Sight


Anatomy & Phsiology:

Eye Muscles

  1. Inferior Oblique- Elevates eye and turns it medially
  2. Inferior Rectus- Depresses eye
  3. Lateral Rectus- Moves eye laterally
  4. Medial Rectus- Moves eye medially
  5. Superior Oblique- depresses eye and turns it laterally
  6. Superior Rectus- Elevates eye

Cranial Nerves Controlling Muscles

  1. (III) Oculomotor
  2. (III) Oculomotor
  3. (VI) Abducens
  4. (III) Oculomotor
  5. (III) Oculomotor
  6. (IV) Trochlear
external image eye%20muscles%203.gif

Structure of Eyeball:

Fibrous layer-Outermost part of eye.
Sclera
  • White of the eye
  • Also known as the fibrous tunic
  • Tendonlike
  • Shapes and Protects the eyeball
  • Provides an anchoring site for extrinsic eye muscles
  • Pierced by optic nerve (Continuous with dura mater of brain)
Cornea
  • Crystal Clear
  • Bulges anteriorly
  • Allows light to enter the eye
  • Covered by two Epithelial sheets- External sheet is straitified squamos that helps protect the cornea from abrasion. Deep corneal endothelium is composed of simple squamos epithelium, it is the inner face of cornea, and its cells have active sodium pumps that maintain the clarity of the cornea by keeping the water content low in the cornea.
  • Supplied with nerve endings, most are pain receptors
  • There are no blood vessels, and because of this it is out of reach for the immune system.
Vascular Layer-Middle coat of the eye
Choroid
  • Blood vessel rich
  • Forms 5/6 of vascular layer
  • Brown pigments, produced by melanocytes helps asorb light
  • Forms two smooth muscle structures-The ciliary body where the lens is attached. Ciliary zonule is a suspensory liament
  • The iris has a rounded opening (pupil)
  • The pupil lets light to enter the eye
  • In close vision and bright light circular muscles contract and the pupil constricts
  • In distant vision and dim light radial fibers contract to dilate the pupil, which allows more light to enter.
Innermost part of the eye
  • Retina extends to ciliary body
  • Contains millions of receptor cells, Rods and cones (photoreceptors)
  • Photoreceptors respond to light
  • Electrical signals pass from photoreceptors through bipolarcels and ganglion cells (neuron chains)
  • The nerve impulses that are transmitted to optic cortex, then leaves the retina
  • Photoreceptor cells are distributed over the entire retina except where the optic nerve leaves the eyeball, Optic disc (blind spot)
  • Optic disc is when the light from an object is focused on the optic disc, it dissapears from our view and we can't see the rods and cones evenly distributed in retina.
  • Cones are most dense at the edge of retina and decrease in number as center of retina is approached
  • Rods allow us to see in gray tones in dim light and provide our peripheral vision
  • Cones allow us to see in color under bright light conditions densest in center of retina and decrease in number toward retinal edge
  • Fovea centralis-Tinypit that contains only cones, location is lateral to each blind spot
  • Area of greatest visual acuity, or point of sharpest vision
  • 3 varieties of cones, each is sensitive to particular wavelengths of visible light; 1 responds to blue light, 1 green light, and 1 responds to a range that include red and green wavelengths of light
  • Intermediate colors-Impulses received at the same time from more than 1 type of cone by visual cortex
  • When all cones are being stimulated, we see white
  • Lens- Flexible biconvex crystal-like structure
  • Lens are held upright by ciliary zonule, attached to ciliary body. Divided in two segments-Anterior (aqueous) segment, anterior to lens, contains a clear watery fluid called aqueous humor. Posterior (vitreous) segment, is filled with a gel-like substance called vitreous humor, or vitreous body
  • Vitreous humor helps prevent the eyeball from collapsing inward by internal reinforcement.
  • Aqueous humor is secreted by a special area of choroid, it helps maintain intraocular pressure (pressure in the eye).
  • Aqueous humor provides nutrients for lens and cornea, which lackblood supply
  • Aqueous humor is reasorbed into venous blood through scleral venous sinus, located in junction of sclera and cornea
external image vision-eye.jpeg

Light and Visual Pathway

  • Light rays are bent in the eye as they encounter the cornea, aqueous humor, lens, and vitreous humor
  • When an eye is resting, it needs no change in the lens for distant vision.
  • In closer vision, the light scatters, and spread out and lens must bulge out more to make close vision possible. This happens because the ciliary body contract, allowing the lens to be more convex.
  • Axons carrying impulses from retina are bundled together at posterior aspect of eyeball tissue from back of the eye as the optic nerve
  • Optic Chiasma are fibers tracts that result
  • Optic Chiasma contains fibers from lateral side of eye on the same side and medial side of opposite eye
  • Optic tract fibers synapse with neurons in thalamus, whose axons form optic radiation, which runs to occipital lobe of brain.
  • The synapse with cortical cells, and seeing occurs
  • Each side of the brains receives visual input from both eyes
  • Humans have binocular vision, 2 eyed vision,visual fields overlap. This creates depth perception.

external image eye-brain.jpg
Eye anatomy and phsiology