Rhythmical synaptic control of axonal conduction in a lobster motor neuron.

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Abstract
The main dilator motor neuron of the esophageal nervous system (ODI), which has a spike-initiating zone in 3 centers, sends an axon into each output nerve of the symmetrical commissural ganglia. The somatofugal spikes generated by the esophageal spike-initiating zone of OD1 (located in the esophageal ganglion) are rhythmically blocked in the commissural ganglia and regularly fail in the output nerves. The conduction block does not result from the repetitive firing of the OD1 axons, as spikes can propagate without block for several seconds at frequencies up to 100 Hz when synaptic activity is blocked in the commissural ganglia. It also does not depend on the discharge of the commissural spike-initiating zones of OD1, and does not act as a low-pass filter. A slight influence of the geometry of the axons of OD1 is shown by the fact that spikes pass through the site of block more easily in the orthodromic direction than in the antidromic one, and that for orthodromic spikes the extent of block differs in the different output nerves of a given side. The conduction block in the commissural part of the axons of OD1 is essentially synaptically controlled by the oscillators that generate the esophageal rhythm. The esophageal oscillators exert a true control on impulse conduction in OD1 axons. They not only regularly impede the spike propagation, but also alternately enhance it. Several important functional implications are discussed; among these the most fundamental is that a rhythm generator can induce a rhythmical muscular activity either by phasically stimulating a silent motor neuron or by rhythmically blocking the axonal conduction in a tonically firing motor neuron.