40 

 L.A. 

 cm 





 250 

 Aa 

 mm 

 Hg 







40 



LV- 0. 

 cm 







+ 

 MSL 



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FIG. 10. LA = left atrial pressure; Ao = aortic pressure; LV-D = left ventricular diastolic pres- 

 sure; MSL = changes in myocardial segment length; + = elongation; — = shortening. 



ular filling "'because the atrioventricular pressure 

 difference is small." It would appear from the data 

 shown above that the more likely explanation is the 

 position of the ventricle at that time (late diastole) 

 on a higher part of its pressure-\olume as well as its 

 pressure-length curve. 



The extent to which atrial systole can contribute 

 to the end diastolic elongation of x'entricular myocar- 

 dium, when coupled with the dependence of the ven- 

 tricle's stroke work on initial fiber length, indicates 

 the substantial extent to which variations in the 

 \igor of atrial systole could pro\ide the stimulus for 

 greater or lesser amounts of ventricular stroke work. 



MITRAL v.ALVE CLOSURE. The mechanism of closure 

 (jf the atrioventricular valves has been the subject of 

 considerable speculation. It has recently been es- 

 tablished that not only can the mitral \alve be closed 

 solely as the result of atrial activity but that auto- 

 nomically induced changes in atrial activity can in- 

 fluence whether or not the valve closes (io6a). This 

 will be discussed below. 



h. neuron.'kl effects on the performance 

 ch.^ractertstics of the he.art 



(extrinsic influences) 



A. Influence of Cardiac Sympathetic Nerve 

 Stimulation on the Ventricle 



contractility. Supramaximal stimulation of the 

 isolated left stellate ganglion, while heart rate is 



held constant by atrial pacing, produces the changes 

 shown in figure 13^4. During the interval between 

 the arrows at top of the first channel, the isolated 

 left stellate ganglion was stimulated. The proinpt 

 elevation of cardiac output and systolic, mean 

 and diastolic aortic pressures is accompanied by a 

 fall of mean left and right atrial pressures and a 

 widening of the PA-LA pressure difference. As 

 found by .Shipley & Gregg (io8), left stellate ganglion 

 stimulation sometimes does not cause an appreciable 

 change in heart rate. In such animals when observa- 

 tions on mvocardial contractility are made either 

 with or without a controlled rate the results are the 

 same. In some experiments the elexation of arterial 

 pressure is small during stellate ganglion stimulation, 

 the increased contractility being evidenced primarily 

 by a fall in mean atrial pressure (105). 



The fall in left atrial pressure during stellate gan- 

 glion stimulation is consistently accompanied by a 

 lowering of left ventricular end diastolic pressure. 

 The more rapid development of tension, the more 

 rapid myocardial shortening, the augmented aortic 

 pressure, the shorter duration of ejection, and the 

 more rapid relaxation are consistent and noteworthy 

 (105). Changes of the type observed can be seen in 

 figure 19 (lower). 



In figure 1 •^B the dashed lines show the relation be- 

 tween mean left atrial pressure and left ventricular 

 stroke work during control period (C) and during 

 stellate ganglion stimulation {SS); the solid lines 

 show the relation between left ventricular end diastolic 

 pressure and stroke work during control period (C) 



