Retina

• Consists of single-cell-thick pigmented epithelium

• Photoreceptor neurons:

• Rods and cones.

• Layers of other neurons

• Neural layers are forward extension of the brain.

• Neural layers face outward, toward the incoming light.

• Light must pass through several neural layers before striking the rods and cones.

• Rods and cones synapse with other neurons.

• AP conducted outward in the retina.

• Outer layers of neurons that contribute to optic nerve called ganglion cells.

• Neurons receive synaptic input from bipolar cells, which receive input from rods and cones.

• Horizontal cells synapse with photoreceptors.

• Amacrine cells synapse with several ganglion cells.

Effect of Light on Rods

• Rods are activated when light produces chemical change in rhodopsin.

• Bleaching reaction:

• Rhodopsin dissociates into retinene (rentinaldehyde) and opsin.

• 11-cis retinene is converted to all-trans form.

• Initiates changes in ionic permeability to produce AP in ganglionic cells.

• Provide black-and-white vision.

Dark Adaptation

• Gradual increase in photoreceptor sensitivity when entering a dark room.

• Maximal sensitivity reached in 20 min.

• Increased amounts of visual pigments produced.

• Slight increased pigment in cones.

• Greater increased rhodopsin in rods.

• 100,00-fold increase in light sensitivity in rods.

Category: Ophthalmology Notes