GIANT NERVE FIBER SYSTEM OF THE CRAYFISH. A CONTRIBUTION TO COMPARATIVE PHYSIOLOGY OF SYNAPSE

Abstract

The four giant fibers in the central nervous system of the crayfish, Cambarus clarkii, were isolated in the oesophageal commissures and stimulated. The same peripheral mechanisms, namely those involved in the flight reflex, were brought into action in every case. The two medial giants, measuring 200-250 [mu] in the commissures, conduct at a rate of 15-20 m./sec. They are independent of each other and behave like peripheral nerve fibers. The segmented lateral fibers measure 70-90 [mu]. in the commissures; in the abdominal cord they are, like the medial giants, about 150 [mu]. in diam. Their overall conduction speed is 10-15 m./sec. Transmission from one segment to the next of the same fiber is accomplished within 0.1 msec, though each segment is completely separated from the next by a membrane. The lateral giants are interconnected in each ganglion posterior to the 4th thoracic, the crossing time from one to the other exhibiting a delay of [plus or minus]0.5 msec. By cutting the lateral giants between different ganglia, the impulse is delayed by repeated crossing. Nerve fibers in the first and third root of the abdominal ganglia are, in fresh preparations, excited by a single impulse in any of the 4 giants. Blocking of this transmission occurs with high frequencies of stimulation. This is not due to lengthening of the refractory period, since stimulation with double shocks with the same frequency results in activation of the root by each 2d impulse. In aged prepns., lengthening of the refractory period occurs, a 2d giant fiber impulse failing to elicit a root potential. Whereas the connections of the different elements of the lateral giants conduct with equal facility in both directions, the transmission from the giants to the roots is one way. It is concluded that transmission between different nervous elements may vary in "synaptical" proper-ties.