Circulation of Body Fluids 565 



charge gradually stops. Arthropod hearts, with nervous pacemakers and con- 

 duction systems, are more sensitive than molluscan hearts. 



Calcium antagonizes the effect of potassium and sodium on the pacemaker, 

 and slows heart beats. In the perfused vertebrate heart the predominant cal- 

 cium action is a marked enhancement of contraction resulting in systolic ar- 

 rest; in the absence of calcium the vertebrate heart relaxes and stops in dias- 

 tole. Calcium is essential for contraction; a heart stopped in zero calcium 

 continues rhythmic action potentials.^ ^-^ In molluscs, however, the inhibiting 

 effect of calcium on the pacemaker is predominant, and in excess Ca+ + 

 the heart stops in diastole by pacemaker inhibition. In the absence of calcium 

 some molluscan hearts stop in diastole QPecten, Ostrea, Anoniia'); in others 

 CHelix, Octopus^ the pacemaker stimulation predominates, and in zero cal- 

 cium the hearts stop in systole. 



In the arthropods the action of calcium on the pacemaker is predominant, 

 and high calcium slows the heart and stops it in diastole, whereas low Ca+ + 

 accelerates the heart, and zero calcium stops it in systole. In Limulus the 

 systolic action of low calcium results from an increase in gross frequency of 

 the ganglionic discharge, an increase in nerve unit frequency, and in 

 the number of active units; in zero calcium there is a tendency to asynchroni- 

 zation.^^^ The pacemaker-inhibiting action of high calcium predominates 

 over the muscle-contracting action in the arthropods and molluscs (except 

 PectenX but not in the vertebrates. The pacemaker stimulation of low cal- 

 cium predominates over the muscle-relaxing effect in arthropods and in Helix, 

 Aplysia, and Octopus, but not in Pecten and Ostrea and not in vertebrates. 

 In the oyster the pacemaker effect of high calcium predominates, whereas 

 the muscular effect of low calcium (or high potassium) is predominant. 



Magnesium appears essential for rhythmic beating of the hearts of marine 

 pelecypods and gastropods. Magnesium lacks the muscle-contracting effect 

 of Ca+ + , but like calcium in excess it inhibits the pacemaker, bringing about 

 diastolic arrest. In molluscan hearts which require Mg+ + the omission of 

 this element results in acceleration and arrest in systole. 



Many of the above effects are referable not to single ions but to ionic ratios; 

 e.g., some high potassium effects are prevented by simultaneously increasing 

 calcium. Although the cellular nature of these ionic effects is unknown, salt 

 studies are useful in separating pacemaker, conducting, and contractile 

 systems. 



Effects of Electrical Stimulation on Heart Muscle. The muscle of a verte- 

 brate heart is a syncytium of branched striated libers. Such a heart (adult 

 or embryonic) is absolutely refractory to electrical stimulation during most of 

 systole; it can be excited (is relatively refractory) at the end of systole and in 

 diastole, but an extra contraction elicited during this time is of submaximal 

 height. Recovery is complete by the end of diastole. Any extra contraction 

 is followed by a compensatory pause longer than the normal diastolic pause 

 (Fig. 211). Any contraction, rhythmic or in response to an electrical stimu- 

 lus, is maximal for the state of the heart, although a distended heart is capable 

 of enhanced contractions and quiescent strips show "staircase" responses to re- 

 peated shocks. The vertebrate heart, therefore, contracts in an all-or-none 

 fashion. When tetanized the heart shows spasmodic uncoordinated contrac- 



