The recognition of transmitter substances

A transmitter substance is present in, and in many instances it is synthesized by, nerves it is released from the presynaptic terminals of the nerve fibres when they are stimulated. On release, it produces changes leading to the excitation or inhibition of neurones standing in synaptic relationship with those from which it has been liberated and after exerting its physiological action, it is inactivated by enzymatic destruction, by recombination into an inactive form in the presynaptic terminals or by diffusion from the region of the synapse. 

The criteria now to be considered are based on these concepts of the processes of synaptic transmission. 

A study of the distribution, in the mammalian central nervous system, of the various substances discussed in this review reveals that none—except y-aminobutyric acid which has general metabolic as well as humoral functions—is as widespread as acetylcholine and there is no general tendency for fibres deficient in acetylcholine to contain correspondingly large amounts of any of the other substances. This is an important point, since the non-cholinergic excitatory transmitter should be found in at least a number of acetylcholine-free nerve tracts. Instead, it appears that some areas of the nervous system contain a multiplicity of humoral substances while others have none at all. 

The distribution of the various humoral factors discussed is not considered in detail here since the information is available elsewhere. The interested reader, wishing to confirm the validity of the assertions just made, is referred particularly to the recent book by Garattini and Valzelli in which all the available data are collected together in the most comprehensive tables yet published. 

It must also be remembered that in the brain, non-neuronal elements (the glial tissue) outnumber neurones at least tenfold and that some of the substances found in the nervous system might well occur in glial cells. The intrinsic activity of the glial cells, the local control of blood vessels and the production of local hormones for the hypothalamo-hypophyseal system might all involve the participation of humoral factors, to which class some of the substances currently classed as transmitters might belong. 

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