86 INVERTEBRATE PHYSIOLOGY 



the step is not only very much more marked but the final hump of the action 

 potential, the part occurring after the step, is greatly reduced. At about 

 8° C it is completely absent. There remains a potential with a smooth, un- 

 stepped rise and an exponential decay. This potential is similar in shape to 

 the vertebrate end-plate potential and may be similarly designated in in- 

 sects. There is, however, a marked difference between the insect and the 

 vertebrate muscle. In the former the end-plate potential is not just confined 

 to a single site as in vertebrate muscle, and the latency of the response does 

 not differ in different parts of the fiber. These observations reflect the 

 nature of the innervation, i.e., the distributed end plates of the insect 

 muscle ; and the logical interpretation is that in the insects end-plate po- 

 tentials analogous to the vertebrate end-plate potentials are produced 

 nearly simultaneously in the several end-plate regions of the fiber. The 

 area involved is so large that depolarization occurs synchronously over the 

 whole surface of the fiber. 



If the membrane potential is raised or lowered by passing polarizing or 

 depolarizing current across the muscle-fiber membrane through a second 

 intracellular electrode inserted in the same fiber, the magnitude of the 

 end-plate potential is correspondingly raised or lowered. There is a simple 

 linear relationship between end-plate potential and resting potential, the 

 line passing through the origin. This evidence strongly suggests that the 

 end-plate potential is due to the formation of a temporary short circuit of 

 the resting membrane, probably produced by a chemical substance released 

 under the end plate and increasing the permeability of the membrane to 

 several ions (cf. Fatt and Katz, 1951 ) . The effect of reducing the tempera- 

 ture is to show that the normal response has two components, a primary 

 junctional response or end-plate potential and a secondary, spike-like re- 

 sponse. The end-plate potential is probably due to a partial short circuit 

 of the membrane in the several areas on each muscle fiber in the immediate 

 vicinity of the end plates. If no further response occurred there would be 

 a simple exponential decay of the end-plate potential after the peak of the 

 transmitter action. Instead the depolarizing action affects the properties 

 of the resting membrane. This leads to a transient additional change in po- 

 tential which sums with the end-plate potential, thus generating the 

 secondary response. Typical responses from Calliphora, Periplaneta, and 

 Schist ocerca muscle fibers are illustrated in Fig. 5. The time courses of 

 pure end-plate potentials are indicated by dotted lines. 



The secondary responses can be studied independently of the end-plate 

 potentials by passing depolarizing current across the muscle-fiber mem- 

 brane. They appear at and above a critical level and consist either of 

 oscillatory responses of small amplitude with frequencies from a few to 

 over 100 per second, or spike-like responses reaching a height up to 25 mV 



