88 INVERTEBRATE PHYSIOLOGY 



gation of the somewhat similar though larger responses of some fibers in 

 Crustacea, and as occurs always in ordinary vertebrate skeletal muscle. In 

 vertebrate muscle the spike response may reach a height of 70 mV, three 

 times that of the locust. 



Further information on insect transmission can be obtained by studying 

 the effects of potassium, calcium, and magnesium on the process. Raising 

 the magnesium or lowering the calcium in the bathing fluid has an effect 

 similar to cooling, in that the step in the rising phase of the action potential 

 which is probably due to a delay in the start of the spike responses is 

 markedly increased. This effect is partly due to a reduction in the magni- 

 tude of the end-plate potential. If the magnesium is raised or the calcium 

 lowered sufficiently, only the pure end-plate potential remains since it 

 becomes too small to evoke any spike response. The pure end-plate po- 

 tentials show considerable facilitation and summation. Potassium in excess 

 lowers the magnitudes of both the end-plate potentials and the secondary 

 responses. Its effect is partly indirect, due to the reduction of the resting 

 potential. 



A typical locust response from a fiber with a 60 mV resting potential has 

 an overshoot of 13 mV. The total action potential of 7Z mV is composed 

 of 48 mV junctional response (peak height) and 25 mV spike response. 

 A similar fiber not showing an overshoot might have an action potential of 

 58 mV. This would be composed of about 40 mV junctional response and 

 18 mV spike response. The presence or absence of the overshoot is not an 

 important matter functionally, for fibers without an overshoot undoubtedly 

 twitch quite vigorously. When treatment with high magnesium, high 

 potassium, or calcium-free saline has reduced the response to a pure end- 

 plate potential of no more than 20 mV, there is still a small twitch contrac- 

 tion. On the other hand, a single muscle fiber shows a brisk local twitch 

 when current sufficient to evoke only a local response is passed. Evidently 

 either the end-plate potential or the local spike can activate the contractile 

 mechanism. It has not yet proved possible to affect the secondary response 

 experimentally without at the same time affecting the magnitude of the 

 end-plate potential, so an exact estimate of the part played by this response 

 in eliciting contraction is not possible at present. It seems probable that in 

 arthropods generally the link between membrane depolarization and ten- 

 sion is direct and progressive. A wide range of tension can be produced in 

 the same fiber, provided its membrane potential is lowered in graded steps. 

 Even the large "fast"-fiber action potentials, which are all-or-nothing 

 events, may nevertheless be required in quick trains before the muscle con- 

 tracts fully. Otherwise it is difficult to account for the high tetanus/twitch 

 ratio encountered even in short-fibered insect muscles where there is vir- 

 tually no connective tissue and the tendons are inelastic. 



