546 Comparative Animal Physiology 



of animals. The cellular basis for rhythmic activity may be essentially simi- 

 lar in all those tissues where a continuous energy source (e.g., oxidation) 

 results in repetitive activity— a relaxation oscillator. 



Myogenic Hearts. In the vertebrate heart the beat is myogenic; it originates 

 in muscular tissue. In an adult frog or fish the beat can be seen to start in 

 the sinus venosus, and in an adult bird or mammal it can be seen to start 

 in the sino-auricular node and spread to auricle and ventricle. In tissue cul- 

 tures of embryonic heart muscles the fibers may develop rhythmic contrac- 

 tions, and in a developing chick embryo contractions occur in the myocar- 

 dium of the bulboventricle after only 29 hours of incubation.^'* With devel- 

 opment, the anterior regions of the heart gradually take over control of the 

 rhythm. If the sinus venosus is removed from an adult amphibian heart the 

 normal rhythm stops, but other regions may take over initiation of regular 

 but slower contractions. Similarly in mammals destruction of the sino-auric- 

 ular (S-A) node causes the heart to stop, at least until some other region 

 initiates slow contractions. Local cooling or warming is most effective in 

 altering rate when applied in the sino-auricular region; electrical stimulation 

 elicits normal electrocardiograms only when applied at the S-A node, and 

 the first trace of the action potential in the heart appears in this region. By 

 warming regions of fish hearts, Skramlik^^^ found three pacemakers— the 

 sinus, the auricular floor (canal between sinus and ventricle), and the auric- 

 uloventricular junction, and three groups of fishes were described according 

 to the distribution of these centers. Vertebrate heart muscle, then, is ap- 

 parently capable of spontaneous rhythmicity, but the muscle of the sinus 

 region differs from other regions in rapid recovery of excitability, and this 

 becomes the pacemaker. 



The wave of excitation which originates in the sinus is conducted over 

 the atrial myocardium and then spreads in the specialized Purkinje tisssue 

 through the ventricle. The rapidly conducting Purkinje tissue appears to 

 be an adaptation to high body temperature, since it is present only in birds 

 and mammals; in other vertebrates ventricular conduction is in the myo- 

 cardium. The vertebrate heart contains nerve cells, particularly in the region 

 of the pacemaker, but these are secondary neurones of the vagus system. 



Certain drugs act upon pacemaker mechanisms and are useful in distin- 

 guishing between neurogenic and myogenic hearty (Table 68). The drug 

 acetylcholine inhibits the adult vertebrate heart with respect to both ampli- 

 tude and rate. Embryonic vertebrate hearts prior to vagal innervation are 

 either insensitive to ACh, as in Fundtdus/' or inhibited only by very high 

 concentrations, as in the chick. ^"" Apparently innervation sensitizes the heart 

 to the drug. 



MoUuscan hearts also are myogenic. Nerve cells occur on or near the 

 heart in cephalopods, but these are secondary neurones of regulating 

 nerves."'' In certain gastropods nerve cells have been described by some 

 observers and denied by others.^-''^ Several histologists failed to find any 

 nerve cells in the hearts of clams and mussels. The beat can originate at 

 any point over the heart.-' A contraction may be local or may be complete. 



The action of drugs also supports the hypothesis that the origin of the 

 beat in molluscan hearts is mvogenic. Acetvlcholine inhibits the hearts of 



