Physiology of vision

• Palpebral fissure is a gap that separates free margins of upper and lower eyelids.
• Medial canthus and lateral canthus is where two eyelids are connected.
• Eyelashes are robust hairs that prevent foreign matter from reaching surface of eye.
• Tarsal (Meibomian) glands secrete lipid-rich product that helps keep eyelids from sticking together.
• Lacrimal caruncle contains glands producing thick secretions and contributes to gritty deposits that appear after a good night’s sleep.
• Conjunctiva is the epithelium covering the inner surfaces of eyelids (palpebral conjunctiva) the outer surface of the eye (ocular conjunctiva).
• Lacrimal apparatus produces, distributes and removes tears.
• Fornix is the pocket where the palpebral conjunctiva meets the ocular conjunctiva.
• Lacrimal gland secretions contain lysozyme, an antibacterial enzyme.
• The aqueous humour is divided into two chambers: the anterior (which extends from the cornea to the iris) and the posterior (which is between the iris and the ciliary body and lens).
• Functions of the vascular layer (Uvea) of the eye:

  1. Provides a route for blood vessels and lymphatics that supply the eye with nutrients.
  2. Regulates the amount of light that enters the eye: iris.
  3. Secretes and reabsorbs aqueous humour in the posterior and anterior chambers.
  4. Controls the shape of the lens, which is important in focusing: ciliary body and ligaments.
• Ciliary Body extends posteriorly to the level of the ora serrata.
• The ora serrata is the serrated anterior edge of the thick, inner portion of the neural tunic.
• The choroid is the vascular layer that separates fibrous and inner layers posterior to ora serrata.
• The aqueous humour is the fluid that circulates within the eye and diffuses through the walls of the anterior chamber into the scleral venous sinus (canal of Schlemm).
• Functions of the fibrous layer:

  – Supports and protects the eye

  – Serves as an attachment site for the extrinsic eye muscles

  – Contains structures assisting in the focusing process
• Tears pass through the: lacrimal puncta, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct.
• Bipolar cells are located in the retina and are responsible for the perception of light intensity.
• Amacrine cells are found in the retina and are responsible for the perception of color.
• Horizontal and amacrine Cells facilitate or inhibit communication between photoreceptors and ganglion cells. They also alter sensitivity of retina.
• Lens transparency depends on a precise combination of structural and biochemical characteristic.
• When that balance is disturbed, the lens loses its transparency resulting in a condition called Cataract.
• In the human eye, the greatest amount of refraction occurs when light passes from the air through the corneal tissue which has a density close to that of water.
• Lens fibres are cells in the interior of the lens. They have no nuclei or organelles and are filled with transparent proteins, called crystallin. Crystallin provides clarity and focusing power to the lens.
• In the human eye, the greatest amount of refraction occurs when light passes from the air through the corneal tissue which has a density close to that of water.
• Additional refraction occurs when light passes from the aqueous humour into the relatively dense lens.
• The lens provides the extra refraction needed to focus the light rays from the object towards a focal point on the retina.
• Focal point is a specific point of intersection on retina.
• Focal distance is the distance between centre of lens and its focal point.
• Focal distance is determined by two factors: the distance of the object from the lens (the closer the object is to the lens, the greater the focal distance) and the shape of the lens (the rounder the lens, the more refraction occurs)
• Accommodation refers to the ability of the eye to change the shape of the lens so as to maintain clear images at different distances
• Astigmatism is a condition where the cornea is not spherical, causing light rays to focus at different points.
• Hyperopia is when the light rays are focused behind the retina. The eyeball is too shallow, or the lens is too flat. This can be corrected by a converging, convex lens.
• Myopia is when the light rays are focused in front of the retina. The eyeball is too long, or the lens is too thick/convex. This can be corrected with a diverging concave lens.
• Senile cataracts are caused by age-related changes in the lens.
• Glaucoma is:

  • damage to the optic nerve leading to progressive, irreversible vision loss,
  • caused by an increase in intraocular pressure,
  • and caused by a narrowing or blockage of Schlemm’s canal.
• Retinitis pigmentosa is a degenerative disease of the retina that results in progressive loss of vision. It is a genetic disorder.
• Macula densa degeneration is a degenerative disease that causes the macula to become thinner and less sensitive to light.
• Inner segment contains major organelles and is responsible for cell functions.
• Discs of the outer segment in both rods and cones contain special organic compounds called visual pigments. Visual pigments are where light absorption occurs (a key step in photoreception). Derivatives of rhodopsin (opsin protein plus retinal) – Retinal synthesized from vitamin A.