94 THE NERVE IMPULSE 



Exercise XVIII 



All this discussion has been in terms of a 

 single nerve fiber. Nowadays physiologists often 

 work with such single nerve fibers, either the 

 giant nerve fiber of the squid or with the very 

 much smaller nerve fibers of higher animals in- 

 cluding mammals. For the latter one needs to 

 use microelectrodes, which may be only 1/x in 

 diameter. 



You must remember that the nerves that you 

 see in animals and with which you will be work- 

 ing are bundles of many such nerve fibers. 

 Though each individual nerve fiber exhibits all- 

 or-nothing behavior, the nerve bundle does not, 

 since a strong stimulus may excite many nerve 

 fibers whereas a weaker stimulus may excite few. 

 Many other characteristics of the nerve impulse, 

 however, can be studied in such nerve bundles. 



In order to measure nerve impulses, very sensi- 

 tive devices must be used. At the present time 

 all such work is done with electronic amplifiers 

 and recorded with oscilloscopes. The expense 

 and complexity of such equipment has in the past 

 restricted neurophysiological experimentation to 

 relatively few places and relatively advanced 

 courses. This is a great pity, for the phenomena 

 are of the highest importance, generality, and 

 interest. The experiments we are about to de- 

 scribe involve the use of the equipment described 

 in Appendix B. It is the finest equipment of its 

 kind that is available, quite adequate for ad- 

 vanced research in neurophysiology, yet de- 

 signed for maximum dependability and simplic- 

 ity of operation. Fundamentally it is not very 

 much harder to use than a television set. We 

 hope that it will be a relief to you to see what 

 electronics can do when it is not producing soap 

 operas. 



In this first period, working in groups of four, 

 everyone should learn to operate the electronic 

 equipment and set up and record the impulses 

 from a frog sciatic nerve. The best arrangement 

 is for two of you to begin at once to dissect out 

 the frog sciatic nerve, while the other two learn 

 how to operate the equipment. Then, after the 

 preparation is set up, the students who know 

 how to work the apparatus can teach the others 

 how to do it while recording from the nerve. 



THE ELECTRONIC EQUIPMENT 



This equipment may at first glance seem com- 

 plex and forbidding. It is, in fact, simpler than it 

 looks. Many of the dials are simply multipliers 

 of the same adjustment, or of no concern in the 

 normal operation of the instrument. There are 

 only five or six controls you must learn to 

 operate. 



The general principle for study of such small 

 electrical changes as found in nerve cells is first 

 to amplify the potentials and then to observe 

 their magnitude, duration, and shape by means 

 of a recording device. From the nerve chamber, 

 which will enclose the preparation, note the 

 leads into the preamplifier. We shall speak of the 

 nerve potential and its changes as the signal. The 

 preamplifier amplifies this signal 100 or 1000 

 times. The other dials on the amplifier allow 

 one to select certain frequencies of signal while 

 eliminating others. These need not concern us 

 today, but next week we will have opportunity 

 to use these controls. 



To observe and measure the response, the 

 magnified signal is led into the indicator, an 

 oscilloscope with its amplifier. In the indicator, 

 the impact of a narrow beam of electrons on a 

 fluorescent screen makes a spot of light, as in a 

 TV set. The beam of electrons on its way to the 

 screen passes between two metal plates, charged 

 with the amplified biological potential. The beam 

 of electrons, themselves negatively charged, is de- 

 flected toward the positively charged plate and 

 away from the negatively charged plate, so dis- 

 placing the spot of light on the screen from the 

 zero position which it occupies when both plates 

 are equally charged. In this way the polarity 

 and magnitude of the signal are reflected in the 

 direction and extent of the deflection of the spot 

 of light on the screen. 



The controls on the indicator {intensity, focus, 

 position, etc.) are familiar to anyone who has 

 ever adjusted a TV set. The calibrated amplifier 

 in the indicator allows further magnification or 

 attenuation of the input signal. 



Besides polarity and amplitude, we are in- 

 terested also in the change of biological potential 



