Sensory Systems — MCQs

Question 471: Initiation of visual impulse is associated with which of the following?

• A. Condensation of opsin with vitamin A aldehyde
• B. Photoisomerization of 11-cis-retinal (Correct Answer)
• C. NADP
• D. NAD

Explanation: ***Photoisomerization of 11-cis-retinal*** - The initiation of a visual impulse begins with the **photoisomerization of 11-cis-retinal** to **all-trans-retinal** upon absorption of light by rhodopsin. - This is the **primary photochemical event** that triggers the entire phototransduction cascade. - The conformational change in retinal subsequently causes a conformational change in the associated **opsin protein**, activating the **rhodopsin molecule** (visual purple) and initiating signal transduction through the G-protein cascade. - This process is also known as **photoactivation** or the first step in **bleaching of rhodopsin**. *Condensation of opsin with vitamin A aldehyde* - **Condensation of opsin with 11-cis-retinal** (vitamin A aldehyde derivative) occurs in the **dark** to form rhodopsin (visual purple). - This process represents the **regeneration of the photopigment**, not the light-triggered initiation of the visual impulse. - This is the reverse process that restores rhodopsin after bleaching. *NADP* - **NADP (nicotinamide adenine dinucleotide phosphate)** is primarily involved in **anabolic reactions** and oxidative stress response, acting as a cofactor in various metabolic pathways. - It is **not directly involved in the phototransduction cascade** that initiates a visual impulse. *NAD* - **NAD (nicotinamide adenine dinucleotide)** is a coenzyme crucial for **catabolic reactions** and energy metabolism, particularly in glycolysis and the citric acid cycle. - Similar to NADP, it plays **no direct role in the primary photochemistry** or signal transduction mechanisms that initiate a visual impulse.