146 INVERTEBRATE PHYSIOLOGY 



tained from muscle fibers show differences that make a classification neces- 

 sary. Instead of end-plate potentials the presumably purely local effects 

 around the nerve endings will be called junctional potentials as was done 

 by Kuffler and Gerard (1947) for the comparable potentials in the slow 

 muscle fibers of the frog. The secondary response which arises from these 

 potentials after they surpass a certain value will be called a spike. This will 

 not imply that the process is conducted over the whole membrane ; when 

 the latter takes place it will be called a conducted spike. The conducted 

 spike always shows, as far as known, an overshoot of the membrane po- 

 tential, whereas spikes may vary from just visible enhancements of the 

 junctional potentials to the maximum level with overshoot. Since there are 

 strong indications that the junctional potentials (but not the spikes) of the 

 different axons innervating the muscle fiber have different mechanical 

 results, which do not depend on the shape or size of the potential, they must 

 also be named with regard to the axon bringing them about. Therefore, in 

 a double motor innervated muscle it will be necessary to distinguish be- 

 tween "slow" junctional potential and "fast" junctional potential. 



The potentials of the contractile part of the muscle-stretch receptor 

 organs have been studied by Kuffler (1954) and Furshpan (1955), using 

 one or two nerve impulses only, which will presumably make the contribu- 

 tion of any "slow" motor axon, if present, negligible. These structures are 

 favorable for this type of work because they constitute thin isolated strands, 

 which we consider as single muscle fibers. They offer a clear picture of the 

 motor innervation, the motor fiber (s) running along the length of the 

 structure, giving off branches into it at many points (Alexandrowicz, 

 1951). Both Kuffler and Alexandrowicz are inclined to consider them as 

 consisting of bundles of muscle fibers rather than single units. However, 

 Furshpan, using two microelectrodes, failed to find any sign of high-re- 

 sistance membranes between them, indicating the absence of charged 

 membranes. The only exception found was when one electrode was in the 

 anterior muscular segment, the other in the posterior one, separated by the 

 intercalated region in which the sense cell has its endings. 



Using the anterior muscular section, Kuffler and Furshpan have both 

 observed spikes which were not conducted. Kuffler found that, when no 

 spikes were present at a given time, stretching the organ by pulling it to 

 one side would bring one about, but this spike was confined to the stretched 

 region. Furshpan, using two internal electrodes, found that one locus 

 might spike at a time that the other gave only a large junctional potential. 

 When a second impulse was delivered shortly afterwards, the other locus 

 would also spike. He could show that the speed of spread of the spike, 

 when spiking took place all along the structure, would be the same as that 

 of the junctional potential, and concluded that the nerve fiber plays the 



