98 THE NERVE IMPULSE 



Exercise XVIII 



ACTION POTENTIALS OF HUMAN 

 NERVE AND MUSCLE 



It is possible to record indirectly the nerve and 

 muscle action potentials in your own hand by 

 means of remote electrodes when the hand is in 

 a nonconducting medium such as distilled water. 

 For this experiment, fill a porcelain pan with dis- 

 tilled water and suspend about 6 inches of the 

 two heavy copper wire electrodes in the water at 

 the two ends of the pan with clamps. Make sure 

 the exposed ends of the electrodes are entirely 

 under the surface of the water. Connect the 

 leads from the electrodes to the binding post 

 terminals on the side of the copper cage, and 

 connect the input leads from the amplifier to 

 these terminals. Use the following settings: 



Amplification = 1000 



Indicator amplifier = 0.05 volts/division 



Waveform duration = 1000 msec 



Hold your hand relaxed in the water. When 

 the baseline has settled down to a steady trace, 

 clench your fist. Individual action potentials of 

 200 to 300 ^v should be seen. Remember that in 

 the frog nerve, you were stimulating all the 

 nerve fibers simultaneously, so that it appeared 

 as if you were eliciting just one large action 



potential. Here the action potentials in the vari- 

 ous fibers (both nerve and muscle) are staggered 

 in time, and consequently appear as smaller 

 single spikes. To see the single action potentials 

 more clearly, increase the sweep speed by de- 

 creasing the waveform duration to 100 msec. 

 Note that the action potentials continue as long 

 as the fist is clenched. One nerve impulse serves 

 only to twitch a muscle. For a muscle to remain 

 contracted, it must receive a continuous train of 

 impulses. Alternately clench and relax your fist 

 as quickly as you can ; note the bursts of impulses 

 that excite each tightening of the fist. 



From this preparation, you can also learn 

 something about the anatomy of the hand. 

 Many of the movements of the hand, such as 

 flexion and extension of the fingers are performed 

 by muscles in the forearm which connect to the 

 fingers by tendons. Flex your fingers under the 

 water and note that you see no action potentials. 

 Try spreading your fingers sideways, and bringing 

 them together again. What do you conclude? 

 Move your thumb and hand in as many ways as 

 possible and try to decide which movements are 

 performed by the muscles in the hand and which 

 by the muscles in the forearm. Check your con- 

 clusions in an anatomy atlas. 



EQUIPMENT 



Per 2-4 students 



electronic recording and stimulating setups 



frog 



dissecting pan 



goose-neck lamp 



thread (1 spool) 



plasticene 



beaker (400 ml) 



medicine dropper 



plastic ruler 



nerve chamber 



2 glass dissecting needles 



mineral oil (to fill nerve chamber) 



frog ringers (25 ml) 



porcelain pan (approx. 12" X 8" X 8") 



2 heavy copper electrodes (heavy copper wire 

 flattened about 6" at one end and otherwise insu- 

 lated works very well) 



2 electrode clamps 

 distilled water 



