Module 4 – Muscle Physiology: Structure, Function, and Contraction Mechanisms

Muscle Physiology Overview

Introduction

This module covers the macro- and micro-architecture of skeletal muscle, the molecular basis of muscle contraction, and the mechanisms underlying excitation-contraction coupling. Understanding these principles is essential for comprehending how muscles generate force and movement in the human body.

Types and Structure of Muscle Tissue

Classification of Muscle Tissue

• Striated Muscle: Includes skeletal muscle and cardiac muscle.

• Unstriated Muscle: Includes smooth muscle.

• Voluntary Muscle: Skeletal muscle, under conscious control.

• Involuntary Muscle: Cardiac and smooth muscle, not under conscious control.

Muscle Composition and Organization

• Muscle Fibre (Myofibre): Formed by fusion of myoblasts; a single, elongated, multinucleated cell.

• Connective Tissue Layers:

  • Epimysium: Surrounds entire muscle.

  • Perimysium: Surrounds groups of fibres (fascicles).

  • Endomysium: Surrounds individual muscle fibres.

• Hierarchy: Muscle → Fascicle → Muscle fibre → Myofibril

Microscopic Structure: Striations and Sarcomeres

Striations and Filament Arrangement

• Myofibrils: Long chains in the cytoplasm, comprising 80% of muscle volume.

• Contractile Proteins:

  • Actin: Thin filaments

  • Myosin: Thick filaments

• Sarcomere: Functional unit of muscle, repeating pattern along myofibril.

Sarcomere Bands and Zones

• A Band: Wide, dark band (thick filaments).

• I Band: Light band (thin filaments, no overlap with thick).

• Z Line: Anchors thin filaments, marks sarcomere boundaries.

• H Zone: Center of A band, only thick filaments.

Key Proteins in Muscle Contraction

Anchoring and Regulatory Proteins

• Titin: Largest protein, anchors myosin, provides elasticity.

• Troponin and Tropomyosin: Regulatory proteins that bind to actin and control contraction.

Actin and Myosin Structure

• Actin: Globular molecules polymerize into helical filaments; each has a myosin-binding site.

• Myosin: Two heavy chains, four light chains; globular heads with binding sites for actin and ATP.

Mechanism of Muscle Contraction

Sliding Filament Theory

• Muscle contraction is due to the sliding of thin (actin) filaments over thick (myosin) filaments.

• Cross-Bridge Cycle:

  1. Myosin head binds to actin.

  2. Power stroke: Myosin head pivots, pulling actin filament.

  3. ADP and Pi are released.

  4. ATP binds to myosin, causing detachment from actin.

  5. ATP hydrolysis re-cocks the myosin head.

ATP Role in Contraction

Function	Description
Hydrolysis by myosin	Energizes cross-bridge for force generation
Binding to myosin	Dissociates cross-bridge from actin
Hydrolysis by Ca2+ ATPase	Transports Ca2+ into SR, ending contraction

Excitation-Contraction Coupling

Process Overview

• Action Potential: Travels along muscle fibre membrane.

• Transverse Tubules (T-tubules): Conduct action potential into muscle interior.

• Sarcoplasmic Reticulum: Releases Ca2+ in response to action potential.

• Calcium: Binds to troponin, shifting tropomyosin and exposing myosin-binding sites on actin.

Key Steps

1. Action potential triggers Ca2+ release from SR.

2. Ca2+ binds to troponin, moving tropomyosin.

3. Myosin binds to actin, initiating cross-bridge cycling.

4. Muscle contracts; relaxation occurs when Ca2+ is pumped back into SR.

Motor Units and Muscle Force

Motor Unit Structure and Function

• Motor Unit: One motor neuron and all the muscle fibres it innervates.

• Neuromuscular Junction: Site where motor neuron stimulates muscle fibre.

Recruitment and Control of Force

• Motor Unit Recruitment: All fibres in a motor unit contract when activated.

• Henneman's Size Principle: Motor units are recruited from smallest to largest, allowing fine control and minimizing fatigue.

• Frequency of Activation: Increased stimulation frequency increases muscle tension.

Muscle Twitch and Contraction

• Twitch: Brief, weak contraction from a single action potential.

• Latent Period: Time between excitation and contraction, includes Ca2+ release and cross-bridge cycling.

Motor Unit Table

Muscle	Number of Muscle Fibres per Motor Unit
Finger muscles	Few (greater control)
Back muscles	Many (less control)

Summary and Key Points

• Muscle contraction is driven by the interaction of actin and myosin filaments, regulated by calcium and ATP.

• Excitation-contraction coupling links electrical signals to mechanical contraction via calcium release.

• Motor units and their recruitment patterns determine the strength and precision of muscle contractions.

Additional info: These notes expand on the provided slides with definitions, hierarchical structure, and academic context for key terms and mechanisms.