126 



J. W. WOODBURY AND W . E. CRILL 



2C mV 

 Parallel to 

 Trabeculum 



1 



Perpendicular to 

 Trabeculum 



Fig. 1. (Above) — Schematic diagram of experimental arrangement. A constant 

 current, /,, is applied to a cell of a trabecula of a rat atrial appendage through one 

 intracellularly placed microelectrode and the changes in transmembrane potential, 

 Urn, measured at different distance and angles to the current electrode with 

 another intracellular electrode. (Below) — Records of the applied current (lower 

 trace) and the resulting voltage changes (upper trace). In most of the voltage 

 records there is "anodal break" excitation. The upper row shows the decrement 

 of voltage with distance parallel to the fiber direction and lower row, the fall-otf 

 perpendicular to fiber direction. Note that the decrement is much steeper in the 

 latter direction. The S-shape of the potential rise at large distances can barely be 

 distinguished because the time base is too slow. 



cular direction show the much greater spatial decrement in the perpendicular 

 direction. The curves are roughly symmetrical about the voltage axis. The 

 dashed lines were drawn by eye. 



The phenomena shown in Figs. 1 and 2 have appeared consistently in a 

 number of experiments and are considered not to have been seriously dis- 

 torted by electrode-inflicted membrane damage. Thus, some trabeculae have 

 been completely mapped without the current electrode becoming dislodged, 

 the resting potential at the current electrode being sufficiently high to main- 

 tain excitability at all times. Most of the mapping was done with hyper- 

 polarizing currents since even small depolarizations excited the tissue. This 

 is not surprising because the atria were frequently spontaneously active. 

 Hyperpolarizing break excitation was usually seen (Fig. 1). A further argu- 

 ment for the validity of the results is their reproducibility from point to point 



