# PowerPhys:Activity 2 Twitch Contractions and Summation

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## Predictions

Effect of Muscle Fiber Length on Contraction

As muscle fiber length increases: contraction force increases, becoming maximum at an optimal length, then decreasing at longer lengths.

Effect of Stimulation Frequency on Contraction
As the frequency of stimulation increases, the force of contraction: increases.

### Materials and Methods

• Measurement of Threshold Stimulus
•  Dependent Variable: contraction force.
•  Independent Variable: stimulation voltage
•  Controlled Variables: temperature, frequency of stimulation and muscle fiber length. Effect of Muscle Length on Contraction
• Dependent Variable: contraction force.
• Independent Variable: muscle fiber length.
•  Controlled Variables: temperature, stimulation voltage, frequency of stimulation. Effect of Stimulation Frequency on Contraction
• Dependent Variable: contraction force.
•  Independent Variable: frequency of stimulation.
•  Controlled Variables: temperature, stimulation voltage.
• What structure was stimulated to cause a muscle contraction? The motor unit was stimulated.
• Explain why the temperature of the water bath was 35ºC (95 ºF). The water bath was 35ºC because is near the mouse’s normal body temperature. 6. At a stimulation frequency of 15 Hz how many stimuli were there per second? There are 15 pulses (stimuli) per second.

## Results

1. What is the maximum force generated at the threshold stimulus? The maximum force generated was 0.084 gf.
2. How does increasing voltage above threshold stimulus affect force development? The stimulus is measured in volts, and its role is to cause depolarization of the muscle fiber or the nerve. A minimum voltage is needed in order to induce an action potential. This minimal voltage is called a threshold stimulus. Once the depolarization reaches the threshold stimulus, the muscle fiber or the nerve will generate an action potential, and will cause a contraction. As the stimulus voltage is increased past the muscle’s threshold voltage, an increase in the amount of force in the entire muscle can be observed. By increasing the voltage above the threshold stimulus, also the force generated from the muscle increases. An increase in voltage means that it is delivered to the whole muscle, and more motor units are being activated. As a result the total force produced by the muscles increases.
3. What was the optimal muscle length (muscle length that generated the maximum force)? It was between 8.0mm-9.0mm.
4. What was the maximum force generated at optimal length? 0.084 gf. 7. How does increasing muscle fiber length affect force of muscle contraction? A muscle fiber will develop its greatest tension when there is an optimal zone of overlap between thick and thin filaments. The length of a muscle impacts its force-producing capability because of the corresponding overlap of thick and thin filaments at varying lengths. When a muscle fiber is stimulated to contract and that muscle is at an optimal length, indicated by the greatest possible overlap of thick and thin filaments, maximal strength is produced. If a muscle is too short or too long, less than optimal force is produced due to the lack of filament overlap and binding site availability.
5. What was the force of contraction at a stimulation frequency of 22.5? In table 6 I do not have a value for 22.5. I have 15 stimuli per second and 30 stimuli per second. So the value for 22.5 stimuli per second would be between greater than 0.120 gf and less 0.220 gf.
6.  At what frequency of stimulation was the maximum force generated? 60 Hz. 10. What was the maximum force generated in this experiment? 0.277 gf
7. How does increasing stimulation frequency affect force production? The tension produced during a sustained contraction is greater than that produced by a single twitch. The more stimuli per second, the greater the force generated by the muscle due to a sustained contraction. Muscle tension depends on the frequency of stimulation of motor units and the number of motor units stimulated. By increasing the frequency of muscle stimulation, a sustained force generation is being produced.

## Discussion

1. Explain why muscle contraction does not occur below the threshold stimulus. It does not occur below the threshold stimulus because it needs a minimum voltage that can induce an action potential. The stimulus causes depolarization in either the nerve or muscle. If depolarization doesn’t reach the threshold, then the nerve or the muscle will not generate that necessary action potential to cause a muscle contraction. A subthreshold stimulus does not cause an action potential because it does not bring the membrane potential to the threshold.
2. The maximum stimulation voltage used was 500 mV. If the stimulation voltage was increased to 600mV volts, would the force of contraction decrease, stay the same, or increase? Explain your choice. The force of contraction will decrease. Once the stimulation voltage rises above the threshold, force generation increases as the stimulation voltage increases. However, this trend does not continue past the optimal amount of voltage that generates the maximum force. Maximum force occurs when all the motor units of a muscle are stimulated and all muscle fibers are contracting. After reaching this optimal threshold, there is a voltage above which any further increase in stimulation intensity results in no further increase in force generation. Thus a stimulus to the muscle greater than the maximal stimulus (voltage) does not produce a greater force.
3. Explain why the force of contraction changes with muscle fiber length. There is a muscle length-tension relationship between the length of the fiber and the force produced by that fiber at that length. When an isolated muscle fiber is stretched to the point of minimal or maximal overlap of actin and myosin and then stimulated to contract, the force of contraction measured is minimal. On the other hand, if an isolated muscle fiber is stimulated to contract when there is an optimal overlap of actin and myosin the force produced will be maximal.
4. Compare the maximum force generated when stimulation frequency was increased with the maximum force generated at threshold stimulus. In your opinion, which experiments resulted in the highest level of intracellular calcium? The maximum force generated, when stimulation frequency was increased, was 0.227 gf (for 60Hz) vs. the maximum force generated at threshold stimulus was 0.084 gf (for 400 mV). The experiment that resulted in the highest level of intracellular Ca was when stimulation frequency was increased.
5. Explain why differences in intracellular calcium levels result in differences in force production. Calcium has an important role in the regulation of muscle contraction. An increase in the Ca level in the sarcoplasm starts the sliding of thin filaments, but when the level of Ca in the sarcoplasm declines, sliding will stop. By increasing the Ca concentration in the sarcoplasm, muscle contraction starts. A decrease in it stops the contraction. Also, when a muscle fiber is relaxed, the Ca concentration in the sarcoplasm is very low. The lower the Ca concentration will result in lesser force production. If more force is needed, then it triggers an increase in Ca concentration in the sarcoplasm.
6.  In the experiment Effect of Stimulation Frequency on Contractile Force, what were the muscle fiber length and the stimulation voltage? Why was this muscle fiber length and stimulation voltage used? The muscle length was from 6.5, increasing by .5 to 10.5, and the stimulation voltage was 400 mV. In the first experiment the measurements were made at a muscle length of 9mm. The stimulation voltage of 400 mV was used because this value represents the threshold stimulus. Different muscle lengths were used in order to find the optimal balance between length and force of the muscle fiber.
7. Restate your predictions that were correct and give data from your experiment that support them. My predictions were supported by the data from the experiment. As the length of the muscle increased so did the force until it reached an optimum balance between length and maximum force obtained. The optimal length was between 8 and 9 mm resulting in a maximum force of 0.084 gf. At a 9.5 mm length, the force declined to 0.076gf. My second prediction was also right. An increase in frequency results in an increase in force. At a frequency of 7.5 Hz the force was 0.112 gf, compared to a frequency of 60 Hz where the force has increased to 0.277gf.

## Application

1. Botulinum toxin is taken up by axon terminals and inhibits the ability of a motor neuron to stimulate a skeletal muscle motor unit to contract by preventing the release of acetylcholine from the axon terminal. Explain how this prevents skeletal muscle contraction. This toxin blocks exocytosis of synaptic vesicles at the neuromuscular junction. Therefore Ach (responsible for signaling muscles to contract) is not released, and muscle contraction does not occur.
2. Explain why injecting Botox (derived from botulinum toxin) into a superficial facial muscle reduces the appearance of deep facial wrinkles called muscle lines. Hint: Superficial facial muscles are attached at one end to skin. When Botox is injected deep to the wrinkles, it causes a temporary paralysis of the muscles whose excessive contraction are causing the wrinkles.

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