PhysioEX Exercise 2

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Skeletal muscles are connected to bones by


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


motor unit refers to

one motor neuron and all skeletal muscle fibers it innervates

motor neuron and muscle fiber intersect at

neuromuscular junction

a twitch

contractile response to a single AP

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


skeletal muscle fiber refers to

individual skeletal muscle cell

graded depolarization in the skeletal muscle fiber that is elicited in response to one AP from motor neuron is

an end-plate potential

which is not a phase of skeletal muscle twitch?

hyperpolarization phase

a skeletal muscle twitch is

one contractile response to a single AP

which of the following matches the twitch phase with its definition

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

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

Acetylcholine is released at end-plate potential, diffuses into sarcolemma, attaches to receptors on motor-end plate causing changes in ion permeability that results in the start of muscle contraction

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

Motor units are connected at neuromuscular junction. Specifically, the neuronal axon and the sarcolemma of muscle meet, forming the motor end-plate. Acetylcholine is released from axon down into muscle membrane causing a graded depolarization of the end-plate potential. Depolarization of end-plate leads to increase in calcium released by sarcoplasmic reticulum, and more calcium is used by action and myosin of sarcomeres to cause contraction.

Describe the 3 phases of muscle twitch

Latent phase – the lag time between when the stimulus is given to the time of muscle contraction. Contraction phase – the time from the start of muscle contraction to peak intensity of muscle contraction, this is where the highest muscle tension is generated by muscle contraction Relaxation phase – the time from peak muscle contraction to end of muscle contraction.

Does the duration of the latent period change with different stimulus voltages?

No the latent period did not change when diferent voltage of stimulus was applied.

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

Sodium ions move in to cause a change in membrane potential towards the positive trend which is depolarization.

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

sarcoplasmic reticulum

During the latent period for an isometric contraction

the cellular events involved in excitation-contraction coupling occur.

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

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?

Increasing the stimulus voltage on isolated skeletal muscle increases the amount of active force produced by the muscle. This happens because more fibers and motorneurons are activated and leads to an increase in muscle force.

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

In vivo, the body uses acetylcholine to change the electrochemical gradient causing changes in membrane potential. The current must surpass the threshold voltage and as it increases past the threshold, the force generated by the muscle gets larger because more fibers are activated through the process called motor unit recruitment. The potential then reaches a plateau which is called the maximal voltage where no matter how many muscle fibers are activated, the amount of force produced remains the same.

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

All muscle fibers are recruited by the motor neurons to generate maximal force. However, any additional stimulus past this maximal voltage will not generate any additional force.

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 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.

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


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

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.

The term tetanus refers to

sustained muscle tension due to very frequent stimuli.

Describe how increasing the stimulus frequency affected the force developed by the isolated whole skeletal muscle in this activity.

Increasing stimulus frequency increases muscle tension generated by each successive force however there will be a plateau.

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 stimuli/sec, the force generated is in a state called unfused tetanus, the muscle tension increase and decrease at different values At 140 stimuli/sec, the force generated is in a state called complete (fused) tetanus, the muscle tension are not wavering anymore. At above 146 stimuli/sec, the force generated is in a state called maximal tetanic tension which means any higher frequency will not change the force generated.

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

Beyond 146 stimuli/sec, there is no further increase in the peak force. This muscle tension is called maximal tetanic tension.

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.

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


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

buildup of Ca2+ in the muscle fibers

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

power stroke of the myosin heads.

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.

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

Contractile force decrease over time because no additional force is generated after reachin maximal tetanic tension by previous contractile activity.

What are some proposed causes of skeletal muscle fatigue?

Build-up of lactic acid, ADP, and Pi, as well as low blood calcium level.

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.

Turning off the stimulator, a period of rest is introduced which can delay the onset of fatique because that allows the increase in the intracellular ADP, lactic acid, Pi. Lower concentrations of these things let the muscle sustain muscle tension longer. So length of rest period is proportinoal to length of time for sustained muscle tension.

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. Allow for multiple rest periods, exercising frequently can build endurance for the muscles to withstand higher tension.

During an isometric contraction

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

The force that results from muscles being stretched is

passive force.

Active force

is determined by the amount of myosin bound to actin.

When you generate the isometric length-tension curve, which of the following forces will not be indicated on your screen?

tetanic force

Passive force in skeletal muscle is largely caused by

protein titin

When a skeletal muscle is stimulated and generates force but remains at a fixed length

the muscle is contracting isometrically.

Which protein is mostly responsible for the development of passive force in a muscle?


In skeletal muscle, active force stimulated through a range of muscle lengths

will utilize ATP hydrolysis to drive the cross bridge cycle.

Which of the following is not depicted in a typical skeletal muscle isometric length-tension curve?


Maximal active tension will be produced in a skeletal muscle fiber when

the fiber is at its resting length.

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 of the muscle. If muscle is lengthened past its resting length, the passive force is increased. If the muscle is shortened to its resting length, the active force increases.

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

Muscle length because in isometric contraction, the muscle length does not change.

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 depends on the degree of association between actin and myosin. If the resting length of the muscle is shortened, there are more actin bound to myosin which increases the active force.

What skeletal muscle lengths generated passive force?

80-100 mm muscle lengths generated passive force.

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

At the point where the bicep muscles remain at fixed length.

During an isotonic concentric contraction, the

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

During an isotonic concentric contraction

the latent period increases with heavier loads.

During the latent period for an isotonic concentric contraction

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

Muscle shortening velocity

decreases with heavier loads.

An isotonic contraction of a muscle is one in which

the length of the muscle changes.

Which of the weights allowed the fastest muscle shortening velocity?


Which of the weights induced the longest latent period of the muscle contraction?


Which weight did the muscle contraction move the greatest distance?


Which of the weights allowed the longest duration of muscle contraction?


When lifting a heavy load

he muscle shortening velocity is decreased in comparison with lighter loads.

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