Neural circuits for generating rhythmic movements.

Inasmuch as the identified neural circuits discussed in this review pertain only to the nervous systems of two invertebrate species, one may ask whether or not these findings are generally applicable to central nervous oscillators that generate rhythmic movements in animals of other species and phyla, particularly in the vertebrates. This question is not easy to answer at this time, because detailed cellular network analyses thus far have been possible only in a very few neurophysiologically favorable preparations, such as those presented by the cardiac and stomatogastric ganglia of the lobster and the segmental ganglion of the leech. Nevertheless it is significant that the mechanisms according to which these invertebrate circuits are now thought to generate their oscillations--endogenous rhythmic polarization, reciprocal inhibition, and recurrent cyclic inhibition--were all first proposed to account for generation of rhythmic movements in vertebrate animals (7-9, 51, 71, 79). Moreover, the pattern of motor neuron activity in rhythmic movements of vertebrates is not necessarily more complex than the corresponding pattern in analogous movements of invertebrates. Therefore, the very much greater number of neurons in the central nervous system of vertebrates does not necessarily imply a greater complexity of the central oscillators that generate their rhythmic movements; it may only place greater obstacles in the way of identifying the underlying neuronal circuitry. In any case, it is worthy of note that the current list of fundamentally different and theoretically plausible types of neuronal oscillators is not only quite short but also of long standing. Thus, on these grounds, it seems reasonable to expect that the identified circuits discussed here will prove to be of general applicability to the generation of rhythmic movements in the whole animal kingdom.