What is the role of acetylcholine in a skeletal muscle contraction?
answer
Acetylcholine binds to receptors in the motor end plate, initiating a change in ion permeability that results in the end-plate potential.
question
Describe the process of excitation-contraction coupling in skeletal muscle fibers.
answer
Excitation-contraction coupling is the release of calcium which binds to troponin, removing the blocking action of tropomyosin so that myosin can bind to actin.
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Describe the three phases of a skeletal muscle twitch
answer
Latent period is the preparing for contraction.
Contraction is when muscle tension peaks
.
Relaxation period is at the end of muscle contraction.
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Does the duration of the latent period change with different stimulus voltages? How well did the results compare with your prediction?
answer
The latent period did not change in stimulus voltage.
question
At the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane depolarization?
answer
Sodium would move into the cell to bring about membrane depolarization.
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Describe the effect of increasing stimulus voltage on isolated skeletal muscle. Specifically, what happened to the muscle force generated with stronger electrical stimulations and why did this change occur?
answer
The active force increased as predicted to the point in which it reached a plateau and was no longer able to increase.
question
How is this change in whole-muscle force achieved in vivo?
answer
This is achieved by the recruitment of more muscle fibers over time.
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What happened in the isolated skeletal muscle when the maximal voltage was applied?
answer
All of the muscle fibers have been recruited and so the maximal force has been achieved.
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What is the difference between stimulus intensity and stimulus frequency?
answer
The stimulus intensity in the electrical changes that relate to the action potential. The frequency is the number of action potentials per minute.
question
After observing the effect of stimulating the isolated skeletal muscle multiple times in a short period with complete relaxation between the stimuli, describe the force of contraction with each subsequent stimulus. Are these results called treppe or wave summation?
answer
With complete relaxation, it would be treppe. This is staircase effect, where you see an increase in the forece/tension produced.
question
How did the frequency of stimulation affect the amount of force generated by the isolated skeletal muscle when the frequency of stimulation was increased such that the muscle twitches did not fully relax between stimuli? Are these results called treppe of wave summation?
answer
The voltage needed to increase because the tension wasn't great enough at the lower voltage. This is consistent with wave summation.
question
To achieve the active force of 5.2 g, did you have to increase the stimulus voltage above 8.5 volts? If not, how did you achieve an active force of 5.2g?
answer
Yes, it was necessary to increase the voltage above 8.5 volts to achieve their active force of 5.2 grams.
question
Compare and contrast frequency -dependent wave summation with motor unit recruitment (previously observed by increasing stimulus voltage). How are they similar? How was each achieved in the experiment? Explain how each is achieved in vivo?
answer
Frequency-dependent wave summation is dependent upon stimulation by the nervous system. The motor recruitment depends upon the number of motor fibers available.
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Describe how increasing stimulus frequency affected the force development by the isolated whole skeletal muscle in this activity.
answer
The force developed increases as the stimulus frequency increases - to a point.
question
Indicate what type of force was developed by the isolated skeletal muscle in this activity at the following stimulus frequencies: at 50 stimuli/sec. at 140 stimuli/ sec, and above 146 stimuli/sec
answer
At 50 stimuli/sec: 5.12g
At 140 stimuli/sec: 5.91g
Above 146 stimuli: 5.95g
question
Beyond what stimulus frequency is there not further increase in the peak force? What is the muscle tension called at this frequency?
answer
After 146 stimuli/sec there is no further increase in force. This is the maximal tetanic tension
question
When a skeletal muscle fatigues, what happens to the contractile force over time?
answer
When skeletal muscle fatigues, the contractile force decreases over time.
question
What are some proposed causes of skeletal muscle fatigue?
answer
The buildup of lactid acid, ADP, and inorganic phosphate are thought to be involved in muscle fatigue.
question
Turning the stimulator off allows a small measure of muscle recovery. Thus, the muscle will produce more force for a longer time period if the stimulator is briefly turned off than if the stimuli were allowed to continue without interruption. Explain why this might occur.
answer
When you increase the rest periods, you see an increase in the muscle tension produced.
question
List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal muscles.
answer
They could periodically rest during vigorous exercise.
question
What happens to the amount of total force the muscle generates during the stimulated twitch?
answer
Total force can increase or decrease depending upon the starting resting length. This is due to the length-tension relationship of the sarcomere.
question
What is the key variable in an isometric contraction of a skeletal muscle?
answer
The length-tension relationship. The passive force is important in determining the active force produced.
question
Based on the unique arrangement of myosin and actin in skeletal muscle sarcomeres, explain why active force varies with changes in the muscle's resting length.
answer
The active foreces vary with the number of crossbridges fromed, which changes with the resting length in the muscle.
question
What skeletal muscle length generated passive force? (provide a range)
answer
The muscle lengths from 80 -100 mm generated passive force.
question
If you were curling a 7kg dumbbell, would your muscle be contracting isometrically?
answer
No, they would be changing in length, so this would not be isometric contraction
question
If you were using your bicep muscles to curl a 7kg dumbbell, when would your muscles contracting isotonically?
answer
Yes, because your muscles are changing in length.
question
Explain why the latent period became longer as the load became heavier in the experiment.
answer
The latent period became longer because it takes more time to generate the force required.
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Explain why the shortening velocity became slower as the load became heavier in this experiment.
answer
When the weight is heavy, the speed in which the muscle lifts the weight decreases in speed at a slower velocity.
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Describe how the shortening distance changed as the load became heavier in this experiment.
answer
With an increase in weight, the shortening distance became shorter.
question
Explain why it would take you longer to perform 10 repetitions lifting a 10-kg weight than it would to perform the same number of repetitions with a 5-kg weight.
answer
It takes longer to perform 10 reps lifting 10-kg than 5-kg, because with 5-kg, maximal shortening velocity is attained much faster. The heavier, the load, the slower the muscle twitch and the lighter the load, the faster the muscle twitch.
question
Describe what would happen in the following experiment: A 2.5-g weight is attached to the end of the isolated whole skeletal muscle used in these experiments. Simultaneously, the muscle is maximally stimulated by 8.5 volts and the platform supporting the weight is removed. Will the muscle generate force? Will the muscle change length? What is the name for this type of contraction?
answer
This is an isometric contraction. The muscle would generate passive force when given an electrical stimulus but with the weight pullin down on the muscle, the muscle would not visibly move. Since the weight is puling down on the muscle, the muscle is going to lengthen but not because of a contraction, but because of the weight just pulling down on it.
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