The autonomic nervous system (ANS) is that part of the nervous system that controls the visceral activities of the body. This control is involuntary and enables the body to adjust to varying physiological demands. For example, one cannot consciously increase cardiac output, but when physical threat is detected, the ANS will initiate changes in various systems of the body which enable the individual to deal with the physical demand. The ANS is controlled mainly by centres in the brain stem and hypothalamus. Sensory inputs are relayed to these areas and reflex responses are effected in the visceral organs. Acting in concert with the hypothalamus and the endocrine system, the ANS is largely responsible for the control of the internal environment of the body.
There are two subdivisions: sympathetic and parasympathetic. In general, the two systems are antagonistic to each other. It is not always predictable whether sympathetic or parasympathetic stimulation will produce inhibition or excitation in a particular organ, but most organs are predominantly controlled by one or the other system. This background activity is known as the sympathetic or parasympathetic tone. For example, arteriolar smooth muscle has a predominant sympathetic tone, whereas the basal tone in the gut is mainly parasympathetic.
Neurones of the sympathetic nervous system originate in the thoracic and lumbar segments (from T1 to L2) of the spinal cord, the so called thoraco-lumbar outflow (Figure NE.23). These synapses in a paired chain of ganglia, the sympathetic ganglia, are situated on either side of the vertebral column. The nerve fibres that run from the spinal cord to the sympathetic ganglia are known as pre-ganglionic fibres, while those which leave the ganglia to reach their effector organs are known as post ganglionic fibres. A few of the pre-ganglionic fibres pass through the sympathetic chain without forming synapses until they arrive at a more peripheral location in the coeliac and mesenteric ganglia or the adrenal medulla. The adrenal medulla is unique in that it is innervated by sympathetic pre-ganglionic fibres but has no post ganglionic nerve fibres.
The parasympathetic nervous system leaves the CNS via cranial nerves (III, VII, IX and X) and sacral nerves (S2-4). This is called the cranio-sacral outflow (Figure NE.23). Approximately three-quarters of all parasympathetic fibres are located in the two vagus (X) nerves. Like the sympathetic pathway, the parasympathetic system has both pre- and post ganglionic neurones. However, the cell bodies of the parasympathetic ganglia are located within the effector organs themselves, and so the pre-ganglionic fibres travel long distances from the spinal cord and the post ganglionic fibres are, therefore, relatively short.
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