Physiology of Heat and Cold Practice Test

The key idea is that brown adipose tissue is activated through sympathetic nervous system signaling during cold exposure. When you’re cold, sympathetic nerves release norepinephrine onto brown adipocytes. This norepinephrine binds to beta-adrenergic receptors, raising cyclic AMP and activating protein kinase A, which promotes lipolysis and the thermogenic program. The fatty acids released from lipolysis both fuel the process and directly enhance the activity of UCP1. UCP1 uncouples oxidative phosphorylation from ATP production in mitochondria, dissipating the energy as heat instead of storing it, which is the mechanism behind non-shivering thermogenesis. This explains why cold exposure triggers brown fat to generate heat.

The parasympathetic system would not drive this process, so acetylcholine signaling is not responsible. Brown adipose tissue can be activated in adults during cold exposure, not kept inactive, and while exercise can increase overall sympathetic tone, the direct neural signaling to brown fat described here is the primary mechanism, not a simpleHeat produced by exercise without neural input.