Proceedings of The Physiological Society

University of Bristol (2001) J Physiol 536P, S102

Communications

Two escape swim synaptic inputs in the jellyfish Aglantha digitale

R.W. Meech and G.O. Mackie

Department of Physiology, University Walk, Bristol BS8 1TD, UK and Biology Department, University of Victoria, Victoria, British Columbia, Canada, V8W 2Y2


Following Gladfelter's observation in 1973 that the hydrozoan jellyfish Aglantha digitale has two swimming modes, observations in the wild show that it can escape contact with crab larvae, amphipods and copepods (Mackie & Mills, 1983). Fast jet-propelled escape swimming can be elicited by stimuli at either the base or the apex of the animal. In one pathway, which originates at the vibration-sensitive receptors at the base of the animal (Arkett et al. 1988), signals are conducted around the margin by a ring giant axon within the outer nerve ring (Roberts & Mackie, 1980). Excitation spreads to the contractile myoepithelium via carrier neurons and a series of eight giant motor axons that run inside the bell of the animal. A second pathway carries stimuli from the internal surface of the bell. Impulses are propagated down the motor axons themselves and spread through fine rootlet neurons (Mackie & Meech, 2000).

Experiments were carried out on specimens of Aglantha pinned out in seawater containing 90 mM Mg2+ to block contraction. All intracellular recordings were from motor axons at a position within about 150 µm of the nerve ring. The following is a summary of data obtained from 20 motor axons. Cutting through the rootlet system, which runs in the inner nerve ring, isolated the ring giant input. A stimulating electrode near the margin was used to elicit a sharply rising postsynaptic potential (PSP) in the motor axon and a bridge circuit was used to change the membrane potential in the vicinity of the recording electrode. The peak amplitude of the PSP depended on the membrane potential and extrapolated to zero at 0 mV, which suggested that the membrane permeability increase during PSP was non-selective.

To isolate the rootlet input the stimulating electrode was placed on a motor axon adjacent to the recording site. When the recording site was hyperpolarised a fast rising PSP became evident; it decreased in amplitude with further hyperpolarisation. This rootlet PSP may represent an attenuated action potential spread by electrical synapses between the rootlet neurons and the motor axon.We acknowledge the support of NSERC of Canada and The Wellcome Trust. We thank the Director and staff of Friday Harbor Marine Station for excellent facilities.

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    Mackie, G.O. & Meech, R.W. (2000). J. Exp. Biol. 203, 1797-1807.

    Mackie, G.O. & Mills, C.E. (1983). Can J. Fish. Aquat. Sci. 40, 763-776.

    Roberts, A. & Mackie, G.O. (1980). J. Exp. Biol. 84, 303-318.

Where applicable, experiments conform with Society ethical requirements