Chapter 6

Contraction of Skeletal Muscle

Dr. Marko LjubkovićDepartment of Physiology

Anatomy of Skeletal Muscle

• Muscle fibers

• Myofibrils

• Sarcomeres

Sarcomere

Myofibrils

SR

Anatomy of Skeletal Muscle

Actin and Myosin Filaments

Cross-bridge Cycle

Muscle Length-Force Relationship

Velocity-Load Relationship

Determined by isotonic contraction

Sources of Energy for Contraction1. ATP pool

2. Creatine phosphate

3. Carbohydrates

• Glycolysis

• Oxidative phosphorylation

4. Fatty acids

• β-oxidation

Oxygen debt• Strenuous ex. (mostly fast

motor units)• Endurance ex. (slow units)

Fatigue• Accumulation of lactic acid

Types of Muscle Fibers

1. Fast twitch fibers• Thicker muscle fibers and nerves• Extensive SR• Abundance of glycolitic enzymes• Lower mitochondrial content• Different myosin isoform

2. Slow twitch fibers3. Both types found in individual muscle

Mechanics of Muscle Contraction

1. Motor unit

• α-motor neuron + muscle fibers

• Small (100 to 500 fibers, slow twitch) and Large (1000 to 2000 fibers, fast twitch)

2. Recruitment

• Slow twitch units recruited first

• Spatial summation

• Temporal summation (frequency depends on muscle type)

Muscle Growth