CHAPTER 10 



BIOELECTRIC MEASUREMENTS 



'Action potentials.' Cell physiologists have often hoped — and 

 sometimes persuaded themselves to believe — that there is some 

 common denominator in the responses of cells to stimuli. Heil- 

 brunn (1952), for example, is a proponent of the 'calcium-release' 

 theory of stimulation. According to this theory, such varied cells 

 as muscle fibres, plant cells and unfertilized eggs respond to their 

 specific stimuli by the intracellular release of calcium. R. S. Lillie 

 (1924) placed the emphasis elsewhere. He believed that the first 

 and most important response of the unfertilized egg to activation 

 was the propagation of an action potential over the egg surface. 

 Such an action potential would probably diff"er from those which 

 occur in nerve fibres and cylindrical plant cells, in not having a 

 recovery phase. A nerve fibre can propagate action potentials 

 along its length repeatedly and at a very high frequency. The 

 ability to do this depends upon the membrane reconstitution or 

 recovery travelling along the nerve immediately behind the elec- 

 trical depolarization which is one of the characteristics of the 

 action potential. We can predict, on purely biological grounds, 

 that if an egg responds to fertilization by propagating an action 

 potential over its surface, the form of the electric change will be 

 quite different from that observed in active nerve or muscle fibres, 

 because fertilizatio7i is, under ordinary cotiditions, irreversible. The 

 action potential would not, therefore, be expected to look like the 

 normal one in Fig. 26a, but more like that shown in Fig. 26b, 

 which has no recovery phase. Before considering the possibility 

 of action potentials being propagated over egg surfaces at fertiliza- 

 tion or activation and the claims which have been made to this 

 effect, one should be clear as to what an action potential is, and 

 how it is recorded. Two electrodes, connected to a voltmeter, are 

 shown on the intact surface of a nerve fibre in Fig. 27a, with an 

 action potential coming towards them from left to right. At this 

 stage, the electrodes are equipotential as they are both on inactive 

 parts of the nerve surface. When the action potential reaches 

 electrode 1 , Fig. zyb, a transient potential difference develops be- 



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