Neuromuscular Transmission in the Longitudinal Layer of Somatic Muscle in the Earthworm

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Abstract
The properties of the miniature inhibitory junction potentials (M.I.J.P.) and the inhibitory junction potentials (I.J.P.) elicited by nerve stimulation were investigated in longitudinal muscle fibres of the earthworm. Histograms of the amplitudes (mean, 0·71 mV.) and the intervals (mean, 101 msec.) of the M.I.J.P. showed skew curves. The polarity of the M.I.J.P. was reversed at about —60 mV. When the external chloride was substituted by glutamate the M.I.J.P. disappeared as an external chloride concentration of 15-20 mM, and further reduction reversed their polarity. Picrotoxin blocked generation of the M.I.J.P. and the I.J.P. The cross-over point of the current-voltage relation curves, with and without presence of GABA, occurred at a membrane potential of − 54 mV. in potassium-free solution, and at − 56 mV. in potassium-excess solution. Iontophoretic application of GABA produced slow hyperpolarization. The equilibrium potential of the GABA-potential was about − 60 mV. During the time course of the GABA-potentials an increase in the membrane conductance was observed. Miniature excitatory junction potentials (M.E.J.P.) and excitatory junction potentials (E.J.P.) could be recorded from the longitudinal muscle, but the M.E.J.P. were rare. D-tubocurarine, but not atropine, completely blocked the M.E.J.P. and E.J.P. Prostigmine enhanced their amplitude and duration. The reversal potential level for the E.J.P. was about o mV. Sodium-free solution lowered the reversal potential level for the M.EJ.P. to −20 mV.