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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



records (fig. 12) a step-like prepotential of 40 to 100 

 msec duration precedes the rapid depolarization 

 phase. Is this prepotential due to current flow from 

 one or more bordering cells? Can the nodal cells 

 actually respond to a stimulus of over 40 msec dura- 

 tion, or is the effective threshold, like that of ordinary 

 myocardium, no different for a i o msec stimulus than 

 for a longer one (22)? If this latter condition were 

 true, the step would not be functional although it 

 might be characteristic. Also, are we to believe that 

 one cell along the A-V pathway requires 40 msec to 

 become depolarized when the entire period between 

 the firing of atrial cells in the nodal region and the 

 firing of ventricular cells may normally be no longer 

 than 60 msec in some of the animals used? Can the 

 step be due to distant cells? Similar speculation 

 applies to the diastolic prepotential seen in other 

 records. The diastolic prepotential of the nodal cells 

 would be expected were they functioning as pace- 

 maker cells, but like the "step" discussed above, it 

 does not appear that this characteristic serves an 

 essential function in some records. Strangely, the 

 diastolic prepotential of the nodal cells has reached 



what might well be threshold when the cell is fired 

 by a conducted impulse. 



The possibility further exists that the small size of 

 nodal cells makes it difiicult to pierce them without 

 causing a '"leak" which produces some characteristics 

 of these shapes. As will be indicated below, similar 

 critical questions exist with respect to the extra- 

 cellular potentials recorded in this region. All of the 

 mentioned characteristics of intracellular A-V nodal 

 potentials are consonant with the concept, discussed 

 below, of slow conduction in small fibers which may 

 have difficulty in exciting large fibers. Although no 

 measurements of velocity were presented in these 

 sttidies, Hofifman et al. (58) indicate that when an 

 intracellular electrode was moved i mm along the 

 slowest conducting portion of the A-V nodal system, 

 time intervals of 20 to 70 msec were noted between 

 the rapid phases of depolarization at the two sites. 

 This might be construed as indicating a conduction 

 velocity as low as I7Q or ^20 ^n per sec. 



Extracellular action potentials have been recorded 

 from the A-V nodal region by several groups of 

 investigators. Van der Kooi and co-workers (138) 



FIG. 13. A sketch of the atrioventricular conduction system in the cow showing the sites at which 

 recording electrodes were placed and the electrograms recorded at these sites. At position i, a 

 small multiphasic atrial potential is found followed by a small positive-going potential. This positive 

 "hump"' increases slightly in positions 2 and 3 and is much bigger in position 4, where it begins to 

 approach a positive-negative potential. In position 5 the potential is markedly positive-negative 

 and at position 6 markedly positive. Here it is followed by a rapid negative -going deflection which 

 appears to be the influence at this locus of the common bundle potential. Position 7 shows a positive 

 common bundle potential, perhaps superimposed on some A-V nodal activity; the common bundle 

 potential becomes positive-negative at positions 9 and 10. A similar figure has been published by 

 Scher and co-workers (116). [From Pruitt & Essex (91).] 



