Which signaling pathway initiates non-shivering thermogenesis in response to cold exposure?

• A. Parasympathetic to BAT.
• B. Endocrine cortisol signaling to adipose tissue.
• C. Somatic motor input to brown fat.
• D. Sympathetic nervous system activation to BAT via norepinephrine leads to UCP1 activation.

Answer: (D)

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.