Appendix C 609 



which is called the impedance. Equation 2 has meaning only for 

 sinusoidal currents and potentials or for Fourier transforms of currents 

 and potentials. The ratio Z is called the impedance. It can be repre- 

 sented in the form 



Z = R+jX 



where both R and X are real numbers. If the impedance Z is real 

 (that is, X = 0), the voltage and current are said to be in phase. In 

 this case, at all times, the instantaneous ratio of V to / is the same 

 constant. The real part of Z is called the resistance R. However, if R 

 is zero, but the reactance X is different from zero, V and / will be 90° 

 out of phase. An a-c circuit is illustrated in Figure 2 of Chapter 4. 



Only the resistance R contributes to the power dissipated by an ele- 

 ment in an alternating circuit. In a direct current circuit, one may 

 write 



P = VI 



This same formula may be used for an alternating current circuit, pro- 

 viding one uses the real instantaneous values of V and / and averages 

 over a period. For a sinusoidal current 



D \I \*R \V \*R 



2 2|Z| 2 



where the vertical lines indicate the absolute values of the complex 

 quantities. In the neuron, the power dissipated is so small compared 

 to metabolic heat losses that it is in general unimportant. 



Other electrical terms and symbols are discussed in Section 2 of 

 Chapter 1 1 . For references, see Chapters 4 and 1 1 . 



