PowerPhys Action Potentials Lab Activity

Laboratory Report Top of Form Activity: Twitch Contractions and Summation Name: Estrella Salazar Bottom of Form Instructor: Professor Rejeana Stephens Date: Top of Form 11.04.2022 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 contraction force: Increases Materials and Methods Measurement of Threshold Stimulus Dependent Variable: contraction force Independent Variable: stimulation voltage Controlled Variables: temperature frequency of stimulation 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? Muscle contraction was caused by stimulating the motor unit. Explain why the temperature of the water bath was 35ºC (95 ºF). To match the mouse's natural body temperature, the water bath was heated to 35 C (95 F). At a stimulation frequency of 15 Hz how many stimuli were there per second? There were 15 stimuli presented every second at a stimulation frequency of 15 Hz. Results Measurement of Threshold Stimulus   Voltage (mV) 100 mV 200 mV 300 mV 400 mV 500 mV Maximum Force (gf) (Motor unit 1) 0 0 0 0.098 0.098 Effect of Stimulation Value on Twitch Contraction Force What is the threshold stimulus? The stimulus threshold is 400 mV. What is the maximum force generated at the threshold stimulus? The threshold stimulus produces a maximum force of 0.098 gf. How does increasing voltage above threshold stimulus affect force development? A threshold stimulus is required to generate an action potential. The force produced by the muscle also increases as the voltage is raised above the threshold stimulus. Muscle Length and Contraction Force Muscle Length (mm) 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 Maximum Force (gf) (Motor unit 1) 0.081 0.088 0.097 0.099 0.099 0.099 0.090 0.079 0.072 0.081 0.087 0.096 0.098 0.098 0.098 0.089 0.079 0.072 0.080 0.087 0.096 0.098 0.098 0.098 0.089 0.078 0.071 Average 0.081 0.087 0.096 0.098 0.098 0.098 0.089 0.079 0.072 Effect of Muscle Fiber Length on Contraction Force What was the average force of contraction at a muscle length of 7.0 mm? At a muscle length of 7.0mm, the average contraction force was 0.087 gf. What was the optimal muscle length (muscle length that generated the maximum force)? The optimal muscle length was between about 8 mm and 9 mm. What was the maximum force generated at optimal length? The maximum force generated at optimal length was 0.099 gf. How does increasing muscle fiber length affect force of muscle contraction? After a certain point, lengthening the fiber results in a reduction in the maximum force generated. Stimulation Frequency and Contractile Force   Stimuli per second 7.5 15 30 45 60 Maximum Force (gf) (Motor unit 1) 0.104 0.125 0.220 0.245 0.261 Effect of Stimulation Frequency on Contraction Force 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 0.18 gf as estimated from graph 6. At what frequency of stimulation was the maximum force generated? Maximum force was produced at a stimulation frequency of 60 Hz, resulting in 0.261 gf. What was the maximum force generated in this experiment? The maximum force generated in this experiment was 0.261 gf. How does increasing stimulation frequency affect force production? An increased frequency of muscle stimulation results in a sustained force generation. Discussion Explain why muscle contraction does not occur below threshold stimulus. Because the minimum voltage required to produce an action potential was not reached, muscle contraction does not take place in response to stimuli below the threshold. The nerve or the muscle will not produce an action potential to cause a muscle contraction if depolarization falls short of the threshold. When stimulation voltage was increased above threshold, did the force of contraction increase, decrease, or stay the same? Explain why this occurred. The force of contraction decreased as stimulation voltage was raised above threshold. A stimulus that is stronger than the stimulus' maximum does not result in more force. When a muscle's motor units are all stimulated, the force will be at its maximum. However, once the optimal threshold is reached, a stimulus above it will not cause the force output to increase any further. Explain why force of contraction changes with muscle fiber length. The length of the muscle fibers and the force of contraction are correlated. The force of contraction is minimal when a muscle fiber is stretched to the point where actin and myosin overlap minimally or maximally. But if a muscle fiber is stimulated when actin and myosin are optimally overlapped, the force generated will be the greatest. 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? Increased stimulation frequency resulted in a maximum force of 0.261 gf at 60Hz. At a stimulus threshold, the highest force produced was 0.098 gf at 400 mV. The experiment in which the stimulation frequency was increased produced the highest level of intracellular calcium Explain why differences in intracellular calcium levels result in differences in force production. Myosin and actin filaments move as a result of calcium concentrations in the sarcoplasm. Muscle contraction results from an increase in calcium. The contraction is halted and the filaments are prevented from exerting any force by a decrease in it. In the experiment Effect of Stimulation Frequency on Contractile Force, why was muscle fiber length set to optimal length? Because this number indicates the threshold stimulus, the muscle fiber length was set to optimum length in the experiment Effect of Stimulation Frequency on Contractile Force. 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. The contraction force of the muscle increases with muscle length, reaching a maximum at an ideal length before declining at longer lengths. The greatest force was 0.098 gf when the length was between 8 and 9 mm. The force of contraction decreased as the length increased. Increased force is the result of increased frequency. The force was 0.103 at a frequency of 7.5 Hz and 0.261 at a frequency of 60 Hz. Application 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. At the neuromuscular junction, this toxin prevents synaptic vesicles from being exocytosed. As a result, acetylcholine can be released to tell muscles to contract, preventing actual contraction. 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 temporarily paralyzes the muscles in the wrinkles when it is injected deeply into them. As a result, these muscles won't be able to contract excessively like they once could.