Physiological aspects of paired stimulation

Abstract

Starling's law of the heart states that "the energy of contraction, however measured, is a function of the length of the muscle fibre" (Starling, 1915). This physiological property of myocardial and skeletal muscle enables the heart, within certain limits, to eject during each systole the amount of blood that enters it during diastole. It follows that this mechanism plays an important role in the regulation of the circulation. The significance of Starling's law has tended to obscure other important adaptive mechanisms of the heart. Almost everyone has experienced the feeling of a premature beat; as a rule, it is not the premature beat which is felt but rather the contraction following it, the so-called post-extrasystolic beat. Until recently, this enhanced beat was explained on the basis of Starling's law, name1y that the long pause following the extrasystole increased diastole filling and thus enhanced the post-extrasystolic beat. However, it has been demonstrated time and again that the same effect takes place without filling playing any part in it (Meijler, Van de Bogaard, van der Tweel and Durrer, 1962; Siebens, Hoffman, Cranefield and Brooks, 1959).