Chapter 9: The Muscular System Part 2 incomplete

What is a motor unit?

One motor neuron and all the muscle fibers it supplies.

How many muscle fibers can a motor unit contain?

Four to several hundred.

Which muscles have small motor units?

Muscles controlling fine movements (fingers and eyes).

Which muscles have large motor units?

Large weight-bearing muscles (thighs and hips).

Where are muscle fibers from one motor unit located?

Spread throughout the muscle

What happens when a single motor unit contracts?

It causes a weak contraction of the entire muscle.

What is a muscle twitch?

A muscle's response to a single, brief threshold stimulus.

What are the 3 phases of a muscle twitch?

1. Latent period

2. Period of contraction

3. Period of relaxation

What happens during the 3 phases of a muscle twitch?

1. Latent period: EC coupling occurs.

2. Contraction: Cross-bridges form; muscle shortens.

3. Relaxation: Ca²⁺ is reabsorbed; muscle tension returns to zero.

What are graded muscle responses?

Variations in the degree of muscle contraction needed to control skeletal movement.

How are muscle responses graded?

• Changing the frequency of stimulation

• Changing the strength of the stimulus

What happens when stimulus strength increases beyond the threshold?

The muscle contracts more vigorously as stimulus strength increases.

What is muscle tone?

The constant, slightly contracted state of all muscles that does not produce active movement.

What is an isotonic contraction?

The muscle changes length and moves a load. It can shorten or lengthen during contraction.

What is muscle tone?

The constant, slightly contracted state of all muscles that does not produce active movement.

What happens during an isometric contraction?

Muscle tension increases to its maximum capacity, but the muscle neither shortens nor lengthens.

When does an isometric contraction occur?

When the load is greater than the tension, the muscle can develop.

Isometric = tension changes, muscle length stays the same.

What is the only source of energy used directly for muscle contraction?

ATP

How long do the available ATP stores last during muscle activity?

About 4–6 seconds.

How is ATP regenerated in muscle cells?

1. Interaction of ADP with creatine phosphate (CP)

2. Anaerobic glycolysis

3. Aerobic respiration

Which energy source helps regenerate ATP by transferring a phosphate to ADP?

Creatine phosphate (CP)

What is muscle fatigue?

A state of physiological inability of a muscle to contract.

What causes muscle fatigue?

• ATP production fails to keep pace with ATP use

• Relative deficit of ATP, causing contractures

• Lactic acid accumulates in the muscle

• Ionic imbalances occur

What happens during intense exercise in relation to muscle fatigue?

Rapid muscle fatigue occurs, followed by rapid recovery.

Why does intense exercise cause rapid muscle fatigue?

Na⁺-K⁺ pumps cannot restore ionic balances quickly enough.

What happens during low-intensity exercise in relation to muscle fatigue?

Fatigue develops slowly.

What causes fatigue during low-intensity exercise?

The sarcoplasmic reticulum (SR) becomes damaged, disrupting Ca²⁺ regulation.

What must happen for a muscle to return to its resting state after vigorous exercise?

• Oxygen reserves must be replenished.

• Lactic acid must be converted to pyruvic acid.

• Glycogen stores must be replaced.

• ATP and CP reserves must be resynthesized.

What is oxygen debt?

The extra amount of oxygen needed for restorative processes after exercise.

How much of the energy released during muscle activity is used for work?

40%

What happens to the remaining 60% of energy released during muscle activity?

It is given off as heat.

How does the body prevent dangerous heat levels during muscle activity?

By radiating heat from the skin and sweating.

What three factors affect the force of muscle contraction?

1. Number of muscle fibers contracting.

2. Relative size of the muscle.

3. Degree of muscle stretch.

How does the number of muscle fibers contracting affect muscle strength?

The more muscle fibers contracting, the stronger the contraction.

How does muscle size affect muscle strength?

The bulkier the muscle, the greater its strength.

At what percentage of normal resting length do muscles contract most strongly?

80–120% of their normal resting length.