292 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



and indeed, in tlie studies of atrial excitation to be 

 cited below, a great deal of success has been achieved 

 with this technique. In contrast, this use of positive 

 and negative potentials is of much more restricted 

 value in the case of a larger and more complicated 

 mass of myocardium like the ventricles. Indeed some 

 serious errors are possible (fig. 6). The use of potential 

 shape as an indication of sequence of activity is 

 unreliable, since if an electrode records a positive 

 potential at a particular instant we can only say that 

 activitv is approaching, we do not know from which 

 direction, or how close. In the unipolar potential we 

 would expect to see a rapid negative-going potential 

 as the wave passed the recording site, and a con- 

 tinuation of negativity as it retreated from the record- 

 ing site. The attempt has been made to use this 

 negative-going portion of an extracellularly recorded 

 lead to determine the instant of local activity. This 

 particular instant, however, is often not clearly and 

 unequivocally discernible on such a unipolar record. 

 Further, the negative-going phase can be super- 

 imposed on a negative or on a positive potential. 



In the above discussion the unipolar record is 

 considered as taken between an exploring electrode 

 and a trulv distant point which averages all potentials. 

 In the open-chest animal {30) the "unipolar" record 

 mav in fact be a "bipolar" between the exploring 

 electrode and the place where the heart makes contact 

 with the body. Also, the conductivity of the surround- 

 ing media may grossly alter unipolar potentials. 



Ideally, an intracellular record may be substituted 

 for the extracellular unipolar record. Here the rapid 

 depolarization phase, lasting less than i msec, is an 

 accurate indication of local activity. Unfortunately, 

 use of intracellular electrodes is not a practical method 

 of studying thick masses of muscle. One substitute for 

 attempting to read the inflection point on a unipolar 

 extracellular record would be to take the first deriva- 

 tive of the record. This derivative should indicate the 

 most rapid rate of change of potential at the recording 

 point, and should give a brief spike potential which 

 reaches its peak as the wave passes the recording 

 point. An approximation of this derivative can be 

 achieved by recording the potential between two 

 electrodes which are very close to one another in the 

 muscle. The derivative of the unipolar record is, of 

 course, the rate of change of voltage with respect to 

 time (dv/dt). The approximation which is made by 

 taking the voltage change between two close electrodes 

 is to determine as closely as possible the rate of change 

 of voltage with distance (dv/dx). The accuracy of 

 this approximation depends entirely on the assump- 



FiG, 7. Upper trace: potentials recorded by ultramicroelectrode 

 in the pacemaker region. Lower trace: potentials simultaneously 

 recorded by second ultramicroelectrode in normal atrial tissue. 

 [After West et al. (142).] 



tion that a propagated wave of activity passes the 

 two points and moves in a consistent direction not 

 parallel to the line joining the points. It has been 

 verified several times that this space derivative 

 indicates the instant of local activity (40, 75, 106, 1 14). 



The Cardiac Pacemaker; Excitation of Atrium 



One property of cardiac tissue is automaticity — the 

 ability to beat rhythmically without external stimuli. 

 The cells with the most rapid inherent rhythm are 

 called pacemaker cells. In cold-blooded animals, 

 pacemaker activity seems to be possible for all parts 

 of the heart, but in intact warm-blooded animals 

 pacemaker activity is normally confined to the S-A 

 and the A-V nodes (fig. 7). Other parts of the Purkinje 

 system may also normally generate impulses, but it is 

 iiot certain that all portions of the mammalian heart 

 can do so normally. However, it is clear that with even 

 minor departures from the normal state extrasystolic 

 (i.e., abnormal) beats may originate at both atrial 

 and ventricular sites. The pacemaker with the highest 

 inherent rhythm will ordinarily dominate the heart 

 rate, and the impulses conducted from it will de- 

 polarize slower pacemakers faster than they can 

 generate impulses. Normally, the dominant pace- 

 maker is the S-A node. The A-\' node is the pace- 

 maker with the second highest inherent rate; if the 

 S-A node fails or is abnormally slowed, the A-V 

 node will usually control the heart rate. 



As has been previously indicated, the sinus node 

 has a characteristic intracelhilar potential with a 



