Neurohumoral transmitter

Neurohumoral transmitters are chemicals that facilitate the transmission of nerve impulses across nerve synapses and neuroeffector junctions. Acetylcholine is a neurohumoral transmitter that is present in the peripheral autonomic nervous system, in the somatic motor nervous system, and in some portions of the central nervous system. 

Acetylcholine (ACh, 51-84-3) is a molecular ion which functions as a neurohumoral transmitter in the peripheral nervous system. It is released at the end of one cell, diffuses across the synapse, induces permeability changes in the next cell, and is then rapidly removed from the synaptic region by a hydrolytic reaction catalvzed by the enzyme acetylcholinesterase (AChE,... 

Acetylcholine, supposedly a neurohumoral transmitter, is a compound effective in amounts of the same order of magnitude as vitamins or hormones. It was to be expected that the study of the enzyme mechanisms connected with this ester, as well as other enzymes involved in nerve activity, should yield much information. In this way the electrical changes during the activity could eventually be correlated with chemical reactions. 

The demonstration that a substance is released on stimulation of a tract of nerve fibres provides useful evidence that it is present in nervous rather than in glial tissue. Its transmitter function becomes more likely if it can be shown that the amount released is sufficient to stimulate or inhibit neighbouring nerve cells. Recent advances in histochemistry have permitted the more accurate localization of neurohumoral substances not only between neurones and glial tissue, but also within the subcellular components of individual cells. 

Acetylcholine occupies a unique position among neurohumoral substances for it is the only one which has been conclusively shown to have a transmitter function at central synapses. The relevant evidence has been extensively... 

Adrenaline is the major component of the hormonal secretion of the adrenal medulla and in the adrenal gland noradrenaline can be regarded simply as a precursor of the active substance. In sympathetic nerves, however, catecholamine synthesis proceeds no further than noradrenaline, which exists there as a transmitter substance in its own right, with properties different from those of adrenaline. Noradrenaline and small amounts of adrenaline are present in the brain also, but in addition, certain areas of the brain contain amounts of dopamine quite out of prop>ortion to their noradrenaline content and there is suggestive evidence that dopamine has an independent neurohumoral function. The unique situation thus arises that one, or probably two of the precursors of adrenaline have specialized functions of their own. 

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