The nervous system may affect metabolism in various ways: (1) through direct effects on metabolically active tissues (e.g. stimulation of lipolysis), and also on the digestive system (e.g. stimulation of salivary flow, gastric acid secretion); (2) through stimulation of muscle contraction, which is in turn linked to various metabolic adjustments; (3) through indirect effects mediated by changes in hormone secretion (especially modulation of insulin and glucagon release); and (4) through indirect effects on other bodily systems, particularly the circulatory system (e.g. changes in cardiac output and distribution of blood flow to different organs and tissues).

The indirect effects are operative continuously, maintaining normal operation of the body. The importance of the direct effects of the nervous system on metabolism in everyday life is probably a matter of 'fine tuning', but becomes more apparent in acutely stressful situations such as exercise, mental stress or physical injury. The adrenal medulla works in many ways like an extension of the sympathetic nervous system (which it is), and it is often difficult to distinguish effects of noradrenaline released at sympathetic nerve terminals from those of circulating adrenaline.

Activation of the sympathetic nervous system has a generally catabolic function, whereas the parasympathetic has a more anabolic role, although these generalisations do not hold true for all specific effects.

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