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# BIOS252 Week 1 PowerPhys2 Lab Report 2

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LABORATORY REPORT

Activity 2: Twitch Contractions and Summation
Name: Trisha Mora
Instructor: Professor Vais
Date: 7/15/14

PREDICTIONS

Effect of Muscle Fiber Length on Contraction

1. As muscle fiber length increases: Contraction Force Increases.

Effect of Stimulation Frequency on Contraction

2. As the frequency of stimulation increases, the force of contraction: Increases.

MATERIALS AND METHODS

Measurement of Threshold Stimulus

1. Dependent Variable is the contraction force

2. Independent Variable is the stimulation voltage

3. Controlled Variables are the muscle fiber length, temperature, freq of stimulation.

Effect of Muscle Length on Contraction

1. Dependent Variable is the contraction force.

2. Independent Variable is the muscle fiber length

3. Controlled Variables are the stimulation voltage, temp, and freq of stimulation.

Effect of Stimulation Frequency on Contraction

1. Dependent Variable is the contraction force

2. Independent Variable is the frequency of stimulation.

3. Controlled Variables are the temperature and stimulation voltage.

4. What structure was stimulated to cause a muscle contraction?
The motor unit was stimulated

5. Explain why the temperature of the water bath was 35ºC (95 ºF).
Because 95 degrees is the perfect temp for optimal functioning of a mouse.

6. At a stimulation frequency of 15 Hz how many stimuli were there per second?
15 per second

RESULTS

See Table 4: Measurement of Threshold Stimulus
See Graph: Effect of Stimulation Value on Twitch Contraction Force

1. What is the threshold stimulus?
The stimulus that is just strong enough to depolarize the membrane to threshold. (400mV)

2. What is the maximum force generated at the threshold stimulus?
0.093 (gf)

3. How does increasing voltage above threshold stimulus affect force development?
By increasing the voltage above threshold stimulus the force increases. The more the voltage increase the more motor units are stimulated and activated thus increasing the force development.

See Table 5: Muscle Length and Contraction Force
See Graph: Effect of Muscle Fiber Length on Contraction Force

4. What was the force of contraction at a muscle length of 7.0 mm?
An average of 0.082 gf

5. What was the optimal muscle length (muscle length that generated the maximum force)?
The optimal muscle length is between 8-9 mm

6. What was the maximum force generated at optimal length?
0.093 (gf)

7. How does increasing muscle fiber length affect force of muscle contraction?
There is an increase in the force generated but after the threshold, the plateau occurs. Increasing the length and have a muscle length that is too long will affect the overlap of thick and thin filaments, and binding site availability.

See Table 6: Stimulation Frequency and Contraction Force
See Graph: Effect of Stimulation Frequency on Contraction Force

8. What was the force of contraction at a stimulation frequency of 22.5? Approximately 0.16 (gf) (Between 0.125 and 0.124 were 15 and 30 stimuli)

9. At what frequency of stimulation was the maximum force generated?
60 Hz

10. What was the maximum force generated in this experiment?
0.268 gf

11. How does increasing stimulation frequency affect force production?
The greater amount of stimuli per second the greater the force generated by that muscle due to sustained contraction. As the stimulation frequency increases the force production increases.

DISCUSSION

1. Explain why muscle contraction does not occur below threshold stimulus.
This is all-or-nothing theory. The motor unit works as many in one and until the force produced from the stimulation is high enough and the ca+ is through all of the motor units, they will all do nothing. When the minimal voltage needed meets threshold then a reaction takes place. So until the Ca+ is released to all triads the contraction will not occur.

2. The maximum stimulation voltage used was 500mV_____. 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 contraction would stay the same until it decreased. This happens because once the maximum force to create contraction is reached the muscles plateau (staying the same) until the force decreases. So it cannot go past maximum. The contraction goes no further, it is maxed out.

3. Explain why force of contraction changes with muscle fiber length.
There is a muscle length – tension relationship between the length and the force required between the muscles fiber. This id depending on the placement of the actin and myosin in the sarcomere. Then there is a perfect overlapping and the maximum contraction then occurs.

4. Compare the maximum force generated when stimulation frequency was increased with the maximum force generated at threshold stimulus. In your opinion, which experiment resulted in the highest level of intracellular calcium? The maximum force generated when stimulation was increased was 0.268gfat 60 mH, and the maximum force generated at threshold stimulus was 0.093gf at 400mZ. I believe the highest level of intracellular ca+ was when the stimulation frequency was increased resulting in 0.268 gf at 60 mH.

5. Explain why differences in intracellular calcium levels result in differences in force production. Intracellular calcium is needed for force production to stimulate contraction. The increase of ca+ causes a sliding of the filaments and when the ca+ decreases the sliding will cease. The increase starts the contraction. Thus, the more ca+ the more the force and the more the contraction.

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 fiber length was from 6.5 to 10.0 with a 0.5 increase increments. The stimulation voltage was 400 mZ. They gave a range of muscle fiber length to show the optimal length and force. The 400 mZ was used because this is the threshold stimulus from experiment 1.

7. Restate your predictions that were correct and give data from your experiment that support them. Restate your predictions that were not correct and correct them with supporting data from your experiment. As muscle fiber length increases: Contraction Force Increases. This was a correct prediction. As the length went from 6.5-9 mm the contraction force also increased from 0.075 to 0.092 gf. As the frequency of stimulation increases, the force of contraction: Increases. This too was also correct prediction. The experiment proved the increase. As the stimulation increased from 7.5 to 60 the force also increased to 0.268 from 0.107 gf.

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. The Botulinum prevents release of acetylcholine from the axon terminal. Therefore Ach is not released preventing a muscle contraction. I got help from a reference because after I studied switch contraction more these past two days, I realized there was more to it than I had previously thought.
Reference: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2856357/

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. Botox paralyzes the muscles and it relaxes them dissapating the wrinkle, however when the botox wears off, the muscle will strengthen and then pull causing a wrinkle.

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