Plateau potentials drive axonal impulse bursts in insect motoneurons

Abstract

Intracellular recordings were made from the soma of the `fast' coxal depressor motoneuron of the hind leg of the cockroach Periplaneta americana under current- and voltage-clamp. In response to depolarizing current injection, this neuron was able to generate plateau potentials that were able to far outlive the duration of the applied depolarization. These events constitute an inherent membrane property of the neuron because they could be evoked in somata that had been surgically isolated from other parts of the neuron (the soma is devoid of synaptic contacts); these experiments also showed that the soma of this neuron can participate in the generation of plateau potentials. The amplitude of these events at the spike-initiating zone was sufficient to evoke action potentials that correlated 1:1 with axonal impulses recorded extracellularly from the axon of the neuron. The ability to generate plateau potentials is associated with a region of negative slope resistance in the current--voltage relation of the neuron. Ion substitution experiments showed that the plateau potentials recorded from this neuron are calcium dependent. Our observations show for the first time that an insect motoneuron can produce plateau potentials. Accordingly, bistable membrane properties may play a role in shaping motor output in insects.