FREDERICK CRESCITELLI 



183 



peratures, 3.9-16.0 minutes with u mean of 9.7 minutes. At the low tem- 

 peratures the time to 50% block averaged 2.4 minutes and the range was 

 0.6-5.0 minutes. Recovery of conduction through the treated segment 

 after the addition of Ringer's solution was faster at the normal tempera- 

 tures. In the same six experiments (table 1 ) recovery of the A spike to the 

 50% level required an average time of 40 seconds at the normal tempera- 

 ture and the range was 12-66 seconds. At the low temperatures the av- 

 erage recovery time was 194 seconds and the range was 66-456 seconds. 



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Fig. 5. Conduction block of A fibers produced by excess potassium, i.e. 15.3 niM KCl. 

 Height of A wave (in percentage) plotted against time (min.). The four steps were 

 carried out consecutively as numbered using the .same desheathed nerve preparation. 

 Temperatures were as indicated on graph and washing out with Ringer's solution is 

 indicated at points shown by R. 



A typical experiment and the manner in which it was carried out are 

 illustrated in figure 5. Curve 1 depicts the course of lilock and recovery of 

 the A potential at 5°C. The test solution, containing all the constituents 

 of Ringer's fluid except for KGl which was at 8.5 times the normal concen- 

 tration, was added at zero time to a 20 mm segment of the nerve between 

 the central and peripheral portions of the nerve trunk. The central end 

 was stimulated using a shock at 14 times the strength of the stimulus 

 needed to elicit maximal A activity. Complete block occurred in 3 minutes 

 and at 7 minutes Ringer's solution at 5°C was added to the nerve segment 

 in place of the test fluid. Recovery to well over the 90% level followed. The 



