Muscular system BIO233

Muscular System

Muscle Overview

  • Muscle: Latin for "little mouse"; primary tissue types include:

    • Cardiac Muscle Tissue: Found in the heart.

    • Smooth Muscle Tissue: Located in the walls of hollow organs.

    • Skeletal Muscle: Makes up nearly 50% of the body’s mass.

Properties of Muscle Tissue

  • Myofilaments: Responsible for muscle cell shortening, composed of actin or myosin proteins.

Key Properties

  • Contractility: Nerve signals excite muscle cells, generating electrical impulses along plasma membrane.

  • Excitability: Ability to respond to stimuli.

  • Extensibility: Muscle can be stretched (e.g., smooth muscle stretch occurs with contents).

  • Elasticity: Muscle recoils after being stretched.

Terminology in Muscle Tissue

  • Myo/mys: Prefix meaning "muscle."

  • Sarco: Prefix meaning "flesh."

    • Sarcolemma: Plasma membrane of muscle cells.

    • Sarcoplasm: Cytoplasm of muscle cells.

Primary Functions of Muscle Tissue

  1. Movement: Muscles create movement by contracting and pulling on tendons attached to bones.

  2. Support: Helps support visceral organs and shields tissues from injury.

  3. Posture: Maintains body position through continuous muscular contraction.

  4. Temperature Regulation: Contributes to body temperature through muscle activity.

  5. Communication: Facilitates non-verbal communication through facial expressions, gestures, and vocalization.

Types of Muscle Tissue

  • Skeletal Muscle Tissue:

    • Voluntary control.

    • Striated cells make up 40% of body weight.

  • Cardiac Muscle Tissue:

    • Involuntary control located in the heart.

    • Striated cells.

  • Smooth Muscle Tissue:

    • Involuntary control located in the walls of hollow organs.

    • Non-striated cells.

Gross Anatomy of Skeletal Muscle

  1. Connective Tissue and Fascicles:

    • Epimysium: Dense connective tissue surrounding the entire muscle.

    • Perimysium: Surrounds each fascicle (group of muscle fibers).

    • Endomysium: Fine sheath of connective tissue wrapping each muscle cell.

  2. Nerves and Blood Vessels: Each muscle receives a nerve, an artery, and one or more veins for supply.

  3. Muscle Attachments:

    • Origin: Less movable attachment point.

    • Insertion: More movable attachment point.

  4. Tendons: Tough fibrous connective tissue connecting muscle to bone, providing support to local nerves and blood vessels.

Microscopic Anatomy of Skeletal Muscle Fiber

  • Myofibrils: Long rods making up 80% of the cytoplasm; contain repeating segments called sarcomeres. Striations arise from the internal structure of myofibrils.

  • Sarcomeres: Basic units of contraction in skeletal muscle, defined by:

    • Z Disc: Boundaries of each sarcomere.

    • Actin (Thin Filaments): Extend from Z disc toward sarcomere center.

    • Myosin (Thick Filaments): Located at the center of the sarcomere, containing ATPase enzymes.

Muscle Contraction Mechanism

  • Sliding Filament Model:

    1. Calcium ions bind to troponin on actin’s active site.

    2. Myosin binds to actin, forming cross-bridges.

    3. Phosphate release causes myosin head to change shape, pulling actin fiber toward M line.

    4. A new ATP molecule replaces ADP, disconnecting the cross-bridge.

    5. Repeat cycle with new ATP.

Types of Muscle Actions

  1. Isotonic Contractions: Muscle length changes with constant tension.

    • Concentric: Muscle shortens against resistance.

    • Eccentric: Muscle lengthens under tension.

  2. Isometric Contractions: Muscle contracts without changing length (e.g., during static holds).

Muscle Fiber Alignment and Fascicle Arrangement

  • Pennate Muscles: Fibers attach at an angle, enhancing force generation; various patterns include:

    • Unipennate: Fibers on one side of tendon.

    • Bipennate: Fibers on both sides of a centrally positioned tendon.

    • Multipennate: Fibers attach from multiple directions to several tendons.

  • Fascicle Types: Arrangement impacts muscle function, force, and speed capabilities.

Aging and Muscle Fibers

  • Inactivity Effects: Leads to muscle atrophy, particularly in Type II fibers which are replaced by fat/connective tissue.

  • Maintenance Programs: Resistance training and anaerobic exercise can mitigate aging effects on muscle tissue.

Disorders of Muscle Tissue

  • Common Disorders:

    • Muscular Dystrophy: Inherited disorders leading to muscle degeneration, typically in Type II fibers.

    • Myofascial Pain Syndrome: Pain due to tight muscle bands.

    • Fibromyalgia: Chronic pain syndrome primarily affecting women.