Skeletal Muscle Force Generation, Energy Usage and Fibre Types

Question 1

What is meant by excitation contraction coupling ?

Answer 1

Linkage between excitation of the muscle fibre membrane and the onset of contraction

Question 2

What occurs as a result of contraction in the Sliding-Filament Mechanism ?
Explain what specifically happens on a microscopic level.

Answer 2

Shortening of a skeletal muscle fiber
The overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges.

Question 3

Which one(s) of A band, I band H zone change upon shortening of the the muscle fiber and which one(s) stay unchanged ?

Answer 3

A band- unchanged
I band- reduced
H zone- reduced

Question 4

What is the energy source for contraction ?

Answer 4

ATP being converted to ADP + Pi

Question 5

How much ATP is contained in muscles ?

Answer 5

Around 4 mM/kg of ATP

Question 6

What is rigor mortis ?

Answer 6

Stiffening of skeletal muscles after death

Question 7

When does rigor mortis begin ?

Answer 7

Begins 3-4 hours after death

Question 8

How long after death is rigor mortis maximised ?

Answer 8

Maximum after 12 hours

Question 9

What is the molecular explanation of rigor mortis ?

Answer 9

Cells accumulate calcium

In the absence of ATP, cross bridges can bind to actin but the cross-linkage is irreversible

Question 10

What are the 4 functions of ATP in skeletal muscle contraction ?

Answer 10

1. Hydrolysis of ATP by Na+/K+ -ATPase in the plasma membrane maintains Na+ and K+ gradients, which allows the membrane to produce and propagate actions potentials
2. Hydrolysis of ATP by Ca2+ ATPase in the sarcoplasmic reticulum provides the energy for the active transport of Calcium ions into the reticulum, lowering cytosolic Ca2+ concentrations to prerelease concentrations, ending the contraction, and allowing the muscle fiber to relax
3. Hydrolysis of ATP by myosin-ATPase energizes the cross-bridge, providing the energy for force generation.
4. Binding of ATP to myosin dissociates cross-bridges bound to actin, allowing the bridges to repeat their cycle of activity.

Question 11

Are contraction and shortening always equivalent ? If not, explain what contraction refers to, and give a situation in which one occurs but not the other.

Answer 11

No
Refers to the activation of the force-generating sites within muscle fibers (the cross-bridges)
Holding a dumbbell at a constant position requires muscle contraction, but not muscle shortening.

Question 12

What are the three ways a muscle fibre can form ATP ? Which is used in different intensity levels/lengths of times ?

Answer 12

– Phosphorylation of ADP by creatine phosphate (MOST RAPID LEVEL OF ACTIVITY)
– Oxidative phosphorylation of ADP in the mitochondria (MODERATE LEVEL OF ACTIVITY)
– Phosphorylation of ADP by the glycolytic pathway in the cytosol (AT HIGHER INTENSITY LEVEL OF ACTIVITY, e.g. distance running)

Question 13

What are the two main bases upon which skeletal muscle fibers can be classified ?

Answer 13

Maximal velocities of shortening (fast or slow)

Major pathway they use to form ATP

Question 14

Which microscopic property of skeletal muscle causes different muscle fibers to have different maximal velocities of shortening ?

Answer 14

Fast and slow fibers contain forms of myosin that differ in the maximal rates at which they use ATP.
This determines the maximal rate of cross-bridge cycling

Question 15

What are the two types of skeletal muscle fibers, classified according to the type of enzymatic machinery available for synthesizing ATP ?

Answer 15

Oxidative or Glycolytic

Question 16

What are some of the cellular features of oxidative fibers ?

Answer 16

Contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation

Question 17

What does ATP production from oxidative fibers depend on ?

Answer 17

Blood flow to deliver oxygen and fuel molecules to the muscle

Question 18

Which molecule keeps oxygen in muscle ?

Answer 18

Myoglobin

Question 19

Are oxidative fibres darker or lighter than their glycolytic counterparts ? What is another name for them ?

Answer 19

Darker

Dark Meat

Question 20

What are some of the cellular features of glycolytic fibers ?

Answer 20

Few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen

Question 21

Are glycolytic fibres darker or lighter than their oxidative counterparts ? What is another name for them ?

Answer 21

Lighter

White Meat

Question 22

What is a major function of muscles containing many oxidative fibres ?
many glycolytic fibers ?

Answer 22

Long term contraction (e.g. standing)

Quick burst of activity

Question 23

On the basis of these two characteristics (maximal shortening velocity and major pathway to form ATP), what are the three principal types of skeletal muscle fibers which can be distinguished ?

Answer 23

• Slow-oxidative fibers (Type I): combine low myosin- ATPase activity with high oxidative capacity.
• Fast-oxidative-glycolytic fibers (Type IIa): combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.
• Fast-glycolytic fibers (Type IIb): combine high myosin- ATPase activity with high glycolytic capacity.

Question 24

What are the two factors upon which the tension a muscle can develop depend on ?

Answer 24

AMOUNT OF TENSION DEVELOPED BY EACH FIBER:

• AP frequency
• Fiber length
• Fiber diameter
• Fatigue

NUMBER OF FIBERS CONTRACTING AT ANY TIME:

• Number of fibers per motor unit
• Number of active motor units

Question 25

What does the shortening velocity of a whole muscle depend on ?

Answer 25

• LOAD ON THE MUSCLE
• TYPES OF MOTOR UNITS IN THE MUSCLE
• NUMBER OF MOTOR UNITS RECRUITED TO WORK AGAINST THE LOAD

Question 26

How does muscle adapt to exercise ?

Answer 26

Increase in the amount of contractile activity increases the size of muscle fibers and increases their capacity for ATP production.

Question 27

What is muscle atrophy ?

Answer 27

“decrease in the mass of the muscle”

Question 28

What are the types of atrophy ?

Answer 28

Disuse atrophy (like an arm in a cast)
Denervation atrophy (nerve damage = loss of function)