SYNAPTIC FACILITATION IN THE CRAYFISH

Abstract

By leading off from motor fibers of the 3d root of an abdominal ganglion in Cambarus clarkii, it is shown that mutual facilitation exists between locally separated synapses on a motoneurone. Four preganglionic pathways have been stimulated; the 3 isolated giant fibers of the cord and the homo-lateral first root of the same ganglion from which the 3d root is used. All 4 pathways can facilitate each other as shown with prepns. in which 2 impulses are necessary to produce a root potential. With most combinations no summation occurs when 2 impulses arrive at 2 synapses of the postganglionic fiber simultaneously or at short intervals. This period is called the "inert" period and its length is directly dependent on the distance between the 2 synapses stimulated. It varies from 2 to 5 msec. Transmission occurs at the 2d synapse only after an increase in excitability has spread to this place from the synapse first stimulated. The increase in excitability is shorter and arrives later the further the 2 synapses used for summation are separated. In certain combinations summation occurs also when the impulses arrive simultaneously.