CARDIAC MUSCLE CONTRACTILITY 



'55 



to consider it again in relation to the internal 

 dynamics of shortening and the development of the 

 active state. For purposes of this argument the muscle 

 can be considered as a contractile element in series 

 with a passive elastic element. If the series elastic 

 element were not there, and the contractile element 

 were fixed at either end, stimulation would result in 

 a rise and fall in tension which would faithfully re- 

 flect the contraction-relaxation cycle within this 

 muscular element. Such is not the case when the 

 elastic element is in series, since now the tension rise 

 is slower by virtue of the extension of the elastic ele- 

 ment which occurs when the muscular element is 

 stimulated, and likewise the fall in tension is prolonged 

 because of the release of potential energy that was 

 imparted to the elastic element during stretching. 

 This is shown diagrammatically in figure 3, which is 

 taken from Gasser and Hill's description in 1924 (87). 

 The behavior of the muscular element without the 

 influence of the series elasticity is observed at various 

 points in the contraction cycle by raising the tension 

 of the elastic component by a quick stretch, thus es- 

 tablishing isometric conditions for the contractile 

 element. [For the most elegant of the experimental 

 methods for measuring active state, see reference 

 (139).] The mechanical response under these circum- 

 stances is indicated by the solid curve in figure 3, and 

 has been called the "active state" of the muscle. The 

 active state in skeletal muscle rises extremely quickly 

 to peak level very shortly after stimulation and then 

 rapidly declines. The twitch tension (dotted curve) 

 rises more slowly because the series elastic element 

 allows the contractile component to shorten. It like- 

 wise falls more slowly since the series elastic elements 

 stretched by the contractile component return to their 

 resting length only after the active state in a single 

 twitch has declined to zero. Tetanization of the muscle 

 prolongs the active state, without increasing its in- 

 tensity, the difference between twitch and tetanus 

 tension being due simply to adequate time for elastic 

 element tension to be raised to the le\el of the 

 muscular element at the active state peak. Therefore, 

 whereas all these complexities are avoided in the 



' This introduces a complication into our definition of con- 

 tractility. The measurement of maximum work capacity and its 

 relation to isometric tension development in striated muscle 

 was worked out with tetanized fibers, so that the active state 

 peak was being measured. .Since the heart cannot be tetanized, 

 and since measurements of active state are impractical as a 

 general procedure, one must be satisfied with isometric twitch 

 tension as a reasonable measure of cardiac muscle contractility 

 in all e.xcept detailed biophysical studies. 



tetanized muscle, in the case of single twitches, be they 

 in striated or heart muscle, isometric tension is a func- 

 tion a) of the intensity of the active state, and b) of 

 the duration of the active state, in relation to the 

 elasticity of the series noncontractile element.^ We will 

 consider in a later section (see section vi) an example 

 of a decline in isometric twitch tension of heart muscle 

 which is thought to be due to a decline in active state 

 duration but not intensity. 



Efficiency 



A comment should be made about the use of 

 efficiency (work output/energy input) for evaluating 

 myocardial function. It should be clear from the 

 foregoing discussion that there need be no relationship 

 between efficiency and contractility, since the work 

 output of a muscle depends on the conditions of load- 

 ing. An experimental curve relating efficiency to load 

 for frog sartorius is shown in figure 4 (138). EflSciency 

 is of course zero for both zero load and zero shorten- 

 ing, since no work is done at these extremes. It rises 

 to a maximum when the load is 48 per cent of the 

 isometric value. Thus any value of efficiency for this 

 muscle of given contractility can be obtained de- 

 pending on the load. Figure 5 shows the variation of 

 work done bs' dog cardiac muscle as a function of 

 load. There are no data on energy input for calcula- 

 tion of efficiency, but the study illustrates the main 

 point made in this paragraph and is an example of a 

 good biophysical investigation on heart muscle (249). 



A/ijiliaitions to Cardiac Muscle 



A discussion of cardiac contractility would not be 

 complete without leaving the relatively idealized 

 frog sartorius and mentioning in passing certain prob- 

 lems peculiar to real heart muscle. Instead of a straight 

 strip of parallel fibers the heart is a hollow viscus made 

 up of a syncytium of fibers running in several different 

 directions and stimulated by a complex conduction 

 system. The relationships between wall tension and 

 hydrostatic pressure in tubes have recentiv been re- 

 introduced to physiologists by Burton (41 ), and will 

 be considered in ex/enso by other authors in this vol- 

 ume. They add another variable to the relation be- 

 tween contractility of the myocardial cell and work 

 done by the heart as an organ. The conduction svstem 

 plays an important role in the contractility of the 

 whole heart so that, for example, abnormalities in 

 conduction leading to marked asynchrony in con- 

 traction of different portions of the ventricle will cause 



