Giant multimodal heart motoneurons of Achatina fulica: a new cardioregulatory input in pulmonates

The regulation of the heartbeat by the two largest neurons, d-VLN and d-RPL N, on the dorsal surface of visceral and right parietal ganglia of Giant Af rican snail, Achatina fulica, was examined. Using the new method of animal preparation, for the first time, discrete biphasic inhibitory-excitatory ju nction potentials (I-EJPs) in the heart and several muscles of the visceral sac were recorded. The duration of hyperpolarizing phase (H-phase) of biph asic I-EJPs was 269 +/- 5.6 ms (n = 5), which is 2-3 times less than that o f the cholinergic inhibitory JPs (682 +/- 68.5 ms, n = 5). The H-phase of I -EJPs was not altered by the application of atropine, picrotoxine. succinyl cholinchloride, D-tubocurarine and tetraethylammonium or substitution of Cl - ions. Even the low-frequency neuronal discharges (1-2 imp/s) evoked signi ficant facilitation and potentiation of the H-phase. Between the multimodal neurons d-VLN/d-RPLN and mantle or visceral organs there is evidence of di rect synaptic connections. These neurons were found to have no axonal branc hes in the intestinal nerve as once suspected but reach the heart through s everal other nerves. New giant heart motoneurons do not interact with previ ously identified cardioregulatory neurons. (C) 2001 Elsevier Science Inc. A ll rights reserved.