Having considered the basic plan of the autonomic nervous system, we may now examine some further details of interest about autonomic neurons.
1. The neurotransmitter acetylcholine is liberated at the terminals of preganglionic neurons, both sympathetic and parasympathetic. Acetylcholine is also liberated at the terminals of parasympathetic postganglionic neurons. However, the neurotransmitter liberated at the terminals of sympathetic postganglionic neurons is (as a rule) noradrenalin or adrenalin. Cells of the adrenal medulla which receive terminals of preganglionic sympathetic neurons, and produce noradrenalin and adrenalin, may be regarded as modified sympathetic postganglionic neurons. It may be noted that cells of the sympathetic ganglia and of the adrenal medulla have a common embryological origin from the neural crest.
Postganglionic sympathetic neurons innervating sweat glands are exceptional in that their terminals liberate acetylcholine.
2. It is possible that some sympathetic preganglionic neurons may be located in spinal segments above T1 or below L3, and may leave the cord through corresponding spinal nerves.
3. Some preganglionic sympathetic fibres may leave the spinal cord through dorsal nerve roots.
4. Some sympathetic postganglionic neurons may be located in intermediate gangliapresent on the trunks of spinal nerves, in the ventral rami or in rami communicantes.
5. The number of postganglionic sympathetic neurons (or fibres) is much greater than that of preganglionic neurons, each preganglionic fibre synapsing with many postganglionic neurons. This results in considerable dispersal of the nerve impulse. A similar, but much lesser, dispersal of impulses also takes place in the parasympathetic nervous system. This is to be correlated with the fact that sympathetic stimulation produces widespread effects, whereas the effects of parasympathetic stimulation are much more localised.
6. Some of the neurons in sympathetic ganglia are interneurons. Some of these are described as small intensely fluorescent (SIF) neurons. Some others are chromaffin.
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