4 : 2/ The Conduction of Impulses by Nerves 



73 



Qualitatively, one may think of the potential as similar to an electrical 

 pressure or force driving positive charges to regions of lower potential 

 (and negative ones in the opposite direction). The ratio of the potential 

 difference to the current flowing through a conductor is called the 

 resistance R. For many substances, R is a constant independent of the 

 current. In these cases, one can easily analyze direct-current circuits, 

 such as those shown in Figure 1 . 



r = internal resistance of battery 



% — emf of battery 



R — load resistance 



V = potential difference across R 



I = current through R 



(a) 



(b) 



^C 



C = capacitator 



Charge flows only while capacitator is 

 becoming charged 



(c) 



Figure I. (a) Direct current circuit, (b) Direct current circuit with capacitor, 

 (c) Simplified circuit representing a resting axon (see Chapter 24). 



Most bioelectrical phenomena involve changes which occur quite 

 rapidly in time. As stated in the first chapter, events with complex 

 time dependence can be analyzed in terms of simple harmonic changes 

 (alternations) at one frequency. In electricity, a-c circuits are more 

 complex than d-c inasmuch as elements other than resistances can 

 impede the flow of an alternating current. In an a-c circuit of fixed 

 frequency, the ratio of the potential to the current is called the impedance. 

 The ratio of the component of the potential in phase with the current, 

 to the current, is called the resistance, whereas the ratio of the out-of- 

 phase component of the potential to the current is the reactance. React- 

 ances arise due to capacitors, C, which do not pass direct current, and 



