Coupled motoneurones are part of the crayfish swimmeret central oscillator [25]

SWIMMERETS in the crayfish are the paired ventral abdominal appendages which beat in a metachronal rhythm during behaviours such as swimming and burrow ventilation. Each swimmeret is driven by alternating bursts of impulses in antagonistic power- and return-stroke motoneurones. Although proprioceptive input affects the detailed structure of the rhythm, the basic motor programme can be elicited from the deafferented abdominal central nervous system (CNS) by tonic stimulation of single 'command fibres' in the ventral nerve cord ^1-3. Each swimmeret is controlled by a hemiganglion capable of generating the rhythm in isolation, although normally the most posterior swimmeret ganglion (the 5th) acts as a pacemaker, regulating the activity of the other ganglia through a series of coordinating fibres located in the medial region of the ventral nerve cord⁴. I report here the preliminary results of an investigation into the structure of the central neural oscillator controlling swimmeret movements in crayfish. The results suggest that motoneurones in this system do not act merely as output elements which passively relay the activity of an oscillator formed from pre-motor neurones within the CNS, but that the motoneurones themselves are an integral part of the pattern generating circuits. There is both excitatory and inhibitory central coupling between motoneurones mediated either by electrical or graded chemical synapses. Furthermore, the activity of some motoneurones can directly influence the intensity, period, and phase of the oscillation.