PhysioEX Skeletal Muscle Physiology

Which of the following terms are NOT used interchangeably?

muscle cell - muscle fiber

muscle tension - muscle force

motor unit - motor neuron

in vivo - in the body/ the living

motor unit - motor neuron

Which of the following is NOT a phase of a muscle twitch?

latent period

shortening phase

contraction phase

relaxation phase

shortening phase

Where does the acetylcholine that stimulates muscle contraction bind?

receptors in the sarcoplasmic reticulum

receptors in the motor end plate

receptors in troponin

receptors in the axon terminal

receptors in the motor end plate

When acetylcholine binds to its receptors, it results in __________.

a graded depolarization

the end plate potential

a change in ion permeability

the end plate potential, a graded depolarization, and a change in ion permeability

the end plate potential, a graded depolarization, and a change in ion permeability

Which of the following occurs during the latent period of muscle contraction?

An action potential is generated.

Acetylcholine is released at the axon terminal and an action potential is generated.

Acetylcholine is released at the axon terminal.

Calcium is released from the sarcoplasmic reticulum.

Calcium is released from the sarcoplasmic reticulum.

As the stimulus voltage was increased in this activity, which of the following occurred?

The latent period increased in length.

The muscle force generated decreased.

The latent period decreased in length.

The muscle force generated increased.

The muscle force generated increased.

Which of the following describes the relaxation phase?

The sarcomeres are increasing in length, and the force generated decreases.

The sarcomeres are decreasing in length, and the force generated increases.

The sarcomeres are increasing in length, and the force generated increases.

The sarcomeres are decreasing in length, and the force generated decreases.

The sarcomeres are increasing in length, and the force generated decreases.

Which of the following can trigger a muscle twitch?

release of acetylcholine or electrical stimulation

release of acetylcholine

electrical stimulation

release of acetylcholine or release of calcium from the sarcolemma

release of calcium from the sarcolemma

release of acetylcholine or electrical stimulation

Skeletal muscles are connected to bones by

tendons

Skeletal muscles are composed of hundreds to thousands of individual cells called

fibers

The term motor unit refers to

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

The motor neuron and muscle fiber intersect at what is called

the neuromuscular junction.

A twitch is

one contractile response to a single action potential.

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.

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.

What occurs during the latent period of these isometric contractions?

All the steps of excitation-contraction coupling occur.

An action potential in a motor neuron triggers the release of which neurotransmitter?

acetylcholine

The term skeletal muscle fiber refers to

an individual skeletal muscle cell.

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).

A skeletal muscle twitch is

one contractile response to a single action potential

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.

Which of the following is proportional to the amount of tension produced by a skeletal muscle?

the threshold voltage

the length of the latent period

the length of the relaxation phase

the number of motor units activated

the number of motor units activated

NO force is generated during which of the following?

a muscle twitch

the relaxation phase

the latent period

the contraction phase

the latent period

What is the minimum voltage needed to generate active force in the skeletal muscle?

threshold voltage

recruitment voltage

stimulus voltage

contraction voltage

threshold voltage

A motor unit is defined as _______.

the axon terminals of a single motor neuron and all of the muscle fibers that it stimulates

a single neuron and a single muscle fiber

a single muscle fiber and all its axon terminals

a nerve and a muscle

the axon terminals of a single motor neuron and all of the muscle fibers that it stimulates

In the lab, a researcher finds that the threshold stimulus to induce an action potential in a muscle fiber's sarcolemma was 3.0 volts. Which of the following would result in muscle tension?

1.0 volt

3.0 volts and 4.0 volts

3.0 volts

4.0 volts

2.0 volts

3.0 volts and 4.0 volts

Increasing the applied voltage in the simulation corresponds to which in vivo event?

muscle twitch recruitment

motor unit recruitment

motor neuron recruitment

muscle fiber recruitment

motor unit recruitment

As the stimulus voltage increased, the resulting muscle tension _______.

increased proportionally

remained the same

increased to a point until it reached a plateau

decreased proportionally

increased to a point until it reached a plateau

Which of the following occurs during the contraction phase?

Acetylcholine binds to the sarcolemma.

The sarcomeres are increasing in length.

Cross-bridge cycling is taking place.

Ca2+ release from the sarcoplasmic reticulum is blocked.

Cross-bridge cycling is taking place.

Skeletal muscle fibers are innervated (stimulated) by

motor neurons.

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

In resting skeletal muscle, calcium is stored in

the sarcoplasmic reticulum.

During the latent period for an isometric contraction

the cellular events involved in excitation-contraction coupling occur.

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?

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

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?

0.00; there was no activation of skeletal muscle fibers by this stimulus

What is the lowest stimulus voltage that induces active force in the skeletal muscle?

threshold voltage.

Enter the threshold voltage for this experiment.

0.8 volts.

Enter the maximal voltage for this experiment.

8.5 volts

Motor unit recruitment refers to

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

Active tension (or force) in a skeletal muscle fiber results from

activation of cross bridge cycling via increased intracellular calcium levels

The is the minimal stimulus needed to cause a depolarization of the muscle plasma membrane (sarcolemma)

threshold voltage

By definition, the is the amount of stimulus required to successfully recruit all the muscle fibers into developing active force.

maximal voltage.

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)

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

Treppe is the phenomenon that results in _______.

a progressive increase in the force generated with repetitive stimulation

an increase in the stimulation frequency

motor unit recruitment

maximal force production by a skeletal muscle

a progressive increase in the force generated with repetitive stimulation

When the frequency of stimulation of a muscle is great enough, _______.

full relaxation of the muscle occurs

full relaxation of the muscle occurs and wave summation results

muscle twitches overlap

wave summation results

wave summation results and muscle twitches overlap.

wave summation results and muscle twitches overlap.

When wave summation occurs, _______.

the muscle force generated increases

the muscle fiber has received a single stimulus event

the muscle force generated decreases

individual muscle twitches are separated

the muscle force generated increases

Why does wave summation occur?

A large amount of time elapses between stimulations.

Muscle fibers have been allowed to fully relax.

Muscle twitches are not yet overlapping.

Muscle fibers are partially contracted when the next stimulus arrives.

Muscle fibers are partially contracted when the next stimulus arrives.

Which of the following produced an increase in stimulus intensity?

allowing the muscle to completely relax in between contractions

clicking the mouse in rapid succession

increasing the voltage applied

clicking the mouse in rapid succession and allowing the muscle to completely relax in between contractions

increasing the voltage applied

Which of the following does NOT describe treppe?

A period of relaxation can be seen on the muscle twitch tracing.

It is also referred to as the “staircase” effect.

An incremental increase in force occurs.

An increase in stimulus intensity is required to see the effect.

An increase in stimulus intensity is required to see the effect.

During a single twitch of a skeletal muscle

maximal force is never achieved

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

Wave summation is achieved by

increasing the stimulus frequency (the rate of stimulus delivery to the muscle)

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

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 increase. There will be a limit to this increase.

Was there any change in the force generated by the muscle during the second stimulated twitch?

Yes, the second twitch generated more muscle force

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.

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.

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

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

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

Wave summation is achieved by

increasing the rate of stimulus delivery (frequency) to the muscle.

Which of the following does NOT describe tetanus?    

It can be fused.

It can be unfused.

The muscle relaxes fully between contractions.

The maximum force produced has reached a plateau.

The muscle relaxes fully between contractions.

Which of the following is true of the maximum stimulus frequency?    

Muscle force will decrease.

Muscle has reached maximal tetanic tension.

No greater muscle force can be generated.

No greater muscle force can be generated and muscle force will decrease.

No greater muscle force can be generated and the muscle has reached maximal tetanic tension.

No greater muscle force can be generated and the muscle has reached maximal tetanic tension.    

Muscle twitches overlap with _______.    

both fused and unfused tetanus

fused tetanus only

unfused tetanus only

threshold voltage applied

both fused and unfused tetanus

Which of the following would result in fused or unfused tetanus compared to a single muscle twitch?

increasing the stimulus frequency

decreasing the stimulus frequency

decreasing the stimulus frequency or increasing the stimulus frequency

threshold stimulus and single stimuli

increasing the stimulus frequency

To transition from unfused tetanus to fused tetanus, _______.

the muscle fibers were allowed to relax

stimulus frequency increased

stimulus frequency decreased

wave summation was not allowed

stimulus frequency increased

Which of the following describes the relationship between stimulus frequency and muscle tension?

When stimulus frequency increases, muscle tension decreases.

When stimulus frequency increases, muscle tension increases indefinitely.

When stimulus frequency decreases, muscle tension increases indefinitely.

When stimulus frequency increases, muscle tension increases to a maximum value.

When stimulus frequency increases, muscle tension increases to a maximum value.

At what stimulus frequency was maximal tetanic tension developed?

140 stimuli/sec

130 stimuli/sec

146 stimuli/sec

50 stimuli/sec

146 stimuli/sec

Stimulus frequency refers to

the rate that stimulating voltage pulses are applied to an isolated whole skeletal muscle

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

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.

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?

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

What begins to happen at around 80 msec?

unfused tetanus develops.

How does the trace at 130 stimuli/sec compare with the trace at 50 stimuli/sec?

Fused tetanus develops at this greater stimulus frequency.

How do the traces with 146-150 stimuli per second compare with the trace at 130 stimuli per second?

Maximal tetanic tension develops with these very high stimulation frequencies.

The term tetanus refers to

sustained muscle tension due to very frequent stimuli.

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

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

Which of the following is matched correctly?

complete tetanus - fused tetanus

incomplete tetanus - fatigue

complete tetanus - unfused tetanus

incomplete tetanus - fused tetanus

complete tetanus - fused tetanus

Which of the following situations delay the development of muscle fatigue the longest?

Alternating sprints with up-hill jogging

High intensity exercise

Consistent interruptions of muscle stimulation

Prolonged muscle contraction

Repetitive stimulation of a muscle

Consistent interruptions of muscle stimulation

Which of the following molecules is NOT thought to accumulate during muscle fatigue?

ADP

lactic acid

inorganic phosphate

ATP

ATP

During muscle fatigue, the ____________ decreases.

stimulus frequency

force of contraction

rest period

number of muscle fibers

force of contraction

In this activity, why did the muscle force decrease over time?

The stimulation frequency was decreased.

The stimulation frequency was increased.

Intracellular calcium built up.

Muscle fatigue developed.

Muscle fatigue developed.

Which of the following describes how increasing the duration of the rest period affected muscle tension?

Increasing the duration of the rest period increased the duration of the subsequent maximal tension.

Increasing the duration of the rest period decreased the amount of the subsequent maximal tension.

Increasing the duration of the rest period decreased the duration of subsequent maximal tension.

Increasing the duration of the rest period had no effect on the subsequent maximal tension.

Increasing the duration of the rest period increased the duration of the subsequent maximal tension.

What might be occurring on a molecular level during the rest periods?

Intracellular lactic acid is increasing.

Intracellular ADP and inorganic phosphate are decreasing.

Intracellular calcium is decreasing.

Muscle fatigue is increasing.

Intracellular ADP and inorganic phosphate are decreasing

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

complete (fused) tetanus.

When the stimulus frequency reaches a value beyond which no further increase of skeletal muscle force can occur, the muscle has reached its

maximal tetanic tension.

A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called

fatigue

Which of the following is not thought to be a contributing factor to the development of fatigue?

buildup of Ca in the muscle fibers.

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?

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

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

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 P declined during the rest period.

During cross bridge cycling in skeletal muscle, force is created by the

power stroke of the myosin heads.

The term tetanus refers to

sustained muscle tension due to repetitive stimuli.

A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called

fatigue

During fatigue

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

If an intervening rest period is imposed on active skeletal muscle

the development of fatigue will be delayed.

With both ends of the muscle fixed, which of the following occurs if the load attempted is equal to or greater than the force generated by a muscle?    

Isotonic muscle contraction occurs.

The muscle shortens.

The muscle lengthens.

Isometric muscle contraction occurs.

Isometric muscle contraction occurs.    

The resting length of the muscle _______.    

describes the length of the muscle during contraction

occurs when peak tension develops

occurs when the muscle is relaxed

describes the muscle during stimulation

occurs when the muscle is relaxed