PhysioEx Exercise #2

In resting skeletal muscle, calcium is stored in A. The sarcomere B. The myofibril C. The sarcoplasmic reticulum D. The cytosol

C. The sarcoplasmic reticulum

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' A. Frappe B. Wave summation C. Treppe D. Motor unit recruitment

C. Treppe

What begins to happen at around 80 msec? A. Summation of force ceases B. Fused tetanus develops C. Unfused tetanus develops D. Maximal tetanic tension is reached

C. Unfused tetanus develops

Which of the weights allowed the fastest muscle shortening velocity? A. 0.5 g weight B. 1.0 g weight C. 1.5 g weight D. 2.0 g weight

A. 0.5 g weight

Which of the weights allowed the longest duration of muscle contraction? A. 0.5 g weight B. 1.0 g weight C. 1.5 g weight D. 2.0 g weight

A. 0.5 g weight

Motor unit recruitment refers to A. An increase in the number of active muscle fibers to increase the force developed in a muscle B. An increase in the number of active muscle fibers to decrease the work a muscle must perform C. A decrease in the number of active muscle fibers to increase the simplicity of force development in a muscle D. A decrease in the number of active muscle fibers to decrease the force developed in a muscle

A. An increase in the number of active muscle fibers to increase the force developed in a muscle

The term skeletal muscle fiber refers to A. An individual skeletal muscle cell B. An isolated skeletal muscle, such as the biceps or the triceps C. An individual myosin fiber D. An individual sarcomere

A. An individual skeletal muscle cell

Muscle shortening velocity A. Decreases with heavier loads B. Increases with heavier loads C. Decreases with lighter loads D. Is constant regardless of the load

A. Decreases with heavier loads

Wave summation is achieved by A. Increasing the rate of stimulus delivery (frequency) to the muscle B. Removing intracellular calcium faster between stimuli C. Summation action potentials so that their depolarizing magnitude is greater D. Allowing the skeletal muscle to completely relax between stimuli

A. Increasing the rate of stimulus delivery (frequency) to the muscle

Wave summation is achieved by A. Increasing the stimulus frequency (the rate of stimulus delivery to the muscle) B. Decreasing the stimulus frequency C. Adding action potentials together so that their depolarizing magnitude is greater D. Allowing the muscle to completely relax between stimuli

A. Increasing the stimulus frequency (the rate of stimulus delivery to the muscle)

The force that results from muscles being stretched is A. Passive force B. Active force C. Total force D. Tetanic force

A. Passive force

Maximal active tension will be produced in a skeletal muscle fiber when A. The fiber is at its resting length B. The fiber is shorter than its resting length C. The fiber is longer than its resting length D. The fiber is at any length because tension is not dependent on muscle length

A. The fiber is at its resting length

Is the total muscle force generated by the higher frequency simulation greater than the force generated in previous simulations? A. Yes, it is greater than the previous stimulations B. No, it is the same as before C. No, it is less than the previous stimulations

A. Yes, it is greater than the previous stimulations

Skeletal muscles are composed of hundreds to thousands of individual cells called A. Sarcomeres B. Myofibrils C. Fibers D. Myofilaments

C. Fibers

Define electrochemical stimulus.

Elicitation of muscle contraction using electrical impulses.

How is this change in whole-muscle force achieved in vivo?

In vivo, each fiber in the muscle receives direct stimulation via a nerve ending. Recruitment of additional motor units increases the total muscle force produced, this is called motor unit recruitment.

Will changes to the stimulus voltage alter the duration of the latent period?

No, changing the stimulus voltage will not change the latent period duration.

When a skeletal muscle fatigues, what happens to the contractile force over time?

Over time, the contractile force decreases

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?

The latent period will increase, the shortening velocity will decrease, the distance will decrease, and the contraction duration will decrease

What skeletal muscle lengths generated passive force?

80-100 mm

Define motor unit.

A motor unit consists of a motor neuron and all the muscle cell fibers it innervates.

What do you observe in the active force display when the stimulus voltage is set to 0.0, and why does this observation make sense? A. 0.00; there was no activation of skeletal muscle fibers by this stimulus B. 0.00; this low stimulus voltage activates only a few skeletal muscle fibers C. 0.00; once skeletal muscle is isolated from the body and mounted on the transducer stand, it can no longer generate active force when stimulated

A. 0.00; there was no activation of skeletal muscle fiber by this stimulus

Which weight did the muscle contraction move the greatest distance? A. 0.5 g weight B. 1.0 g weight C. 1.5 g weight D. 2.0 g weight

A. 0.5 g weight

Which of the weights induced the longest latent period of the muscle contraction? A. 0.5 g weight B. 1.0 g weight C. 1.5 g weight D. 2.0 g weight

A. 1.5 g weight

Active tension (or force) in a skeletal muscle fiber results from A. Activation of cross bridge cycling via increased intracellular calcium levels B. Stretching the muscle before applying the stimulus voltage C. Moving the muscle on its support stand D. Detachment of the tendons from the bones in the body

A. Activation of cross bridge cycling via increased intracellular calcium levels

Which of the following is not thought to be a contributing factor to the development of fatigue? A. Buildup of Calcium in the muscle fibers B. Buildup of lactic acid in the muscle fibers C. Buildup of ADP in the muscle fibers D. Buildup of Pi in the muscle fibers

A. Buildup of calcium in the muscle fibers

When the stimulus frequency reaches a value beyond which no further increases in force are generated by the muscle, the muscle has reached its A. Maximal tetanic tension B. Threshold voltage C. Unfused tetanic tension D. Latent period

A. Maximal tetanic tension

A twitch is A. One contractile response to a single action potential B. One contractile response to a series of action potentials C. Multiple contractile responses to a series of action potentials D. Multiple contractile responses to a single action potential

A. One contractile response to a single action potential

An isotonic contraction of a muscle is one in which A. The length of the muscle changes B. The length of the muscle stays the same C. The muscle tension increases, but the muscle cannot lift the load D. Muscle shortening velocity equals zero

A. The length of the muscle changes

Which of the following is not depicted in a typical skeletal muscle isometric length-tension curve? A. Time B. Active force C. Total force D. Passive force

A. Time

What is the role of acetylcholine in a skeletal muscle contraction?

Acetylcholine causes a change in the ion permeability by binding to receptors in the motor end plate.

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 varies with the size of the muscle's resting length because the size of the resting length determines the ability of the myosin and actin's filaments to overlap. At a normal resting length, the myosin and actin filaments overlap the most. During a decreased length, the actin filaments are so close together that they overlap and at a long resting length, the actin filaments stretch out a lot while the myosin filaments stay in place which decreases the area of actin-myosin overlap.

Describe the process of excitation-contraction coupling in skeletal muscle fibers.

An action potential in a motor neuron triggers the release of acetylcholine from its terminal. Acetylcholine then diffuses onto the muscle fiber's plasma membrane (sarcolemma) and binds to receptors in the motor end plate, initiating change in ion permeability that results in a graded depolarization of the muscle plasma membrane (the end-plate potential). The end-plate triggers a series of events that results in contraction of a muscle cell.

What is the key variable in an isometric contraction of a skeletal muscle?

An important factor in determining the amount of force a muscle can develop when stimulated is resting length

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: an active force of 5.12, at 140: an active force of 5.91 and at 146: an active force of 5.95

During an isotonic concentric contraction A. The latent period decreases with heavier loads B. The latent period increases with heavier loads C. The latent period is always the same regardless of the load D. There is no latent period

B The latent period increases with heavier loads

As the resting length of the muscle is changed, what will happen to the amount of total force the muscle generates during the stimulated twitch? A. An increase in muscle length will increase total force B. A decrease in muscle length will decrease total force C. Total force can increase or decrease depending upon the starting resting length D. An increase in muscle length will decrease the total force

B. A decrease in muscle length will decrease total force

An action potential in a motor neuron triggers the release of which neurotransmitter? A. Glutamate B. Acetylcholine C. Glycine D. Norepinephrine

B. Acetylcholine

What occurs during the latent period of these isometric contractions? A. The length of the muscle fiber is sliding into and optimal length B. All the steps of excitation-contraction coupling occur C. Muscle fiber cross bridges are cycling at sub-maximal rates D. The required amounts of ATP are being generated

B. All the steps of excitation-contraction coupling occur

Does the force generated by the muscle change with each additional stimulus? A. As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increase and there was no limit to this increase B. As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increased, and a limiting maximum value was observed C. As the stimulus frequency increased, the muscle tension generated by each successive stimulus remained the same D. As the stimulus frequency increased, the muscle tension generated by each successive stimulus always decreased.

B. As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increased, and a limiting maximum value was observed

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? A. As the stimulus frequency increases, the muscle force generated by each successive stimulus will increase. There will be no limit to this increase B. As the stimulus frequency increases, the muscle force generated by each successive stimulus will increase. There will be a limit to this increase C. As the stimulus frequency increases, the muscle force generated by each successive stimulus will remain the same D. 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

B. As the stimulus frequency increases, the muscle force generated by each successive stimulus will increase. There will be a limit to this increase

As the stimulus frequency increases further, what will happen to the muscle tension and twitch appearance with each successive stimulus? Will there be a limit to this response? A. As the stimulus frequency increases, the muscle tension generated by each successive stimulus will increase. There will be no limit to this increase B. As the stimulus frequency increases, the muscle tension generated by each successive stimulus will increase. There will be a limit to this increase C. As the stimulus frequency increases, the muscle tension generated by each successive stimulus will remain the same D. As the stimulus frequency increases, the muscle tension generated by each successive stimulus will first increase and then decrease as the stimulus frequency becomes very high

B. As the stimulus frequency increases, the muscle tension generated by each successive stimulus will increase. There will be a limit to this increase

Why was a maximal voltage observed in this experiment? A. The maximal voltage induces the most calcium release from a muscle fiber's sarcoplasmic reticulum B. 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) C. The maximal voltage induces the most acetylcholine release from a muscle fiber's sarcolemma (or plasma membrane) D. The maximal voltage induces the fastest cross bridge cycles

B. 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)

During an isotonic concentric contraction, the A. Force generated by the muscle is less than the weight of the attached load B. Force generated by the muscle is greater than the weight of the attached load C. Force generated by the muscle is equal to the weight of the attached load D. Muscle remains at a fixed length

B. Force generated by the muscle is greater than the weight of the attached load

How does the trace at 130 stimuli/sec compare with the trace at 50 stimuli/sec? A. Summation of force ceases at this greater stimulus frequency B. Fused tetanus develops at this greater stimulus frequency C. Unfused tetanus fails to develop at this greater stimulus frequency D. Maximum tetanus sinking occurs

B. Fused tetanus develops at this greater stimulus frequency

Which of the following is not a phase of a skeletal muscle twitch? A. Contraction phase B. Hyperpolarization phase C. Latent period D. Relaxation phase

B. Hyperpolarization phase

Active force A. Is determined by the weight of the load that the muscle is trying to move B. Is determined by the amount of myosin bound to actin C. Equals total force D. Is greatest when muscles are stimulated at short lengths

B. Is determined by the amount of myosin bound to actin

During a single twitch of a skeletal muscle A. calcium is not released from the sarcoplasmic reticulum B. Maximal force is never achieved C. No force is produced unless the muscle shortens D. Maximal force is always achieved

B. Maximal force is never achieved

In order to produce sustained muscle contractions with an active force value of 5.2, will you need to increase the stimulus voltage? A. Yes B. No

B. No

The term tetanus refers to A. The contractile response of a muscle to one action potential B. Sustained muscle tension due to repetitive stimuli C. Sustained muscle tension due to a single stimulus D. One twitch within the muscle

B. Sustained muscle tension due to repetitive stimuli

The term tetanus refers to A. The contractile responses of a muscle to one action potential B. Sustained muscle tension due to very frequent stimuli C. Sustained muscle tension due to a single stimulus D. One simultaneous twitch of all the fibers contained within the muscle

B. Sustained muscle tension due to very frequent stimuli

Skeletal muscles are connected to bones by A. Joints B. Tendons C. Ligaments D. Motor axons

B. Tendons

If the stimulator is briefly turned off for defined periods of time, what will happen to the length of time that the muscle is able to sustain maximal developed tension when the stimulator is turned on again? A. The length of the rest period will not substantially impact the length of time for sustained muscle tension B. The length of the rest period will proportionately increase the length of time for sustained muscle tension C. The length of the rest period will proportionately decrease the length of time for sustained muscle tension

B. The length of the rest period will proportionately increase the length of time for sustained muscle tension

Stimulus frequency refers to A. The rate that the data tracings move across the oscilloscope screen B. The rate that stimulating voltage pulses are applied to an isolated whole skeletal muscle C. The rate of force development in stimulated twitches D. The number of stimulating electrodes in contact with the isolated whole skeletal muscle

B. The rate that stimulating voltage pulses are applied to an isolated whole skeletal muscle

What is the lowest stimulus voltage that induces active forces in the skeletal muscle? A. Menial stimulus B. Threshold voltage C. Minimus voltage D. Adequate stimulus

B. Threshold voltage

In skeletal muscle, active force stimulated through a range of muscle lengths A. Will be heavily influenced by the mass of the muscle being studied B. Will utilize ATP hydrolysis to drive the cross bridge cycle C. Will always be the dominant contributing to the total force of the muscle D. Will depend on the load attached to the free end of the muscle

B. Will utilize ATP hydrolysis to drive the cross bridge cycle

Why is the shortening velocity slower as the load became heavier?

Because there is slower muscle twitch so it takes longer for the contraction to occur.

Explain why it would take 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 your muscle has to use more force to lift heavier weight, causing it to take longer to achieve that force as well as it needs a longer relaxation phase

Beyond what stimulus frequency is there no further increase in the peak force? What is the muscle tension called at this frequency?

Beyond the stimulus frequency of 146 stimuli/sec there is no further increase in peak force. The muscle tension at this frequency is called maximal tetanic tension.

Passive force in skeletal muscle is largely caused by A. The actin thin filaments B. The myosin thick filaments C. The protein titin D. The bungee cords

C. The protein titin

During the latent period for an isotonic concentric contraction A. There is no development of muscle force B. The sliding filament mechanism causes the muscle to shorten C. Cross bridges cycle and, when muscle tension exceeds the load, muscle shortening occurs D. Calcium remains stored inside the sarcoplasmic reticulum

C. Cross bridges cycle, and when muscle tension exceeds the load, muscle shortening occurs

A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called A. Tetanus B. Summation C. Fatigue D. Rigor mortis

C. Fatigue

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 again? A. Intracellular concentrations of lactic acid increased during the rest period B. Intracellular concentrations of acetylcholine increased during the rest period C. Intracellular concentrations of ADP and Pi declined during the rest period D. Calcium levels in sarcoplasmic reticulum declined during the rest period

C. Intracellular concentrations of ADP and Pi declined during the rest period

What kind of contraction did you observe when you attached a 2.0 gram weight to the skeletal muscle and stimulated a contraction? A. Isotonic concentric B. Isotonic eccentric C. Isometric D. Isomesmeric

C. Isometric

By definition, the ________ is the amount of stimulus required to successfully recruit all the muscle fibers into developing active force A. Latent voltage B. Contractile stimulus voltage C. Maximal voltage D. Motor unit voltage

C. Maximal voltage

Skeletal muscle fiber are innervated (stimulated) by A. Parasympathetic neurons B. Sympathetic neurons C. Motor neurons D. Interneurons

C. Motor neurons

The term motor unit refers to A. All of the motor neurons that innervate one skeletal muscle B. All of the activated muscle fibers within one skeletal muscle C. One motor neuron and all of the skeletal muscle fibers it innervates D. One motor neuron that innervates several skeletal muscles

C. One motor neuron and all of the skeletal muscle fibers it innervates

During cross bridge cycling in skeletal muscle, force is created by the A. Power stroke of the tropomyosin molecules B. Power stroke of the actin molecules C. Power stroke of the myosin heads D. Power stroke of the troponin molecules

C. Power stroke of the myosin heads

When you generate the isometric length-tension curve, which of the following forces will not be indicated on your screen? A. Total force B. Active force C. Tetanic force D. Passive force

C. Tetanic force

As the stimulus voltage is increased from 1.0 volt up to 10 volts, what will happen to the amount of active force generated with each stimulus? A. The active force will continually increase B. The active force will not increase above the value observed when 1.0 volt is applied C. The active force will first increase and then plateau at some maximal value as the stimulus voltage increases D. The active force will first increase and then decrease

C. The active force will first increase and then plateau at some maximal value as the stimulus voltage increases

During the latent period for an isometric contraction A. The actin and myosin filaments slide past each other, causing the muscle to shorten B. Force is generated by the muscle C. The cellular events involved in excitation-contraction coupling occur D. A single twitch occurs

C. The cellular events involved in excitation-contraction coupling occur

If an intervening rest period is imposed on active skeletal muscle A. The development of fatigue will be prevented B. The stimulus frequency will fatigue C. The development of fatigue will be delayed D. The muscle will become refractory to subsequent stimulation

C. The development of fatigue will be delayed

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? A. As the strength of the electrical stimulus is increased, the latent period, changes length B. The electrical stimulus only influences the skeletal muscle fibers that are in immediate contact with the electrodes C. The electrical stimulus mimics acetylcholine release at a neuromuscular junction D. As the strength of the electrical stimulus is increased, the amount of force each individual muscle fiber generates also increases

C. The electrical stimulus mimics acetylcholine release at a neuromuscular junction

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? A. The relaxed period B. The contractile period C. The latent period D. The recess period

C. The latent period

During fatigue A. Myosin heads become locked with the thin, actin filament B. Muscles go from a state of unfused tetanus to a state of complete (fused) tetanus C. The number of active cross bridges begins to decline although the rate of stimulus delivery (frequency) remains constant D. Intracellular calcium becomes depleted; therefore, cross bridges cannot cycle any longer

C. The number of active cross bridges begins to decline although the rate of stimulus delivery (frequency) remains constant

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 A. Skeletal muscle tone (or tonus) B. Treppe C. Wave summation D. Fatigue

C. Wave summation

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 A. Skeletal muscle tone (or tonus) B. The latent period C. Wave summation D. Fatigue

C. Wave summation

Was there any change in the force generated by the muscle during the second stimulated twitch? A. Yes, the second twitch generated less muscle force B. No, the second twitch was the same as the first twitch C. Yes, the second twitch generated more muscle force

C. Yes, the second twitch generated more muscle force

The graded depolarization in the skeletal muscle fiber that is elicited in response to one action potential from the motor neuron is called A. An EPSP (excitatory postsynaptic potential) B. An IPSP (inhibitory postsynaptic potential) C. An EPP (end-plate potential) D. A threshold potential

C. an EPP (end-plate potential)

A single action potential propagating down a motor axon results in A. Several action potentials and several contractile events in the muscle fibers it innervates B. Several action potentials and as single contractile event in the muscle fibers it innervates C. A single action potential and several contractile events in the muscle fibers it innervates D. A single action potential and a single contractile event in the muscle fibers it innervates

D. A single action potential and a single contractile event in the muscle fibers it innervates

Which of the following is not one of the ways that the body can increase the force produced by a skeletal muscle? A. Recruitment of more motor units and, thus, activation of more muscle fibers B. Application of high-frequency stimulation by a motor neuron C. Maintenance of elevated intracellular calcium levels, thus inducing wave stimulation D. Application of higher voltages to the whole muscle

D. Application of higher voltages to the whole muscle

Note that dip in total force at a muscle length at 90 mm as compared to the total force at a muscle length of 80 and 100 mm. Why does this occur? A. At this muscle length, there is too much overlap of myosin and actin, which impairs active force production B. At this muscle length, titin is stretched beyond its natural limits, which impairs active force production C. At this muscle length, passive force is the dominant value in the total force equation D. At this muscle length, active force has decreased in value and passive force has not yet increased to a significant value

D. At this muscle length, active force has decreased in value and passive force has not yet increased to a significant value

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 A. Treppe B. Unfused tetanus C. Ultimate summation D. Complete (fused) tetanus

D. Complete (fused) tetanus

Wave summation increases the force produced in the muscle. Another way to increase the force produced by a muscle is to A. Slow action potential propagation down the motor neurons that innervates the muscle B. Block calcium release from the sarcoplasmic reticulum C. Decrease the number of activated motor units D. Increase the number of activated motor units

D. Increase the number of activated motor units

How do the traces with 146-150 stimuli per second compare with the trace at 130 stimuli per second? A. Summation of force ceases with these very high stimulation frequencies B. Fused tetanus ceases with these very high stimulation frequencies C. Unfused tetanus returns with these very high stimulation frequencies D. Maximal tetanic tension develops with these very high stimulation frequencies

D. Maximal tetanic tension develops with these very high stimulation frequencies

Why does the stimulated muscle force begin to decrease over time despite the maintained stimuli? A. Intracellular acid is accumulating B. Intracellular ADP and Pi are accumulating C. Intracellular calcium movement is impaired D. More than one of these answers could be correct

D. More than one of these answers could be correct

Which of the following distinguishes a state of unfused tetanus from a state of complete (fused) tetanus? A. The rate of stimulus delivery (frequency) is greater during a state of unfused tetanus B. The muscle tension that develops is greater during a state of unfused tetanus C. Muscle tension oscillates between different values during a state of complete (fused) tetanus D. Muscle tension increases and decreases during a state of unfused tetanus

D. Muscle tension increases and decreases during a state of unfused tetanus

Which of the following correctly matches the twitch phase with is definition? A. The latent period: The time from peak tension until the end of the muscle contraction B. The contraction phase: The time between an action potential in a muscle cell and the start of muscle tension C. The relaxation phase: The time between the electrical stimulus D. The contraction phase: The time between the end of the latent period and peak muscle tension

D. The contraction phase: The time between the end of the latent period and peak muscle tension

When a skeletal muscle is stimulated and generates force but remains at a fixed length A. The muscle is contracting isotonically B. There is insufficient electrical stimulation to move the load C. There is no active force developed within the muscle D. The muscle is contracting isometrically

D. The muscle in contracting isometrically

When lifting a heavy load A. The latent period is shorter than the lighter loads B. There are less cross bridges cycling during an isotonic contraction C. The load will be lifted for a longer duration that lighter loads D. The muscle shortening velocity is decreased in comparison with lighter loads

D. The muscle shortening velocity is decreased in comparison with lighter loads

The motor neuron and muscle fiber intersect at what is called A. The excitatory synapse B. The inhibitory synapse C. The pre-synaptic nerve terminal D. The neuromuscular junction

D. The neuromuscular junction

During an isometric contraction A. The force generated by the skeletal muscle is greater than the load it is moving B. Skeletal muscles undergo the sliding filament mechanisms, which causes them to shorten C. There is no active contraction within the skeletal muscle D. The skeletal muscle is generating force, but it remains at a fixed length

D. The skeletal muscle is generating force, but it remains at a fixed length

Which protein is mostly responsible for the development of passive force in a muscle? A. Actin B. Myosin C. Troponin D. Titin

D. Titin

Describe the three phases of a skeletal muscle twitch.

First is the latent period, this is the period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction. No force is generated during the latent period, chemical changes occur intracellularly to prepare for contraction. Next is the contraction phase, this starts at the end of the latent period and ends when muscle tension peaks. Finally is the relaxation phase, this is the period of time from peak tension until the end of the muscle contraction.

How was frequency dependent wave summation and motor unit recruitment achieved in the experiment? How are these achieved in vivo?

Frequency dependent wave summation was achieved by having multiple stimulus before the muscle had replaced. Motor unit recruitment was achieved by increasing the voltage of the stimulus. In vivo, wave summation occurs when muscle fivers that are developing tension are stimulated again before the fibers have replaced. In vivo, motor unit recruitment occurs because more voltage is delivered to the whole muscle, more muscle fibers are activated, creating an increase in the total force produced by the muscle.

Compare and contrast frequency-dependent wave summation with motor unit recruitment.

Frequency-dependent wave summation is changed based on the amount of times a stimulus is applied while motor unit recruitment is changed based on the voltage of the stimulus. These are both similar because they are both ways to increase the force of the muscle.

If you were curling a 7-kg dumbbell, when would your bicep muscles be contracting isometrically?

It would be contracting isometrically at the bottom of the curl, when your arm is facing down

What is one way that human can delay the onset of fatigue when they are vigorously using their skeletal muscles?

One way is to rest during exercise to delay onset of fatigue

At the threshold stimulus, do sodium ions move into or out of the cell to bring about the membrane depolarization?

Sodium ions move into the cell

What is the difference between stimulus intensity and stimulus frequency?

Stimulus intensity is the amount of voltage that is being applied to the muscle. Stimulus frequency is how many times a stimulus is being applied to a muscle

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?

The force of contraction after each one is slightly bigger than the first one, but not as big if there was no relaxation. These results are called treppe as there is a step like formation.

Define 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.

If you were using a bicep muscle to curl a 7-kg dumbbell, when would your muscle be contracting isotonically?

When your arm is up at the top of the lift

How was an active force of 5.2 achieved?

To achieve an active force of 5.2, the frequency of the stimulus had to be increased

What happened in the isolated skeletal muscle when the maximal voltage was applied?

When maximal voltage is applied, maximal tension in the whole muscle occurs as all the muscle fibers have been activated by a sufficiently strong stimulus. When the stimulation voltage is greater than the maximal voltage, an increase in the force of contraction will not occur.

Describe what would happen in the following experiment: A 2.5 kg 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?

Yes your muscle would still generate force and change length. This would be an isotonic contraction