Abstract
The morphology, cable electric properties and synaptic responses of skeletal motor units in fourth-instar Heliothis virescens are described. The ventral internal lateral (VIL 1) muscle has an input impedance of approx. 150 kΩ and a length constant that is 60% of the mean fibre length. Significant electrotonic coupling, indicated by a coupling coefficient of approx. 0.4, is evident between VIL 1 in adjacent segments. Each VIL 1 is innervated by a single “fast” excitatory motor neurone which generates a composite neuromuscular response consisting of an excitatory junctional potential (EJP) and overshooting action potential. The neuromuscular response is rapidly desensitized when exposed to bath-applied glutamate and related analogues. The order of effectiveness is quisqualate > glutamate > aspartate. The EJP amplitude is reduced and the action potential abolished in low calcium (0.75 mM) saline. Quantum content measured under normal conditions is approx. 17, but drops to 3 when extracellular calcium is reduced to 0.75 mM. MEJC durations are unusually long in physiological saline and are further prolonged to tens of milliseconds when extracellular calcium is lowered, suggesting a pronounced influence of calcium on transmitter-activated channel kinetics.
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