Master the Heat & Cold Challenge 2026 – Ignite Your Physiology Skills!

Cold exposure triggers a rapid response from the sympathetic nervous system that targets brown adipose tissue. Norepinephrine released from sympathetic nerves binds to beta-adrenergic receptors on brown adipocytes, raising cAMP and activating protein kinase A. This signaling promotes lipolysis and makes fatty acids available as fuel; these fatty acids also directly stimulate UCP1. UCP1 then uncouples oxidative phosphorylation, letting protons leak across the mitochondrial membrane and releasing energy as heat instead of producing ATP. This mechanism is what drives non-shivering thermogenesis. The other options don’t initiate this rapid, targeted BAT response: parasympathetic input to BAT isn’t thermogenic, endocrine cortisol signaling is slower and not the immediate trigger, and somatic motor input to brown fat doesn’t directly activate the thermogenic pathway.