PRESYNAPTIC INHIBITION AT THE 

 NEUROMUSCULAR JUNCTION IN CRAYFISH 



Josef Dudel and Stephen W. Kuffler 



Neurophysiology Laboratory, Department of Pharmacology, Harvard Medical School, 



Boston, Massachusetts 



In crayfish neuromuscular junctions the inhibitory transmitter acts by 

 increasing the permeabihty of the muscle fiber to certain ions while the 

 membrane potential remains near the resting level (Fatt and Katz, 1953). 

 This postsynaptic inhibitory mechanism seems widespread in different species. 

 In recent experiments on the abductor muscle of the dactyl of the crayfish 

 Orconectes virilis a new second mechanism was found. If inliibitory impulses 

 are timed to arrive at the neuromuscular junction from 1 to 6 msec before 

 the excitatory impulses they reduce the excitatory junctional potentials 

 (e.j.p.'s). Under specific conditions it was shown that this e.j.p. reduction 

 could not be brought about solely by a postsynaptic conductance increase 

 and therefore one had to postulate an additional mechanism (Dudel and 

 Kuffler, 1960). 



In close analogy with neuromuscular transmission in vertebrates (del 

 Castillo and Katz, 1956) it was shown in the crayfish junction that the e.j.p.'s 

 are made up of quantal units, which are of the same size as the spontaneous 

 excitatory miniature potentials. These units represent a quantal release of 

 transmitter from the excitatory nerve terminals. The quantum content was 

 analyzed by recording through extracellular microelectrodes from single 

 junctional spots which are distributed over the muscle surface. 



In Fig. 1a the upper sweep shows e.j.p.'s recorded with an intracellular 

 electrode at a stimulation rate of 1/sec. The lower sweep presents the 

 simultaneous activity at a single junctional area recorded with an extracellular 

 microelectrode. Series of several hundred extracellular e.j.p.'s were analyzed 

 statistically and the size of the quanta and the probabihty of release of the 

 quanta were determined. For instance, the last extracellular e.j.p. in Fig. lA 

 was composed of two quanta. At times the single junction fails to release any 

 quanta (arrow). If, as in Fig. 1b, inhibitory impulses preceded the excitatory 

 ones by 2 msec, both the intra- and extracellular e.j.p.'s were reduced. At the 

 same time the number of failures of transmission was increased at the single 

 junctional area (three arrows). In Fig. 2 the same events are illustrated as in 

 Fig. 1, recorded with a fast sweep. In Fig. 2a the extracellular e.j.p. has a 



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