[The multiplicity of neurotransmitters: the functional significance].

The generally accepted synaptic concept is an idealization based on two major assumptions essential for a system of neurone to be orderly organized: firstly, specificity of anatomical connections between neurones is assumed, and, secondly, it is postulated that the signal is transmitted via isolated compartment of extracellular space, the synaptic cleft. In the framework of this conceptual model, systems can be built using a single sort of signal molecules, and knowledge on diversity of neurotransmitters remains unconceptualized. To provide that the signal is properly delivered to a due address, neurones can however equally utilize the chemical mode of addressing. According to an alternative conceptual model proposed by the author, orderliness of a neuronal system is based on plurality of signal molecules. To describe the idealized elementary unit of heterochemical integration, the term "Heteron" is introduced. Heteron is defined as the network of individually specific neurones differing in their respective transmitters. It is postulated that (i) the transmitter situation is changed evenly throughout the heteron and (ii) responses of sensitive targets to a specific transmitter situation are composed into a well co-ordinated whole. The functional repertory of a heteron containing n neurones (i.e. n transmitters) will thus include at least n integrated states. The conceptual synapse corresponds to a theoretical limit of the heteron (when n = 1). It is suggested that primitive, simpler nervous systems roughly correspond to a single heteron while evolved brains and cords include multiple units of heterochemical integration.