Muscle Tissue

Muscle Tissue

Chapter Overview

  • Textbook: Anatomy & Physiology by Saladin, 10th Ed

  • Focus on the unity of form and function in muscle tissue.

11.1 Types and Characteristics of Muscular Tissue

  • ### Expected Learning Outcomes:

    • Describe physiological properties common to all muscle types.

    • List defining characteristics of skeletal muscle.

    • Discuss elastic functions of connective tissue components in muscle.

11.1a Universal Characteristics of Muscle

  • Characteristics all muscle cells share:

    • Excitability (responsiveness):

    • Ability to respond to chemical signals, stretch, and electrical changes across the plasma membrane.

    • Conductivity:

    • Local electrical excitation triggers a wave of excitation that travels along the muscle fiber.

    • Contractility:

    • Muscle fibers shorten when stimulated.

    • Extensibility:

    • Capability of being stretched between contractions.

    • Elasticity:

    • Ability to return to its original resting length after being stretched.

11.1b Skeletal Muscle

  • Definition:

    • Voluntary, striated muscle typically connected to bones.

  • Striations:

    • Alternating light and dark transverse bands; arise from the arrangement of internal contractile proteins.

  • Voluntary Control:

    • Generally subject to conscious control; other muscle types are involuntary.

  • Dimensions:

    • Average diameter: 100 μm, length: 3 cm, can be up to 500 μm thick and 30 cm long.

  • Terminology:

    • Skeletal muscle cells are also referred to as muscle fibers or myofibers.

11.1c Skeletal Muscle Connective Tissues

  • Components of skeletal muscle:

    • Endomysium:

    • Surrounds each muscle fiber. Also provides a chemical environment for muscle fiber and nerve ending interaction, important for guiding excitation.

    • Perimysium:

    • Bundles muscle fibers into fascicles. Contains larger blood vessels, nerves, and stretch receptors (muscle spindles).

    • Epimysium:

    • Envelops the entire muscle. Further protection, merges with deep fascia to form tendons.

  • Functions:

    • Collagen:

    • Extensible and elastic; it stretches slightly under tension and recoils when released, resists excessive stretching, protects muscles from injury, and contributes to power output and efficiency.

11.2 Skeletal Muscle Cells

  • ### Expected Learning Outcomes:

    • Describe structural components of a muscle fiber.

    • Identify major proteins of a muscle fiber and their functions.

    • Connect striations of a muscle fiber to the arrangement of protein filaments.

11.2a The Muscle Fiber

  • Components:

    • Sarcolemma:

    • Plasma membrane of muscle fiber. Contains voltage-gated ion channels crucial for action potential propagation.

    • Sarcoplasm:

    • Cytoplasm of muscle fiber. Enriched with unique components like glycogen, myoglobin, and large number of myofibrils.

    • Myofibrils:

    • Long protein cords filling the sarcoplasm.

    • Glycogen:

    • Carbohydrate stored for muscle energy during exercise.

    • Myoglobin:

    • Red pigment providing oxygen for muscle activity.

    • Mitochondria:

    • Numerous organelles for energy (bean-shaped near sarcolemma, tubular and dynamic deeper).

    • Nuclei:

    • Muscles contain 30 to 80 nuclei per millimeter for fiber repair.

    • Sarcoplasmic Reticulum (SR):

    • Network around myofibrils; contains terminal cisterns as calcium reservoirs; calcium binds to calsequestrin. Forms a network that closely associates with T-tubules, crucial for regulating intracellular Ca^{2+} levels.

    • Transverse (T) Tubules:

    • Penetrate the cell; associated with terminal cisterns on each side. Invaginations of the sarcolemma that penetrate deep into the cell, ensuring rapid transmission of electrical signals to the entire fiber interior. They are essential for synchronous contraction.

11.2b Myofilaments

  • Types:

    • Thick Filaments:

    • Composed of myosin molecules.

    • Myosin structure: club-shaped molecules with a shaft-like tail and a double globular head, arranged in a helical array. Each myosin head has an ATP-binding site and an actin-binding site. The heads are referred to as "cross-bridges" during contraction.

    • Thin Filaments:

    • Comprising fibrous (F) actin, tropomyosin, and troponin.

    • F-actin: A polymer of G-actin (globular actin) monomers; each G-actin has an active site that can bind to myosin heads.

    • Tropomyosin: A regulatory protein that covers the active sites on actin when the muscle is at rest, preventing myosin binding.

    • Troponin: A complex of three polypeptides. One binds to actin, one binds to tropomyosin, and one (Troponin C) binds to Ca^{2+} ions, initiating the conformational change that uncovers active sites.

    • Elastic Filaments:

    • Made of titin, which stabilizes thick filaments and provides recoil.

11.2c Striations and Sarcomeres

  • Organization:

    • Striations arise from A-bands (dark) and I-bands (light).

    • Main components: A band, I band, H band, M line, and Z disc.

11.3 The Nerve–Muscle Relationship

  • ### Expected Learning Outcomes:

    • Define a motor unit and its role in muscle contraction.

    • Describe the structure of the neuromuscular junction (NMJ).

    • Explain electrical charge differences across the plasma membrane.

11.3a Motor Neurons and Motor Units

  • Skeletal muscle contraction: Requires nerve stimulation; without it, paralysis can occur (denervation atrophy).

  • Motor Unit Definition:

    • A motor unit is a single nerve fiber and all the muscle fibers it innervates.

    • Muscle fibers in a motor unit contract together, contributing to uniform movement.

  • Size Variation:

    • Motor units can be small (fine control in eye/hand muscles) or large (powerful contractions in thigh muscles).

11.3b The Neuromuscular Junction

  • Definition:

    • The synapse where a nerve meets a muscle fiber.

  • Components:

    • Axon terminal: Contains voltage-gated Ca^{2+} channels; arrival of action potential opens these, allowing Ca^{2+} influx, which triggers ACh release.

    • Synaptic vesicles: Undergo exocytosis to release ACh into the synaptic cleft.

    • Synaptic cleft: The space between the axon terminal and the muscle fiber.

    • Junctional folds: Increase surface area for ACh receptors, ensuring efficient binding and response.

  • ACh Role:

    • Acetylcholine released from vesicles binds to receptors on the muscle cell to stimulate contraction.

  • Myasthenia Gravis Implication:

    • ACh receptor impairment causing muscle weakness.

11.3c Electrically Excitable Cells

  • Definitions:

    • Electrically excitable cells react to stimulation with voltage changes.

  • Resting Membrane Potential (RMP):

    • Cell’s interior is negatively charged compared to the outside due to ion distributions.

  • Action Potentials:

    • Sequence of depolarization (Na^+ in) and repolarization (K^+ out) in response to stimulation.

11.4 Behavior of Skeletal Muscle Fibers

  • ### Expected Learning Outcomes:

    • Explain excitation, excitation–contraction coupling, contraction, and relaxation phases in muscle function.

11.4a Phases Overview

  • Excitation:

    • Action potentials in motor nerves lead to muscle action potentials.

  • Excitation–Contraction Coupling:

    • Linking action potentials to myofilament activation.

  • Contraction:

    • Muscle develops tension and may shorten.

  • Relaxation:

    • Muscle returns to resting state once stimulation ceases.

11.4b Excitation of a Muscle Fiber

  • Process:

    • 1. Nerve Signal Arrival: Action potential arrives at the axon terminal, opening voltage-gated Ca^{2+} channels.

    • 2. ACh Release: Ca^{2+} influx stimulates exocytosis of synaptic vesicles, releasing acetylcholine (ACh) into the synaptic cleft.

    • 3. ACh Binding: ACh diffuses across the cleft and binds to ligand-gated ion channels on the sarcolemma.

    • 4. End-Plate Potential (EPP): Binding opens the channels, allowing Na^+ to flow into the muscle fiber and K^+ to flow out, leading to a rapid depolarization called an end-plate potential.

    • 5. Action Potential Propagation: If the EPP reaches threshold, voltage-gated Na^+ channels nearby open, generating an action potential that propagates along the sarcolemma and into the T-tubules.

11.4c Contraction Mechanism

  • Steps of Myofilament Interactions:

    • 1. Calcium Release: Action potential in T-tubules opens Ca^{2+} channels in SR, releasing Ca^{2+} into sarcoplasm.

    • 2. Active Site Exposure: Ca^{2+} binds to troponin C, causing a conformational change in troponin-tropomyosin complex, moving tropomyosin away from actin's active sites.

    • 3. Cross-Bridge Formation: Myosin heads (energized with ADP + Pi from previous ATP hydrolysis) bind to the exposed active sites on actin, forming cross-bridges.

    • 4. Power Stroke: Myosin heads pivot, pulling the thin filaments past the thick filaments towards the M line (releasing ADP and Pi). This shortens the sarcomere.

    • 5. Cross-Bridge Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.

    • 6. Myosin Reactivation: ATP is hydrolyzed into ADP and Pi, re-energizing ("recocking") the myosin head, preparing it for another cross-bridge cycle. This cycle continues as long as Ca^{2+} is present and ATP is available.

11.4d Relaxation Mechanism

  • Steps Involved:

    • 1. Cessation of Nerve Signal: Nerve impulses stop, and no more ACh is released.

    • 2. ACh Breakdown: Acetylcholinesterase (AChE) in the synaptic cleft breaks down remaining ACh, stopping muscle stimulation.

    • 3. Ca^{2+} Reabsorption: Active transport (SERCA pumps) in the SR membrane pump Ca^{2+} back into the SR cisterns (often binding to calsequestrin), lowering sarcoplasmic Ca^{2+} concentration.

    • 4. Active Sites Blocked: Without Ca^{2+} bound to troponin, tropomyosin moves back to cover the active sites on actin, preventing further cross-bridge formation.

    • 5. Muscle Relaxation: Myosin can no longer bind to actin, and the muscle fiber passively returns to its resting length, aided by the elastic components.

11.4e Length–Tension Relationship

  • Overview:

    • Muscle tension depends on how stretched or shortened it is before stimulation; optimal length leads to maximum force generation.

11.5 Behavior of Whole Muscles

  • ### Expected Learning Outcomes:

    • Discuss muscle twitch phases, contraction strength variations, and types of contractions (isometric, isotonic, concentric, eccentric).

11.5a Muscle Twitches

  • Definition:

    • Quick cycle of contraction and relaxation, measured in a myogram showing latent period, contraction phase, and relaxation phase.

11.5b Stimulus Intensity and Contraction Strength

  • Key Concepts:

    • Influence of various factors on contraction strength including muscle length, frequency of stimulation, and motor unit recruitment.

11.5c Isometric and Isotonic Contraction

  • Definitions:

    • Isometric Contraction: No change in muscle length; produces internal tension against resistance.

    • Isotonic Contraction: Change in muscle length with constant tension; includes concentric (muscle shortening) and eccentric (muscle lengthening) contractions.

11.6 Muscle Metabolism

  • ### Expected Learning Outcomes:

    • Explain muscle energy demands, fatigue, oxygen debt, and muscle fiber types.

11.6a ATP Sources

  • Pathways:

    • Immediate Energy: Provided by the phosphagen system using creatine phosphate and ATP.

    • Short-term energy: Involves anaerobic fermentation, yielding 2 ATP per glucose.

    • Long-term energy: Aerobic respiration meeting demands after approximately 40 s of exercise.

11.6b Fatigue and Endurance

  • Muscle Fatigue:

    • Results from prolonged use; identified causes differ based on exercise intensity.

11.6c Excess Postexercise Oxygen Consumption (EPOC)

  • Definition:

    • Elevated oxygen consumption after exercise to restore ATP, myoglobin oxygen reserves, and remove lactate.

11.6d Physiological Classes of Muscle Fibers

  • Types:

    • Slow-twitch (Type I): Endurance muscles with high oxidative capacity.

    • Fast-twitch (Type II): Quick response fibers that utilize glycolysis.

    • Intermediate (Type IIa): Fast and fatigue-resistant, adaptable with training.

11.6e Muscular Strength and Conditioning

  • Factors Influencing Strength:

    • Muscle size, fascicle arrangement, motor unit activation, and muscle length effects.

  • Exercise Effects:

    • Resistance training leads to fiber hypertrophy; endurance training increases fatigue resistance and aerobic capacity.

11.7 Muscular Dystrophy and Myasthenia Gravis

  • Muscular Dystrophy (MD):

    • Group of hereditary diseases causing muscle degeneration, often due to dystrophin gene defects. Common form: Duchenne MD.

  • Myasthenia Gravis:

    • Autoimmune disease affecting neuromuscular junctions, leading to muscle weakness, particularly in facial muscles.