rods and cones

rods

• bipolar photoreceptor cells

• higher convergence with bipolar cells

• located over periphery of retina

• become denser as you get closer to macula

• involved in non-color vision

• vision under conditions of low light = mesopic

• inactive

• contains rhodopsin, opsin, and retinal

rhodopsin

• purple pigment

• 1000x more sensitive to light

• too much light → deactivates → can’t absorb light at all

opsin

• protein

• like a channel protein → invoves G protein and cyclic GMP

retinal

• yellow photosensitive pigment

• cis-retinal shape: tightly bound to internal surface of opsin

  • in the dark state

  • cyclic GMP attached to Na+ channel → keeps it open

• trans-retinal shape:

  • when rod cells absorb light

  • opsin changes → activates transducin G protein → activates cyclic GMP phosphodiesterase

cones

• Conical

• Molecule iodopsin → visual pigment

  • Retinal combined with opsin protein

  • 3 types of opsin → blue, red, green

  • Each iodopsin is sensitive to a narrow spectrum of visible light

  • But because of the overlap → now can perceive color over a wide range

• Involved in color vision and vision acuity

  • Not light sensitivity

• Located in fovea centralis and macula

• Distribution

  • Densest in the macula

  • Most dense in the middle

• There are enough photons of light to activate all the cones

• Cones are always activated

• Bipolar photoreceptor cells with conical part → light sensitive

• Require bright light to function

  • Light dimes → few cone cells respond

• Light of a given wavelength strikes retina → cone cells with photopigments that can respond to that wavelength → generate APs in retina

• Color interpreted in visual cortex of occipital lobe

• One cone cell synapses with only one bipolar cell