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		Bio 220 Lecture 24: Muscle II I. Muscle (continued) I'd just like to mention a few factors that influence muscle function, then discuss the types of muscle fibers and compare the function of the two muscle systems in fish. A. Mechanics49.31 A twitchis the mechanical response to a single action potential. Following stimulation, cross-bridges begin to form, but the muscle doesn't shorten until the tension (the force exerted on an object by contracting muscle) is greater than the load. There is a considerable delay between the action potential and active fiber shortening because it takes time for the Ca++ to be released and to cause the regulatory proteins to move out of the way. If another action potential is generated, more Ca++ is released and more cross-bridges form, so the contraction is stronger. 1. Length-tension relationship49.27 The amount of tension that can be generated by a muscle is a function of the degree of overlap between the thick and thin filaments. The greatest tension is generated when there is the greatest number of cross bridges formed. When the muscle is stretched, parts of the filaments don't overlap. When the muscle is maximally shortened, the filaments are moving past each other, so they can't form as many cross-bridges. The maximal tension is generated in the middle of the length range. When relaxed, the length of most muscle fibers is close to their optimal length, so they are able to immediately generate at least 50% of their maximal power. 2. Work and power production The workthat a muscle produces = the force exerted xchange in length The powerthat the muscle generates = Work/time. Velocity= change in length/time However, the velocity of shortening affects the force exerted. Maximal force is exerted at approximately 1/3 maximum velocity.