Muscular System: Physiology of Muscle Contraction Notes

Module 2: Muscular System - Physiology of Muscle Contraction

Overview

  • Focus on the physiology of muscle contraction in skeletal muscle.
  • Key learning outcomes include:
    • Understanding microanatomy of skeletal muscle cells and significance of striations.
    • Features and function of the neuromuscular junction.
    • Four stages of excitation-contraction coupling.
    • Stages of the cross-bridge cycle.

Types of Muscle Tissue

  • Skeletal Muscle:
    • Long cylindrical cells, multinucleate.
    • Striated appearance due to the arrangement of myofilaments.
    • Under voluntary control.
    • Found attached to bones or skin.
  • Cardiac Muscle:
    • Striated, branched cells.
    • Found in heart walls, under involuntary control.
  • Smooth Muscle:
    • Nucleated, fusiform shape without striations.
    • Found in walls of hollow organs, under involuntary control.

Structure of Skeletal Muscle Fibre

  • Connective Tissue Layers:
    • Muscle fibre (cell) surrounded by sarcolemma.
    • Muscles are made of bundles of muscle fibres (fascicles).
  • Myofibrils and Myofilaments:
    • Muscle fibres contain many myofibrils (~80% volume).
    • Myofibrils composed of myofilaments (actin and myosin).

Myofilaments

  • Thick Filaments (Myosin):
    • Double-headed structure, heads bind actin and ATP.
  • Thin Filaments (Actin):
    • Composed of G-actin subunits forming F-actin chains.
    • Tropomyosin and troponin regulate contractions by managing access to myosin-binding sites.
  • Striations in Myofibrils:
    • A bands (dark) = overlapping actin and myosin.
    • I bands (light) = actin and titin.

Sarcomere Structure

  • Functional Unit:
    • Sarcomere is the smallest contractile unit, measured from Z disc to Z disc.
    • Contains A band, I band, H zone, and M line.
  • Contraction Mechanism:
    • During contraction, overlapping increases, Z discs move toward M line.

Excitation-Contraction Coupling (ECC)

  1. Neuromuscular Junction:
    • Motor neuron releases acetylcholine (ACh), generates end plate potential.
  2. Action Potential Generation:
    • Depolarization triggers an action potential, propagating along the sarcolemma and T-tubules.
  3. Calcium Release:
    • AP triggers release of Ca²⁺ from sarcoplasmic reticulum into cytosol.
  4. Muscle Contraction Triggering:
    • Ca²⁺ binds to troponin, shifting tropomyosin to expose myosin-binding sites on actin.

Cross-Bridge Cycle

  1. Attachment:
    • Energized myosin head binds to actin, forming cross bridge.
  2. Power Stroke:
    • ADP and phosphate release causes head to pivot, pulling actin inward.
  3. Detachment:
    • New ATP binds to myosin, causing detachment from actin.
  4. Cocking:
    • Hydrolysis of ATP re-energizes myosin head to high-energy state, ready to bind again.

Key Points for Understanding

  • Striations in Skeletal Muscle: Vertical dark and light bands indicate arrangement of myofilaments.
  • Functional Contraction Units: Sarcomeres contract collectively in myofibrils, impacting muscle contraction.
  • Excitation-Contraction Coupling Events: Sequence from action potential to calcium binding leads to the cross-bridge cycle.
  • ATP Importance: Necessary for muscle contraction and relaxation, by facilitating cross-bridge detachment and calcium reuptake into the sarcoplasmic reticulum for relaxation.