Abstract
It is well recognized that tissues have a minimal gradient requirement (lambda) (1,2) which must be satisfied if applied pulses are to stimulate nerve, cardiac or skeletal muscle (2,3). Current pulses which have too slow a rise time do not stimulate, supposedly because accommodation has occurred(2). The term accommodation is also used to describe the decrease in facilitatory effects of long lasting subthreshold conditioning pulses(4). The terms catelectrotonus and anelectrotonus are well known to physiologists and it is generally recognized that the effectiveness of the applied current decreases somewhat because of accommodation. The purpose of the present paper is to give evidence indicating that it is unsafe to assume that “accommodation” is the same in both cases.
Method. Dog ventricle trabecular muscles were used in this work. They were maintained in a tissue bath and perfused with Tyrode's solution. Rectangular pulses and pulses with a controlled rate of linear rise were used in determining thresholds and minimal gradient requirements. Long duration conditioning pulses could also be applied. The methods of such studies have been described fully elsewhere(3) but Fig. 1 gives a diagram of the devices employed.
Results. Fig. 2 shows the changes in threshold to testing stimuli which occurred during and following application of a 280 msec conditioning pulse of half-rheobasic strength. It can be seen that an initial facilitatory action was not fully sustained. Threshold rose slightly during the first 100 msec of the cathodal current flow and then remained relatively constant until break of the current. The postcathodal rise in threshold which lasted somewhat more than 50 msec is the typical postcathodal depression. This decrease in facilitatory action of the conditioning pulse is the accommodation referred to and the experiment demonstrates that the phenomenon does occur in this type of cardiac tissue.
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