Anatomy and Physiology I - Muscle Tissue

Muscle System: Skeletal Muscles

Characteristics and Structure of Skeletal Muscle

• Primary tissue type responsible for skeletal movement.

Functions:

• Voluntary control over actions like swallowing.

• Protects internal organs.

• Maintains homeostasis by generating heat.

• Composed of various tissues: muscle fibers, blood vessels, nerves, and connective tissue.

• More than 600 muscles in the human body.

Key Characteristics of Skeletal Muscle

• Excitability: Responds to neural stimulation.

• Elasticity: Can stretch and recoil.

• Extensibility: Can lengthen.

• Contractility: Can shorten forcefully.

Muscle Terminology

• Sarco-: Root word meaning “flesh.”

Components:

• Sarcolemma: Plasma membrane of muscle fibers.

• Sarcoplasm: Cytoplasm of muscle fibers.

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions.

• Sarcomere: Functional unit of a muscle fiber for contraction.

Organization of Skeletal Muscle

• Hierarchy from smallest to largest:

  • Myofilaments (actin & myosin)

  • Sarcomeres

  • Myofibrils

  • Muscle Fibers

  • Fascicles

  • Whole Muscle

Myofilaments

• Thin Filaments (Actin) and Thick Filaments (Myosin): Special proteins essential for muscle contraction.

  • Actin is blocked by the troponin-tropomyosin complex at rest.

  • Myosin heads bind to actin when tropomyosin is moved.

The Sarcomere

• Smallest functional unit contained in myofibrils.

Regions:

• Z-Discs: Where actin filaments are anchored.

• A-Band: Overlap of actin and myosin.

• I-Band: Only actin.

• H-Zone: Only myosin.

Neuromuscular Junction (NMJ)

• Where motor neuron meets muscle fiber.

• Action of NMJ: Releases acetylcholine (ACh) to initiate muscle contraction.

Excitation-Contraction Coupling

• The process of converting an electrical signal into a muscle contraction involves several key steps:

  1. Action Potential Generation: The motor neuron releases acetylcholine into the synaptic cleft at the NMJ, leading to an action potential in the sarcolemma of the muscle fiber.

  2. Depolarization of Sarcolemma: The binding of ACh results in the opening of sodium channels, causing sodium ions to flow into the muscle fiber and depolarizing the membrane.

  3. Propagation of Action Potential: This depolarization triggers an action potential that travels along the sarcolemma and into the muscle fiber via T-tubules (transverse tubules).

  4. Calcium Release from SR: The action potential reaching the sarcoplasmic reticulum causes it to release calcium ions into the sarcoplasm.

  5. Calcium Binding to Troponin: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites.

  6. Cross-Bridge Formation: Myosin heads attach to the exposed sites on actin, forming cross-bridges and initiating the power stroke.

  7. Contraction Cycle: The myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, which leads to muscle contraction.

Energy Sources for Contraction

• ATP is essential for:

  • Detaching myosin from actin.

  • Recocking myosin heads.

Regeneration Mechanisms:

• Creatine phosphate metabolism (quick energy, lasts ~15s)

• Anaerobic glycolysis (produces ATP without oxygen, but less efficient)

• Aerobic respiration (requires oxygen, more efficient, produces about 36 ATP).

Muscle Fiber Types

• Type I (Red Fibers): Slow, fatigue-resistant for endurance activities.

• Type II (White Fibers): Fast fibers for quick bursts of power but fatigue quicker.

Muscle Fatigue and Recovery

• Caused by:

  • ATP depletion.

  • Lactic acid accumulation.

  • Electrolyte imbalances.

  • Microdamage during prolonged activity.

• Recovery involves replenishing energy sources and repairing muscle fibers.

Conclusion

Understanding muscle structure and function is essential for grasping how movement occurs and the factors influencing muscle performance and recovery.