Curve: % of stretch. so we stretch, then we stimulate it. -Go up to a point where if you go beyond it the tension falls.-The highest point is called Lo- the optimum length of stretch of the muscle. By stretching then stimulating, we increase the tension, up til Lo. Beyond this point, if we keep stretching the muscle, the tension drops off. *Why are we getting this different effect of stretching? Why does this stretching give enhanced tension? It’s at the molecular level this occurs, has to do with interaction between actin-myosin. stretching affects exposed binding sites of actin. myosin must form bridges with actin. 1. No stretch: Overlap of two actins, blocking sites interacting from myosin, not maximizing cross bridges. Actin being blocked by other the actin above it, so theres sometension, but not much. 2. Optimum Stretch: at Lo, the stretching pulls apart the actins and all sites are exposed, maximized tension. 3. Too much stretch: When they are stretched too far apart, Myosin cannot interact with actin, no tension. The length-tension relationship is by virtue of molecular interaction caused by stretching of the muscle. C. Whole Muscle TensionMuscle organ (group, bundles of muscle cells)If were looking at whole muscle, what contributes to the tension exerted by this muscle organ?The # of fibers contracting. A hypothetical muscle with 100 fibers, not all are contracting at that time, only maybe 50 If you do so you get a certain tension from that muscle.But if you contract 75, you get a higher tension and so on. So certainly by regulating the # of muscle fibers contracting, you can alter the tension that muscle organ exerts. Increasing whole muscle tensionThere’s another way, lets say with the same hypothetical muscle with the 100 muscle fibers in it, we don’t want to increase the # of muscle fibers, we want to stay with 50 of them contracting, howcan we still increase the tension? By doing what with those individual muscle fibers? Increasing the tension that each one of those exerts. We can increase whole muscle tension that way, by increasing the # of muscle fibers contracting, and/or increasing the degree of contraction exerted by each muscle cell contracting. How is this done routinely? Motor unit recruitment.Motor unit recruitment: motor unit is a neuron and all the muscle fibers it innervates. A given muscle organ doesn’t have one motor unit, it has many, so you dont have to activate all of them at one time. However, if you increase the number that you activate, youre clearly activating more muscle fibers, and the tension is going to go up. Increasing individual muscle fiber tensionHow can we increase the tension that the individual muscle fibers exert? without changing the # but changing the tension? Summation.