Exercise 2: Skeletal Muscle Physiology: Activity 2: The Effect of Stimulus Voltage on Skeletal Muscle Contraction LabReport

Pre-lab Quiz ResultsYou scored 100% by answering 4 out of 4 questions correctly.

1. Skeletal muscle fibers are innervated (stimulated) by You correctly answered: c. motor neurons.

2. A single action potential propagating down a motor axon results inYou correctly answered: d. a single action potential and a single contractile event in the muscle fibers it innervates.

3. In resting skeletal muscle, calcium is stored inYou correctly answered: c. the sarcoplasmic reticulum.

4. During the latent period for an isometric contractionYou correctly answered: c. the cellular events involved in excitation-contraction coupling occur.

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Experiment ResultsPredict Question:Predict Question: As the stimulus voltage is increased from 1.0 volt up to 10 volts, what will happen to the amount of activeforce generated with each stimulus?Your answer : c. The active force will first increase and then plateau at some maximal value as the stimulus voltageincreases.

Stop & Think Questions:What do you see in the active force display when the stimulus voltage is set to 0.0, and why does this observation makesense?You correctly answered: a. 0.00 g; there was no activation of skeletal muscle fibers by this stimulus.

What is the lowest stimulus voltage that induces active force in the skeletal muscle?You correctly answered: b. threshold voltage

6. Enter the threshold voltage for this experiment in the field below and then click Submit to record your answer in the labreportYou answered: 0.8 volts

12. Enter the maximal voltage for this experiment in the field below and then click Submit to record your answer in the labreport.You answered: 10.0 volts

Experiment Data:

Voltage Length Active Force Passive Force Total Force0.0 75 0.00 0.00 0.000.2 75 0.00 0.00 0.000.8 75 0.02 0.00 0.021.0 75 0.15 0.00 0.151.5 75 0.43 0.00 0.432.0 75 0.66 0.00 0.662.5 75 0.87 0.00 0.873.0 75 1.04 0.00 1.043.5 75 1.19 0.00 1.194.0 75 1.32 0.00 1.324.5 75 1.42 0.00 1.425.0 75 1.51 0.00 1.515.5 75 1.59 0.00 1.596.0 75 1.65 0.00 1.656.5 75 1.70 0.00 1.707.0 75 1.74 0.00 1.747.5 75 1.78 0.00 1.788.0 75 1.81 0.00 1.818.5 75 1.82 0.00 1.829.0 75 1.82 0.00 1.829.5 75 1.82 0.00 1.8210.0 75 1.82 0.00 1.82

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Post-lab Quiz ResultsYou scored 100% by answering 5 out of 5 questions correctly.

1. Motor unit recruitment refers toYou correctly answered: a. an increase in the number of active muscle fibers to increase the force developed in a muscle.

2. Active tension (or force) in a skeletal muscle fiber results fromYou correctly answered: a. activation of cross bridge cycling via increased intracellular calcium levels.

3. The ________ is the minimal stimulus needed to cause a depolarization of the muscle plasma membrane (sarcolemma).

You correctly answered: d. threshold voltage

4. By definition, the ________ is the amount of stimulus required to successfully recruit all the muscle fibers intodeveloping active force.You correctly answered: c. maximal voltage

5. Why was a maximal voltage observed in this experiment?You correctly answered: b. At the maximal voltage, all the muscle fibers contained in this muscle are depolarized and theyall develop active force (that is, they were all successfully recruited).

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Review Sheet Results1. Describe the effect of increasing stimulus voltage on isolated skeletal muscle. Specifically, what happened to the muscleforce generated with stronger electrical stimulations and why did this change occur? How well did the results compare withyour prediction?Your answer:From threshold voltage as the stimulus increases, so does the force of the contraction until it reaches maximum voltage. Atthis level, the more stimulus, the muscle will still contract with the same force.

2. How is this change in whole-muscle force achieved in vivo?Your answer:The increase of electrical current progressively increases the amount of muscle force based on the amount of the electricalcurrent. The electric current must surpass the threshold voltage. Once the stimulus voltage goes beyond the threshold itcreates whole muscle force. The force gets stronger with the higher stimulus voltage until it plateaus at maximal tension,that is the maximum amount of voltage that it will achieve to increase the force, but only to a maximum point. Whole-muscleforce is achieved in vivo by recruitment of additional motor units, increasing the total muscle force produced.

3. What happened in the isolated skeletal muscle when the maximal voltage was applied?Your answer:All of the muscle fibers in the isolated skeletal muscle became depolarized, and all developed active force. Each of thefibers involved in the active force generated from the maximal voltage.