Neuronal control of swimming in the medicinal leech

Summary

The muscle contraction rhythm and the impulse patterns of motor neurons producing that rhythm in the swimming leech were studied over a wide range of swim cycle periods. This cycle consists of a constant time sector lasting about 300 msec and a variable time sector whose changes in length are responsible for the variation in cycle period from about 400 to 2000 msec. The constant time sector consists of the contraction and relaxation of the dorsal and the ventral segmental longitudinal muscles. The variable time sector consists of preparing for the next contractile event. The constancy of the contractile component of the swimming cycle is accounted for by the relative period-independence of the impulse frequency during the bursts of the excitors which command the rhythmic contraction of the dorsal and the ventral longitudinal muscles. Since the impulse frequency of a longitudinal muscle inhibitor does increase as the period shortens and since the burst of this inhibitor occurs during the variable time sector of the cycle, the activity of the inhibitor seems to determine the timing of the next contractile event.

The impulse bursts of bilaterally homologous motor neurons are closely coordinated with respect to burst duration, spike number per burst and burst phase, in agreement with the earlier finding that the homologs are linked by electrotonic junctions. The delay between the bursts of the dorsal excitors and the ventral excitors increases with increasing period in a manner which indicates that about three fourths of the variable time sector and no more than one fourth of the constant time sector occur between the command of dorsal and of ventral longitudinal muscle contraction.

The rearward propagation of the swimming wave along the leech body is reflected in an antero-posterior sequence of bursts of the intersegmental homologs of a dorsal excitor. Periodic spike bursts matching the swimming rhythm can be recorded from the connective linking the ganglia of the cord. It is likely that these spike bursts represent the impulse traffic of axons responsible for the intersegmental coordination of the swimming rhythm. These studies show that the swimming rhythm of leeches and its neuronal control formally resemble three well-studied cases of rhythmic movements of arthropods, namely insect ventilation, decapod swimmeret beating and insect walking.