In isolated abdominal nerve cords of crayfish, the medial or lateral giant axons were stimulated at a position just rostral to the first abdominal ganglion. Recordings of the impulse sequences of the flexor inhibitor (FI) were made from the anterior five ganglia, three ganglia at a time.

In 20% of our preparations, one giant axon impulse caused one to four FI impulses in every abdominal third root. An equal number of FI impulses were usually produced by each abdominal ganglion for any given stimulation.

The earliest FI impulse was observed at the third root of the fourth ganglion. FI impulses occurred with increasing latencies rostrally and caudally from the fourth ganglion.

The FI responses to medial and lateral giant axons stimulation were essentially equivalent.

FI impulses were recorded from the rostral three abdominal ganglia, while the caudal ganglia were cut off one after another from the sixth to the third ganglion. Little change was noted until after the removal of the fourth ganglion, which usually caused all FI impulses to disappear.

From these experimental results, we propose a model of central mechanisms for FI excitation.

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