Exercise 2 Skeletal Muscle Physiology The Muscle Twitch and the Latent Period 1. Define the terms skeletal muscle fiber, motor unit, skeletal muscle twitch, electrical stimulus, and latent period. Motor unit- single ? motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type Skeletal muscle fiber- long and narrow cells that often span the entire length of a muscle Skeletal muscle twitch- an activation of a muscle by firing a motor neuron Electrical stimulus- the application of an electric current by means of a prosthesis to stimulate and restore partial function to a muscle disabled by neurologic lesions Latent period-a seemingly inactive period, as that between exposure to an infection and subsequent illness, or that between the instant of stimulation and the beginning of response. . What is the role of acetylcholine in a skeletal muscle contraction? Acetylcholine is the chemical signal the nerve sends to the muscle to cause contraction 3. Describe the process of excitation-contraction coupling in skeletal muscle fibers. ACH is released at the end-plate potential. ACH diffuses into the sarcolemma, attaches to the receptors in the motor end plate and causes a change in the ions permeability that creates graded depolarization of the end-plate potential. Basically, it attaches to the receptors in the motor-end plate and creates the beginning process of triggering muscle contractions. . Describe the three phases of a skeletal muscle twitch. Latent Phase: During this phase there is no muscle contraction. This phase is the period of time prior to muscle contractions. The latent phase, chemical changes in the cells occur in preparation for muscle contractions. Contraction Phase: Is the period in which the muscle tension is at it’s highest. This is the time where the greatest amount of muscle force is generated. It begins at the end of the latent period and ends when tension peaks at it’s highest.
Relaxation Phase: The period of contraction after the contraction phase, and ends at end of muscle contraction. 5. Does the duration of the latent period change with different stimulus voltages? How well did the results compare with your prediction? No. The latent period remained the same 6. At the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane depolarization? The threshold is the time at which the sodium ions start moving into the cell to bring about the membrane depolarization. A C T I V I T Y 2
The Effect of Stimulus Voltage on Skeletal Muscle Contraction 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 past the muscle’s threshold voltage, the amount of force in the entire muscle increases. Each time the voltage was increased the force generated from the muscle also increased.
This occurs because as the voltage increases, and is delivered to the whole muscle 2. How is this change in whole-muscle force achieved in vivo? The increase of electrical current causes a progressive increase in the amount of muscle force based on the amount of the electrical current. 3. What happened in the isolated skeletal muscle when the maximal voltage was applied? All of the muscle fibers in the isolated skeletal muscle became depolarized and all developed active force. T Y 3 The Effect of Stimulus Frequency on Skeletal Muscle Contraction 1.
What is the difference between stimulus intensity and stimulus frequency? Stimulus intensity describes the amount of force generated to administer the stimulus and stimulus frequency refers to 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?
When stimulating the isolated skeletal muscle multiple times in a short period with complete relaxation between each stimuli the amount of force between each stimulus increased progressively in step like form to a maximum peak in which a plateau of the force occurred. This is known as treppe. 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? When the stimulation frequency was increased to the point in which muscle relaxation did not occur the amount of force progressively increased with each stimulus. When muscle twitches overlap, no muscles relax, and stimuli occurs one after another in a short amount of time, the muscle contraction is higher than if a single stand alone contraction had occurred 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, I did not have to increase the stimulus voltage above 8. 5 volts. In order to achieve an active force of 5. 2g the stimuli needed to be delivered in rapid succession in a short amount of time and not allowing for full relaxation of the muscle 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. A C T I V I T Y 4 Tetanus in Isolated Skeletal Muscle 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 the stimulus frequency was at the lowest (50s/s) the force was at its lowest level out of all of the experiments. As the stimulus frequency was increased to 130 s/s the force increased slightly but fused tetanus developed at the higher frequency.
When the stimulus frequency was increased to the amounts of 146-150 s/s, the force reached a plateau and maximal tetanic tension occurred, where no further increases in force occur from additional stimulus frequency. 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. 50- Unfused Tetanus 140-Fused Tetanus 146+ Maximal Tetanic Tension 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 T I V I T Y 5 Fatigue in Isolated Skeletal Muscle 1. When a skeletal muscle fatigues, what happens to the contractile force over time? When a skeletal muscle fatigues the contractile force declines due to previous contractile activity. Once the muscle has reached maximum tetanic tension, there is no longer an increase in force generated by the muscle. At this stage, the muscle becomes fatigued and contractile force starts to decrease over time. 2. What are some proposed causes of skeletal muscle fatigue?
The buildup of lactic acid, ADP, and Pi in the muscle fibers. One other reason comes from Calcium levels decreasing from muscle contractions. 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? a period of rest is created; thus allowing the concentrations of intercellular concentrations of Lactic Acid, ADP, and Pi to decrease.
With these concentrations being at a decreased level, the length of time that the muscle is able to maintain maximum tension increases. If the stimulator was not turned off for a short time, the muscle would not be allowed to reach a rest period and further fatigue would continue. 4. List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal muscles. Doing multiple sets of low repetition exercise, allowing for multiple times of rest periods, healthy diet, along with adequate exercise. A C T I V I T Y 6 The Skeletal Muscle Length-Tension Relationship . What happens to the amount of total force the muscle generates during the stimulated twitch? How well did the results compare with your prediction? Total force is altered by the starting resting length 2. What is the key variable in an isometric contraction of a skeletal muscle? 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 is generated from myosin thick filaments bind to thin actin filaments, engaging the cross bridge cycle and ATP hydrolysis.
Active force data changes as the resting length of the muscle changes. When the resting length of the muscle is shortened, the active force amount increases. When the resting length of the muscle is lengthened, the active force amount decreases. The change in the active force amount is completely caused by the amount of myosis bound to actin 4. What skeletal muscle lengths generated passive force? (Provide a range. ) 80, 90, and 100 5. If you were curling a 7-kg dumbbell, when would your bicep muscles be contracting isometrically? At any point in which the bicep muscle remains at a fixed length.
My best example is holding 2 dumbbells parallel to the floor and holding them in that position for 30 seconds A C T I V I T Y 7 Isotonic Contractions and the Load-Velocity Relationship 1. If you were using your bicep muscles to curl a 7-kg dumbbell, when would your muscles be contracting isotonically? When your arm extends downwards to the point of it being straight. (but not while it is straight) & when the arm rises from the down position back to the raised position. 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 or contraction of the muscle. Cross bridges cycle and when the muscle tension exceeds the load, muscle shortening happens. The latent period increases as the weight of the load gets heavier 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? The shortening velocity refers to the speed of the contraction from the muscle shortening while lifting a load.
Maximal shortening velocity is only attained with a minimal load. With a light load, the shortening velocity is at its Maximal shortening velocity. When the weight is heavy, the speed in which the muscle lifts the weight decreases in speed at a slower velocity. 4. Describe how the shortening distance changed as the load became heavier in this experiment. How well did the results compare with your prediction? 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.
It would take longer with the heavier weight because as the weight of the load increases, so does the latent period time and the shortening velocity speeds. With the lighter weight, the muscle is contracted quicker in both latent and shortening velocity speeds 6. Describe what would happen in the following experiment: A2. 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 generates force in terms of passive force in where the muscle does react to the electrical stimulus but the weight of the load pulling downwards would not allow the muscle to visibly move. The force is generated from stretching the muscle caused by the recoil of the tissue. If the platform that supports the weight is removed, and the 2. 5g weight is still connected, the weight would pull the muscle downwards causing it to lengthen. The muscle changing length would not be from the result of muscle contractions, but only from the weight pulling down. This type of contraction would be Isometric