In this article about the regulation of body temperature in mammals, I will take humans as a representative of this group of animals and discuss how temperature of their body is regulated. The temperature of the body is tightly regulated by several mechanisms which are controlled by neurons in the hypothalamus and which are called preoptic neurons.
The preoptic neurons receive nerve impulses from the skin surface and from mucus membranes. In turn, the preoptic neurons send impulses to two other neuronal centers in the hypothalamus and which are specialized in elevating and lowering of body temperture respectively. These centers are called heat inducing center and heat dissipating center. These two centers in the hypothalamus are stimulated by nerve impulses from the preoptic neurons also in the hypothalamus.
In the case of low body temperature or hypothermia as occurs in cold places in the winter time mechanisms that function by conserving heat in the body take place. The skin of the body has many temperature sensitive nerve receptors. When it senses cold temperature it sends in turn nerve impulses to the hypothalamus to the preoptic area.
These impulses are then processed and after that the neurons of the preoptic nucleus send nerve signals to the heat promoting center also in the hypothalamus. Also the neurons of the preoptic nucleus stimulate the release of thyrotropin releasing hormone which in turn stimulates the pituitary gland to secrete thyroid stimulating hormone which functions by stimulating the thyroid gland to secrete thyroid hormone. Thyroid hormone in turn induces high metabolic rate in the body the thing that can increase body temperature to its normal value.
Also nerve impulses from the heat promoting center in the hypothalamus stimulate the sympathetic nervous system which can cause constriction of blood vessels in the skin by the action of the neurotransmitters epinephrine and norepinephrine. The constriction of skin blood vessels reduces the amount of heat that is dissipated from the body.
Conservation of heat along with the continued metabolism in the body contribute significantly to restoring body temperature to normal values. Also stimulation of the sympathetic nervous system by the neurons of the heat promoting center in the hypothalamus leads to activation of neurons of the adrenal medulla which in turn increase the secretion of the the hormones epinephrine and norepinephrine to the blood circulation.
In addition to their function in constricting blood vessels of the skin they contribute to increasing the metabolic rate in the body. Thus causing an elevation of body temperature to its normal values. Also the hypothalamus stimulates muscles contraction and relaxation which is called shivering. This process can significantly increase the body temperature to its normal values.
In the case of hyperthermia or increased core body temperature the skin neuronal receptors send nerve impulses to the preoptic neurons which send in turn impulses to the heat dissipating center in the hypothalamus. Nerve impulses from the heat dissipating stimulate blood arteries dilatation which can cause an increased flow of blood to the skin which allows for increased loss of heat from the body to the surrounding. The loss of heat to the surrounding from the skin occurs via infra red radiation and via conduction with air.
Also the metabolic rate in the body is diminished and muscle shivering does not take place. Sweat glands in the skin are stimulated by the elevated temperature of the blood. When water that has exited the body through sweating evaporates large amount of heat is lost through the skin. This thing can cool down the skin surface and subsequently cooling down the body temperature. Usually the body core temperature is affected by several conditions including exercise and hormones production.