Eye and Vision

• The eye is a sensory organ, analogous to a camera.  It focuses visual images on the retina. 
• Retina is a layer of the eye that contains sensory receptors that respond to light stimuli.
  • Visual pathway is optic nerves to optic chiasma to optic tract to lateral geniculate nucleus of thalamus to visual cortex
    • Medial retinal fibers cross over in chiasma, lateral fibers do not
• Visual cortex of the occipital lobe of the brain interprets signals relating to shape, color, and movement & spatial organization.
• Nuclei of origin of oculomotor (III), trochlear (IV) and abducens (VI) cranial nerves coordinate eye movements and changes in lens shape and pupil size.

Three processes involved in focusing:

1. Refraction - bending of light rays as they pass obliquely from one medium into another of different optical density (property of transparent material which is a measure of speed of light through it).
   • Cornea & aqueous humor - responsible for about 0.8 total refraction
   • Biconvex lens - responsible for about 0.2 total refraction, causes convergence, capable of change in curvature*
     1. Focuses image on retina inverted & reversed
2. Accommodation of lens* - change in refractive power due to change from moderately convex (distant vision) to very convex (higher refraction for near vision).
   • Mechanism:

     Lens is elastic and tends to be spherical.  Elastic suspensory ligaments attached to the edge of the lens connect to ciliary processes of the choroid coat and maintain tension on lens causing it to be moderately convex.  A circular layer of smooth muscle (ciliary muscle) lies at the edge of the choroid layer.  When the ciliary muscle contracts, tension on the lens is reduced and the lens becomes very convex for near vision.

   • Ciliary muscle is controlled mainly by parasympathetic nerves
   • Accommodation problems
     • Hyperopia - far-sightedness, eyeball too short or lens too flat, insufficient refraction, object focuses behind retina, convex glass lens will correct
     • Myopia - near-sightedness, eyeball too long or lens too curved, object focuses in front of retina, concave glass lens can correct
     • Astigmatism - irregularities in the curvature of cornea or lens, resulting in different degrees of refraction in different planes
     • Presbyopia - lens loses elasticity with age
3. Depth of focus - distance an object can move & remain in focus, arrowing diameter of the pupil increases depth of focus, also prevents light from passing through the periphery of the lens where curvature is greater. Circular smooth muscle of iris constricts pupil, autonomic response

Retina

light sensitive coat of the eye

Retinal layers from deeper to surface contacting vitreous humor

1. Pigmented layer - epithelium containing melanin, prevents light reflection in eyeball, stores Vitamin A which forms retinal
2. Photoreceptor layer - 120 million rods (black & white) & 6-7 million cones (color)
3. Outer synaptic layer - photoreceptors synapse with bipolar cells, convergence, also horizontal cells* present
4. Bipolar cell layer - fewer cells
5. Inner synaptic layer - bipolar neurons synapse with ganglion cells, convergence, also amacrine cells* present
6. Ganglion cell layer - fewest cells
7. Optic nerve fiber layer - axons of ganglionic cells

(* adjust contrast in different light, facilitate or inhibit)

• Macula lutea - area of retina with elongated cones, no rods
• Fovea centralis - center of macula where layers 3-7 are pushed aside allowing light to reach the cones unimpeded, on visual axis, increases visual acuity or resolution.
• Optic disc - blind spot, area of retina where optic nerve exits, no rods or cones

Photoreceptors - rods & cones

• Inner segment contains cellular organelles, synapses with bipolar cells
• Outer segment contains discs formed by infoldings of the cell membrane.  Constantly being renewed moving from bottom to tip
  • Photopigment - integral protein of disc membrane
    • Retinal - Vitamin A derivative
      • Light absorbing portion, 2 isomers cis and trans
    • Opsin - Glycoprotein, each photopigment has different opsin handling different wavelengths (colors) of light, 1 rod & 3 cone iodopsins

Photochemistry mechanism

1. In the dark, Na channels in the outer segment are held open by cGMP
2. Na influx causes depolarization that triggers continual release of glutamate neurotransmitter in rods
3. Glutamate hyperpolarizes (inhibits) bipolar cells.
4. Inner segment continuously pumps Na out and K in, K diffuses out
5. When light passes through retinal layers 3-7 and reaches rods
6. Reddish cis-retinal is tightly attached to opsin
7. Cis-retinal absorbs light and shifts to trans-retinal form (isomerization)
8. Trans-retinal separates from opsin becoming colorless (bleaching)
9. Opsin activates transducin (G protein) in the cell membrane
10. Transducin breaks down cGMP and closes gated Na channels
11. This decreases Na influx into the rod while pump continues
12. Rod becomes hyperpolarized and ceases glutamate release
13. Bipolar cells are not inhibited and release neurotransmitter at synapse with ganglion cells resulting in action potential being sent along optic nerve
14. Retinal isomerase shifts trans-retinal back to cis-retinal form
15. Cis-retinal rebinds with opsin (regeneration)
16. Transducin is deactivated and Na channels are reopened
17. Rods regenerate at about same rate as bleaching occurs in daylight.  Cones regenerate very fast.