Skeletal Muscle - Overview

Definition of Skeletal Muscle

• Also known as striated muscle due to its striped appearance under a microscope.
• Used to move the skeleton.
• Under direct control of the nervous system.
• Capable of producing a range of contractions from quick twitches to powerful sustained tension.

Muscle Fiber Structure

• Muscle Fibers: Among the largest cells in the human body, range from 10-100 micrometers in diameter.
• Each muscle fiber runs the entire length of the muscle, potentially up to 35 cm (14 inches) in a human thigh.

Myofibrils

• Each muscle fiber contains many contractile subunits called myofibrils, extending from one end of the fiber to the other.
• Sarcoplasmic Reticulum: Surrounds each myofibril, a membrane complex forming a network of interconnected tubular structures.
• Contains a fluid rich in calcium ions.

T Tubules

• Deep indentations of the muscle cell membrane known as transverse (T) tubules.
• Extend down into the muscle fiber, passing close to parts of the sarcoplasmic reticulum.
• Arrangement of T tubules and sarcoplasmic reticulum is crucial for controlling muscle contraction.

Components of Myofibrils

• Sarcomeres: Subunits within myofibrils, made up of precise arrangements of actin and myosin filaments.
  • Z Lines: Junction points where sarcomeres are attached end to end throughout the length of the myofibril.
  • Thin Filaments: Consist of actin strands and two accessory proteins, attached to Z lines.
  • Thick Filaments: Composed of the protein myosin, suspended between thin filaments.
  • The alternating thick and thin filaments give myofibrils their striped appearance.

Mechanism of Muscle Contraction

• Cross Bridges: Small arms extending from myosin strands, make contact with thin filaments.
• Actin: Protein of the thin filament, formed from a double chain of subunits resembling a twisted double strand of pearls.
  • Each actin subunit has a binding site for a myosin crossbridge.
  • In relaxed muscles, binding sites are covered by accessory proteins preventing myosin cross bridges from attaching.
  • During contraction, accessory proteins move aside, allowing myosin cross bridges to bind to actin subunits on thin filaments.