202 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



The discussion will be limited to the experiments on isolated single fiber 

 preparations since the results obtained with whole nerve trunks are often 

 very difficult to interpret unless the behavior of the constitutent fibers is 

 clearly understood. The discrepancy between Gasser's results (3) and the 

 results of Schoepfle and Erlanger (5) on the temperature dependence of the 

 amplitude of the action potential is an example of this kind of difficulty. 

 In this article attempts are made to compare the results obtained with 

 the squid axon with those obtained with the amphibian nerve fiber. In 

 general, experiments on the squid axon are easier and the results obtained 

 are more direct than the corresponding experiments and results on the 

 frog myelinated nerve fiber. Since, however, the membrane constants, 

 cable properties and the potential-impedance relationship of the verte- 

 brate nerve fiber are very different from those of the squid axon, it is not 

 always safe to infer the behavior of the frog nerve fiber from experiments 

 on the squid axon. An effort has been made, therefore, to obtain direct 

 experimental data from both of these types of nerve fibers. A portion of 

 our unpublished work described in this article will be discussed more 

 fully in subsequent papers. 



METHODS USED IN EXPERIMENTS ON MYELINATED NERVE FIBERS 



Isolation of Single Nerve Fibers. The nerve innervating the semi- 

 tendinosus or the sartorius muscle of the toad {Bufo marinus) or the bull- 

 frog {Rana catesbiana) was used. A segment 1-5 mm long of the nerve 

 was desheathed and all except one of the fibers in the desheathed region 

 were cut. The fiber selected was, as a rule, motor, and 10-15 fx in diameter. 

 The dissecting instruments were a pair of needles mounted on wooden 

 holders. The entire operation was carried out under dark field illumina- 

 tion (for greater detail, cf. 15). 



Measurement of Action Current. Three different methods w^ere em- 

 ployed for measuring the action current of the myelinated nerve fiber. In 

 the hridge-inmlator 7nethod (fig. lA), the isolated single fiber preparation 

 was mounted on a platform (bridge-insulator) made of two glass plates 

 fixed side by side on a Incite stage. The gap between the two plates meas- 

 ured 0.1-0.2 mm. The segment of the nerve on one side of the dissected 

 region was placed on one glass plate and the portion on the other side was 

 mounted on the other plate. In this manner a thickly myelinated internode 

 of the isolated fiber 'bridged' the gap between the two plates. On either 

 side of the gap the isolated fiber preparation was bathed in a pool of 

 Ringer's solution. An electrode of the Ag-AgCl-Ringer (agar) type was 

 immersed in each pool of Ringer. One of the electrodes was grounded either 

 directly or through a 1000-ohm resistor and the other was led to the input 

 of a preamplifier. A 300-kilohm resistor was connected across the terminals 



