THE PROBLEM OF IMPULSE CONDUCTION IN THE ATRIUM 125 



cells, i.e. the tissue will show electrotonic properties. If electrotonus is not 

 detectable in other cells, impulses cannot spread by local current flow. On 

 the other hand, if potential changes occur in other cells, local circuit spread, 

 although not proven, is rendered highly likely as the simplest mechanism. The 

 data presented here show that there is electrotonic spread of current in tissue 

 and hence support the local circuit mechanism. In 1952, Weidmann showed 

 that electrotonus in Purkinje fibers is accurately described by the cable 

 equations (Weidmann, 1952). He also discussed the problem of impulse 

 conduction in cardiac tissue. 



In comparison with investigations of nerve or skeletal muscle fibers, the 

 study of current spread in cardiac muscle is complicated not only by the 

 complex structure of the tissue but also by the spread of the current into two 

 or three dimensions. The structure makes it difficult to decide on an appro- 

 priate equivalent electrical circuit for the tissue, while the multidimensional 

 spread makes the theoretical analysis more difficult and the obtaining of 

 adequate experimental data more tedious. Atrial tissue was chosen for this 

 study because it was assumed that current spread in this thin, flat tissue would 

 be limited to two dimensions; i.e. the tissue is thin with respect to a space 

 constant. Tliis assumption appears to be valid, but for somewhat different 

 reasons. > 



The experimental design is schematized in the upper part of Fig. 1. An 

 electrode is impaled in a cell near the center of a trabecula of an excised rat 

 atrial appendage and a current pulse of OT sec duration is passed through the 

 membrane. The changes in membrane potential produced by the current are 

 measured successively at different distances and angles with another intra- 

 cellularly placed microelectrode. A series of records obtained at various 

 distances parallel and perpendicular to the edge of the trabecula are shown 

 in the lower part of Fig. 1 . Fiber direction is roughly parallel to the trabecula 

 edge. Only the steady-state voltages have been studied. It can be seen in 

 Fig. 1 that the steady-state voltage falls off rapidly with distance at small 

 radii and more rapidly in the perpendicular than in the parallel direction. The 

 decrement of voltage with distance is not exponential, so the space constant 

 cannot be directly estimated. Nevertheless, since potential changes cannot 

 be detected more than a few hundred micra from the current source, the space 

 constant must be of the order of 100 /x. Despite this astonishingly rapid 

 decrement, it is apparent that current applied in one cell appreciably affects 

 the voltage in adjacent cells, for cell dimensions are roughly 15 ■< 15 x 

 100 /i. 



Steady-state voltage as a function of distance is plotted in Fig. 2. The 

 abscissa is the radial distance from the stimulating electrode to the recording 

 electrode. The ordinate is the ratio of the change in the steady-state voltage 

 (in millivolts) to the applied current (in microamperes) and therefore has the 

 dimensions of kilo-ohms. The curves for both the parallel and the perpendi- 



