CARDIAC TRAXSMEMBRAXE POTEXTIALS— HOFFMAN 



303 



the iKiniial relationslii]) l)ctwcen fibers. Furthermore, the absolute value of the 

 transnienibrane potentials can be determined with certainty and the time-course of 

 potential changes dnring- activity clearly delineated. Finally, observed changes in 

 threshold and refractoriness can be related to alterations in the state of polarization 

 of the fiber membrane. 



Technique. The transniemlirane potentials of single cardiac fibers are recorded 

 by means of a fine glass capillary microelectrode of the type developed by Ling and 

 Gerard.'' The microelectrode employed is drawn from capillary tubing to a tip 

 diameter of less than one micron and filled with a concentrated solution of KCl 

 (3 ]\I) to minimize junction potentials between myo])lasm and electrolyte.^* It has 

 been demonstrated that insertion of an electrode of this size through the fiber 

 membrane does not. of itself, result in measurable injury.^*' ^' Although movement 

 resulting from contraction often dislodges the tip of the microelectrode from within 

 the fiber, in most preparations it is possible to record many cycles of activity from 

 a single fiber without change in the magnitude or time-course of the transmembrane 

 potential. ^^' '^^ In contrast to the motoneuron of the cat spinal cord which is 

 strongly affected by leakage of CI from within the microelectrode,^' the resting and 

 action potentials of single heart fibers are unchanged even after several hours of 

 recording from the same area of the membrane. '''• '" 



In practice the microelectrode is paired with an indifferent electrode of similar 

 composition. The potential difference recorded when both electrodes are extracellu- 

 lar in position is the reference or zero potential. When the tip of the microelectrode 

 is inserted through the membrane and into a resting fiber a potential difference of 

 approximately 90 mv is recorded (fig. 1 i. The sign of this potential difference is 

 such that the inside of the membrane is negative with respect to the outside. This 

 potential is commonly called the resting (transmembrane) potential. With the onset 

 of activity there is an abrupt change in the transmembrane potential which consists 

 of a rapid depolarization and reversal of membrane polarity (inside positive with 



+ 20 mv. 

 mv. i 



-100 mv. 



Fig. 1. — Diagrammatic record of transnienibrane potentials of aurieular muscle. Line a. f = 

 zero potential; b = resting potential; c, d, e = action potential. Time and voltage calibrations 

 shown in figure. 



