Contraction of Whole Muscle

 

Graphics are used with permission of :

adam.com (http://www.adam.com/)

Benjamin/Cummings Publishing Co (http://www.awl.com/bc)

Page 1.  Introduction

There is a wide variation in the level of muscle tension that can be developed within a given muscle.

 

Page 2.  Goals

To examine the component parts of a single muscle contraction.

To understand the relationship between muscle tension and frequency of stimulation, motor unit recruitment, and degree of muscle stretch.

 

Page 3.  Factors Affecting Muscle Tension

Three factors which affect muscle tension in a whole muscle are:

1. Frequency of stimulation

2. Number of motor units recruited

3. Degree of muscle stretch

 

Page 4.  Muscle Twitch

A muscle twitch is a muscle contraction in response to a single stimulus of adequate strength.

 

Page 5.  Three Phrases of a Muscle Twitch

Three phases of a muscle twitch:

1. Latent period

the sarcolemma and the T tubules depolarize

calcium ions are released into the cytosol

cross bridges begin to cycle but there is no visible shortening of the muscle

2. Contraction phase

myosin cross bridge cycling causes sarcomeres to shorten

3. Relaxation

calcium ions are actively transported back into the terminal cisternae

cross bridge cycling decreases and ends

muscle returns to its original length

Each different muscle has different actual time periods for each phase.

The speed with which the contraction phase occurs depends on

the weight of the load being lifted

the type of fibers contracting (slow-twitch fibers or fast-twitch fibers)

Label this diagram:

Page 6.  Temporal Summation of Two Stimuli

Temporal Summation (wave summation)

Occurs when a second stimulus of the same intensity is applied to a muscle before the completion of the relaxation period of the first stimulus.  This results in increased muscle tension. 

 


Page 7.  Graph of Temporal Summation

In temporal summation, the second peak is higher than the first because the additional influx of calcium ions promotes a second contraction, which is added to the first contraction.

 

Page 8.  Effect of Time Interval on Second Contraction

Explain how the time interval between stimuli will affect the height of the second contraction:

 

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Page 9.  Questions on Effect of Time Interval

If you wait until relaxation is complete from the first stimulus, then give a second stimulus to the same muscle, temporal summation will not occur.

 

Page 10.  Summation of Multiple Stimuli

A muscle is repeatedly stimulated with stimuli of equal intensity.  A plot is made of muscle tension vs. time.  As time proceeds, the interval between stimuli is gradually decreased.

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Page 11.  Graph of Multiple Stimuli

Label the five parts to this graph:

Treppe:

The frequency of stimulation was so slow here that relaxation was complete between contractions.  Note that the curve goes down to the baseline after each contraction

The strength of contraction did increase, because muscle contraction causes heat to build up in the muscles and muscles work better when they are warmer. Enzymes can work faster and more efficiently when a muscle is "warmed up."

 

Temporal summation:

Now the frequency of stimulation is increased to the point where relaxation cannot totally occur.

The result is a continual increase in tension which may result from increased availability of intracellular calcium.

 

Incomplete tetanus:

Now the frequency of stimulation is increased to the point where the muscle exhibits even shorter contraction-relaxation cycles, but there is still some degree of relaxation after each contraction.

 

Complete tetanus:

When the frequency of stimulation becomes fast enough, the contractions fuse into a smooth, continuous, total contraction with no apparent relaxation. 

This state is due to a continual depositing of calcium ions in the cytosol.  As a result, the binding sites on actin continually stay exposed.

Fatigue: 

 Tetanus cannot continue forever.  With continued rapid stimulation, there is a build-up of acidic compounds that affect protein functioning, a relative but not total lack of ATP, and ionic imbalances resulting from membrane activities.  This causes muscle fatigue and the gradual inability of the muscle to respond to stimulation.

 

* The animation of the muscle contracting is important here and is best viewed in slow motion.  You may want to click on the  button at the bottom of the screen and adjust the speed.  Note that with treppe, the muscle relaxes after each stimulus.  With temporal summation and incomplete tetanus, the muscle never really relaxes totally, and with complete tetanus, the muscle stays contracted.  With fatigue, the muscle slowly loses its tension.

 

Page 12.  Second Factor Affecting Muscle Tension

Three factors which affect the development of muscle tension are:

1. Frequency of stimulation

2. Number of motor units recruited

3. Degree of muscle stretch

 

Page 13.  Multiple Motor Unit Summation in the Body

We need to recruit fewer motor units to move a light object compared to a heavy object.

 

* Start the animation by clicking on the weight bar that says "80", then place the slider bar on few.  Click the thigh muscle.  Then set the slider between few and many and click the thigh again, finally set the slider on "many" and click the muscle.  Record your data in the following graph, rating the response as:

0. none

1. required much effort

2. required less effort

3. easy

Number of Motor Units Recruited

 

 

few

intermediate

many

 

80

 

 

 

 

 

 

 

120

 

 

 

 

 

 

 

160

 

 

 

 

 

 

 

200

 

 

 

 

 

Page 14.  Lab Simulation of Multiple Motor Unit Summation

In this animation you set the voltage, then record tension vs. time.

Subthreshold stimulus: A stimulus that does not evoke a visible response. The number of motor units responding is not sufficient to cause visible movement.

Threshold: The minimum stimulus that can evoke a response.

Recruitment: A stronger response than threshold because additional motor units join in to generate an increase in tension.

Maximal stimulus: When all motor units are being recruited and more intense stimulus does not evoke greater tension.

 

Page 15.  Third Factor Affecting Muscle Tension

Three factors which affect the development of muscle tension are:

1. Frequency of Stimulation

2. Number of Motor Units Recruited

3. Degree of Muscle Stretch (length-tension relationship)

 

Page 16.  Length-Tension Relationship

The strength of a muscle contraction can be altered by changing the starting length of a muscle.

 

Unstretched muscle:

The overlapping thin filaments from opposite ends of the sarcomere, interfere and conflict with each other.

This restricts productive cross bridge building and less tension develops. The unstretched muscle produces a relatively weak contraction.

Moderately stretched muscle: Maximum tension is developed when there is an optimum overlap of thin and thick filaments so that all cross bridges can participate in the contraction.

Over stretched muscle:

The thin filaments are pulled almost to the ends of the thick filaments where little tension can be developed.

 

 

Page 17.  Summary

A single muscle twitch is composed of latent, contraction, and relaxation periods.

Factors that affect muscle tension:

1. The frequency of stimulation: increasing the frequency provides temporal summation and increased muscle tension.

2. The number of motor units recruited: stimulation of more motor units produces increased muscle tension.

3. The starting length of the muscle: optimum stretch permits maximum binding of cross bridges for maximum muscle tension.

 

Notes on Quiz Questions:

Quiz Question #1: Phase of a Muscle Twitch

This question asks you to watch an animation, then predict if it occurs during the latent period, contraction period, or relaxation period.

 

Quiz Question #2: Phase of a Muscle Twitch

This question asks you to watch an animation, then predict if it occurs during the latent period, contraction period, or relaxation period.

 

Quiz Question #3: Phase of a Muscle Twitch

This question asks you to watch an animation, then predict if it occurs during the latent period, contraction period, or relaxation period.

 

Quiz Question #4: Graph of Multiple Stimuli

In this question, you will be asked to identify the parts of a recording of  muscle tension vs. time when there are multiple stimuli. The frequency of stimuli to the muscle will increase with time.  You will then be asked to match the definitions of treppe, temporal summation, incomplete tetanus, complete tetanus, and fatigue.

 

Quiz Question #5: Number of Motor Units

This question allows you to predict the number of motor units needed to lift a given object.

If we lift a light object, but we think it may be heavy, we use too many motor units and we end up applying too much muscle tension for the job.

 

Quiz Question #6: Thin Filaments at Ends of Thick Filaments

This question asks you to predict the strength of a muscle contraction, given a diagram of a sarcomere.

 

Quiz Question #7: Overlapping Thin Filaments

This question asks you to predict the strength of a muscle contraction, given a diagram of a sarcomere.

 

Quiz Question #8: Ideal Position of Thin Filaments

This question asks you to drag the thin filaments to their position within the sarcomere that will provide the ideal starting position for maximum contractile force.


Study Questions on the Contraction of Whole Muscle:

1. (Page 1.) The level of muscle tension that can be developed within a given muscle is always constant.

a. true                                                 b. false

 

2. (Page 3.)  What are the three factors that affect muscle tension in a whole muscle?

 

3. (Page 4.) A muscle contraction in response to a single stimulus of adequate strength is called a __________________.

 

4. (Page 5.) What are the three phases of a muscle twitch?

 

5. (Page 5.) Tell if the following are characteristic of the latent period, contraction period, or relaxation period of a muscle twitch.

_____________________               a. myosin cross bridge cycling causes sarcomeres to shorten

_____________________               b. no visible shortening of the muscle

_____________________               c. calcium ions are actively transported back into the terminal cisternae

_____________________               d. cross bridge cycling decreases and ends

_____________________               e. the sarcolemma and the T tubules depolarize

_____________________               f. muscle to return to its original length

 

6. (Page 5.) What two factors will cause the speed of the contraction phase of a muscle twitch to vary?

 

7. (Page 6.) What is it called when a second stimulus of the same intensity is applied to a muscle before the completion of the relaxation period of the first stimulus resulting in increased muscle tension? 

 

8. (Page 7.) In temporal summation, why is the second peak higher than the first?

 

9. (Page 8.) If two stimuli are given to the same muscle, which will result in a more intense second contraction?

a. If the stimuli are given 50 milliseconds apart

b. If the stimuli are given 70 milliseconds apart

 

10. (Page 9.) If you wait until relaxation is complete from the first stimulus, then give a second stimulus to the same muscle, why won't temporal summation occur?

 

11. (Page 11.) In which of the following states does the muscle stay contracted, with no apparent relaxation?

a. Temporal summation

b. Fatigue

c. Complete tetanus

d. Treppe

e. Incomplete tetanus

 

12. (Page 11.)  In which of the following states does the muscle relax totally between stimulations?

a. Temporal summation

b. Fatigue

c. Complete tetanus

d. Treppe

e. Incomplete tetanus

 

13. (Page 11.)  In which of the following states does the  strength of contraction increase with each stimulation because of a gradual accumulation of calcium ions in the cytosol?

a. Temporal summation

b. Fatigue

c. Complete tetanus

d. Treppe

e. Incomplete tetanus

 

14. (Page 11.)  In which of the following states does the strength of contraction decrease with each stimulation?

a. Temporal summation

b. Fatigue

c. Complete tetanus

d. Treppe

e. Incomplete tetanus

 

15. (Page 11.)  Which of these is not a reason for muscle fatigue?

a. ionic imbalances resulting from membrane activities

b. relative but not total lack of ATP

c. depletion of calcium ions in the cytosol

d. a build-up of acidic compounds

 

16. (Page 12.) Which of these is not a factor that affects the development of muscle tension?

a. Number of motor units recruited

b. Degree of muscle stretch

c. Frequency of stimulation

d. They are all factors

 

17. (Page 13.) A given muscle always uses the same number of motor units, no matter what the muscle is trying to accomplish.

a. true                                                 b. false

 

18. (Page 14.)  The minimum stimulus (voltage) that can evoke a response is applied to a muscle.  This is called _________.

a. threshold

b. a maximal stimulus

c. a subthreshold stimulus

d. recruitment

 

19. (Page 14.)  The stimulus (voltage) is applied to a muscle and there is no visible response.  This is called  _______.

a. threshold

b. a maximal stimulus

c. a subthreshold stimulus

d. recruitment

 

20. (Page 14.)  The stimulus (voltage) is applied to a muscle. When another, more intense stimulus does not evoke greater tension the original stimulus would be called _______.

a. a threshold stimulus

b. a maximal stimulus

c. a subthreshold stimulus

d. recruitment