Final answer:
The nervous system triggers shivering via the hypothalamus when the body temperature falls below its set point, leading to muscle contractions that generate heat. Other thermoregulatory responses include vasoconstriction to conserve heat and hormonal adjustments to increase metabolism.
Step-by-step explanation:
When you are cold, your nervous system sends messages to your muscles to make you shiver. The hypothalamus in the brain detects a drop in body temperature and compares it to the body's temperature set point. If it falls below this threshold, it stimulates the nerves of the somatic nervous system, which innervate skeletal muscle, leading to shivering. This muscle activity causes the extension and contraction of muscle cells, releasing heat energy that elevates the body temperature.
In addition to muscle contractions, the body has other methods to conserve heat. The cardiovascular system works with the skin to either lose body heat or conserve it through vasodilation or vasoconstriction. Blood vessels constrict to conserve heat, and sweat glands reduce their activity. During severe cold, the brain may also trigger the thyroid gland to release thyroid hormone, increasing metabolic activity and heat production, and signal the adrenal glands to release epinephrine. This hormone promotes the breakdown of glycogen into glucose, which serves as an energy source and further increases metabolism and heat generation.