PHysiology of vision continuned
Phototransduction
-Phototransduction: Process by which photoreceptors activated, photopigments within cell discs altered by light 1.Photoreceptor: responds to light stimulation by changing resting membrane potential, then a change in neurotransmitter release 2.Rhodopsin activation: -before light activation, retinal portion in a bent shape, cis retinal -upon exposure, straightens into trans retinal form, dissociates from opsin as phototransduction occurs -rhodopsin changing to being colorless, termed bleaching reaction reduces rhodopsin amount in rod
Phototrandsuction of light
11/20/2013 Details of phototransduction (light) -In light,plasma membrane of photoreceptor hyperpolarized to 70 mV -Opsin activated by light, causes cis retinal to change in shape to trans retinal, bleaching of photoreceptors and photopigment activation, change in rhodopsin conformation causing drop in cGMP levels, reduced Na + entry due to channels regulated by cGMP - Calcium channels closed, block release of inhibitory neurotransmitter -Bipolar cells no longer inhibited, then depolarize, releasing their own neurotransmitter which binds to receptors in ganglion cells -Generates signals transmitted via optic nerve to brain
Phototransduction continued.
3.rhodopsin regeneration: -dissociated trans retinal transported to pigmented layer, retinal converted back to cis retinal form, transported back to the rod, associates with opsin and reforms rhodopsin -light interferes with relatively slow process (rods nonfunctional in bright light) 4.Photopsin in cone cells undergoes similar process -cis retinal transformed to trans retinal (bleaching reaction) -regeneration more quick than rhodopsin regeneration, not as negatively affected by bright light
Phototransduction-Dark adaptation
Dark adaptation: -return of sensitivity to low light levels after bright light -cones initially nonfunctional in low light, rods still bleached from bright light conditions -may take 20 to 30 minutes to see well Details of phototransduction (darkness) -Photoreceptors depolarized in darkness -Have resting membrane potential of 40 mV -maintained by cyclic guanosine monophosphate (cGMP) -When levels elevated, Na +channels in membrane kept open, Na+ enterscells, called dark current opening of calcium channels due to darkcurrent, continuous release of inhibitory neurotransmitter from photoreceptors -Bind bipolar cells, which are hyperpolarized to 70 mV -Bipolar cells not releasing excitatory neurotransmitter do not stimulate ganglion cell -no nerve signal generated
Color Blindness-what is it?
X-linked recessive condition more common in MALES -Absence or deficit in one type of cone cell -Red and green most commonly affected -results in difficulty distinguishing red and green