28o HANDBOOK OF PHYSIOLOGY ^-^ CIRCULATION I 



RAT ATRIUM 

 EQUIVALUE M CONTOURS 

 Is 



Trobecula boundary 



FIG. 2g. Isopotential contour map of a rat atrial trabecula. A current was applied to the tissue 

 via an intracellular electrode located at the origin. The resulting changes in potential (A £) were 

 measured with another intracellular electrode at the points indicated by dots or X's. Dots indicate 

 measurements made along radii parallel to trabecula edge and X's measurements along radii per- 

 pendicular to the trabecula edge. The number attached to each point is the ratio AS/Is, where V 

 is the size of the applied current, in units of mv//iA or K-ohm. Dashed lines are approximate equi- 

 value contours. From inside to outside they are lO, 5, 3.5, and 2.5 mv/^A contours, respectively. 

 Note the much greater spread of current in the direction parallel to the trabecula boundary than 

 perpendicular to it. (Crill, unpublished data, i960.) 



the fiber direction as in tlie perpendicular direction. 

 Draper & Mya-Tu (34) and Sane et al. (104) have 

 observed conduction velocities two to four times 

 faster parallel to the long axis of the myocardial 

 fiber than perpendicular to it. This correlation is 

 strong evidence for local circuit propagation. There- 

 fore, the principal consideration is the special prop- 

 erties of cardiac cells which allow the flow of myo- 

 plasmic current between cells and thus accomplishes 

 intercellular transmission by local current flow. 



Strurtiirr of Cardiac Muscle 



The essential features of cardiac tissue membrane 

 structure are shown .schematically in figure 31, which 



is based on the work of Muir (93, 94) and Sjostrand 

 el al. (107, 108). Cardiac cells are shaped and parked 

 somewhat like bricks with dimensions of the order 

 of 15 ^ by 15 /i by 100 /x. Myofibrils (not shown in 

 fig. 31) run the length of the cell and terminate on 

 the cell membrane at places corresponding to Z 

 bands on other sarcomeres. Cell surfaces parallel 

 to the fibril axis are separated from their surrounding 

 cells by 20 to 30 nm, whereas the membranes per- 

 pendicular to the fil:)rils are only 8 to 10 nm apart. 

 The opposing meinbranes in these regions are thick- 

 ened and greatK folded, the surface area being about 

 10 times the cross-sectional area. The.se thickened and 

 tortuous membranes constitute the microscopically 



