[The biological substrate for the generation of behavioral acts]

There is a growing recognition that Central Pattern Generators (CPGs) play the fundamental role in the production of motor commands. A CPG is defined as a set of neurons involved in joint production of patterned sequenced output. It is generally believed that the sequence arises from the pattern of synaptic connections among these neurons. Alternatively, it was suggested (Sakharov, 1985) that the orderly organization of pattern-generating units might be due to the chemical diversity of their constituent neurons representing different phenotypes. Recent researches demonstrate that a given CPG can be reorganized to produce a different pattern of output activity. This multifunctionality can hardly be explained in terms of the anatomical (= synaptic) organization. The heterochemical approach appears to be more flexible. We hypothesize that dynamic fluctuations of the local extracellular milieu determine the physiological properties and receptor profile of individual neurons, and thus the self-organisation of the latter into a CPG.