Muscular System

Types of Muscle

  1. Skeletal Muscle

    • Voluntary control

    • Striated appearance

    • Multinucleated fibers

  2. Cardiac Muscle

    • Involuntary control

    • Striated appearance

    • Single nucleus, interconnected fibers

    • Contains intercalated discs

  3. Smooth Muscle

    • Involuntary control

    • Non-striated appearance

    • Single nucleus

    • Found in walls of hollow organs

Connective Tissue Layers Associated with Skeletal Muscle

  • Epimysium: Outermost dense regular connective tissue covering a muscle.

  • Perimysium: Fibrous CT that covers surrounding fascicles, a group of muscle fibers (10-100 fibers).

    • Note: Fascicles give meat its “grain”.

  • Endomysium: Fine areolar CT surrounds individual muscle fibers (cell).

Microscopic Anatomy of Skeletal Muscle Fibers

Each muscle fiber (myofiber) contains numerous myofibrils.

  • A muscle consists of hundreds to thousands of muscle cells, plus connective tissue wrappings, blood vessels, and nerve fibers.

  • Covered externally by epimysium.

  • Myofibrils: Grouping of myofilaments.

    • Definition: Myofibrils are densely packed, rod-like elements within individual myocytes (muscle cells).

    • They constitute 80% of cell volume.

    • Exhibit striations: aligned repeating series of dark A bands and light I bands.

    • Sarcolemma: Plasma membrane of a muscle fiber.

  • Myofilaments:

    • Thick Filaments: Mainly myosin.

    • Thin Filaments: Include actin, tropomyosin, and troponin.

    • Elastic Filaments: Composed of titin, providing elasticity.

  • Structural Hierarchy:

    • Muscle → Fascicle → Myofiber → Myofibrils → Myofilaments

Sarcomere

  • Definition:

    • Smallest structural and functional unit of muscle, defined from Z line to Z line.

  • Features:

    • Contains organized arrangements of thick and thin myofilaments.

      • Thick Filaments: Run the entire length of A band.

      • Thin Filaments: Run length of I band and partway into A band.

      • Z Disc: Anchors the thin filaments.

      • H Zone: Lighter midregion where filaments don’t overlap.

      • M Line: Holds adjacent thick filaments together.

Muscle Contraction and Sarcomere Length

  • Muscle contraction results in the shortening of the sarcomere length, causing the muscle to contract.

    • Calcium ions released bind to troponin, removing tropomyosin from myosin-binding sites on actin.

    • Contraction begins as myosin heads bind to actin, detach, and bind again, propelling thin filaments toward M line.

      • Sarcomere shortens, muscle cells shorten, whole muscle shortens.

Interaction of Actin and Myosin

  • Key Roles in Muscle Contraction

    • Actin:

      • Thin filament that provides binding sites for myosin.

    • Myosin:

      • Thick filament that pulls actin during contraction.

  • Regulatory Proteins

    • Troponin:

      • Regulatory protein that binds calcium and shifts tropomyosin, exposing binding sites on actin.

    • Tropomyosin:

      • Covers binding sites on actin in muscle at rest.

Excitation-Contraction Coupling

  • Step-by-step process:

    • Motor neuron fires, releasing acetylcholine at the neuromuscular junction.

    • Acetylcholine binds to receptors on the sarcolemma, initiating an action potential.

    • Action potential travels down the T-tubules, reaching the sarcoplasmic reticulum.

    • Calcium ions are released from the sarcoplasmic reticulum into the cytosol.

    • Calcium binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin.

    • Myosin heads bind to actin, triggering the contraction.

  • Important Anatomical Features

    • Neuromuscular Junction: Where motor neuron communicates with muscle fiber.

    • Sarcoplasmic Reticulum: Stores calcium ions.

    • T-Tubules: Transmit action potentials deep into muscle fibers.

Cross Bridge Cycle

  • Step-by-step process:

    1. ATP is hydrolyzed (ATP → ADP + Pi), re-cocking the myosin head.

    2. The cocked myosin head binds to the myosin-binding site on actin.

    3. Power stroke occurs, pulling actin filaments.

    4. New ATP binds, causing release of the myosin head.

    5. Cycle repeats as long as calcium and ATP are present.

Motor Units

  • Definition:

    • A motor unit is composed of a motor neuron and all the muscle fibers it innervates.

  • Large Motor Units:

    • Innervate many fibers, suited for strength and power.

  • Small Motor Units:

    • Innervate fewer fibers, suited for fine movements.

Muscle Twitches and Summation

  • Muscle Twitch:

    • A single contraction response to a stimulus.

    • Energy Source: ATP

  • Phases of a Muscle Twitch:

    1. Latent phase

    2. Contraction phase

    3. Relaxation phase

  • Summation:

    • Muscles can reach tetanus through continuous stimulation leading to sustained contractions.

Muscle Metabolism

  • Various types of metabolism include:

    • Aerobic metabolism: Produces ATP in the presence of oxygen.

    • Anaerobic metabolism: Produces ATP without oxygen (e.g., lactic acid fermentation).

Muscle Fiber Types

  • Type 1 (Red, Slow Oxidative Muscle):

    • High mitochondria density, red coloration, slow calcium release, more endurance.

  • Type 2 (White, Fast-Twitch Muscle):

    • Quick bursts of energy, rapid fatigue, genetic predisposition influences fiber ratios.

Clinical Alterations in Homeostatic Balance in the Muscular System

  • Relevant clinical alterations may include muscle atrophy, hypertrophy, and conditions affecting muscle function such as myopathies.

Smooth Muscle Structure and Function

  • Tonic Contraction:

    • Sustained contractions for extended periods (e.g., sphincters).

  • Phasic Contraction:

    • Quick contractions that occur in waves (e.g., peristalsis in the digestive tract).