Chapter 17.5 - Eyesight Pathway & Transduction

What is the visual pathway?

  1. Axons of retinal ganglion cells exit eyeball at optic chiasm and form optic nerve on each side

  2. Axons from the temporal half of each retina at the optic chiasm go directly to the thalamus on the same side

  3. Axons from the nasal half of each retina cross the optic chiasm and go to the opposite thalamus

  4. Axon branches project to the midbrain - pupil constriction and head and eye movement coordination

  5. Axon branches extend to hypothalamus which regulates sleep and other activities in response to light/darkness

  6. Axons project from thalamus to occipital lobe of cerebrum on same side

Photoreceptors and Photopigments

  • located in pigmented layer of retina

  • rods: cylindrical or rod-shaped and contain photopigment rhodopsin

  • cones: tapered or cone-shaped and contain photopigments (blue, green, and red)

What are all photopigments made up of?

Part 1: glycoprotein known as opsin

Part 2: retinal molecule (derivative of vitamin A which has carotene in it and absorbs light)

How does phototransduction occur?

  • occurs in outer segment of a photoreceptor

  • activation by a light stimulus causes a hyperpolarizing receptor potential NOT A DEPOLARIZATION POTENTIAL

  • retinal has 2 forms: cis and trans

  • trans: retinal form separated from opsin (bleaching)

  • cis: retinal binds to opsin again (regeneration)

Steps of phototransduction in DARKNESS

  1. Darkness: cis-retinal form of photopigment

  2. Darkness: high cGMP in the cytosol of photoreceptor outer segment

  3. Darkness: cGMP binds and opens cation channels so Na+ enters cell

  4. Darkness: photoreceptor depolarizes

  5. Depolarization spreads to synaptic terminal which has Ca+ channels in its membrane - Ca+ enters cell

Steps of Phototransduction in LIGHT

  1. cis-retinal converts to trans-retinal

  2. G protein called transducin is activated

  3. Transducin activates cGMP phosphodiesterase

  4. cGMP is broken down

  5. Lower cGMP means reduced Na+ inflow

  6. Decreases Na+ inflow causes a hyperpolarizing potential - more negative and much closer to resting potential of -70 mV

  7. Hyperpolarization spreads causing decrease in Ca+ entry

  8. Decreased Ca+ = decreases of the inhibitory neurotransmitter