Phototransduction in rod photoreceptors involves a signal amplification cascade. When light activates rhodopsin, the G-protein (transducin) activates phosphodiesterase (PDE), which hydrolyses cGMP. The fall in cGMP causes closure of CNG channels. Which electrophysiological consequence immediately follows CNG channel closure in the outer segment?

• A Membrane depolarisation; increased Ca²⁺ influx through voltage-gated channels triggering synaptic vesicle fusion
• B Membrane hyperpolarisation; increased glutamate release activating ON bipolar cells via mGluR6 receptors
• C Membrane hyperpolarisation (from –40 mV toward –70 mV); reduced glutamate release from the synaptic terminal ✓
• D Membrane depolarisation; reduced K⁺ outflow causing prolonged depolarisation and sustained glutamate release

Explanation

In darkness, CNG channels are open (kept open by high cGMP), admitting Na⁺ and Ca²⁺ into the outer segment, maintaining the 'dark current' and a relatively depolarised membrane potential of approximately –40 mV, with continuous glutamate release. Light → rhodopsin activation → transducin → PDE activation → cGMP hydrolysis → CNG channel closure → cessation of dark current → membrane hyperpolarisation to approximately –70 mV → reduction in glutamate release. This hyperpolarisation-based signalling is unique among sensory receptors. ON-centre bipolar cells express mGluR6 and are depolarised when glutamate is removed.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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