Muscle Contraction - Mechanics - Lecture 8

Chapter Links and Quizzes

 

Mechanisms of contraction

Latent period

Innervation pattern

Tension development

Internal vs. External

Summation

Length - Tension

Control of contraction

Energy

Audio - Lecture - - Review 

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1. Work performed by muscle - myofilaments

isometric contraction - increase tension, constant length, no muscle shortening

isotonic contraction - shortening, constant tension

  most muscle work is a combination of these two -

a. All-or-none: a single AP releases enough Ca⁺⁺ to saturate all troponin binding sites - opens all active sites on actin - maximal interaction with myosin because AP is all-or-none

How is contraction controlled?  Is it true you get maximal contraction of myofilaments or no contraction?

following repolarization of AP, sarcoplasmic reticulum sequesters Ca⁺⁺ - decrease in tension as Ca⁺⁺ comes off troponin and tropomyosin covers actin sites.

b. relation between internal-myofilament - and external-muscle - tension.

1-Why difference?

-due to elastic components of the muscle

Z line connections

connective tissue in tendon membrane

these act like series elastic elements

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2-Analogy (get a rubber band)

a. when contractile element shortens, must first stretch series elastic elements, therefore initially the load does not move; takes time to stretch elastic element.

How then is contraction varied?

b. Summation of contractions

• 1) if second AP arrives before series elastic element recoils to initial state - more internal tension can be applied to the external tension, therefore force of contraction is greater

as AP frequency increases, external tension increases until there is no time between AP for the series elastic element to relax.

This allows internal tension to be directly applied to external tension

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fatigue

- ATP - probably not

- maybe decreases in glycogen

- toxic build up - lactic acid

- also important - psychological fatigue -- pain -- will to win

think about this in laboratory

c. effect of fiber length on tetanus tension - easy experiment - isometric conditions - set length, record tension

studied in laboratory

1- Mechanism

-actin myosin interactions

Resting length of skeletal muscle - is near length needed for maximum tension; heart is a different

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d. how is contraction development controlled in a muscle of set length?

1 - alter frequency of AP - summation - vary tension up to tetanus tension.

2 - call upon other motor units - recruitment = this increases # of muscle fibers firing. This is done by increasing stimulus, therefore by increasing G.P. more axons will reach threshold and fire.

• For example - move 100 vs. move 1 pound, brain determines how many muscle fibers needed. Also how far to kick a ball.

2. Energy requirements

ATP is needed:

A) to recycle myosin heads

B) pump Ca⁺⁺ into sarcoplasmic reticulum

C) operate Na-K⁺ pump in sarcolemma.

D) exocytosis of NT at N-M junction

a. some ATP is stored, ready for use, not enough to last for more than 1 minute of maximum exercise. -- 10-15 sec maximum

b. some energy is stored in creatine phosphate

another 10-15 seconds

during exercise

ADP + CP --- ATP + creatine quickly produces ATP

c. mitochondria uses oxygen and carbohydrates (glucose) to produce ATP.

1 glucose - 36 ATP --- aerobic respiration

d. anaerobic - no oxygen - sprinting

1 glucose - 2 ATP + lactic acid --- glycolysis

(very fast)

3. Skeletal muscle fiber types

a. fast oxidative - fast ATPase on myosin has many mitochondria and many blood vessels, lots of myoglobin - oxygen binding - red in color; intermediate fatigue - maintained contraction

b. fast glycolytic - fast ATPase on myosin few mitochondria - (glycolytic - no oxygen) and few capillaries, little myoglobin - white in color - fast fatigue - short lived contraction

c. slow oxidative - slow ATPase on myosin - many mitochondria and many capillaries - lots of myoglobin - slow fatigue - maintained contractions

Myoglobin - by binding oxygen - this lowers conc. of oxygen in muscle; increases diffusion into muscle.

Most muscles are mixed - the ratio depends on the demand

4. Effect of exercise training

muscle fibers do not reproduce, therefore no change in number

only increase in size - hypertrophy - due to increase # myofibrils

(small increase in # of muscle cells - not from division but from splitting)

a. short - max exercise - increase diameter - more actin and myosin

increase % fast glycolytic - bulky muscle

b. long-less exertion - no change diameter

increase % fast oxidative, increase # blood vessels, use more oxygen

REVIEW QUESTIONS

1. Muscle fatigue can be due to:
   1. increases in Ca⁺⁺
   2. decreases in lactic acid.
   3. loss of elasticity within the muscle
   4. decreased creatinine phosphate.
   5. increased glycolysis.
2. The difference between a twitch and tetanus is that:
   1. Elastic elements in the muscle are more stretched during a twitch.
   2. Elastic elements in the muscle are more stretched during tetanus.
   3. The stimulus strength is greater during a twitch.
   4. The stimulus strength is greater during tetanus.
   5. Two or more are correct.
3. During a muscle twitch, which process is responsible for a longer muscle relaxation than muscle contraction?
   1. Depletion of Ach within the nerve terminal.
   2. Re-uptake of Ca⁺⁺ from the myoplasm by the sarcoplasmic reticulum.
   3. Enzymatic degradation of Ach.
   4. Desensitization of receptors.
   5. all of the above
4. The contractile response of a skeletal muscle fiber in situ to a neural stimulus is an all-or-none property. This is attributed to the fact that:
   1. calcium-troponin binding is an all-or-none phenomenon
   2. sarcomere length cannot be altered
   3. the actin and myosin only form cross-bridges once per contraction
   4. the action potential of the sarcolemma is an all-or-none phenomenon
   5. all of the above are correct
5. Slow oxidative muscle:
   1. is a white muscle
   2. has very few capillaries
   3. can contract for only a short period of time
   4. is found in the leg muscles of sprinters
   5. None of the above are correct.
6. Muscle tetanus is the result of:
   1. a muscle twitch.
   2. saltatory conduction.
   3. electrotonic propagation.
   4. summation.
   5. nerve block.
7. Which of the following statements about skeletal muscle is false?
   1. Calcium ions are released as an action potential is propagated along a skeletal muscle cell.
   2. Calcium ions are actively collected and stored in the sarcoplasmic reticulum as the muscle returns to the resting state.
   3. Ach provides energy to propel actin filaments past myosin filaments during muscle cell activity.
   4. The interaction of Ach with receptors on the sarcolemma produces a graded potential called an EPP.
   5. all of the above are true

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