308 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



which the increase in permealjiHty of the nienihrane to Na is great enough to permit 

 a self-sustaining depolarization. In other words, depolarization progressively in- 

 creases Na permeability and at a critical level of membrane potential the net mem- 

 brane current is inward and positive. Under this condition a rapid depolarization 

 and reversal of the membrane potential results. This relationship between mem- 

 brane potential and Na current has been studied in detail in the squid giant axon f~ 

 studies of isolated Purkinje fibers suggest that in the case of heart muscle the rela- 

 tionship is essentially as outlined.^^ 



The major factors which influence the strength of stimulus required to elicit a 

 propagated response in cardiac muscle can be summarized quite briefly. As shown 

 in figure 4, an effective stimulus must lower the transmembrane potential from the 

 normal resting value to the critical or threshold level in order to elicit a propagated 

 action potential. The stimulus requirement will thus depend upon both the absolute 

 value of the resting potential as well as the value of the critical threshold potential. 

 An additional factor of importance, especially when the stimulus duration is long, 

 is the value of membrane resistance. 



(b) Pacemaker activity. It is perhaps pertinent to discuss the nature of spon- 



20- 



+ - 



mv 0- 



20- 



40- 



60- 



80 



100- 



lyri 



100 msec 



A 



THRE_SHOLD__POT_E_NTI_AL 

 RESTING POTENTIAL 



20- 



H- - 



mv 0- 



20- 



40- 



60- 



80 



B 



:^ 



^ H RE S HOL p_ _POTE NJiA L_ 



"resti'ng potential" 



Fig. 4. — Diagrammatic representation of effect of changes in resting potential on the stimulus 

 intensity required to elicit an action potential. Compare with figure 3. In A, a larger stimulus 

 (Sj) is required to depolarize from resting potential (90 mv) to threshold potential (65 mv) 

 than in B, where smaller stimulus (S2) depolarizes from resting potential of 80 mv to thresh- 

 old potential of 65 mv. 'J'imc and voltage calibrations sliown in figure. 



