HN220 Unit 3 (1)

Chapter 12: Skeletal Muscle Structure

Skeletal Muscles

• Definition: Skeletal muscles are muscles that are attached to at least two bones and are responsible for movement of the skeleton. There are exceptions for certain muscles that are connected to skin, cartilage, or other structures (e.g., external anal sphincter).

• Tendons: Tendons are made of dense, elastic connective tissue that plays a crucial role by linking muscles to bones. They transmit the force generated by muscles to facilitate joint movement. Tendons are adaptable and can withstand substantial tensile forces.

Structure at the Cellular Level

• Body of the Muscle: This is the main part of the muscle that generates force during contraction. It consists of various muscle fibers bundled together, surrounded by connective tissues.

• Connective Tissues:

  • The epimysium is the outer layer of connective tissue that encases the entire muscle.

  • The perimysium extends inward from the epimysium, organizing the muscle into bundles called fascicles, which are groups of muscle fibers.

  • Individual muscle fibers within each fascicle are encased in a thin sheath called endomysium.

• Muscle Fibers: Each muscle fiber is an elongated cell, multi-nucleated due to the fusion of myoblasts during embryonic development. The nuclei are positioned beneath the sarcolemma (the muscle fiber plasma membrane), which is essential for muscle repair and maintenance.

Sarcoplasm and Myofibrils

• Sarcoplasm: This is the semi-fluid cytoplasm found within muscle fibers, rich in mitochondria, which supply ATP for muscle contraction. It also contains the myofibrils, which are essential for muscle contraction due to their banded appearance that is characteristic of striated muscles.

• Myofibrils: Composed of an arrangement of myosin (thick filaments) and actin (thin filaments), myofibrils are the contractile units of the muscle.

• Sarcoplasmic Reticulum: This organelle wraps around myofibrils and is crucial for calcium ion storage. It works in concert with transverse tubules (T tubules) that help transmit electrical signals from the sarcolemma into the muscle cell, initiating contraction.

• Lateral Sacs/Terminal Cisternae: These are enlarged areas of the sarcoplasmic reticulum that store calcium ions, which are essential for initiating muscle contractions.

• Triad: A functional unit formed by one T tubule associated with two terminal cisternae, crucial for facilitating calcium release upon stimulation.

Function in Muscle Contraction

• Muscle contraction is initiated by neural signals that cause the sarcoplasmic reticulum to release calcium ions, essential for the crossbridge cycle and muscle shortening. Striated muscles are characterized by the arrangement of sarcomeres, which are the basic functional units of muscle contraction.

Sarcomere Structure

• Z Lines: These structures anchor the thin actin filaments and delineate the borders of each sarcomere.

• M Lines: Located in the center of the sarcomere, the M line anchors thick filaments together.

• Banding Patterns:

  • A Band: The dark striation where thick and thin filaments overlap. The length of the A band remains constant during contraction.

  • H Zone: Located within the A band, this lighter region shortens during contraction as actin filaments slide past myosin filaments.

  • I Band: The light striation that contains only thin filaments. This band shortens during contraction as they are pulled towards the M line.

Thin Filament Structure

• Actin Monomers (G actin): These monomers polymerize to form F actin, creating double helical strands of actin filaments.

  • Regulatory Proteins:

    • Tropomyosin: It blocks myosin-binding sites on actin filaments when the muscle is in a relaxed state.

    • Troponin: This complex consists of three proteins that regulate muscle contraction. When calcium binds to troponin, it undergoes a conformational change that moves tropomyosin, exposing the binding sites on actin allowing contraction to occur.

Troponin Structure

• Troponin is comprised of three subunits:

  • One binds to actin, anchoring the troponin to the thin filament.

  • The second binds to tropomyosin, stabilizing the position of tropomyosin over the myosin binding sites (Troponin T).

  • The third has a calcium-binding site that, upon receiving calcium ions, triggers the contraction mechanism by shifting tropomyosin.

Crossbridges and Force Generation

• Crossbridges: Myosin projections interact with actin to facilitate muscle contraction. Each myosin head operates as a crossbridge, converting chemical energy from ATP into mechanical force to generate movement.

Myosin and Force Generation

• Myosin Head: This is the mechanism that drives muscle contraction. It has two important sites:

  • Actin-Binding Site: This site allows myosin to attach to actin filaments.

  • ATPase Site: This site hydrolyzes ATP into ADP and inorganic phosphate (Pi), releasing energy necessary for muscle contraction.

• Critical Components: Thick filaments also contain titin, a spring-like protein essential for maintaining the structural integrity of the sarcomere during stretching and contraction.

Mechanism of Force Generation

• Sliding Filament Model: During contraction, the I band shortens due to the sliding of thin filaments past thick filaments, while the A band length remains unchanged. The Z lines move closer, resulting in the shortening of the sarcomere and, consequently, the overall muscle.

Crossbridge Cycle

1. Binding of Myosin to Actin: In the energized form, myosin binds to actin in the presence of calcium.

2. Power Stroke: The release of Pi upon binding releases energy that pivots the myosin head, pulling actin toward the center of the sarcomere.

3. Rigor State: Following the power stroke, ADP is released, and myosin remains tightly bound to actin (as happens in rigor mortis).

4. Unbinding: A new ATP molecule binds to the ATPase site on myosin, reducing its affinity for actin and allowing for detachment.

5. Cocking: Hydrolysis of ATP to ADP and Pi resets the myosin head to a high-energy state, ready for another cycle.

Cycle Continuation

• The crossbridge cycle continues repeatedly as long as calcium ions and ATP are present. Regulatory proteins, troponin and tropomyosin, modulate the availability of myosin binding sites on actin, thereby controlling the contractions and relaxation of muscle fibers.

Excitation-Contraction Coupling

• Muscle cells share excitability characteristics similar to neurons:

  • Neuromuscular Junction: This is the synapse between a motor neuron and a muscle fiber where the neurotransmitter acetylcholine (ACh) is released, leading to depolarization and the initiation of an action potential.

  • End-Plate Potential (EPP): This potential is larger than typical postsynaptic potentials, generating action potentials that travel along the T tubules, resulting in the release of calcium from the sarcoplasmic reticulum, which is crucial for muscle contraction.

Phases of Muscle Twitch

1. Latent Period: This is the time delay between the onset of the muscle action potential and the beginning of muscle contraction. During this time, calcium is being released from the sarcoplasmic reticulum.

2. Contraction Phase: Muscle tension rises, reaching a peak as the muscle fibers shorten.

3. Relaxation Phase: This phase lasts from the peak of tension until the muscle returns to its resting state. It is the longest phase as calcium is reabsorbed and muscle fibers stretch back to their original length.

Muscle Twitch Characteristics

• Twitch reproducibility is explained by the all-or-nothing principle, where muscle fibers either reach the strength needed for contraction or do not contract at all. Variations in twitch strength can arise from differences in individual muscle fiber properties and stimulation frequency.

Isometric vs. Isotonic Contractions

• Isometric Contraction: The muscle generates tension without changing length; ideal for stabilizing joints (e.g., holding a heavy object in place).

• Isotonic Contraction: The muscle shortens while maintaining a constant force, allowing movement (e.g., lifting a dumbbell).

Isometric and Isotonic Twitch Measurements

• Isometric Twitch: Measures tension without any movement; useful for assessing pure force generation.

• Isotonic Twitch: Measures muscles that are able to shorten during contraction, displaying characteristic tension plateaus during movement.

Twitch Summation

• Summation: Occurs when consecutive muscle twitches overlap due to high-frequency stimulation, leading to greater tension.

• Tetanus: Characterized by a plateau phase where tension remains constant amidst rapid stimulation.

  • Types of Tetanic Contraction:

    • Unfused Tetanus: Exhibits small oscillations with brief periods of relaxation.

    • Fused Tetanus: Represents a state of continuous contraction without relaxation.

Factors Affecting Muscle Force Generation

• Motor Unit Recruitment: The process of activating additional motor units to produce greater overall force.

• Size Principle: Smaller motor units are recruited for lower force tasks, while larger units are activated for high force requirements.

• Velocity of Shortening: Influenced by the load; greater loads result in slower contraction velocities.

Length-Tension Relationship

• Muscle force generation is closely related to the initial length of muscle fibers before contraction. An optimal starting length allows for maximal force production, while stretching beyond or shortening significantly leads to decreased force-generating capability.

Muscle Activity Control

• Origin and Insertion: The origin is the stationary bone, while the insertion is the moving bone that muscle acts upon.

• Antagonistic Muscles: Pairs of muscles that work oppositely (e.g., biceps causing flexion while triceps cause extension).

Muscle Receptors

• Muscle Spindles: Sensory receptors that detect changes in muscle length, providing feedback for muscle reflexes through afferent pathways.

• Golgi Tendon Organs: Sensors that respond to muscle tension, contributing to protective reflexes to prevent excessive force.

Smooth Muscles

• Structure: Structural differences exist as smooth muscles lack the striations found in skeletal and cardiac muscles. They possess thick and thin filaments that are arranged diagonally across the muscle cell.

• Excitation-Contraction Coupling: Predominantly regulated by calcium sourced both externally and from the sarcoplasmic reticulum.

Contractile Mechanics in Smooth Muscle

• Calcium ions bind to calmodulin, which subsequently activates myosin light chain kinase (MLCK). This activation phosphorylates myosin, facilitating muscle contraction.

Neural Control in Smooth Muscle

• Smooth muscle contraction is autonomously regulated (sympathetic and parasympathetic control), exhibiting varied responses to stimuli.

• Although direct synaptic connections are absent, coordinated contractions occur via gap junctions that allow electrical impulses to propagate between cells.

Cardiac Muscles

• Similarities to Skeletal Muscles: Cardiac muscles are striated, contain a sarcomere structure, and utilize a troponin-tropomyosin regulatory system similar to skeletal muscle.

• Gap Junctions: These structures are vital for the propagation of action potentials, enabling synchronous heart contractions.

• Pacemaker Activity: Specialized cardiac cells generate heart rhythm independently from neural input, thus managing cardiac control in a myogenic fashion.