84 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



I y/// //////// //T 



FIG. 8. .1. Resting and action potential of the squid giant axon recorded witli an intracellular glass 

 pipette electrode. Time marker (0.5 msec.) indicates the potential level observed when the recording 

 electrode was in the surrounding sea water. Temperature, 23°C. B. Exponential variation in the 

 meiBbrane potential caused by passage of a constant current through the membrane of a squid giant 

 axon with a long intracellular silver wire electrode. The thick portions of the wire in the diagram 

 on the top show the exposed surface of the electrodes. Time marker, 1000 cycles per sec. Temperature, 

 20°C. (The axons in the diagrams are disproportionately thick and short.) 



The fact that the potential level is the same every- 

 where in the axoplasm indicates, according to Ohm's 

 law, that there is no measurealjle flow of electric cur- 

 rent in the axoplasm at rest. It also proves that the 

 resting potential represents, as in the frog muscle 

 fiber (76) and in other nervous elements, a sharp drop 

 of electric potential across the space occupied by the 

 thin surface memijrane of the cell. The resting poten- 

 tial of an excised squid giant axon is known to be 50 

 to 60 mv; it is considerably smaller than that of verte- 

 brate skeletal muscle and nerve cells. 



When a pulse of stimulating current is applied to 

 a giant axon with an internal recording electrode, 

 there occurs a transient rise of 100 to 120 mv in the 

 potential of the axoplasm referred to ground (fig. 8.-1). 

 The magnitude of the action potential measured by 

 this method is practically independent of the position 

 of the electrode tip in the axoplasm. If the tip of the 

 internal electrode touches or pierces the surface mem- 

 brane, both the resting and action potentials are pro- 

 foundly diminished or completeh' eliminated. The 

 action potential represents, therefore, a transient 

 variation of the potential difference across the surface 



membrane of the axon. It is important to distinguish 

 this 'memljrane action potential' from those recorded 

 with external electrodes. 



When it was discovered that the membrane action 

 potential is suisstantially larger than the resting po- 

 tential of the membrane (22, 56), some investigators 

 who believed the membrane hypothesis of Bernstein 

 (10) were greatly surprised. In 1902 Bernstein postu- 

 lated, without clear supporting e\-idcnce, that the 

 action potential may be a mere diminution or disap- 

 pearance of the resting memijrane potential (see 

 p. 117). The finding that the inside potential rises 

 above the outside potential near the peak of the ac- 

 tion potential, therefore, conflicts with this postulate 

 of the membrane hypothesis. 



Besides the role in maintaining a potential differ- 

 ence, the surface membrane of the resting axon plays 

 another important part in electrophysiology of the 

 nerve fiber. The resting membrane has a high re- 

 sistance to a direct current. This can be shown by 

 the use of the arrangement of figure Bfi, in which a set 

 of two metal wire electrodes was used instead of a 

 glass pipette. 



