In phototransduction, the 'dark current' maintains photoreceptors in a slightly depolarised state in the absence of light. Which ion channel and second messenger are responsible for maintaining this dark current?

• A cAMP-gated K+ channels kept open by high cAMP from adenylyl cyclase in darkness
• B cGMP-gated cation channels (CNG channels) held open by high cGMP generated by guanylyl cyclase in darkness ✓
• C Voltage-gated Ca2+ channels sustained by tonic glutamate release from bipolar cells
• D TRPV1 channels activated by physiological temperature sustaining Na+ influx

Explanation

In darkness, phosphodiesterase (PDE) activity is low and guanylyl cyclase (GC) continuously generates cGMP, keeping cGMP levels high in rod outer segments. High cGMP holds cyclic nucleotide-gated (CNG) channels open, allowing Na+ and Ca2+ influx — the dark current — that maintains the photoreceptor at approximately -40 mV (partially depolarised), sustaining glutamate release onto bipolar cells. Light activates rhodopsin → transducin → PDE activation → cGMP hydrolysis → CNG channel closure → hyperpolarization → reduced glutamate release → signal to bipolar and ganglion cells.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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