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Muscle Physiology

Muscle Physiology

Overview

  • Focuses on the structure and function of muscle fibers and mechanisms of muscle contraction.

Anatomy of Muscle Fibers

  • Components:

    • Nucleus

    • Myofibrils

    • Sarcoplasm

    • Myofilaments

    • Triad Structure:

      • Terminal Cisternae (overlapping with adjacent parts)

      • Transverse Tubule (T tubule)

      • Sarcoplasmic Reticulum

Muscle Fiber Organization

  • Sarcolemma: Membrane surrounding the muscle fiber.

  • Openings: Channels for ion exchange and metabolic processes.

Myofibril Structure

Banding Pattern

  • I bands: Light bands containing only actin filaments.

  • A bands: Dark bands containing myosin and overlapping actin filaments.

  • H Zone: Center of A band with only thick filaments; shortens during contraction.

  • M Line: Middle of A band connecting filaments.

  • Z-disk: Structural unit connecting adjacent myofibrils.

  • Cross-Bridges: Projections from myosin filaments that interact with actin.

  • Sarcomere: Functional unit between two Z disks.

Muscle Contraction Mechanism

Steps in Skeletal Muscle Contraction

  1. Acetylcholine (ACh) Release: From the motor neuron.

  2. Na+ Influx: ACh binds to receptors, opening Na+ channels.

  3. Calcium Release: Ca++ released from sarcoplasmic reticulum.

  4. Cross-Bridge Formation: Interaction between actin and myosin.

  5. ATP Role: Hydrolysis and reformation of cross-bridges; muscle shortens with contractions.

  6. Result: Muscle contraction through synchronous shortening of fibers.

Excitation-Contraction Coupling

  • Action Potential (AP): Propagation along the sarcolemma and down T tubules.

  • Role of Calcium:

    • Calcium released from sarcoplasmic reticulum (SR) activates muscle contraction.

    • Binding to troponin removes tropomyosin blockage from myosin binding sites.

Muscle Energy Requirements

ATP Functions

  • Necessary for:

    • Cross-bridge cycling.

    • Active transport of Ca++ back into SR.

    • Maintenance of Na+ and K+ gradients for action potentials.

Energy Sources for Muscle Contraction

  • Phosphocreatine: Provides quick ATP regeneration for short bursts (5-8 sec).

  • Glycogen Breakdown: Produces pyruvic and lactic acid for moderate duration (1 min).

  • Oxidative Metabolism: Sustained ATP production for longer contractions.

Force Summation in Muscle Contraction

  • Methods:

    • Twitch Contractions: Adding individual muscle twitches for intensity.

    • Multiple Motor Unit Summation: Recruitment of larger motor units.

    • Frequency Summation & Tetanization: Increasing stimulation frequency leads to merged contractions.

Muscle Contraction Types

  • Isometric Contraction: Muscle does not shorten during tension generation.

  • Isotonic Contraction: Muscle shortens while maintaining constant tension.

Muscle Fiber Types

  • Type I Fibers: Slow fibers (red muscle); high endurance, rich in vascular supply, mitochondria and myoglobin.

  • Type II Fibers: Fast fibers (white muscle); larger, rapid energy access, lower endurance.

Neuromuscular Junction Pharmacology

Drugs Inducing Effects

  • Similar to ACh: Methacholine, Carbachol, Nicotine.

  • Blockers: Curariform drugs; inhibit neuromuscular transmission.

  • AChE Inhibitors: Neostigmine, Physostigmine.

Rigor Mortis

  • Ca++ release causes sustained muscle contraction post-death.

  • Presence of ATP is necessary to break cross-bridges, leading to stiffness and inability to relax.

Tetanus Disease

Cause

  • Clostridium tetani: Anaerobic bacillus releasing toxins, causing muscle contractions.

Clinical Findings

  • Symptoms: Lockjaw, painful contractions, opisthotonos (back arching).

  • Mortality: 10-30%; prevention through vaccination.