CARDIAC TRANSAII'MBRANE POTENTIALS— HOFFMAN 309 



taneous rh3thniicity in cardiac muscle at this time. Early studies of pacemaker ac- 

 tivity in cardiac muscle suggested that spontaneous activity was associated with 

 slow changes in membrane potential^^^' ^' A direct demonstration of the membrane 

 potential changes in a pacemaker was provided by microelectrode studies of isolated 

 Purkinje fibers. ^^ This work revealed a consistent difference between records ob- 

 tained from pacemakers and from other areas of the membrane. Records of the 

 transmembrane potential obtained from a pacemaker showed that the resting po- 

 tential begins to decrease immediately after the end of repolarization (fig. 5). When 

 this slow diastolic depolarization reaches the critical level of membrane potential 

 (the threshold potential) a rapid depolarization and formation of an action potential 

 result. Typical records obtained from Turkinje fiber pacemakers show, in addition 



Fig. 5.— Transnienibi-ane action potentials recorded from a single Purkinje fiber. Top: Rec- 

 ords from potential pacemaker, showing slow diastolic depolarization. Bottom: Pacemaker 

 records showing gradual transition from slow diastolic depolarization into action-potential 

 upstroke. 



