Responses or Higher Animals: The Effeciors - 439 



media yield preparations in which there is 

 only a small residue of thin fibers. There is 

 some evidence that indicates that small 

 amounts of a third muscle protein, called 

 tropmyosin, may be represented in the struc- 

 ture of the thin fibers. 



As mentioned previously, actin and myo- 

 sin, extracted from muscle, unite chemically, 

 forming actomyosin; and threads of acto- 

 myosin contract forcibly when treated with 

 ATP. In the intact fibril, such an myosin- 

 actin complex may be indicated by the nu- 

 merous cross bridges that connect the thick 

 and thin filaments (Fig. 24-8). The cross 

 bridges originate from each of the thick fila- 

 ments at very regular intervals (every 65 

 Angstroms) along its length, and the points 

 of origin of the bridges display a spiral ar- 

 rangement, such that one helical turn is 

 completed in a distance of about 400 Ang- 

 stroms. This arrangement permits a thick 

 filament to establish a connection with each 

 of the six surrounding thin filaments once 

 in every complete spiral turn. The thickness 

 of a thick filament is just great enough to 

 accommodate four chains of myosin mole- 

 cules, lined up in parallel; and it is reason- 

 able to assume that the bridges may represent 

 side branches from the regimented myosin 

 molecules. 



The precise mechanism of contraction is 

 still far from clear, however. Formerly it was 

 thought that shortening and lengthening, 

 respectively, represented a coiling and un- 

 coiling of the helical structure (p. 87) of the 

 actomyosin complex, but this view seems 

 scarcely tenable in view of recent studies. 

 When a muscle is stretched, or when it is 

 caused to contract in moderate degree, 

 neither the thick nor the thin filaments show 

 any change of length. Rather, the filaments 

 slide past each other, as is shown in Figure 

 24-8. Only toward the end of a very strong 

 contraction is there any detectable thicken- 

 ing and shortening of the filaments, and this 

 appears to be confined to the ends, where 

 the thin filaments are brought into contact 

 with each other and where the thick fila- 



ments are finally brought into contact with 

 the Z-partitions (Fig. 21-8). 



The view proposed by a number of cur- 

 rent workers is that contraction is mediated 

 by the cross linkages between the myosin and 

 actin filaments. Energy from the hydrolysis 

 of ATP appears to be utilized in shifting 

 the bonding sites along the filaments, causing 

 them to be pulled along each other. And 

 conversely a resynthesis of ATP is postulated 

 to have the opposite effect, thus causing re- 

 laxation. However, many unanswered ques- 

 tions still remain. For example, how is 

 contraction achieved in visceral muscle that 

 is devoid of striated structure? And precisely 

 how does ATP operate in causing the bond- 

 ing sites to be shifted progressively along 

 the length of the filaments? 



VISCERAL AND CARDIAC MUSCLE 



As to metabolism, cardiac muscle resem- 

 bles skeletal muscle, and presumably visceral 

 muscle is also similar, although very little 

 information is available in this regard. But 

 just as there are plain differences of struc- 

 ture among the three types of muscle, so 

 there are differences in their physiological 

 behavior. Skeletal muscle acts most rapidly, 

 completing a single contraction and relaxa- 

 tion in 0.1 second or less — in comparison to 

 1 to 5 seconds for cardiac muscle, and 3 to 

 1 80 seconds for visceral muscle from various 

 organisms. 



A continuous mass of either visceral or 

 cardiac muscle responds definitely in an all- 

 or-none fashion. An excitation of one group 

 of fibers keeps spreading until it involves all 

 the fibers. In the heart there is a specialized 

 system of modified fibers, which serves as a 

 conducting system. In visceral muscle, how- 

 ever, the spread of a contraction may depend 

 upon nervous conduction. Visceral muscle is 

 permeated by a fine network of nerve fibers 

 that cannot be removed by dissection. 



An outstanding feature of cardiac muscle 

 is the rhythmicity of its action. A frog's 

 heart, for example, if skillfully handled, may 



