Skeletal Muscle: Structure and Function

Overview

• Skeletal Muscle: Also known as striated muscle due to its striped appearance when viewed under a microscope.
• Function: Used to move the skeleton and is under the direct control of the nervous system.
• Contractions: Can range from quick twitches to sustained powerful tension.

Muscle Fibers

• Size: Among the largest cells in the human body, 10 to 100 micrometers in diameter.
• Length: Each fiber can run the entire length of the muscle, up to 35 centimeters (about 14 inches) in the human thigh.
• Components: Contains multiple subunits called myofibrils.

Myofibrils

• Structure: Cylindrical and extend throughout the muscle fiber.
• Surrounding Membrane: Sarcoplasmic reticulum, a membrane complex forming a network of interconnected tubes.
• Calcium Ions: The sarcoplasmic reticulum contains a fluid rich in calcium ions, essential for muscle contraction.

T-tubules

• Definition: Transverse tubules that extend into the muscle fiber and are close to the sarcoplasmic reticulum.
• Importance: Crucial for controlling muscle contraction.

Sarcomeres

• Arrangement: Made up of actin and myosin filaments with Z-lines marking the junction points.
• Components:
  • Thin Filaments: Consist of actin and two accessory proteins attached to Z-lines.
  • Thick Filaments: Composed of the protein myosin.
• Appearance: The alternating thick and thin filaments give myofibrils a striped appearance.

Cross Bridges

• Structure: Small arms extending from strands of myosin to contact thin filaments.
• Function:
  • Actin proteins form a double chain resembling twisted strands of pearls.
  • Each actin subunit has a binding site for a myosin cross bridge.

Muscle Contraction

• Relaxed Muscle: Binding sites on actin are covered by accessory proteins, preventing myosin attachment.
• Contracted Muscle: Accessory proteins move aside, allowing myosin cross bridges to attach to actin binding sites on thin filaments.