Muscle Physiology Chapter 5

Types of Muscle Tissue

• Muscles are classified based on their location, histological structure, and control mechanism.

Skeletal Muscle

• Composed of striated muscle fibers that contain myofibrils and myofilaments.
• Myofibrils consist of thick and thin filaments; thick filaments contain myosin, while thin filaments are made of actin.
• Each muscle fiber is encased in a cell membrane known as the sarcolemma, which conducts action potentials.
• The sarcoplasmic reticulum (sER) stores calcium ions (Ca²⁺).
• T-tubules conduct action potentials from the sarcolemma to the sarcoplasmic reticulum.

Myofilaments Structure

• Thick myofilaments are primarily composed of myosin, featuring a head, tail, and flexible hinge (neck).
• The myosin head has an actin-binding site and an ATP-binding site essential for muscle contraction.
• Thin myofilaments include three proteins: actin, tropomyosin, and troponin.
  • Each actin protein has a binding site for myosin.
  • Tropomyosin wraps around actin and blocks myosin-binding sites during relaxation.
  • Troponin, a multi-subunit protein, regulates contraction by binding Ca²⁺.

Excitation-Contraction Coupling

• Muscle contraction involves two main steps: excitation and contraction.
• The neuromuscular junction is where the nervous system communicates with muscle fibers.
• Nerve endings at the neuromuscular junction release acetylcholine (ACh) in response to action potentials.
• ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma, triggering muscle fiber depolarization.

Excitation Steps

• Depolarization occurs when Na⁺ enters the muscle fiber, leading to action potential propagation along the sarcolemma and T-tubules.
• This triggers the release of Ca²⁺ from the sarcoplasmic reticulum.

Contraction Steps

• Ca²⁺ binds to troponin C, exposing binding sites on actin for myosin.
• Myosin heads, energized by cleaving ATP, attach to actin, initiating muscle contraction.
• The binding of a new ATP molecule leads to detachment of myosin from actin, resulting in muscle relaxation.

Smooth Muscle

• Characterized as involuntary and non-striated muscle tissue.
• Found in various systems:
  • Cardiovascular: located in blood vessels.
  • Digestive: forms sphincters in the alimentary canal.
  • Respiratory: regulates airway diameter.
  • Urinary: forms sphincters for the urinary bladder.
  • Reproductive: comprises the male reproductive tract.

Explore the intricacies of muscle physiology in Chapter 5. This quiz covers the different types of muscle tissues, including their histological structure and control mechanisms. Test your understanding of skeletal, smooth, and cardiac muscles.