- In the dark, rods show a large inward “dark” current which is suppressed by a flash of light. Which one of the following statements, explaining the effect of light, is true?
(1) Sodium channels in the outer segment of rods are closed.
(2) Cytoplasmic cGMP concentration increases.
(3) Sodium channels in the inner segment of rods are closed.
(4) Transducin dissociates from beta-arrestin.
In darkness, rod photoreceptors maintain a steady inward current—the “dark current”—flowing mainly through cGMP-gated sodium channels in their outer segment membrane. This current depolarizes the rod and enables continuous neurotransmitter release.
Effect of Light on Rod Photoreceptors
-
Exposure to light activates rhodopsin, triggering a phototransduction cascade.
-
Activated transducin stimulates phosphodiesterase to hydrolyze cGMP.
-
The cytoplasmic concentration of cGMP decreases, leading to closure of the cGMP-gated sodium channels in the outer segment membrane.
-
Closure of these channels stops the inward sodium current, causing hyperpolarization of the rod cell membrane.
-
This hyperpolarization reduces glutamate release, signaling light detection to downstream neurons.webvision.med.utah+1
Evaluating the Statements
-
(1) Sodium channels in the outer segment of rods are closed: True and describes the primary mechanism of light effect on dark current.
-
(2) Cytoplasmic cGMP concentration increases: False; cGMP concentration decreases upon light exposure.
-
(3) Sodium channels in the inner segment are closed: False; sodium channels responsible for dark current are located in the outer segment.
-
(4) Transducin dissociates from beta-arrestin: False, transducin dissociates from rhodopsin but not beta-arrestin; beta-arrestin is involved in rhodopsin inactivation.
Conclusion
The true explanation for the effect of light on the rod dark current is the closure of sodium channels in the outer segment due to cGMP breakdown.
The correct answer is:
(1) Sodium channels in the outer segment of rods are closed
-
