Central circuitry in the jellyfish Aglantha. I: The relay system.

1. The relay system is an interneuronal pathway in the margin of the jellyfish Aglantha digitale. It excites a second interneuronal pathway, the carrier system, and is itself excited by pacemaker neurones concerned with slow swimming. It also excites a slow conduction pathway in the tentacles causing graded, tonic contractions of all the tentacles during slow swimming. 2. The pacemakers, the carrier system and the relay system all contribute to the production of excitatory postsynaptic potentials (EPSPs) in a giant axon that runs in the outer nerve ring (ring giant axon). These EPSPs may cause the latter to spike during slow swimming. If it does so, it will fire tentacle giant axons, producing twitch contractions of the tentacles. Such contractions probably help to contract the tentacles rapidly at the start of slow swimming. This is an unusual case of a giant axon that normally mediates escape behaviour being appropriated for use during a non-escape activity. 3. The relay system can conduct impulses on its own but their conduction velocity is greatly increased when preceded by either pacemaker or ring giant spikes. This phenomenon, termed the 'piggyback effect', may be due to extracellular field effects rather than to actions mediated by chemical or electrical synapses. 4. Recordings from the epithelial cells that ensheath the ring giant and outer nerve ring neurones show miniature synaptic potentials and other events that seem to reflect events in the nervous system, but no functions can be assigned to them. 5. There is no obvious counterpart to the relay system in medusae lacking escape circuitry.