Nervous regulation of metabolism

As described previously, the regulatory roles played by the autonomic nervous system on blood glucose homeostasis is well documented in comparison with those on fat and protein metabolism. It can be summarized as follows: It was shown that an increase in blood glucose concentration produced an increase in the activity of the pancreatic branch of the vagus nerve whereas it induced a decrease in the activity of the pancreatic branch of the splanchnic nerve and adrenal nerve. It was also shown that a decrease in blood glucose concentration activated the sympatho-adrenal system and suppressed vago-pancreatic system. It seems rational that these responses are involved in the maintenance of blood glucose level. Studies on the innervation of the liver led us to a conclusion that sympathetic innervation of the liver plays a role in eliciting a prompt hyperglycemic response through liberation of norepinephrine from the nerve terminals, and that the vagal innervation synergically worked with the humoral factor (insulin) for glycogen synthesis in the hyperglycemic condition. The glucose-sensitive afferents from the liver seem to initiate a reflex control of blood glucose level. The gustatory information on early insulin response (EIR), reported by Steffens (1976), is supported by the electrophysiological observations. Mei's reports (1981) also indicated the importance of information from the intestinal glucoreceptors in the reflex control of insulin secretion via the vagus, which has been proved electrophysiologically. The role of integrative function of the hypothalamus and brainstem through neuronal networks on neural control of blood glucose homeostasis is evident. On the neural control of fat metabolism, it is evident that the sympathetic outflows to the brown adipose tissue as well as white adipose tissue predominantly play important roles. However, there is still little information on the central mechanism played by the hypothalamus on the fat metabolism and the neural pathways from the hypothalamus to the sympathetic motoneurons innervating adipose tissues. The reports on the neural control of protein metabolism are very few. Only several studies were reported on the liver function in relation to the protein metabolism. Further extensive studies should be expected.