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Laboratory Report

LABORATORY REPORT Activity: Name: Instructor: Date:

Twitch Contractions and Summation

03.01.2021

Predictions Effect of Muscle Fiber Length on Contraction 1. 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 2. As the frequency of stimulation increases, the force of contraction: increases

Materials and Methods Measurement of Threshold Stimulus 1. Dependent Variable contraction force

2. Independent Variable stimulation voltage

3. Controlled Variables temperature, frequency of stimulation, muscle fiber length

Effect of Muscle Length on Contraction 1. Dependent Variable contraction force

2. Independent Variable muscle fiber length

3. Controlled Variables temperature, stimulation voltage, frequency of stimulation

Effect of Stimulation Frequency on Contraction 1. Dependent Variable contraction force

2. Independent Variable frequency of stimulation

3. Controlled Variables temperature, stimulation voltage

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

5. Explain why the temperature of the water bath was 35 C (95 F).

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Laboratory Report

The temperature of the water bath was 35ºC (95 ºF) to mimic the mouse's normal body temperature.

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

Results Table 4: Measurement of Threshold Stimulus 100 mV 0

Maximum Force (gf) (Motor unit 1)

Voltage (mV) 300 mV 0

200 mV 0

400 mV 0.082

500 mV 0.082

Effect of Stimulation Value on Twitch Contraction Force Force (gf) 0.11 0.088

1. 100 mV 2. 200 mV 3. 300 mV 4. 400 mV 5. 500 mV

0.066 0.044 0.022 0 1

2

3

4

5

1. What is the threshold stimulus? Threshold stimulus is 400 mV. 2. What is the maximum force generated at the threshold stimulus? The maximum force generated at the threshold stimulus is 0.082 gf 3. How does increasing voltage above threshold stimulus affect force development? A minimum voltage (threshold stimulus) is needed in order to produce an action potential. By increasing the voltage above the threshold stimulus, also the force generated from the muscle increases.

Table 5: Muscle Length and Contraction Force Maximum Force (gf) (Motor unit 1) Trial 1 Trial 2 Trial 3 Averages

6.5

7.0

7.5

8.0

Muscle length (mm) 8.5

9.0

9.5

10

10.5

0.068 0.067 0.068 0.068

0.074 0.073 0.073 0.073

0.081 0.080 0.081 0.081

0.083 0.082 0.082 0.082

0.083 0.082 0.082 0.082

0.083 0.082 0.082 0.082

0.075 0.074 0.075 0.075

0.066 0.065 0.066 0.066

0.060 0.059 0.060 0.060

Effect of Muscle Fiber Length on Contraction Force Force (gf) 0.11 0.1 0.09 0.08 0.07 0.06 6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

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Muscle length (mm)

Laboratory Report

4. What was the average force of contraction at a muscle length of 7.0 mm? The average force of contraction at a muscle length of 7.0mm was 0,073 gf.

5. What was the optimal muscle length (muscle length that generated the maximum force)? The optimal muscle length (muscle length that generated the maximum force) was between about 8 mm and 9 mm. 6. What was the maximum force generated at optimal length? The maximum force generated at optimal length was 0.083 gf. 7. How does increasing muscle fiber length affect force of muscle contraction? Increasing muscle fiber length increases force of muscle contraction up to a certain point. After a certain point, increasing fiber length actually decreases the maximum force produced.

Table 6: Stimulation Frequency and Contraction Force

Maximum Force (gf) (Motor unit 1)

7.5 0.103

15 0.120

Stimuli per second 30 45 0.217 0.264

60 0.263

Effect of Stimulation Frequency on Contraction Force Maximum force (gf) 0.35 0.3 0.25 0.2 0.15 0.1 7.5

15

30

45

60

pulses/sec (Hz)

8. What was the force of contraction at a stimulation frequency of 22.5? The force of contraction at a stimulation frequency of 22.5 is about 0.17 gf as estimated from graph 6.

9. At what frequency of stimulation was the maximum force generated? Maximum force generated at a frequency of stimulation of 45hz, producing 0.264 gf. 10. What was the maximum force generated in this experiment? The maximum force generated in this experiment was 0.264 gf. 11. How does increasing stimulation frequency affect force production? Increasing stimulation frequency increases the force produced up to a certain point where it plateus. By increasing the frequency of muscle stimulation, a sustained force generation is being produced.

Discussion 1. Explain why muscle contraction does not occur below threshold stimulus. Muscle contraction does not occur below threshold stimulus because the minimum voltage needed to produce an action potential was not reached. To explain more, if depolarization doesn’t reach the threshold, then the nerve or the muscle will not generate an action potential to cause a muscle contraction. 2. When stimulation voltage was increased above threshold, did the force of contraction increase, decrease, or stay the same? Explain why this occurred.

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Laboratory Report

When stimulation voltage was increased above threshold, the force of contraction decreased. A stimulus greater than maximal stimulus does not produce more force. Maximal force will occur when all the motor units of a muscle are stimulated. However, after reaching the optimal threshold, a stimulus above that will not result in further increase in the force output. 3. Explain why force of contraction changes with muscle fiber length. There is a relationship between force of contraction and muscle fiber length. When a muscle fiber is stretched where it reaches a minimal/ maximal overlap of actin and myosin the force of contraction will be small. However, if a muscle fiber is stimulated when there is 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 experiment resulted in the highest level of intracellular calcium? The maximum force generated when stimulation frequency was increased was 0.264 gf at 40Hz. The maximum force generated at threshold stimulus was 0.082 gf at 400 mV. The experiment that resulted in the highest level of intracellular calcium was when stimulation frequency was increased.

5. Explain why differences in intracellular calcium levels result in differences in force production. Calcium concentrations in the sarcoplasm are responsible for the movement of myosin and actin filaments. Therefore, an increase in calcium leads to muscle contraction. A decrease in it stops the contraction and prevent the filaments from creating any force. 6. In the experiment Effect of Stimulation Frequency on Contractile Force, why was muscle fiber length set to optimal length? In the experiment Effect of Stimulation Frequency on Contractile Force, the muscle fiber length was set to optimal length because this value represents the threshold stimulus. 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 the length of the muscle increased contraction force increases, becoming maximum at an optimal length, then decreasing at longer lengths The optimal length was between 8 and 9 mm resulting in a maximum force of 0.082 gf. Lengths past this resulted in decrease in force of contraction. An increase in the frequency results in an increase in force. At a frequency of 7.5 Hz the force was 0.103 while at frequency 45 hz the force was 0.264.

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 the exocytosis of synaptic vesicles at the neuromuscular junction. Therefore acetylcholine will be able to be released to signal the muscles to contract which results in no contraction.

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 located there. Therefore these muscles will not be able to excessivley contract like they once did.

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