Activity 1 The Muscle Twitch and the Latent Period
1. 1.Define the terms skeletal muscle fiber, motor unit, skeletal muscle twitch, electrical stimulus, and latent period 2.What .is the role of acetylcholine in a skeletal muscle contraction? Skeletal muscle fiber-Skeletal muscle fibers are the individual cells that are bound together in fascicles. Many fascicles are bound together to make a muscle
Motor unit-A motor unit consists of a motor neuron and all of the muscle fibers in innervates.
Skeletal muscle twitch-A skeletal muscle twitch is a mechanical response to a single action potential.
Electrical stimulus- used to mimic the contractions of skeletal muscles Latent period-Latent period is the period of time between the generation of an action potential in a muscle cell and the start of muscle contraction.
The role of acetylcholine in a skeletal muscle contraction is to bind specific receptors on the sarcolemma to open sodium channels so calcium ions can be released into the sarcoplasm
2.Describe .the process of excitation-contraction coupling in skeletal muscle fibers The process of excitation-contraction coupling in skeletal muscle fibers is that the end plate potential triggers a series of events that results in the contraction of a muscle cell. End plate potential is caused by the events that occur at the neuromuscular junction.
3. Describe the three phases of a skeletal muscle twitch.
The first phase is the latent period- this is the period of time that elapses between the generations of an action potential in a muscle cell. The second phase is the contraction phase- in this phase, the muscle fibers shorten. The third phase is the relaxation phase- in this phase, the muscle is going back to its original stae of relaxation and it will once again lengthen.
4. Does the duration of the latent period change with different stimulus voltages? How well did the results compare with your prediction? With different stimulus voltages, the duration of the latent period stays the same no matter what. I though the latent period would change and my results proved otherwise. 5. At .the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane depolarization? At the threshold stimulus, sodium ions start to move into the cell to bring about the membrane depolarization.
Activity 2 The Effect of Stimulus Voltage on Skeletal Muscle Contraction
1. 1.Describe the effect of increasing stimulus voltage on isolated skeletal muscle. Specifically, what happened to the muscle force generated with stronger electrical stimulations and why did this change occur? How well did the results compare with your prediction? As the stimulus voltage is increased, the amount of force in the muscle also increases. Each time the voltage was increased more fibers activate and it causes the total force produced to increase.
1. 2.How is this change in whole-muscle force achieved in vivo? It is achieved by recruitment of more motor units that increase the total muscle force.
1. 3.What happened in the isolated skeletal muscle when the maximal voltage was applied? When maximal voltage was applied, all the muscle fibers became depolarized and developed active force.
Activity 3 The Effect of Stimulus Frequency on Skeletal Muscle Contraction 1. 1.What is the difference between stimulus intensity and stimulus frequency? Stimulus intensity is the amount of force generated to start the stimulus. Stimulus frequency is the rate of delivered stimulus to the muscle
2. .In this experiment you observed the effect of stimulating the isolated skeletal muscle multiple times in a short period with complete relaxation between the stimuli. Describe the force of contraction with each subsequent stimulus. Are these results called treppe or wave summation? These results are a treppe. This is because there was a isolated skeletal muscle stimulated multiple times in a short period. Each stimulus increased progressively in step like formation to a maximum limit where a plateau in force occurred.
1. 3.How did the frequency of stimulation affect the amount of force generated by the isolated skeletal muscle when the frequency of stimulation was increased such that the muscle twitches did not fully relax between subsequent stimuli? Are these results called treppe or wave summation? How well did the results compare with your prediction this was a wave summation. This is because the stimulation frequency was increased to the point in which relaxation did not occur. The muscle twitches overlapped.
1. 4.To achieve an active force of 5.2 g, did you have to increase the stimulus voltage above 8.5 volts? If not, how did you achieve an active force of 5.2 g? How well did the results compare with your prediction? No, in order to achieve an active force of 5.2 g the stimuli just needed to be sent in a rapid amount in a short time. This is known as wave summation.
5.Compare and contrast frequency-dependent wave summation with motor unit recruitment (previously observed by increasing the stimulus voltage). How are they similar? How was each achieved in the experiment? Explain how each is achieved in vivo. Frequency-dependent wave summation is dependent upon stimulation by the nervous system. The motor recruitment depends upon the number of motor fibers available
Activity 4 Tetanus in Isolated Skeletal Muscle
1. 1.Describe how increasing the stimulus frequency affected the force developed by the isolated whole skeletal muscle in this activity. How well did the results compare with your prediction? when stimulus frequency is low, the force is also low. As the stimulus frequency was increased, the forces increased also.
1. 2.Indicate what type of force was developed by the isolated skeletal muscle in this activity at the following stimulus frequencies: at 50 stimuli/sec, at 140 stimuli/sec, and above 146 stimuli/sec. at 50-unfused tetanus
at 140- fused tetanus
at 146+- maximal tetanic tension
1. 3.Beyond what stimulus frequency is there no further increase in the peak force? What is the muscle tension called at this frequency? maximal tetanic tension
Activity 5 Fatigue in Isolated Skeletal Muscle
1. 1.When a skeletal muscle fatigues, what happens to the contractile force over time? the contractile force declines. Once the muscle reaches maximum tetanic tension, there is no longer an increase in force generated. At this stage, the muscle is fatigued and contractile force decreases
1. 2.What are some proposed causes of skeletal muscle fatigue? the build up of lactic acid, ADP and Pi in the muscle fibers.
1. 3.Turning the stimulator off allows a small measure of muscle recovery. Thus, the muscle will produce more force for a longer time period if the stimulator is briefly turned off than if the stimuli were allowed to continue without interruption. Explain why this might occur. How well did the results compare with your prediction? Turning the stimulator off, creates a period of rest. This allows concentrations intracellular concentrations of lactic acid to decrease. With that at a decreased level, the length of time for maximum tension increases. If the .
1. 4.List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal muscles. We cold delay onset fatigue by doing multiple sets of low repetition in exercise, allowing for multiple times of rest and a healthy diet and good amount of exercise.
Activity 6 The Skeletal Muscle Length-Tension Relationship
a. 1.What happens to the amount of total force the muscle generates during the stimulated twitch? How well did the results compare with your prediction? The total amount of force increases when the muscle is lengthened or shortened.
a. 2.What is the key variable in an isometric contraction of a skeletal muscle? Muscle length is the key variable in isometric contraction
b. 3.Based on the unique arrangement of myosin and actin in skeletal muscle sarcomeres, explain why active force varies with changes in the muscle’s resting length. Active force data changes as the resting muscle length changes. When the resting length is shortened, the active force increases. When the resting length is lengthened, the active force decreases. The change of active force is caused by the amount of myosis bound to actin.
a. 4.What skeletal muscle lengths generated passive force? (Provide a range.) 80-100
b. 5.If you were curling a 7-kg dumbbell, when would your bicep muscles be contracting isometrically? When the bicep muscle remains at a fixed length the bicep muscles will be contracting isometrically.
Activity 7 Isotonic Contractions and the Load-Velocity Relationship
a. 1.If were using your bicep muscles to curl a 7-kg dumbbell, when would your muscles be contracting isotonically? When one is extending their arm downwards and when lifting the dumbbell to the raised position is when the muscles will be contracting isotonically.
a. 2.Explain why the latent period became longer as the load became heavier in the experiment. How well did the results compare with your prediction? The latent period occurs when there is a rise in muscle tension but no movement of the muscle. The latent period increases as the weight of the load gets heavier, it is necessary for force to be generated by the muscle.
a. 3.Explain why the shortening velocity became slower as the load became heavier in this experiment. How well did the results compare with your prediction? Maximal shortening velocity is only occurring with minimal load. When it is a heavier weight, the speed in which the muscle lifts decreases with a slower velocity.
a. 4.Describe how the shortening distance changed as the load became heavier in this experiment. How well did the results compare with your prediction? The shortening distance decreased with a heavier load.
a. 5.Explain why it would take you longer to perform 10 repetitions lifting a 10-kg weight than it would to perform the same number of repetitions with a 5-kg weight. Because as the weight of the load increases, so does the latent period and the shortening velocity speeds. With the lighter weight, the muscle is able to contract quicker in latent and shortening velocity speeds. a.
6.Describe d happen in the following experiment: A 2.5-g weight is attached to the end of the isolated whole skeletal muscle used in these experiments. Simultaneously, the muscle is maximally stimulated by 8.5 volts and the platform supporting the weight is removed. Will the muscle generate force? Will the muscle change length? What is the name for this type of contraction? The muscle will still generate force and change length. This type of reaction is an isotonic contraction.
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