Muscle Physiology and Cellular Structure

Classification and General Principles of Muscle Tissue

  • Overview of Muscle Tissue:

    • Muscle tissue is one of the four primary tissue types that constitute the human body.
    • The ability to convert chemical energy into mechanical force and movement exists to a limited degree in most cells, but it is the dominant specialized function in muscle cells.
    • Muscle contractions generate force and movement necessary to regulate the body's internal environment and execute external physical movements in relation to the environment.

  • Three Types of Muscle Tissue:

    • Skeletal Muscle:
    • Primarily attaches to bones; its contraction supports and moves the skeleton.
    • Contraction is initiated by action potentials in motor neurons belonging to the somatic motor division of the peripheral nervous system.
    • Operates under voluntary control in most cases.
    • Smooth Muscle:
    • Arranged in sheets surrounding hollow internal organs and tubular structures (e.g., stomach, intestines, urinary bladder, uterus, blood vessels, and lung airways).
    • Functions to propel luminal contents through hollow organs or regulate internal fluid/air flow by altering tube diameter.
    • Controls additional functions such as erecting hair follicles in the skin and adjusting pupil diameter in the eye.
    • Involuntary control; operates autonomously or in response to signals from the autonomic nervous system, hormones, autocrine factors, or paracrine factors.
    • Cardiac Muscle:
    • The muscle tissue of the heart wall.
    • Contraction generates intraventricular pressure to propel blood throughout the circulatory system.
    • Involuntary control; regulated by the autonomic nervous system, hormones, autocrine/paracrine signals, and intrinsic electrical pace-making capabilities (spontaneous contraction).
    • Combines structural and functional features of both skeletal and smooth muscle.

  • General Physiological Principles Exemplified by Muscle:

    • Structure dictates function: Cellular and tissue-level specializations directly determine force generation and movement capacity.
    • Controlled exchange across membranes: Transmembrane flux of Ca2+\text{Ca}^{2+} coordinates muscle activation and relaxation mechanisms.
    • Physical and chemical principles: Physical laws of mechanics govern muscle-bone lever systems, while biochemical laws dictate energy conversion.
    • Metabolic utilization: Muscle cells generate, store, and utilize energy across multiple biochemical pathways.

Cellular Structure and Regeneration of Skeletal Muscle

Striated skeletal muscle fiber tissue micrograph and diagram

  • Skeletal Muscle Fiber Structure:

    • Skeletal muscle exhibits a distinct striated appearance under light microscopy due to alternating light and dark bands perpendicular to the cell's long axis.
    • A single skeletal muscle cell is termed a muscle fiber due to its elongated shape.
    • Dimensions:
    • Fiber diameter ranges from 10\text{ to }100\text{ }̣\mu\text{m} (10 μm10\,\mu\text{m} to 100 μm100\,\mu\text{m}).
    • Fiber length can reach up to 20 cm20\,\text{cm}.
    • Multinucleated Structure:
    • Muscle fibers are formed during embryonic development through the fusion of multiple undifferentiated, single-nucleated cells called myoblasts.
    • Differentiation completes around the time of birth, after which fibers grow in size from infancy to adulthood.
    • Retained nuclei are distributed along the length of the fiber just beneath the plasma membrane, each governing gene expression and protein synthesis within its local intracellular domain.

  • Muscle Repair and Satellite Cells:

    • Muscle damage or destruction post-birth triggers repair mediated by satellite cells, a population of quiescent, undifferentiated stem cells located between the plasma membrane and the surrounding basement membrane along muscle fibers.
    • Strain or injury activates satellite cells, causing them to undergo mitotic proliferation.
    • Proliferated daughter cells differentiate into myoblasts, which either:
    • Fuse together to form entirely new muscle fibers.
    • Fuse with damaged or stressed muscle fibers to repair and reinforce them.
    • Severe damage may not restore the exact original count of muscle fibers.

  • Muscle Hypertrophy:

    • Hypertrophy refers to an increase in the size of muscle fibers in response to heavy resistance exercise or as compensation for lost muscle tissue.
    • Cellular mechanisms underlying hypertrophy include:
    • Expansion (enlargement) of existing fibers.
    • Splitting of existing fibers.
    • Proliferation, differentiation, and fusion of satellite cells.
    • Hormonal regulators of muscle growth and hypertrophy include growth hormone, insulin-like growth factor (IGF), and sex hormones.

Connective Tissue and Structural Hierarchy

Hierarchical structure of skeletal muscle

  • Connective Tissue Layers:
    • A named muscle consists of numerous individual muscle fibers bound together by connective tissue sheaths.
    • Muscle fibers are bundled into functional groups called fascicles.
    • Epimysium: Dense collagenous connective tissue layer enveloping the entire outer surface of a whole muscle.
    • Perimysium: Connective tissue layer surrounding each bundle of muscle fibers (fascicle).
    • Endomysium: Delicate connective tissue layer surrounding individual muscle fibers.

Skeletal muscle connective tissue layers

  • Tendons and Attachments:
    • Skeletal muscles are attached to bones via bundles of collagen fibers forming tendons.
    • Fiber length relative to muscle length: Some muscle fibers extend the full length of a muscle, but most are shorter and arranged at an angle relative to the muscle's long axis.
    • Some tendons extend long distances from the muscle belly to the bone attachment site (e.g., forearm muscle bellies controlling finger movements via long distal tendons in the hand).

Filament and Sarcomere Architecture

  • Myofibrils:
    • Cylindrical protein structures within the cytoplasm, measuring 1 to 2 μm1\text{ to }2\,\mu\text{m} in diameter.
    • Myofibrils run continuously from one end of the muscle fiber to the other and anchor to tendons at the fiber ends, occupying most of the intracellular volume.

Structure of thick filaments, thin filaments, and myosin molecule

  • Thick Filaments:

    • Composed mainly of the protein myosin.
    • Myosin Molecule Structure:
    • Composed of six polypeptide chains: two large heavy chains and four smaller light chains.
    • The two heavy chains intertwine to form a long tail that lies along the longitudinal axis of the thick filament.
    • The ends of the heavy chains combine with light chains to form two globular heads that project outward from the sides of the filament, forming cross-bridges.
    • Each globular head contains two distinct binding sites:
      1. An actin-binding site for attaching to the thin filament.
      2. An ATP-binding site, which possesses enzymatic activity (myosin-ATPase) to hydrolyze ATP and release energy for force generation.
    • Myosin molecules are oriented in opposite directions in each half of the thick filament, pointing away from the center.

  • Thin Filaments:

    • Have approximately half the diameter of thick filaments.
    • Composed mainly of the protein actin, along with nebulin, troponin, and tropomyosin.
    • Actin:
    • Monomeric actin (G-actin) is a globular monomer consisting of a single polypeptide chain.
    • Monomers polymerize into a helical polymer composed of two intertwined strands (F-actin) that form the backbone of the thin filament.
    • Each actin monomer contains a specific binding site for a myosin cross-bridge.
    • Regulatory and Structural Proteins:
    • Tropomyosin: A fibrous, double-stranded protein that wraps around the actin helix, blocking myosin-binding sites on actin in resting muscle.
    • Troponin: A regulatory protein complex attached to tropomyosin that controls tropomyosin positioning in response to calcium binding.
    • Nebulin: A structural protein associated with thin filaments thought to assist in thin filament assembly and length regulation.

Micrograph and diagram of sarcomere bands and lines

  • Sarcomere Organization:
    • A sarcomere is the repeating functional unit of the contractile apparatus, bounded by two successive Z lines.
    • A band:
    • The dark band located in the middle of the sarcomere containing the full length of the thick filaments.
    • Named from anisotropy, describing its non-uniform optical properties.
    • I band:
    • The light band lying between the ends of the A bands of adjacent sarcomeres.
    • Contains the portions of thin filaments that do not overlap thick filaments.
    • Bisected by the Z line.
    • Named from isotropy, describing its uniform optical appearance.
    • Z line (Z disk):
    • Interconnecting network of structural proteins anchoring thin filaments from adjacent sarcomeres.
    • H zone:
    • A narrow, lighter space in the center of the A band.
    • Represents the central gap between the opposing non-overlapping ends of thin filaments in a resting sarcomere.
    • Named from German heller ("light").
    • M line (M disk):
    • A narrow dark band in the exact center of the H zone.
    • Consists of cross-linking proteins that bind adjacent thick filaments together at their midpoints.
    • Named from German mittel ("middle").
    • Titin Filaments:
    • Formed by the elastic protein titin, extending from the Z line to the M line.
    • Linked to both M-line proteins and thick filaments, acting to center thick filaments within the sarcomere and maintain structural stability.

Cross-sectional filament arrangement in overlap region

  • 3D Cross-Sectional Geometry:
    • Within the region of filament overlap in the A band:
    • Each thick filament is organized in a hexagonal array surrounded by 66 thin filaments.
    • Each thin filament is surrounded in a triangular array by 33 thick filaments.
    • The total numerical ratio of thin filaments to thick filaments in the overlap zone is 2:12:1.

Intracellular Membrane Systems: Sarcoplasmic Reticulum and T-Tubules

Transverse tubules and sarcoplasmic reticulum organization

  • Sarcoplasmic Reticulum (SR):

    • Homologous to the smooth endoplasmic reticulum of non-muscle cells.
    • Forms a network of membrane sleeves surrounding each individual myofibril.
    • Terminal Cisternae (Lateral Sacs):
    • Enlarged blind sacs located at the ends of each SR segment.
    • Interconnected by smaller tubular networks in the central region of the segment.
    • Function as the intracellular storage reservoir for calcium ions (Ca2+\text{Ca}^{2+}).
    • Calsequestrin:
    • A specialized Ca2+\text{Ca}^{2+}-binding protein located inside the lumen of terminal cisternae.
    • Binding to calsequestrin allows high-density storage of Ca2+\text{Ca}^{2+} ions without generating a large free ion concentration gradient across the SR membrane.

  • Transverse Tubules (T-Tubules):

    • Independent tubular invaginations of the cell membrane (sarcolemma).
    • Positioned directly between the terminal cisternae of adjacent SR segments, surrounding myofibrils at the boundary where A bands meet I bands.
    • Luminal space of T-tubules is continuous with the extracellular fluid (ECF).
    • Conduct surface sarcolemmal action potentials into the interior of the muscle fiber, ensuring rapid, synchronized activation of deep myofibrils.