Muscle Contraction

Overview

• Muscle contraction is essential for all skeletal movements.
• Composed of muscle fibers, which include repetitive units called sarcomeres.
• Sarcomeres contain overlapping thin (actin) and thick (myosin) filaments.
• Sliding Filament Theory: Muscle contracts when these filaments slide past each other, shortening the sarcomere, and consequently, the muscle.

Cross-Bridge Cycling

• Molecular basis for the sliding movement.

Initiation of Muscle Contraction

• Triggered by a nerve impulse stimulating muscle fibers.
• Leads to the release of calcium ions.

Role of Calcium and Troponin

• Calcium ions bind to troponin on actin myofilaments.
• Causes displacement of tropomyosin, exposing myosin binding sites on actin.

Myosin Head Binding

• Each myosin head is initially bound to ADP and a phosphate from the previous contraction.
• Myosin heads release these phosphates and bind to exposed actin sites.

Sliding Movement

• Myosin heads propel the actin filaments via a head-first movement.
• Powered by chemical energy stored in myosin heads.
• ADP is released during this movement.

Termination and Reset

• ATP binds to myosin heads, breaking the actin-myosin bonds.
• ATP is then decomposed into ADP and phosphate.
• Energy released is stored in myosin heads for the next cycle.
• Myosin heads return to the starting position on actin, ready for a new cycle.

Role of Continued Calcium Presence

• Presence of calcium ions triggers further cycles of contraction.