CARDIAC TRANSMEMBRANE POTENTIALS— HOFFMAN 



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Trausiucmhrane pntcntials and excitability. A stiuK of tlic rt'lationslii'iis be- 

 tween the transineinl)r;ine ])()teiitials of single cardiac til)er,s and the excitahiiitv of 

 the heart to a])])h'ed stimuli is an interesting extension of the material ])resente<l in 

 the i)receding paper. A series of inxestigations of a similai" nature, based on the re- 

 cording of the monophasic (injury) potential has been summarized b\- Schutz.'" 



(a) Threshold. The concept of "threshold" can be understood best by a con- 

 sideration of the time-course of the transmembrane potential following a series of 

 stimuli which increase in intensity from subthreshold to threshold strength and 

 finally elicit an action potential. In an experiment of this type (fig. 3) rectangular 

 pulses of cathodal current (current passing outward through the membrane) are 

 applied across the membrane of a single fil)er. The weakest current gives rise to a 

 small depolarization that lasts as long as the stimulus ; after the cessation of current 

 flow the membrane potential returns to resting values with an exponential time- 

 course. As the stimulus is increased in intensity, ecjual increments of current give 

 rise to progressively larger and larger depolarizations until, when the membrane 

 potential falls to a certain level, a very rapid depolarization (the upstroke of the 

 action potential) supervenes. These observations show that the "threshold" of the 

 fiber is really a critical level of membrane potential (the threshold potential) at 



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Fig. 3. — Subthreshold and threshold responses. Tracing of transmembrane action potentials 

 of ventricular muscle (solid line) showing progressively larger depolarizations (a, b) result- 

 ing from two subthresliold stimuli and the production of an additional action potential by a 

 threshold depolarization (c). Dotted line represents critical level of membrane potential (the 

 threshold potential) at which self-sustaining depolarization eventuates. Time and voltage 

 calibrations shown in figure. 



