Muscle Tissue Types and Structure

• Muscle tissue accounts for the largest tissue mass in the body

Types of Muscle Tissue

• Skeletal muscle is striated and voluntary, with many nuclei per cell
• Smooth muscle lacks striations, is involuntary, and has a single, spindle-shaped nucleus
• Cardiac muscle is striated, involuntary, and has a single nucleus; it contains intercalated discs and gap junctions that help cells withstand stretching and motion

Skeletal Muscle Structure

• Microstructure includes myofibrils
  • These run the cell's length and bundle thick and thin filaments
• Sarcomeres make up repeating units of myofibrils and are functional until skeletal muscles contract
• Thick filaments are composed of Myosin
• Thin filaments are primarily composed of Actin, but also include troponin, and tropomyosin

Sarcomere Bands and Lines

• M line consists of anchoring proteins that lie in the middle of the H-zone
• A-band contains Myosin and appears as a dark striation
• I-Bands appear as light striations and contain thin filaments, z-line, and titin
• Z-line to z-line defines the length of one sarcomere within the I-band
• H-zone
  • Is a central area lacking thin filaments that disappears when sarcomeres contract
  • Contains anchoring proteins called M-line that anchors thick filaments
• Titin lines anchor Myosin to the M-line
• Sarcomere contraction
  • Myosin heads bind to actin and pull the actin towards the middle of the sarcomere
  • Reduces the H-zone and I-band while the A-band remains constant

Actin and Myosin Interaction

• In a relaxed sarcomere, thick and thin filaments are partially overlapped, with no overlap in the H-zone
• During contraction, thin filaments are "grabbed" by cross-bridges on thick filaments (Myosin)
  • They are subsequently pulled toward the center of the sarcomere
• Z-lines are pulled towards each other, causing the sarcomere to shorten in a stepwise mechanism

Molecular Structure of Actin and Myosin

• Actin molecules are globular and arranged as two twisted chains
  • Each molecule has one binding site for myosin
• Myosin molecules
  • Are complex consisting of several polypeptide chains that collectively form, two twisted cables with a "head" (containing two parts called a cross-bridge) and a "tail
  • Each cross-bridge has two separate binding sites: one for ATP and one for Actin
• Individual myosin molecules in a single filament have opposite orientations: when cross-bridges move, they attach to actin filaments and pull them together from opposite ends of the thick filament
• Each cross-bridge operates independently and requires 1 ATP per cycle
  • A cell may have 100,000 or more

Role of Troponin and Tropomyosin

• Troponin is a globular protein that binds to both tropomyosin and actin
  • It consists of 3 subunits, one of which binds to calcium
• Tropomyosin consists of 2 intertwined chains and a length equivalent to 7 actin molecules
  • It binds to actin and partially covers myosin binding sites, maintained in place by troponin
• When calcium is present, it binds to troponin, causing a shape change that moves tropomyosin away and exposes myosin binding sites; this enables myosin-actin binding and contraction
• Where calcium is removed or reduced, tropomyosin blocks myosin binding sites on actin, causing relaxation