6: Vision

• 3 layers of the human eye:

  1. Cornea and sclera (white)
  2. Choroid, ciliary muscle, and iris
  3. Retina (innermost layer)
• The cornea is the outermost transparent layer of the eye. The majority of refraction occurs here. It is not adjusted by any muscles.
• The iris is made up of 2 pigmented smooth muscle layers that control the size of the pupil, which are affected by the autonomic NS.

  • Parasympathetic activity = contraction of inner muscle = constriction of pupil
  • Sympathetic activity = contraction of outer muscle = dilation of pupil
• The choroid is the dark pigmented layer of the eye that is meant to absorb light.
• Activity of the iris muscle layers is based on adjustments to light. In the dark, we will have predominantly sympathetic stimulation (pupil dilation to allow more light to enter the eye).

  • Vs. in the bright light of day, we will have predominantly parasympathetic stimulation (pupil constriction allows less light to enter the eye).
• Convex lens focus light while concave lens scatter it.
• Nearsightedness is corrected with a concave lens.

  • The issue: light focuses short of the retina
  • The solution: a lens that scatters light before it hits the eye, lengthening the focal point to hit the retina
• Farsightedness, called hyperopia or presbyopia (aging issue), is corrected with a convex lens.

  • The issue: light focuses past the retina
  • The solution: a lens that reduces light scatter before it hits the eye, shortening the focal point onto the retina
• The retina contains 3 types of cells, where electrical is passed in the following order:

  Photoreceptors (rods and cones) > bipolar cells > ganglion cells
• When phototransduction is performed in the dark, cation channels are opened. The depolarization causes neurotransmitter (Glu) release into bipolar cells.
• Cones are primarily concentrated around the fovea, while rods are prevalent around the periphery of the retina.
• When phototransduction is performed in the light, cation channels are closed. The hyperpolarization causes neurotransmitter (Glu) release into bipolar cells to be decreased.
• All cones contain photopigments, also known as opsin, which have 3 variations. All rods contain rhodopsin, another pigment.
• Cones are best used in the daytime because they have a less concentrated arrangement of pigments.

  Rods are best for nighttime due to the arrangement of pigments in disc structures, allowing for the detection of small amounts of light.
• ON bipolar cells respond to Glu binding by closing cation channels, while OFF bipolar cells open them. This affects the release of an excitatory NT.
• Light enters the eye > hyperpolarizes photoreceptor cell > decreased Glu released onto ON bipolar cell > less closing of cation channels > more excitatory NT release > ganglion cell depolarization > more APs
• Light enters the eye > hyperpolarizes photoreceptor cell > decreased Glu released onto OFF bipolar cell > less opening of cation channels > less excitatory NT release > ganglion cell hyperpolarization > less APs