Effect of the tail ganglion on swimming activity in the leech.

In the medicinal leech, Hirudo medicinalis, isolated segmental nerve cords are capable of generating swimming activity. The role played by the head and tail ganglia in regulating the expression of swimming activity by the segmental nerve cord was evaluated by comparing swimming activity in nerve cord preparations with and without the head and tail ganglia attached. Several swim properties were examined, including length of induced swim episodes, ability to initiate swim episodes, swim cycle period, and phase. We found that, in general, the presence of the tail ganglion attached to isolated nerve cords countered the effects produced by the head ganglion on swimming activity. Moreover, we observed that the tail ganglion itself provides excitatory drive to the swim generating system. Thus, the inputs from the head and tail ganglia influence significantly the expression of swimming activity.