the eye composed of dense connective tissue, providing structural support. The anterior portion of the sclera is known as the cornea, which is transparent and helps to refract light entering the eye.
- Choroid: The middle layer of the eye containing numerous blood vessels, giving it a bluish appearance. It becomes thickened in the anterior part to form the ciliary body, which continues forward to form the pigmented iris, the visible colored portion of the eye.
- Lens: A transparent, crystalline structure located behind the iris and held in place by ligaments attached to the ciliary body. It focuses light onto the retina to form clear images.
- Pupil: The aperture in the center of the iris through which light enters the eye. The diameter of the pupil is regulated by the muscles of the iris, adjusting to varying light conditions.
- Retina: The innermost layer of the eye containing three layers of neural cells: ganglion cells, bipolar cells, and photoreceptor cells (rods and cones). Rods and cones contain light-sensitive proteins called photopigments and are responsible for converting light into neural signals.
- Optic Nerve: The bundle of nerve fibers that carries neural signals from the retina to the brain for processing and interpretation.
- Blind Spot: The point on the retina where the optic nerve leaves the eye, lacking photoreceptor cells. This area is devoid of vision and is known as the blind spot.
- Macula Lutea and Fovea: Located at the posterior pole of the eye, lateral to the blind spot, the macula lutea is a yellowish pigmented spot with a central depression called the fovea. The fovea contains densely packed cones and is the area of highest visual acuity.
- Aqueous and Vitreous Chambers: The space between the cornea and the lens is filled with a thin watery fluid called aqueous humor, while the space between the lens and the retina contains a transparent gel called vitreous humor. These fluids help maintain the shape and structure of the eye.
Mechanism of Vision:
- Light Reception: Light rays within the visible wavelength are focused onto the retina by the cornea and lens of the eye.
- Photoreceptor Activation: The light stimulates photoreceptor cells, namely rods and cones, located in the retina. These cells contain photosensitive compounds called photopigments composed of opsin (a protein) and retinal (an aldehyde of vitamin A).
- Photopigment Activation: Upon exposure to light, retinal dissociates from opsin, leading to structural changes in the opsin molecule. This alters membrane permeability in the photoreceptor cells.
- Generation of Signals: Changes in membrane permeability generate potential differences within the photoreceptor cells, producing signals that trigger action potentials.
- Signal Transmission: Action potentials generated in the photoreceptor cells are transmitted to the ganglion cells through intermediary bipolar cells.
- Optic Nerve Transmission: The action potentials, or impulses, are then transmitted along the optic nerves to the visual cortex area of the brain.
- Visual Processing: In the visual cortex, neural impulses are analyzed and interpreted based on previous memory and experience. The brain recognizes the image formed on the retina, allowing for perception and understanding of visual stimuli.
