The Control of Fast and Slow Muscle Contractions in the Siphonophore Stem

Abstract

The stem of the siphonophore Nanomia cara is the only coelenterate preparation so far found which allows intracellular recordings to be made both from muscles (Spencer, 1971) and nerves (Mackie, 1973); as such it offers unique possibilities for the analysis of neuro-muscular interactions at the cellular level. The stem muscle is also interesting as an example of an effector capable of responding by rapid, twitch-like contractions (both local and general) and also by slower, sustained contractions (Mackie, 196.+). Such muscles are found in all three classes of the Cnidaria (reviewed by Josephson, 1974) but the ways in which the two sorts of response (‘fast’ and’ slow’) are differentiated remains problematical. Only one histological type of muscle fibre seems to be involved; it is assumed that there are two excitation pathways. In sea anemones, the electrical correlates of fast contractions are known, but slow contractions can occur in the absence of recordable signals (McFarlane, 1973). In Nanomia, however, both fast and slow contractions have distinctive electrical correlates, the analysis of which is our present concern.