Tritonia Swimming

Abstract

Over the past several decades, the functional organization of motor systems, and in particular those controlling rhythmic movements, has been viewed in the context of two conceptual hypotheses. The observation that many rhythmic motor patterns and behaviors could persist in the absence of phasic sensory feedback led to the idea of a central pattern generator (CPG). The CPG is envisioned as a group of central neurons that generates a sequence of temporally and spatially coordinated activity. It is now clear that most, if not all rhymic behaviors have as their basis a central pattern generator (Delcomyn, 1980). The second major hypothesis has been the concept of the “command” neuron or system. This idea is founded in the work of Wiersma and Ikeda (1964), who observed that stimulation of certain neurons in the crayfish could elicit rhythmic movements of the swimmerets. Despite attempts to define a command neuron by explicit criteria (Kupfermann and Weiss, 1978), the term is most commonly used to describe neurons that, when active, will “turn on” some recognizable, coordinated behavior. The overall organization of rhythmic motor systems can be represented as a series of “black boxes” representing the command and CPG function (Fig. 1) (Grillner, 1977). In this scheme, initiating stimuli would activate an appropriate command neuron or set of command neurons that in turn would activate the central pattern generator for a particular rhythmic behavior.