Muscular System Histology and Physiology Notes

Muscular System Histology and Physiology Notes

9.1 Functions of the Muscular System

  • Types of Muscle Tissue:

    • Skeletal Muscle:

    • Functions: locomotion, facial expressions, posture, respiratory movements, other body movements.

    • Characteristics: Voluntary, controlled by the nervous system.

    • Smooth Muscle:

    • Location: walls of hollow organs, blood vessels, eyes, glands, skin.

    • Functions: propel urine, mix food in digestive tract, pupil contraction, regulate blood flow.

    • Characteristics: Involuntary, autorhythmic in some instances; controlled by endocrine and autonomic nervous systems.

    • Cardiac Muscle:

    • Location: heart.

    • Functions: major source of blood movement.

    • Characteristics: Involuntary, autorhythmic.

9.3 Skeletal Muscle Anatomy

  • Connective Tissue Coverings:

    • Epimysium: surrounds the whole muscle, merges with muscular fascia.

    • Perimysium: surrounds groups of muscle fibers (fascicles), allowing passage for blood vessels and nerves.

    • Endomysium: surrounds individual muscle fibers within each fascicle.

  • Muscle Fiber Structure:

    • Developed from myoblast fusion into multinucleated cells, striated appearance.

    • Various structural adaptations increase muscle efficiency, including extensive capillary beds for nutrient delivery.

9.4 Sarcomere Organization

  • Structure:

    • Sarcomere: basic functional unit of muscle fiber, defined by Z-disks, A-bands, I-bands, H-zones.

    • Components:

    • Actin (thin) Myofilaments: composed of F-actin, tropomyosin, and troponin. Coordinate muscle contraction through active-site exposure.

    • Myosin (thick) Myofilaments: shaped like golf clubs, consist of myosin heavy chains and heads that attach to actin during contraction.

  • Cross-Bridge Cycle:

    • Myosin heads bind to actin, pulling it toward the center of the sarcomere and causing contraction.

9.5 Muscle Fiber Physiology

  • Action Potentials:

    • Generated by the nervous system, leading to muscle contraction.

    • Phases:

    • Depolarization: reduction in membrane potential, opening voltage-gated sodium channels.

    • Repolarization: return to resting potential via potassium outflow.

  • Neuromuscular Junction:

    • Site of synapse between motor neuron and muscle fiber,

    • Acetylcholine (ACh) release triggers the muscle contraction process.

9.6 Muscle Contraction Types

  • Twitches: response of muscle fibers to an action potential.

  • Types:

    • Isometric: no change in length.

    • Isotonic: change in length with constant tension (concentric and eccentric).

9.8 Smooth Muscle Physiology

  • Not striated; responds to various stimuli.

  • Mechanism of Contraction:

    • Calcium binds to calmodulin, activating myosin light-chain kinase, leading to contraction.

9.9 Cardiac Muscle Characteristics

  • Striated, branched fibers with intercalated discs.

  • Autorhythmic and longer action potential durations compared to skeletal muscle.

Review of Key Concepts

  • Neuromuscular Junction: crucial for initiating muscle contraction upon receiving an action potential.

  • Excitation-Contraction Coupling: coupling electrical signals from action potentials to mechanical motion in muscle fibers.

  • Muscle Fatigue: factors include ATP depletion and metabolic accumulation; affects muscle performance and recovery.