The master circadian clock in mammals is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Which of the following provides the PRIMARY input pathway that entrains this clock to the light-dark cycle?

• A Geniculostriate pathway from the lateral geniculate nucleus
• B Medial forebrain bundle from the brainstem raphe nuclei
• C Tectospinal tract from the superior colliculus
• D Retinohypothalamic tract from photosensitive retinal ganglion cells ✓

Explanation

The retinohypothalamic tract (RHT) carries photic information from intrinsically photosensitive retinal ganglion cells (containing melanopsin) directly to the SCN, serving as the primary entrainment pathway for circadian rhythms. The geniculostriate pathway is for conscious vision. The SCN receives photic input independently of the visual cortex, which is why some blind individuals can still entrain circadian rhythms.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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