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Circadian entrainment — MRCPsych Paper A MCQ

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HardNeurophysiology and neurochemistryCircadian entrainmentMRCPsych Paper A

A 37-year-old nurse with shift-work-related insomnia undergoes a laboratory assessment of circadian phase. A brief bright-light pulse during the early biological night delays the onset of endogenous melatonin secretion. Which neural projection most directly conveys the photic signal that phase-shifts the central circadian pacemaker?

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Correct answer: D — Retinohypothalamic projection from the retina to the suprachiasmatic nucleus

Explanation lettering: D = shown as A · C = shown as B · A = shown as C · B = shown as D

Option B is correct. The suprachiasmatic nucleus (SCN) is the central circadian pacemaker. Retinal light information reaches it directly through the retinohypothalamic tract, principally using glutamate and PACAP, and resets SCN activity and clock-gene expression. This produces phase shifts in melatonin and sleep–wake rhythms. The geniculohypothalamic input (A) and median raphe serotonergic input (C) modulate SCN responses and non-photic entrainment but are not the primary retinal conduit. The SCN-to-paraventricular projection (D) is a downstream circadian output. Superior cervical ganglion fibres to the pineal gland (E) regulate melatonin synthesis as the terminal sympathetic limb; they do not convey light to or reset the SCN.

Reference: Golombek DA, Rosenstein RE. Physiology of circadian entrainment. Physiological Reviews. 2010;90(3):1063–1102. https://pubmed.ncbi.nlm.nih.gov/20664079/