Skeletal Muscle Physiology
O B J E C T I V E S 1. To define these terms used in describing muscle physiology: multiple motor unit summation, maximal stimulus, treppe, wave summation, and tetanus. 2. To identify two ways that the mode of stimulation can affect muscle force production. 3. To plot a graph relating stimulus strength and twitch force to illustrate graded muscle response. 4. To explain how slow, smooth, sustained contraction is possible in a skeletal muscle. 5. To graphically understand the relationships between passive, active, and total forces. 6. To identify the conditions under which muscle contraction is isometric or isotonic. 7. To describe in terms of length and force the transitions between isometric and isotonic conditions during a single muscle twitch. 8. To describe the effects of resistance and starting length on the initial velocity of shortening. 9. To explain why muscle force remains constant during isotonic shortening. 10. To explain experimental results in terms of muscle structure.
keletal muscles are composed of hundreds to thousands of individual cells, each doing their share of work in the production of force. As their name suggests, skeletal muscles move the skeleton. Skeletal muscles are remarkable machines; while allowing us the manual dexterity to create magnificent works of art, they are also capable of generating the brute force needed to lift a 100-lb. sack of concrete. When a skeletal muscle from an experimental animal is electrically stimulated, it behaves in the same way as a stimulated muscle in the intact body, that is, in vivo. Hence, such an experiment gives us valuable insight into muscle behavior. This set of computer simulations demonstrates many important physiological concepts of skeletal muscle contraction. The program graphically provides all the equipment and materials necessary for you, the investigator, to set up experimental conditions and observe the results. In student-conducted laboratory investigations, there are many ways to approach a problem, and the same is true of these simulations. The instructions will guide you in your investigation, but you should also try out alternate approaches to gain insight into the logical methods used in scientific experimentation. Try this approach: As you work through the simulations for the first time, follow the instructions closely and answer the questions posed as you go. Then try asking “What if . . . ?” questions to test the validity of your hypotheses. The major advantages of these computer simulations are that the muscle cannot be accidentally damaged, lab equipment will not break down at the worst possible time, and you will have ample time to think critically about the processes being investigated. Because you will be working with a simulated muscle and an oscilloscope display, you need to watch both carefully during the experiments. Think about what is happening in each situation. You need to understand how you are experimentally manipulating the muscle in order to understand your results.
A contracting skeletal muscle will produce force and/or shortening when nervous or electrical stimulation is applied. The force generated by a whole muscle reflects the number of motor units firing at a given time. Strong muscle contraction implies that many motor units are activated and each unit has maximally contracted. Weak contraction means that few motor units are active; however, the activated units are maximally contracted. By increasing the number of motor units firing, we can produce a steady increase in muscle force, a process called recruitment or multiple motor unit summation. Regardless of the number of motor units activated, a single contraction of skeletal muscle is called a muscle twitch (Figure 2.1b). A tracing of a muscle twitch is divided into three phases: latent, contraction, and relaxation. The latent...