Darkness stimulates melatonin secretion from the pineal gland via a pathway involving the suprachiasmatic nucleus. The rate-limiting enzyme converting serotonin to N-acetylserotonin in this pathway is activated by norepinephrine released from which neurons?

• A Postganglionic sympathetic fibers from the superior cervical ganglion ✓
• B Preganglionic parasympathetic fibers of the facial nerve
• C Noradrenergic fibers of the locus coeruleus
• D Retinohypothalamic tract fibers directly innervating the pineal

Explanation

The pineal receives its neural input from postganglionic sympathetic fibers of the superior cervical ganglion. At night, norepinephrine acting on beta receptors raises intracellular cAMP and activates arylalkylamine N-acetyltransferase, the rate-limiting enzyme of melatonin synthesis. The retinohypothalamic tract terminates in the suprachiasmatic nucleus, not the pineal, and the pineal has no direct parasympathetic secretory control.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.