CONDUCTIOiN OF THE NERVE IMPULSE 



11/ 



During the relatively refractory period there is a con- 

 tinuous recovery in the threshold membrane potential. 

 This concept of Adrian seems to explain many facts 

 known al:)out repetitive firing. In tissues with a time 

 constant which is much longer than the duration of 

 the action potential, however, not only the recovery 

 process, but also the time required to charge the 

 membrane capacity is considered to influence the 

 rhythm of repetitive firing (54). It is also known that 

 the oscillation in the membrane potential at sub- 

 threshold levels (8, 9) plays an important role in 

 production of rhythmical activity in some tissues. 



In connection with the pacemaker mechanism, 

 there is an interesting phenomenon which seems to 

 deserve a short discussion. That is 'resetting' of the 

 rhythm of the repetitive response by an ' extra im- 

 pulse' reaching the pacemaker. In 1936 Gilson (41) 

 examined the effect of an artificial (electric) stimula- 

 tion of the sinus of the turtle heart upon the rhythm 

 of the heart beat. He found that the time interval 

 between the artificiallv induced response and the 

 following (natural) response is approximately equal 

 to the normal inter\'al of the automatically induced 

 responses, regardless of the interval between the 

 artificially induced respon.se and the preceding one. 

 Similar phenomena have been demonstrated in 

 natural and artificial pacemakers in the sensory 

 nerve fiber and in the motor nerxe fiber [cf. Tasaki 

 (121)]. 



CURRENT THEORIES OF THE RESTING .-^ND 

 .ACTION POTENTI.ALS 



In the last section of this chapter, we shall briefly 

 discuss the current theories dealing with the mecha- 

 nism whereby the resting and action potential of the 

 nerve or muscle fiber is generated. This problem has 

 been extensively and authoritatively reviewed by 

 many recent inv estigators in a svmposium Electrochemis- 

 try in Biology and Medicine, edited by Shedlovsk\- (l 1 1). 

 The great variety of the views maintained by recent 

 investigators toward the present problem indicates 

 that the current theories to be described below are 

 not yet accepted as unequivocal. We shall make an 

 attempt to explore the sources of equivocalities and 

 controversies in the present problem. 



Resting Potential 



Twenty years before the turn of the century, 

 Biedermann (12, p. 354) discovered that application 



of an isosmotic potassium chloride solution to a por- 

 tion of a muscle generates a large potential difference 

 between the site of application and the remaining 

 surface of the muscle. Later, Hober (49) extended 

 this observation and found that the ability of various 

 cations to affect the resting potential of the muscle 

 increases in the following series: Li, Na, Mg, Cs, 

 NH4, Rb, K. Hober found also that the correspond- 

 ing series for anions is CNS, NO3, I, Br, CI, acetate, 

 HPO4, SO 4, tartarate. 



In 1902 Bernstein (10) published the .so-called 

 ' membrane theory' in which he postulated a) that 

 the resting potential is pre-existent at the plasma 

 membrane of the cell (prior to injury or application 

 oi potassium salts), and h) that the resting potential 

 is maintained by virtue of the semipermealsility of 

 the plasma meinbrane. At that time, the pre-existence 

 of ions in the electrolyte solution (Arrhenius, 1883) 

 was known, and osmotic phenomena in the mem- 

 brane of some plant cells and in artificial membranes 

 (Pfeffer, 1877) were also well understood. Nernst's 

 book on theoretical chemistry dealing with concentra- 

 tion cells had just appeared at that time (1900). 



A present, there is no doubt about the validity of 

 the membrane theory in the form described above. 

 There are in Bernstein's theory two additional postu- 

 lates. He speculated that the resting potential is a diffu- 

 sion potential resulting from the difference in the mo- 

 bility of potassium and phosphate ions through the 

 membrane and also that the action potential is caused 

 by a reduction of the resting potential resulting from 

 a nonspecific increa.se of permeability of the mem- 

 brane during activity. 



Later on, a large volume of work was published 

 showing that, within a certain limit, the relationship 

 between the resting potential, Er, and the external 

 potassium concentration, [K]o, can be expressed by 

 the Nernst equation 



E, = 58 los r— r ^'"^'^ 



(.2-0 



where [K]i represents the concentration of potas- 

 sium in the protoplasm (7, 55, 68, 76, 94). However, 

 the validity of equation (12-1) does not by itself 

 prove that the process of diffusion of potassium ions 

 is responsible for the resting potential. 



Equation (12-1) represents the theoretical maxi- 

 mum (absolute) value of the resting potential that 

 can be attained if the concentration gradient of 

 potassium were the cause of the resting membrane 

 potential. If, therefore, it happens under any circum- 



