The parvocellular neurons of the paraventricular nucleus (PVN) that regulate the HPA axis project to the median eminence and release CRH into the hypophyseal portal blood. Which physiological condition would MOST potently stimulate these neurons?

• A Sustained mild hypoglycemia (glucose 70 mg/dL)
• B Circadian rhythm only, peaking at midnight
• C High serum cortisol levels via positive feedback
• D Acute stressors including severe hypoglycemia, cytokine (IL-1, IL-6) release during infection, and hemorrhagic shock ✓

Explanation

Parvocellular PVN neurons integrate multiple stress inputs: acute psychological stress, severe hypoglycemia (not mild), inflammatory cytokines (IL-1, IL-6, TNF-alpha activating PVN via area postrema and vagal afferents), osmotic stress, and hemorrhagic shock. These stimuli are processed through limbic pathways (amygdala, hippocampus) and brainstem nuclei converging on PVN CRH neurons. Circadian rhythm does exist (peaking around 0800h, not midnight). Cortisol exerts negative feedback on PVN parvocellular neurons (negative, not positive feedback), suppressing further CRH release via glucocorticoid receptors.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.