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md_to_be md_to_be
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7 years ago
PEX-02-04. In Activity 4, you used a multiple stimulus at different rates and recorded a tracing that ran long enough to cross the screen 5 times.
When the muscle was stimulated at 150 stimuli/sec, the trace increased to a maximum value and reached a plateau within the first 80 msecs. Describe what you observed for the remainder of the tracing (before you clicked the "Stop Stimulus" button) and give 2 plausible cellular events that may explain why this situation occurred. (NOTE: WE SAW A DECREASE IN FORCE AFTER THE PLATEAU!(

2)  In the gym, you are attempting to squat a heavy weight, which requires almost 100% effort for you. Describe and explain the 3 factors that allow skeletal muscle to generate maximum tension.

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7 years ago
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In the gym, you are attempting to squat a heavy weight, which requires almost 100% effort for you. Describe and explain the 3 factors that allow skeletal muscle to generate maximum tension.

The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.

Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere. If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree so fewer cross-bridges can form. This results in fewer myosin heads pulling on actin and less muscle tension. As a sarcomere shortens, the zone of overlap reduces as the thin filaments reach the H zone, which is composed of myosin tails. Because myosin heads form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by the myofiber. If the sarcomere is shortened even more, thin filaments begin to overlap with each other, reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.

The primary variable determining force production is the number of myofibers (long muscle cells) within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, for example, the motor cortex of the brain only signals a few neurons of the biceps so only a few myofibers respond. In vertebrates, each myofiber responds fully if stimulated. On the other hand, when picking up a piano, the motor cortex signals all of the neurons in the biceps so that every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more because the tropomyosin is flooded with calcium.
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