Muscle Tissue

Histology of Muscle Tissue - Study Notes

Learning Objectives

  • Identify the microscopic structures of the three muscle types: skeletal muscle, cardiac muscle, and smooth muscle.

  • Understand structural and functional differences among muscle types and recognize their connective tissue organization.

  • Understand the arrangement of myofilaments, specifically actin and myosin, and their roles in muscle contraction mechanisms including transverse tubules, sarcoplasmic reticulum, mitochondria, and contractile filaments.

  • Analyze muscle response to injury and the regenerative capacities of the various muscle types.

Muscle: Overview

  • Embryonic Origin:

    • Derived from the mesoderm, specifically from mesenchyme.

  • Properties of Muscle Tissue:

    • Excitable: Capable of undergoing electric changes and generating action potentials.

    • Contractile: Contains myofilaments which interact to produce contraction.

  • Myofilaments: Comprised of actin (thin filaments) and myosin (thick filaments).

Classification of Muscle Tissue

Morphological Classification

  • Skeletal Muscle: Striated/sarcomeric, with myofilaments arranged in regular, repeating units.

  • Cardiac Muscle: Striated/sarcomeric, but branched and interconnected.

  • Smooth Muscle: Non-striated/nonsarcomeric, with myofilaments arranged diffusely.

Functional Classification of Muscle Contraction

  • Skeletal Muscle:

    • Contraction: Voluntary; attached to bones; involved in movement and posture maintenance.

    • Distribution: Found throughout the axial and appendicular skeleton, and in extraocular muscles for eye movement.

  • Cardiac Muscle:

    • Contraction: Involuntary; forms the myocardium of the heart.

    • Distribution: Found in the heart and base of large vessels.

  • Smooth Muscle:

    • Contraction: Involuntary; involved in various functions including peristalsis in viscera and blood vessel diameter regulation.

    • Distribution: Found in the walls of viscera, vascular system, and skin (arrector pili muscles).

Skeletal Muscle Structure and Features

Longitudinal and Cross Sections

  • Longitudinal Section:

    • Characterized by peripheral nuclei and visible striations; no branching observed.

  • Cross Section:

    • Contains massive cytoplasm and peripheral nuclei; stains red due to myoglobin.

    • Myoglobin binds oxygen, essential for muscle metabolism.

Histological Features

  • Peripheral Nuclei: Observed in longitudinal sections.

  • Striations: Indicative of sarcomeric organization.

Cardiac Muscle Structure and Features

Features

  • Longitudinal Section:

    • Central nuclei which may be elongated; visible striations and branching cells.

    • Intercalated discs identified, facilitating electrical coupling.

  • Cross Section:

    • Central nuclei; relatively high cytoplasm-to-nucleus ratio indicates metabolic activity.

Smooth Muscle Structure and Features

Features

  • Longitudinal Section:

    • Central fusiform nuclei; cells appear organized in a meshwork with no striations.

  • Cross Section:

    • Central nuclei and a lower cytoplasm-to-nucleus ratio when compared to other muscle types.

Connective Tissue Organization in Muscles

  • General Functions of Connective Tissue:

    • Convey neural and vascular components.

    • Transmit the forces generated by myofilaments during contraction.

Connective Tissue Layers

  • Epimysium: Surrounds the entire skeletal muscle.

  • Perimysium: Envelops fascicles, or bundles of muscle cells.

  • Endomysium: Encapsulates individual muscle fibers, includes basal lamina and type III collagen fibers (reticular fibers).

Muscle Development

Myogenesis

  • Origin of Myoblasts:

    • Mesenchymal cells differentiate into myoblasts, which fuse to form longer, multinucleated tubes known as myotubes.

    • Myotubes synthesize proteins for myofilaments and demonstrate cross-striations as they mature.

  • Role of Satellite Cells:

    • Residual myoblasts on the muscle fiber surface that can proliferate and generate new muscle cells in response to injury.

Types of Skeletal Muscle Cells

  • Red Fibers (Type I - Slow Oxidative):

    • Function: Slow contraction, resist fatigue, primarily found in long muscles of the back.

  • White Fibers (Type IIb - Fast Glycolytic):

    • Function: Rapid contraction but fatigue easily; examples include extraocular muscles.

  • Intermediate Fibers (Type IIa - Fast Oxidative Glycolytic):

    • Hybrid characteristics resembling both types I and IIb; commonly found in leg muscles.

  • Myoglobin Presence: Critical for oxygen transport and storage in muscle tissues.

Terminology Related to Muscle Histology

  • Sarcomere: The functional unit of muscle, defined as the region between two Z discs.

  • Sarcoplasm: Equivalent to cytoplasm in muscle cells.

  • Sarcolemma: The plasma membrane of muscle fibers.

  • Transverse Tubules (T Tubules): Invaginations of the sarcolemma that penetrate into the muscle cell’s interior.

  • Sarcoplasmic Reticulum (SR): A specialized type of smooth endoplasmic reticulum characterized by terminal cisternae.

  • Sarcosomes: Refers to mitochondria within muscle fibers.

  • Myofilaments: Composed of thin filaments (actin) and thick filaments (myosin).

Myofibril and Calcium Dynamics

Sarcomere Design and Contraction

  • Myofilaments:

    • Thick filaments (myosin) and thin filaments (actin) are organized in a specific arrangement, producing a banding pattern identified as dark A bands and light I bands.

    • The I band is bisected by Z discs, and the sarcomere represents the functional unit of contraction.

  • Banding Patterns:

    • A Band: Dark, comprises the length of thick filaments.

    • I Band: Light, contains only thin filaments.

    • H Zone: A lighter region within the A band; only thick filaments are present.

    • M Line: Center of the H zone where thick filaments are anchored.

    • Z Discs: Lines that bisect the I band and are critical for aligning myofilaments.

Calcium Regulation in Muscle Contraction

  • Sarcoplasmic Reticulum (SR) Characteristics:

    • Consists of dilated terminal cisternae around myofibrils at the junction of A and I bands, storing calcium ions.

  • Mechanism of Regulation: Depend on calcium release leading to contraction or calcium resequestration leading to relaxation.

  • Triads and Functional Role: Composed of a central T tubule flanked by terminal cisternae, helping facilitate uniform contraction across muscle fibers.

Contraction Cycle of Skeletal Muscle

Steps of Contraction

  1. Cross Bridge Formation: Myosin head binds to actin after ATP hydrolysis.

  2. Power Stroke: Myosin head pivots and pulls the actin filament towards the M line; ADP and Pi are released.

  3. Cross Bridge Detachment: ATP binds to myosin, causing it to release from actin.

  4. Resetting the Myosin Head: ATP is hydrolyzed, re-cocking the myosin head for the next contraction cycle.

    • Cycle Repeat: The process can repeat approximately 200 times in rapid succession.

Muscle Relaxation

  • Mechanism of Relaxation:

    • Reduction of calcium concentration in cytosol, leading to tropomyosin blocking myosin-binding sites on actin,

    • The sarcoplasmic calcium pump removes Ca²⁺ back into SR, facilitated by calsequestrin.

Motor Unit and Innervation of Skeletal Muscle

  • Definition of Motor Unit: The basic functional unit of skeletal muscle, comprising one motor neuron and the muscle fibers it innervates.

  • All-or-None Principle: A muscle fiber contracts fully or not at all; all fibers in a motor unit will react synchronously.

  • Variability in Motor Units: Some muscles (e.g., trapezius) have thousands of fibers per motor unit, while others (e.g., superior oblique) may have only 10-15.

Neuromuscular Junction

  • Composition: A synapse between a motor neuron's axon terminal and a skeletal muscle cell.

  • Key Structural Elements:

    • Presynaptic membrane (axon terminal) containing synaptic vesicles filled with acetylcholine (ACh).

    • Postsynaptic membrane (motor end plate) rich in ACh receptors.

  • Mechanism for Action Potential Propagation:

    • Depolarization opens voltage-gated Ca²⁺ channels, allowing Ca²⁺ influx at the axon terminal, which stimulates the release of ACh into the synaptic cleft.

  • Signal Termination: ACh is degraded by acetylcholinesterase, and choline is recycled to form new ACh.

Clinical Correlates in Muscle Histology

  • Duchenne Muscular Dystrophy (DMD):

    • Caused by mutations in the dystrophin gene, leading to muscle fiber degeneration and replacement by fatty and fibrous tissues.

  • Myasthenia Gravis (MG):

    • An autoimmune disorder targeting ACh receptors leading to muscle weakness that worsens with activity and improves with rest.

  • Cardiac Issues:

    • Myocardial infarction, arrhythmogenic right ventricular cardiomyopathy, and drug-related cardiomyopathies are significant clinical concerns.

  • Smooth Muscle Disorders:

    • Conditions such as asthma, achalasia, leiomyoma, and hypertension highlight the clinical relevance of smooth muscle histology.

Summary of Muscle Types

  • Skeletal Muscle:

    • Characteristics: Multinucleated, striated, under voluntary control, capable of rapid and powerful contraction.

    • Regenerative Capacity: Limited but involves satellite cells.

  • Cardiac Muscle:

    • Characteristics: Uninucleated with intercalated discs, striated, involuntary, spontaneous rhythmic contraction.

    • Regenerative Capacity: Very limited.

  • Smooth Muscle:

    • Characteristics: Nonstriated, involuntary, capable of continuous, slow contractions, and has good regenerative potential.

Conclusion

  • Understanding the histology and biology of muscle tissue is crucial for diagnosing and treating muscle-related disorders, integrating knowledge from basic muscle physiology and pathology.