Exercise XVIII 



THE NERVE IMPULSE 97 



proximal end of the nerve should be in contact 

 with the three closely spaced electrodes used for 

 stimulation. Extend the nerve to its in situ 

 length in the chamber; do not overstretch it. Fix 

 the ends of the threads to the edge of the cham- 

 ber with plasticene. Pour mineral oil into the 

 chamber to cover the nerve. The oil insulates 

 the nerve and prevents it from drying out. 



EXPERIMENTAL PROCEDURE 



Note: Before connecting any leads to the nerve 

 chamber, make sure that the pulse amplitude 

 dial on the pulse generator is turned to 0. If you 

 accidentally put high voltages through the nerve, 

 it will be ruined very quickly. 



Two leads (gray and red) from the pulse gen- 

 erator should be connected to the first two, 

 closely spaced, stimulating electrodes; and two 

 leads (red and black) from the first two recording 

 electrodes to the input plug of the amplifier. The 

 gray ground lead should be connected to the 

 third, closely spaced electrode. (See diagram.) 

 Use the following settings : 



Preamplifier amplification = 100 



Indicator amplifier = 0.5 volts/division 



Waveform duration = 10 msec 



Pulse width =0.1 msec 



Pulse delay = 0.3 



Pulse amplitude = 



Pulse = negative 



Now very slowly increase the pulse amplitude. 

 Note first the stimulus artifact, and then the 

 appearance of the nerve action potential. In- 

 crease the pulse amplitude until the nerve im- 

 pulse is maximal in height. 



The nerve impulse should appear as a di- 

 phasic wave. Why? What is its maximal volt- 

 age? What is its duration? Make a graph of 

 voltage vs. time. Calculate the latency of the 

 nerve impulse by measuring the distance from 

 the start of the stimulus artifact to the beginning 

 of the response (the stimulus artifact is con- 

 ducted down the outside of the nerve and is re- 

 corded instantaneously). Most of the latency is 

 occupied by the time it takes for the nerve im- 



pulse to travel from the stimulating electrode to 

 the first recording electrode. Estimate the speed 

 of conduction of the main group of sciatic nerve 

 fibers from the latency and the distance between 

 these electrodes. Express this velocity in meters 

 per second. 



Determine the minimal stimulus voltage 

 needed to evoke minimal (threshold) and maxi- 

 mal responses. Why can't you increase the 

 amplitude of the nerve potential indefinitely by 

 increasing the stimulus? How does the observed 

 grading of the response fit in with the all-or-none 

 law? Decrease the duration of the stimulus 

 (pulse width) and redetermine the voltages that 

 produce minimal and maximal responses. Try 

 several different pulse widths. Make a plot of 

 pulse duration vs. threshold voltage. What do 

 you conclude? 



Next, crush the distal end of the nerve with a 

 fine pair of metal forceps just before it passes 

 underneath the most distal electrode, and recon- 

 nect the recording leads to the two most distal 

 electrodes (one on each side of the crushed por- 

 tion). Observe that the nerve impulse is now 

 monophasic. Why? 



There are several classes of fibers in the frog 

 sciatic nerve which conduct impulses at different 

 rates. With monophasic recording you may 

 notice humps on the down sweep of the nerve im- 

 pulse. These represent the slower conducting 

 a2, ;S, or 7 fibers separating from the prominent 

 and most rapidly conducting ai fibers. To see 

 these humps clearly, set the "operating mode" to 

 manual, and trigger the impulses by hand at a 

 rate of about 1 per second. Try to estimate the 

 rates of conduction of each of the fiber classes. 

 See if the groups of fibers differ in threshold, by 

 determining the pulse amplitude needed to pro- 

 duce a maximal response in each group. 



Finally, determine the maximum frequency 

 with which the nerve can respond. This is done 

 by progressively decreasing the "waveform dura- 

 tion," so decreasing the intervals between 

 successive stimuli. At what frequency of stimu- 

 lation do the responses begin to decline in am- 

 plitude? What can you conclude concerning the 

 refractory period (see Katz's article) ? 



