96 THE NERVE IMPULSE 



Exercise XVIII 



Gray 



Stimulating leads 



omitted.) Convenient settings to begin with 

 are: 



Indicator: 



"5 volts/division" amplification; 

 Waveform generator: 



"recurrent operating mode," 



100 msec "waveform duration"; 

 Pulse generator: 



0.3 "pulse delay," 



10 msec "pulse width," 



"pulse amplitude." 



Turn the instruments on and allow a few min- 

 utes for them to warm up. Focus, center, and 

 brighten the beam if necessary. With the "wave- 

 form duration" dial of the waveform generator, 

 alter the rate of horizontal sweeping of the 

 electron beam. A setting of 100 msec means that 

 it takes 100 msec (0.1 sec) for the electron beam 

 to complete a sweep. Then, with the wave dura- 

 tion set at 100 msec, very slowly turn up the 

 pulse amplitude of the pulse generator, and ob- 

 serve the square wave deflection on the oscillo- 

 scope screen. (Check that the pulse ampHtude 

 you are putting in is giving a deflection that 

 agrees with the indicator amplifier.) Now vary 

 all the controls mentioned above, and become 



' familiar with them. The only caution is not to 

 put so much pulse amplitude voltage into the 

 indicator that the beam is deflected off the 

 screen; this might cause damage. 



NERVE PREPARATION 



After pithing the frog, strip the skin off one 

 leg. Lay the animal on the dissecting pan, belly 

 down, and expose the sciatic nerve. With 

 glass needles, free as much of the nerve as you 

 can (about 4 cm) from the surrounding tissues, 

 being careful not to stretch or injure it. The more 

 nerve you expose, the better for your experiment. 

 Keep the nerve wet with Ringer solution 

 throughout the dissection. To expose the upper 

 part of the nerve, you will have to remove the 

 overlying pelvic girdle. When the nerve is freed 

 from the surrounding tissues, tie a short length 

 of thread to each end; measure the distance be- 

 tween the threads. Cut beyond the threads, and 

 remove the nerve. Keep track of which end is 

 proximal and which distal. 



Mount the nerve in the nerve chamber so 

 that each end passes underneath an electrode, 

 while the middle section weaves over and under 

 the intervening electrodes. (See diagram.) The 



