ON THE PROBLEM OF IMPULSE CONDUCTION 

 IN THE ATRIUM* 



J. W. Woodbury and W. E. Crill 



Department of Physiology and Biophysics, University of Washington School of Medicine, 



Seattle, Washington 



Considerable evidence supports the view that the action of acetylcholine on 

 pacemaker and atrial tissue is to increase specifically the membrane perme- 

 ability to potassium ions (Hutter. 1957; Trautwein et al., 1956; Trautwein 

 and Dudel, 1958). Since Dr. Hutter has just given the convincing tracer 

 evidence for this view and Dr. Dudel is in the audience, it seems unnecessary 

 to review here the remaining evidence on the nature of the inhibition of the 

 heart by acetylcholine. Rather, we are going to consider another equally 

 important but less explored facet of cardiac electrophysiology, namely, the 

 mechanism of spread of an all-or-nothing impulse over the atrium. This 

 subject is not altogether unrelated to inhibition in the heart, for some evidence 

 substantiating the present concept of the action of acetylcholine has been 

 gathered in the course of these studies. 



An adequate stimulus applied to any point in heart muscle will initiate 

 an impulse which propagates in an all-or-nothing manner throughout the 

 whole muscle. Electrically the tissue beha\es like a single excitable cell. On 

 the other hand, electron micrographs (Muir. 1957; Sjostrand and Andersson- 

 Cedergren, 1960) of cardiac muscle have rather convincingly demonstrated 

 that this tissue is composed of close-packed but discrete cells, each surrounded 

 by a membrane, if the cell membranes have a high electrical resistance, as 

 they do in other tissues, then it is not immediately apparent how an active 

 cell can initiate activity in its neighbors. The simplest possibility is that the 

 mechanism of impulse spread in cardiac tissue is local circuit current flow 

 as it is in nerve and skeletal muscle. Since local circuit spread has been 

 recently brought into question (Sperelakis et al.. 1960a, b; Sperelakis and 

 Hoshiko, 1960), the problem, aside from its fundamental interest, has a 

 certain amount of currency. 



Fortunately, the local circuit hypothesis can be subjected to a simple and 

 fairly conclusive test. If activity spreads by local circuit flow, then a current 

 flowed through the membrane of one cell by means of an intracellularly 

 placed electrode must have a substantial effect on the potentials of adjacent 



* Aided by grants B-1752 and 28-5269 from the National Institutes of Health. 



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