92 THE NERVE IMPULSE 



Exercise XVIII 



electric current is usually stated to be flowing 

 just the other way, from positive to negative. 

 You will have to remember, while using this con- 

 vention, that the positive pole is where there are 

 fewest electrons, and that when a current flows, 

 the electrons flow toward the positive pole. 



In any flow of current we must take account of 

 three factors: 



(1) An intensity factor, the potential, measured 

 in volts. This is the pressure of electrons to 

 flow from the region of higher to that of 

 lower concentration of electrons. It is just 

 like the pressure of water to flow from a 

 higher to a lower level. Just as with water, 

 one could measure this pressure without 

 allowing any flow. It is this pressure of 

 electrons that we measure as the electrical 

 potential or voltage. 



(2) A quantity factor, the current or amount of 

 flow, measured in amperes. There is a 

 certain pressure in the water mains, 

 whether or not you use any water. You 

 can then turn on a tap and allow the water 

 to flow gently or strongly. The same is true 

 of an electric current. 



(3) The third factor is the resistance off'ered 

 by the conductor, measured in ohms. To 

 follow out the water analogy, one might 

 have a narrow pipe, which even when 

 entirely open allows the water to flow 

 through it only slowly, or a wide pipe, 

 which can conduct it very rapidly. Simi- 

 larly a thin copper wire off"ers considerable 

 resistance to the flow of current compared 

 with a thick wire. In these cases the po- 

 tential may be identical; but the flow of 

 current is very different. 



These three quantities are bound together in 

 the simple relation expressed in Ohm's Law: 

 E = IR, in which E is the potential (volts), / is 

 the current (amperes), and R is the resistance 

 (ohms). 



If a very fine electrode is inserted into the in- 

 terior of a nerve fiber, and another electrode 

 touches its surface, one finds a more-or-less con- 



stant electrical potential between these two elec- 

 trodes, called the resting potential. Such an ex- 

 periment is best done on the giant nerve fibers of 

 the squid (see Keynes's article). This resting 

 potential is about 75 to 90 millivolts (mv), with 

 the outside of the nerve fiber positive to the in- 

 side, the nerve membrane forming the interven- 

 ing boundary. The source of the potential is a 

 differential distribution of ions: more K+ ions 

 inside, and more Na+ and Cl~ ions outside. 

 The selective permeability of the nerve mem- 

 brane, which is largely responsible for this 

 diff"erential distribution of ions, is spoken of as 

 its polarization. 



A nerve impulse results from a local depolariz- 

 ation of the membrane, permitting ions to flow 

 through it more freely. The active point on the 

 nerve fiber has momentarily ceased to maintain 

 the diff"erential distribution of ions just described. 

 As a result, the surface of the cell at this point 

 has lost its special positivity; it is therefore nega- 

 tive relative to the remaining cell surface. This 

 change is self-propagating, each such active 

 point stimulating the adjacent region of the fiber. 

 For this reason the nerve impulse appears as a 

 wave of negativity that sweeps down the length 

 of the nerve fiber. It is important to realize that 

 when a nerve fiber carries an impulse, all that it 

 conducts is this excitation. There is no actual 

 flow of either energy or material from one end 

 of the fiber to the other; there is only the passage 

 of excitation. 



This point can be made clear by a simple ex- 

 ample. The conduction of a nerve impulse is not 

 like the conduction of water through a pipe, or 

 of electricity through a wire. It is like the flash 

 that passes down a train of gunpowder if we 

 ignite one end. If you will think of such a train 

 of gunpowder, you will understand readily many 

 important properties of the nerve impulse. So, 

 for example, it is obvious that at each point the 

 gunpowder generates its own response, inde- 

 pendently of all the other points. You would get 

 the same response whether you lit one end with a 

 spark or a bonfire. Where there is a lot of gun 

 powder, there will be a big response; where the 

 gunpowder is thinly sprinkled, there will be a 



