Skeletal Muscle Physiology

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Determining the Latent Period 1. Click Clear Tracings to erase the oscilloscope display. The voltage should be set to 5.0 volts. 2. Drag the 200 msec button to the right edge of the oscilloscope. 3. Click the Stimulate button once, and allow the tracing to complete. 4. When you measure the length of the latent period from a printed graph, you measure the time between the application of the stimulus and the beginning of the first observable response(increase in force). The computer can’t “look ahead,” anticipating the change in active force. To measure the length of the latent period using the computer, click the Measure button. Then click the right arrow button next to the Time window repeatedly until you notice the first increase in the Active Force window. This takes you beyond the actual length of the latent period. Now click the left arrow button next to the Time window until the Active Force window again reads zero. At this point the computer is measuring the time between the application of the stimulus and the last point where the active force is zero (just prior to contraction). How long is the latent period? _2.78_ msec What occurs in the muscle during this apparent lack of activity? The muscle is reacting internally and getting the signals to contract. Investigating Graded Muscle Response to Increased Stimulus Intensity 1. Click Clear Tracings if there are tracings on your screen. 2. Set the voltage to 0.0, and click Stimulate. 3. Click Record Data. If you decide to redo a single stimulus, choose the data line in the grid and click Delete Line to erase that single line of data. If you want to repeat the entire experiment, click the Clear Table button to erase all data recorded to that point. 4. Repeat steps 2 and 3, increasing the voltage by 0.5 each time until you reach the maximum voltage of 10.0. Be sure to select Record Data each time. 5. Observe the twitch tracings. Click on the Tools menu and then choose Plot Data. 6. Use the slider bars to display Active Force on the Y-axis and Voltage on the X-axis. 7. Use your graph to answer the following questions: What is the minimal, or threshold, stimulus? ___0_______ V What is the maximal stimulus? ___8.2_______ V How can you explain the increase in force that you observe? The stronger the stimulus, the more the muscle reacts to that stimulus. 8. Click Print Plot at the top left corner of the Plot Data window to print a hard copy of the graph. When finished, click the X at the top right of the plot window. 9. Click Tools → Print Data to print your data.■ Investigating Treppe When a muscle first contracts, the force it is able to produce is less than the force it is able to produce in subsequent contractions within a relatively narrow time span. A myogram, a recording of a muscle twitch, reveals this phenomenon as the treppe, or staircase, effect. For the first few twitches, each successive stimulation produces slightly more force than the previous contraction as long as the muscle is allowed to fully relax

between stimuli, and the stimuli are delivered relatively close together. Treppe is thought to be caused by increased efficiency of the enzyme systems within the cell and increased availability of intracellular calcium. 1. The voltage should be set to 8.2 volts, and the muscle length should be 75 mm. 2. Drag the 200 msec button to the center of the X-axis time range. 3. Be sure that you fully understand the following three steps before you proceed. • Click Single Stimulus. Watch the twitch tracing carefully. • After the tracing shows that the muscle has completely relaxed, immediately click Single Stimulus again. • When the second twitch completes, click Single Stimulus once more and allow the tracing to complete. 4. Click Tools → Print Graph. What happens to force production with each subsequent stimulus? Investigating Wave Summation As demonstrated in Activity 3 with single stimuli, multiple motor unit summation is one way to increase the amount of force...
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