12.1 Mechanism of muscle contraction

Muscle contraction is a complex process that relies on the precise interaction of various proteins and structures. The sarcomere, the basic unit of muscle fibers, contains myofilaments that slide past each other to generate force.

Calcium ions play a crucial role in initiating and regulating muscle contraction. When released from the sarcoplasmic reticulum, they bind to troponin, triggering a series of events that allow myosin heads to attach to actin filaments and generate force.

Sarcomere Structure and Function

Myofilaments and Sarcomere Organization

• The sarcomere is the basic functional unit of a skeletal muscle fiber, consisting of myofilaments arranged in a repeating pattern
• Myofilaments are composed of two main types of protein filaments:
  • Thick filaments (myosin)
  • Thin filaments (actin, troponin, and tropomyosin)
• The sarcomere is bounded by two Z-lines, which anchor the thin filaments and provide structural support

Sarcomere Zones and Bands

• The H-zone is the central region of the sarcomere where only thick filaments are present
• The M-line is a dense protein structure at the center of the H-zone that helps maintain the alignment of the thick filaments
• The I-band is the region of the sarcomere containing only thin filaments, extending from the Z-line to the edge of the H-zone
• The A-band is the region of the sarcomere where thick and thin filaments overlap, extending the entire length of the thick filaments
• During muscle contraction, the sarcomere shortens as the thin filaments slide past the thick filaments, pulling the Z-lines closer together, resulting in the shortening of the I-band and H-zone while the A-band length remains constant

Calcium Ions in Muscle Contraction

Calcium Ion Storage and Release

• Calcium ions (Ca2+) are essential for initiating and regulating muscle contraction and relaxation
• In a resting muscle fiber, Ca2+ is actively transported and stored in the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum
• When a muscle fiber is stimulated by a motor neuron, an action potential propagates along the sarcolemma and into the transverse tubules (T-tubules)
• The action potential in the T-tubules triggers the release of Ca2+ from the SR through voltage-gated calcium release channels (ryanodine receptors)

Calcium Ion Binding and Muscle Relaxation

• The released Ca2+ binds to troponin C, a subunit of the troponin complex, which is associated with the thin filaments
• The binding of Ca2+ to troponin C causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on the actin filaments, allowing cross-bridge cycling to occur
• During muscle relaxation, Ca2+ is actively pumped back into the SR by Ca2+-ATPase pumps (SERCA), lowering the cytoplasmic Ca2+ concentration
• As Ca2+ is removed from the cytoplasm, it dissociates from troponin C, allowing tropomyosin to block the myosin-binding sites on actin, resulting in muscle relaxation

Sliding Filament Theory vs Cross-Bridge Cycle

Sliding Filament Theory

• The sliding filament theory explains the basic mechanism of muscle contraction, stating that muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere and generating force
• This sliding action is powered by the cross-bridge cycle and results in the shortening of the I-band and H-zone, while the A-band remains constant in length
• The sliding filament theory provides an overall description of the shortening of sarcomeres during contraction

Cross-Bridge Cycle

• The cross-bridge cycle refers to the cyclical attachment and detachment of myosin heads (cross-bridges) to actin filaments, which generates the force for muscle contraction
• The cross-bridge cycle is powered by the hydrolysis of ATP, which provides the energy for myosin head movement and conformational changes
• The cycle consists of several steps:
  1. Attachment of myosin heads to actin
  2. Power stroke (force generation)
  3. Detachment of myosin heads from actin
  4. Recovery stroke (myosin head reset)
• The cross-bridge cycle explains the molecular basis for the sliding action described by the sliding filament theory

Key Proteins in Muscle Contraction

Actin and Myosin

• Actin is the primary component of thin filaments and serves as the binding site for myosin during cross-bridge cycling
  • G-actin monomers polymerize to form F-actin, the backbone of the thin filament
• Myosin is the primary component of thick filaments and is responsible for generating the force of muscle contraction
  • Myosin molecules consist of two heavy chains and four light chains, with the heavy chains forming the rod-like tail and globular heads (cross-bridges)
  • The myosin heads contain actin-binding sites and ATPase activity, which enables the cross-bridge cycle

Troponin and Tropomyosin

• Troponin is a complex of three proteins (troponin C, troponin I, and troponin T) that regulates the interaction between actin and myosin
  • Troponin C binds calcium ions, triggering a conformational change in the troponin complex
  • Troponin I binds to actin and inhibits the actin-myosin interaction in the absence of calcium
  • Troponin T binds to tropomyosin and helps anchor the troponin complex to the thin filament
• Tropomyosin is a long, rod-like protein that lies in the grooves between the two strands of the actin filament
  • In a resting muscle, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation
  • When calcium binds to troponin C, tropomyosin shifts position, exposing the myosin-binding sites on actin and allowing cross-bridge cycling to occur