Exercise 2 Activities 1-7

Put these events in the correct chronological sequence: 1 - end-plate potentials trigger action potentials 2 - T tubules convey potentials into the interior of the cell 3 - acetylcholine binds to receptors on the motor end plate 4 - Ca2+ is released from the sarcoplasmic reticulum CC E2A-14

3, 1, 2, 4

1 Why does the stimulated muscle force begin to decrease over time despite the maintained stimuli? (Note that a decrease in maximal force indicates muscle fatigue is developing.) More than one of these answers could be correct. 2 Why did the length of the intervening rest period affect the length of time the skeletal muscle can maintain maximum tension once the stimulator is turned on again? Intracellular concentrations of ADP and Pi declined during the rest period.

E2 A5

1 During an isometric contraction You correctly answered: the skeletal muscle is generating force, but it remains at a fixed length. 2 The force that results from muscles being stretched is You correctly answered: passive force. 3 Active force You correctly answered: is determined by the amount of myosin bound to actin. 4 When you generate the isometric length-tension curve, which of the following forces will not be indicated on your screen? You correctly answered: tetanic force. 5 Passive force in skeletal muscle is largely caused by You correctly answered: the protein titin.

E2 A6

1 Note the dip in total force at a muscle length of 90 mm as compared to the total force at a muscle length of 80 and 100 mm. Why does this occur? At this muscle length, active force has decreased in value and passive force has not yet increased to a significant value.

E2 A6

1 Skeletal muscle fibers are innervated (stimulated) by motor neurons. 2 A single action potential propagating down a motor axon results in a single action potential and a single contractile event in the muscle fibers it innervates. 3 In resting skeletal muscle, calcium is stored in the sarcoplasmic reticulum. 4 During the latent period for an isometric contraction the cellular events involved in excitation-contraction coupling occur.

E2A-2

Increasing the frequency of stimulation, producing overlapping twitches, results in: CC E2A-14

Elevated Ca2+ levels in the sarcoplasm

Which of the following would occur during in vivo stimulation of muscle contraction via the nervous system but NOT during contraction induced by electrical stimulation applied to whole skeletal muscle? CC E2A-14

Na+ influx through ligand gated channels

which of these should exert the greatest passive force? CC E2A5-7

a sarcomere at 175% of its resting length

When a given muscle contracts isotonically under increasing load, as shown below, which of the following should you expect? CC E2A5-7

an increase in the delay between muscle stimulation and shortening

Which of the following explains why the relaxation phase is longer than the contractile phase? CC E2A-14

contraction involves facilitated diffusion of Ca2+ while relaxation involves the active transport of Ca2+

Rigor mortis occurs 3-4 hours post mortem and generally lasts from 1-4 days. it is characterized by stiffening of the limbs of a corpse resulting from chemical changes in the muscle that cause a state of fixed contraction Which of the following is most likely the cause of rigor mortis? CC E2A5-7

depleted ATP in the sarcoplasm of skeletal muscle

Elevated levels of P1 in the sarcoplasm of skeletal muscle fibers would decrease the release of Pi from myosin. what direct impact would this have on the cross bridge cycle? CC E2A5-7

myosin wouldnt be able to initiate the power stroke if Pi isnt released

Holding voltage and duration of electrical stimulation steady, while increasing the frequency with which it is delivered would stimulate CC E2A-14

summation

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? 2 - How is this change in whole-muscle force achieved in vivo? 3 - What happened in the isolated skeletal muscle when the maximal voltage was applied? E2 A1

1 - As the stimulus voltage on isolated skeletal muscle increased from 1.0 volts up to 10 volts, the active force of the muscle increased as well, just as I had predicted. Additionally, the total force inside the muscle also increased with increasing stimulus voltage. This is due to the fact that as more voltage is delivered to the whole muscle, more muscle fibers are activated and therefore the total force produced by the entire muscle increases. 2 - The change in whole-muscle force reflects the number of active motor units. A strong muscle contraction would imply that many motor units are activated with each unit developing maximum tension/force. This phenomenon is called motor unit recruitment. By increasing. the number of active motor units, we can produce a steady increase in muscle force. 3 - When maximum voltage was applied to the isolated skeletal muscle fibers, maximal tension in the whole muscle occurred. Both the total force and active force remained at the same value.

1 What is the difference between stimulus intensity and stimulus frequency? 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? 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? 4 - To achieve an active force of 5.2, did you have to increase the stimulus voltage above 8.5 volts? If not, how did you achieve an active force of 5.2? How well did the results compare with your prediction? 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. E2 A3

1 - —Stimulus intensity refers to the voltage, for example an increase or decrease in the voltage is referring to the intensity of a particular stimulus.—Stimulus frequency is the rate of stimulus delivery to the muscle. 2 - This process is known as treppe (AKA the staircase effect). The force of contraction with each subsequent is slightly higher then the one before it. When the progressive increase in force generated when a muscle is stimulated in succession, such that the muscle twitches follow another one closely with each peaking slightly higher then than the one before is Treppe. 3 - As the stimulus frequency of stimulation increased, the muscle force generated by each successive stimulus increased at first and then decreased as the stimulus frequency becomes very high, so the results matched my prediction. This process is known as wave summation, and occurs when muscle fibers that are developing tension are stimulated again before the fibers have relaxed. This process occurs because the muscle fibers are already in a partially contracted state when subsequent stimuli are delivered. 4 - No I did not have to increase the stimulus voltage above 8.5 volts. I was able to achieve an active force of 5.3 by wave summation and rapidly increasing the stimulus frequency without the fibers relaxing. My prediction was therefore incorrect. 5 - Frequency dependent wave summation relies on the nervous system for stimulation which differs from motor unit recruitment relies on the number of active motor units. For example if you increase the number of active motor units, we would be able to produce a steady increase in muscle force. This process is achieved by recruiting additional motor units which increases the total muscle force. frequency dependent wave summation occurred by increasing the rate of stimulus delivery to the 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? 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. 3 - Beyond what stimulus frequency is there no further increase in the peak force? What is the muscle tension called at this frequency? E2 A4

1 Increasing the stimulus frequency the muscle tension generated by each successive stimulus will increase but there is a limit to this increase. The results directly matched my prediction. 2 - —At 50 stimuli per second the type of force that was developed by the isolated skeletal muscle was an Unfused Tetanus—At 140 stimuli per second the type of force that was developed by the isolated skeletal muscle was a Fused Tetanus —At 146 stimuli per second the type of force that was developed by the isolated skeletal muscle as a Maximal tetanic tension. 3 - Beyond the stimulus frequency of 146 stimuli per second there was no further increase in the peak force. This is known as the maximal tetanic tension since the stimulus frequency reaches a value beyond which no further increases in force are generated by the muscle.

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? 3 - Describe the process of excitation-contraction coupling in skeletal muscle fibers. 4 - Describe the three phases of a skeletal muscle twitch. 5 - Does the duration of the latent period change with different stimulus voltages? How well did the results compare with your prediction? 6 - At the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane depolarization? E2 A1

1 —Skeletal muscle fiber can be defined as an individual muscle cell.—A motor unit can be defined as a motor neuron and all of the muscle fibers it innervates—Skeletal muscle twitch can be defined as the mechanical response to a single action potential —Electrical Stimulus can be defined as what causes the muscle to contract. A single electrical stimulus will result in a muscle twitch.—Latent period can be defined as the period of time that elapses between the generation of an action potential in a muscle cell and the start of a muscle contraction. 2 - The role of acetylcholine in a skeletal muscle contraction is once it is released into the synaptic cleft acetylcholine will bind to receptors and cause them to open and cause an influx of Sodium in the ion channels which will lead to a depolarization (End-plate potential) and this can lead to an action potential in a motor neuron and trigger the release of acetylcholine from its terminal and will then diffuse onto the sarcolemma and bind to receptors in the motor endplate and cause a graded depolarization of the end plate potential due to the change in ion permeability. 3 - The events that occur at the neuromuscular junction lead to the end plate potential as described in the previous question. If the end-plate potential is sufficient and the depolarization is at threshold level, it will then open up voltage gated channels on the Sarcolemma of the muscle fiber along the T-tubule. When thei action potential arrives at the T-tubule it is propagating into the interior of the muscle fibers and will activate DHP receptors which are voltage-gated receptors. The wave of depolarization that is coming down will alter the conformation of the DHP receptor which will open the associated RyR channel (Ca2+ channels) and allowing Ca2+ to move by facilitated diffusion out of the Sarcoplasmic Reticulum. Ca2+ will then bind to Troponin which causes that to change shape which causes Tropomyosin which binds to actin which will execute its power-stroke. 4 - The three phases of a skeletal muscle twitch are—(1) the latent period: the period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction.—(2) Contraction Phase: starts at the end of the latent period and ends when the muscle tension peaks —(3) Relaxation Phase: Period of time from the peak tension until the end of the muscle contraction. 5 - The duration of the latent period does not change with different stimulus voltages. The results directly matched my prediction. 6 - At the threshold stimulus, sodium ions move INTO the cell to bring about the membrane depolarization.

1 Motor unit recruitment refers to an increase in the number of active muscle fibers to increase the force developed in a muscle. 2 Active tension (or force) in a skeletal muscle fiber results from activation of cross bridge cycling via increased intracellular calcium levels. 3 The is the minimal stimulus needed to cause a depolarization of the muscle plasma membrane (sarcolemma). threshold voltage. 4 By definition, the is the amount of stimulus required to successfully recruit all the muscle fibers into developing active force maximal voltage. 5 Why was a maximal voltage observed in this experiment? At the maximal voltage, all the muscle fibers contained in this muscle are depolarized and they all develop active force (that is, they were all successfully recruited). 6 A sufficiently strong electrical stimulus applied to an isolated, mounted skeletal muscle induces the development of muscle force, or muscle tension. Which of the following statements concerning this observation is true? The electrical stimulus mimics acetylcholine release at a neuromuscular junction.

E2 A2

1 During a single twitch of a skeletal muscle maximal force is never achieved. 2 When a skeletal muscle is repetitively stimulated, twitches can overlap each other and result in a stronger muscle contraction than a stand-alone twitch. This phenomenon is known as wave summation. 3 Wave summation is achieved by increasing the stimulus frequency (the rate of stimulus delivery to the muscle). 4 Wave summation increases the force produced in the muscle. Another way to increase the force produced by a muscle is to increase the number of activated motor units.

E2 A3

1 Predict Question 1: As the stimulus frequency increases, what will happen to the muscle force generated with each successive stimulus? Will there be a limit to this response? As the stimulus frequency increases, the muscle force generated by each successive stimulus will first increase and then decrease as the stimulus frequency becomes very high.

E2 A3

1 Was there any change in the force generated by the muscle during the second stimulated twitch? Yes, the second twitch generated more muscle force. 2 Is the total muscle force generated by the higher frequency stimulation greater than the force generated in previous stimulations? Yes, it is greater than the previous stimulations .3 Does the force generated by the muscle change with each additional stimulus? As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increased, and a limiting maximum value was observed.

E2 A3

1 Which of the following is not one of the ways that the body can increase the force produced by a skeletal muscle? application of higher voltages to the whole muscle. 2 When a muscle receives a stimulus frequency that causes non-overlapping twitches to follow each other closely in time such that the peak tension of each twitch rises in a stepwise fashion up to a plateau value, the result is known as treppe. 3 In this experiment the isolated skeletal muscle was repetitively stimulated such that individual twitches overlapped with each other and resulted in a stronger muscle contraction than a standalone twitch. This phenomenon is known as wave summation. 4 Wave summation is achieved by increasing the rate of stimulus delivery (frequency) to the muscle.

E2 A3

1 - The term tetanus refers to sustained muscle tension due to very frequent stimuli. 2 Which of the following distinguishes a state of unfused tetanus from a state of complete (fused) tetanus? Muscle tension increases and decreases between different values for an unfused tetanus. 3 When the stimulus frequency reaches a value beyond which no further increases in force are generated by the muscle, the muscle has reached its maximal tetanic tension.

E2 A4

1 Stimulus frequency refers to the rate that stimulating voltage pulses are applied to an isolated whole skeletal muscle. 2 Which of the following distinguishes a state of unfused tetanus from a state of complete (fused) tetanus? Muscle tension increases and decreases during a state of unfused tetanus. 3 When the stimulus frequency reaches a value beyond which no further increases in force are generated by the muscle, the muscle has reached its : maximal tetanic tension.

E2 A4

1 During cross bridge cycling in skeletal muscle, force is created by the power stroke of the myosin heads. 2 The term tetanus refers toYou correctly answered: sustained muscle tension due to repetitive stimuli. 3 A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called fatigue. 4 During fatigue the number of active cross bridges begins to decline although the rate of stimulus delivery (frequency) remains constant. 5 If an intervening rest period is imposed on active skeletal muscle the development of fatigue will be delayed.

E2 A5

1 When a skeletal muscle fatigues, what happens to the contractile force over time? Your answer: When a skeletal muscle fatigues, the contractile force over time will decrease. The decrease is due to the fact that when there is increasing frequency of stimulation it will eventually reach a value beyond which not further increase in muscle force can occur (Maximal Tetanic Tension). Also, as muscle fibers fatigue, the number of active cross-bridges decline even though the stimulus delivery is constant and the muscle will not be able to contract. 2 What are some proposed causes of skeletal muscle fatigue? Your answer: Some proposed causes of skeletal muscle fatigue include accumulations of lactic acid, ADP, and Pi in muscles. 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? Your answer: The muscle will produce more force for a longer period of time if the stimulator is briefly turned off compared to if the stimuli continued without interruption due to the fact that during that brief time the causes of fatigue listed in the previous question (accumulations of lactic acid, ADP, and Pi) to become less accumulated and thus making fatigue less and less likely with continuing rest. Therefore, the longer the length of the rest period the greater the increase in the length if time for sustained muscle tension and my results matched. 4 List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal muscles. Your answer: For the reasons discussed in the previous question, by increasing the time (time between sets of a workout for example) before you are using that particular muscle again after undergoing vigorous exercise will help reduce the onset of fatigue. Another way you could reduce the onset of fatigue is by continuing to workout that particular skeletal muscle because over time you will start to create a higher endurance for that muscle and as a result fatigue will develop less quickly.

E2 A5

1 When skeletal muscle twitches fuse so that the peaks and valleys of each twitch become indistinguishable from each other, the muscle is in a state known as You correctly answered: complete (fused) tetanus. 2 When the stimulus frequency reaches a value beyond which no further increase of skeletal muscle force can occur, the muscle has reached itsYou correctly answered: maximal tetanic tension. 3 A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called You correctly answered: fatigue. 4 Which of the following is not thought to be a contributing factor to the development of fatigue? You correctly answered: buildup of Ca2+ in the muscle fibers.

E2 A5

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? Your answer: The amount of total force the muscle generates during a stimulated twitch will either increase or decrease depending upon the starting resting length. My results directly matched my prediction. 2 What is the key variable in an isometric contraction of a skeletal muscle? Your answer: The key variable in an isometric contraction of a skeletal muscle is the length of the muscle because it stays at a fixed length. Iso = same, metric = length (same length). 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. Your answer: Active force varies with changes in muscle's resting length due to the fact that active force is generated when the myosin filaments bind to the actin filaments and engage in the cross-bridge cycle and giving us active contraction when that takes place. 4 What skeletal muscle lengths generated passive force? (Provide a range.) Your answer:The lengths of skeletal muscle that generated passive force were 80-100mm. 5 If you were curling a 7-kg dumbbell, when would your bicep muscles be contracting isometrically? Your answer: Your bicep muscles would be contracting isometrically when there is no muscle movement but you can feel your muscles contracting (muscles are at a fixed length).

E2 A6

1 When a skeletal muscle is stimulated and generates force but remains at a fixed length You correctly answered: the muscle is contracting isometrically. 2 Which protein is mostly responsible for the development of passive force in a muscle? You correctly answered: titin. 3 In skeletal muscle, active force stimulated through a range of muscle lengths You correctly answered: will utilize ATP hydrolysis to drive the cross bridge cycle. 4 Which of the following is not depicted in a typical skeletal muscle isometric length-tension curve? You correctly answered: time. 5 Maximal active tension will be produced in a skeletal muscle fiber when You correctly answered: the fiber is at its resting length.

E2 A6

1 During an isotonic concentric contraction, the You correctly answered: force generated by the muscle is greater than the weight of the attached load. 2 During an isotonic concentric contraction You correctly answered: the latent period increases with heavier loads. 3 During the latent period for an isotonic concentric contraction You correctly answered: cross bridges cycle and, when muscle tension exceeds the load, muscle shortening occurs. 4 Muscle shortening velocity You correctly answered: decreases with heavier loads. 5 An isotonic contraction of a muscle is one in which You correctly answered: the length of the muscle changes

E2 A7

1 If you were using your bicep muscles to curl a 7-kg dumbbell, when would your muscles be contracting isotonically? Your answer: Your biceps would be contracting isotonically when you, for example, when you are raising the dumbbell (contracting) when force is being generated and there is movement. The same could be applied when you are lowering the dumbbell going back down to resting position when the length of the muscle is still changing and you are moving the dumbbell to a different distance. 2 Explain why the delay between muscle stimulation and load movement became longer as the load became heavier in the experiment. How well did the results compare with your prediction? Your answer: The delay between muscle stimulation and load movement become longer as the load become heavier due to the fact that there is a delay between the stimulation of the muscle and movement of the load and during this time ec-coupling occurs there is an increase in the tension of the muscle but there is no movement. Once the tension of the muscle exceeds the weight of the load the isotonic contraction can begin so the heavier the load, the longer the period of delay is. Results did not match my prediction. 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? Your answer: The shortening velocity becomes slower as the load becomes heavier due to the fact that it takes longer to lift a heavier object as compared to if you are lifting a light object. If you contract your biceps with nothing in your hand or no weight, you can do it very quickly. However, if you add a heavy weight and contract your biceps, it will take longer for you to lift that heavy load. My results directly matched my prediction. 4 Describe how the shortening distance changed as the load became heavier in this experiment. How well did the results compare with your prediction? Your answer: Shortening distance decreased as the load became heavier in the experiment. The results I obtained did not match my 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. Your answer: It would take you longer to perform 10 repetitions lifting a 10kg weight as compared to a 5kg weight due to the fact that when you are lifting a heavier load, the shortening velocity decreases and also, your muscles will fatigue quicker and make it harder and harder as the repetitions increase. When doing the same number of repetitions with a lower weight, the shortening velocity is decreased and will allow you to perform these repetitions more quickly then the 10kg load. 6 Describe what would 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? Your answer: —The muscle will generate force during this experiment. The force generated would be passive force and not active force—The muscle will generate force since the weight is still attached and is pulling the muscle down—The name of this type of contraction is Isometric.

E2 A7

1 Predict Question: As the load on the muscle increases, what will happen to the latent period, the shortening velocity, the distance that the weight moves, and the contraction duration? Your answer: The latent period will decrease, the shortening velocity will decrease, the distance will increase, and the contraction duration will increase. Stop & Think Question1 What kind of contraction did you observe when you attached the 2.0 gram weight to the skeletal muscle and stimulated a contraction? You correctly answered: isometric.

E2 A7

1 Which of the weights allowed the fastest muscle shortening velocity? You correctly answered: 0.5-g weight. 2 Which of the weights induced the longest latent period of the muscle contraction? You correctly answered: 1.5-g weight. 3 Which weight did the muscle contraction move the greatest distance? You correctly answered: 0.5-g weight. 4 Which of the weights allowed the longest duration of muscle contraction? You correctly answered: 0.5-g weight. 5 An isotonic contraction of a muscle is one in which You correctly answered: the length of the muscle changes. 6 When lifting a heavy load You correctly answered: the muscle shortening velocity is decreased in comparison with lighter loads.

E2 A7

1 An action potential in a motor neuron triggers the release of which neurotransmitter? You correctly answered: acetylcholine. 2 The term skeletal muscle fiber refers to an individual skeletal muscle cell. 3 The graded depolarization in the skeletal muscle fiber that is elicited in response to one action potential from the motor neuron is called an EPP (end-plate potential). 4 Which of the following is not a phase of a skeletal muscle twitch? hyperpolarization phase. 5 A skeletal muscle twitch is: one contractile response to a single action potential. 6 Which of the following correctly matches the twitch phase with its definition? the contraction phase: the time between the end of the latent period and peak muscle tension.

E2A-1

1 Skeletal muscles are connected to bones by tendons. 2 Skeletal muscles are composed of hundreds to thousands of individual cells called fibers. 3 The term motor unit refers to one motor neuron and all of the skeletal muscle fibers it innervates. 4 The motor neuron and muscle fiber intersect at what is called the neuromuscular junction. 5 A twitch is one contractile response to a single action potential.

E2A-1

1 What is the period of time that elapses between the generation of an action potential and the start of muscle tension development in a muscle fiber? The latent period. 2 What occurs during the latent period of these isometric contractions? All the steps of excitation-contraction coupling occur.

E2A-1