The photoreceptors convert (i.e. transduce) light energy into changes in membrane potential. Rods
outnumber cones in the human retina by 20 to 1.
Phototransduction in Rods
In the photoreceptor, light stimulation of the photopigment activates G-proteins, which in turn
activate an effector enzyme that changes the cytoplasmic concentration of a second messenger
molecule and causes a membrane ion channel to close and alter membrane potential.
IN RESPONSE TO DARKNESS
In complete darkness, the membrane potential of the rod outer segment is about -30
mV. This depolarization is caused by the steady influx of Na . The movement of positive
charge across the membrane occurring in the dark is called dark current.
Sodium channels (gated) are stimulated to open by an intracellular second messenger
called cyclic guanosine monophosphate (cGMP). cGMP is continually produced in the
photoreceptor by guanylyl cyclase keeping the Na channels open.
IN RESPONSE TO LIGHT
Light reduces cGMP, Na channels to close and the membrane potential becomes more
negative. Photoreceptors hyperpolarize in response to light.
The response is initiated by the absorption of electromagnetic radiation by photopigment
in the membrane of the stacked disks in the rod outer segments.
o This pigment is called rhodopsin which is a receptor protein with a pre-bound agonist.
The receptor is opsin which has seven transmembrane alpha helices. The pre-bound
agonist is called retinal (derivative of vitamin A).
o The absorption of light causes a change in conformation of retinal so that it activates
opsin (see Fig. 9.18). This process is called bleaching because it changes the
wavelengths absorbed by the rhodopsin (photopigment changes from purple to yellow).
o Bleaching of rhodopsin stimulates G-protein transducin in the disk membrane which
activates the effector enzyme phosphodiesterase (PDE). PDE breaks down the
cGMP present in the rod at dark. The reduction in cGMP causes the Na channels to
close and the membrane to hyperpolarize.
Phototransduction in Cones
In bright sunlight, cGMP levels in rods fall to the point where the response to light becomes
saturated making vision during the day dependent on cones whose photopigments require more
energy to become bleached. The process