BIOELECTRIC MEASUREMENTS 



127 



are, therefore, equipotential again; while in Fig. 2']d it has reached 

 electrode 2, causing a second transient potential difference, which 

 is the mirror image of Fig. 276, between the electrodes. If the 

 system under examination has not got a muscle attached to one end 

 of it and cannot be repeatedly stimulated, a set-up of the type 

 described above is necessary for a demonstration of conduction. 

 Even then difficulties in interpretation may arise; the reader may 

 like to consider the difference between a record of a propagated 



Outside 



Inside the nerve 

 membrane 



FIG. 28. — Electrical model of a nerve membrane: a, membrane capacitance; b, 

 K+ channel; c, Na+ channel; Gj, membrane K conductance, 05 mmho/cm-; 

 G2, membrane Na conductance, 001 mmho/cm^. The batteries are due to 

 the unequal distribution of Na and K between the inside and outside of the 

 nerve membrane. In the early phases of the action potential, G2 rises 

 (resistance falls), so that the recording system measures the potential due to 

 the sodium battery. A 5-mV. change in the p.d. across the membrane causes 

 an e-fold change in membrane conductance (adapted from Hodgkin & 

 Huxley, 1952). 



action potential with no recovery phase and a record of a non- 

 propagated electrical change, with recovery, at one electrode. 



If a resistance meter is substituted for the voltmeter in the above 

 circuit, the resistance of a square centimetre of membrane can be 

 shown to fall by a factor of 100 or more in the 'active' region, 

 where there is a potential change of lOO mV. There is no capacit- 

 ance change. 



These are, very briefly, the electrical changes which constitute 

 the action potential. Chemically, it consists of an influx of sodium 

 ions during the rising phase, Fig. 26a, possibly due to the transient 



