Neurobiology of Polyorchis. I. Function of effector systems.

The electrical correlates of activity in the effector systems responsible for swimming, crumpling and postural changes have been recorded in the anthomedusan Polyorchis penicillatus. Motor spikes (pre-swim pulses), that initiate swimming contractions, appear without delay at distant sites on the inner nerve-ring in unstimulated preparations. Levels of Mg++ anaesthesia which block the neuromuscular junctions between PSP giant neurons and swimming muscle do not affect PSP activity. Swimming muscle potentials can be recorded from subumbrella and velar muscle sheets using extra- and intracellular electrodes. These action potentials have a distinct plateau and are propagated in a myoid fashion. Resting potentials average -70 mV with spikes overshooting zero by some 62 mV. The effects of repetitive stimulation are described. Extracellular recordings indicate that neuronal pathways may play a major role in mediating crumpling, unlike many other species where epithelial pathways are more important. Endodermal spikes recorded intracellularly from the radial and ring canals have amplitudes of some 92 mV arising from resting potentials that average -55 mV. Repetitive stimulation causes a decrease in amplitude and increase in duration of epithelial action potentials. Tentacle length is controlled by a pacemaker system located in both nerve rings. The frequency of spikes (PTPs) generated by this system determines the length and tonus of tentacles. The neuromuscular junctions between the motor neurons and tentacle muscle are Mg++ sensitive and show facilitating properties.