this dependence of the inotropic response on the timing of the vagal activity within the cardiac cycle indicates that the following conditions must prevail with respect to the vagal innervation of the atrium : ( 1 ) the acetylcholine released from the vagal nerve endings is hydrolyzed at a critically rapid rate in the atrial tissues , ( 2 ) the neurally released acetylcholine must exert its major influence on atrial contraction during some preferential fraction of the cardiac cycle ( presumably , during depolarization ) , and ( 3 ) after a vagal stimulus of a given strength , the concentration of acetylcholine in the region of the myocardial cells will attain its maximum value during this critical phase of the cardiac cycle when the vagal stimuli are given at the optimal time in the cardiac cycle .
C0018792
atrial
C0018787
cardiac