Myology 1 Study Notes

Chapter 10: Myology 1

Types of Muscle Tissue

  • Overview

    • Muscle tissue consists of specialized cells that contribute significantly to nearly half of the body's mass.

    • Function: Transforms chemical energy (ATP) into directed mechanical energy, allowing muscles to exert force.

    • Types of Muscle Tissue:

    • Skeletal muscle

    • Cardiac muscle

    • Smooth muscle

    • Etymology: The prefixes 'myo', 'mys', and 'sarco' relate to muscles.

Anatomy

  • Components of Muscle:

    • Muscle comprises cells filled with myofibrils.

    • Myofibrils are made up of contractile units called sarcomeres.

    • Key proteins in sarcomeres:

    • Actin: Thin filament.

    • Myosin: Thick filament.

    • Mechanism: Contractile proteins slide over each other, facilitating muscle contraction.

Types of Muscle Tissue Details

Skeletal Muscle

  • Location: Attached to bones or skin.

  • Cell Characteristics:

    • Elongated cells termed muscle fibers.

    • Striated appearance (striped).

    • Voluntary control (conscious movement).

    • Rapid contraction but tires easily.

    • Requires stimulation from the nervous system.

Cardiac Muscle

  • Location: Only in the heart, forming most of its walls.

  • Cell Characteristics:

    • Striated with a unique branching structure.

    • Involuntary control (works without conscious thought).

    • Contracts rhythmically without nervous system stimulation.

  • More details to follow in Chapter 18.

Smooth Muscle

  • Location: Found in walls of hollow organs (e.g., stomach, bladder, airways).

  • Cell Characteristics:

    • Non-striated appearance.

    • Involuntary control (functions automatically).

    • Capable of contraction without nervous system stimulation.

Comparison of Muscle Types (Table 9.3)

General Characteristics

  • Body Location:

    • Skeletal: Attached to bones or skin.

    • Cardiac: Walls of the heart.

    • Smooth: Walls of hollow organs.

  • Cell Shape and Appearance:

    • Skeletal: Long cylindrical, multinucleated with striations.

    • Cardiac: Branching, elongated, uni or binucleate, striated.

    • Smooth: Fusiform, uninucleate, no striations.

Connective Tissue Components

  • Skeletal: Epimysium, perimysium, endomysium.

  • Cardiac: Endomysium (attached to heart’s fibrous skeleton).

  • Smooth: Endomysium only.

Myofibrils and Regulation

  • Presence of Myofibrils:

    • Skeletal: Yes, composed of sarcomeres.

    • Cardiac: Yes, with T-tubules present; two at every A-I junction.

    • Smooth: Yes, but irregular thickness; no true sarcomeres, dense bodies present instead.

  • Regulation of Contraction:

    • Skeletal: Voluntary (axon terminals of somatic nervous system).

    • Cardiac: Involuntary with intrinsic system and hormonal control.

    • Smooth: Involuntary with autonomic nerves and hormonal influence.

Function of Skeletal Muscle Tissue

  • Characteristics:

    • Excitability: Ability to respond to stimuli.

    • Contractility: Ability to shorten and produce movement.

    • Extensibility: Ability to stretch and return to resting length.

Structural Components

  • Connective Tissue:

    • Endomysium: Delicate tissue covering individual muscle fibers.

    • Perimysium: Tough tissue binding fascicles together.

    • Epimysium: Coarse sheath covering whole muscle.

    • Fascia: Surrounds muscles outside epimysium.

Microscopic Anatomy of Skeletal Muscle Fibers

  • Fiber Definition: Muscle cells are termed fibers due to their threadlike structure.

  • Key Structures:

    • Sarcoplasm: Cytoplasm of muscle fibers.

    • Sarcoplasmic Reticulum (SR): Network of tubules that manages calcium ion storage and release.

    • Myofibrils: Fine fibers containing contractile units called sarcomeres, organized into A bands (dark) and I bands (light).

Mechanism of Muscle Contraction

Excitation-Contraction Coupling

  • Neuromuscular Junction:

    • Connection point between motor neuron and muscle fiber.

    • Release of Acetylcholine (ACh) triggers impulse in muscle cell membranes.

    • Threshold stimulus: Minimum stimulation required for contraction.

  • Process:

    1. ACh Release: Diffuses across synaptic cleft, stimulating receptors.

    2. Impulse travels across the sarcolemma and along T tubules, causing calcium release.

    3. Calcium binds to troponin, moving tropomyosin and exposing active sites on actin.

Sliding Filament Model

  • Mechanics:

    • Binding and bending of myosin heads (cross-bridges) occur when active sites on actin are exposed.

    • Myosin pulls actin filaments, and as this happens, the sarcomere shortens, leading to overall muscle contraction.

    • Relaxation involves calcium being pumped back into the SR.

Muscle Function and Types of Contractions

Motor Units and Muscle Tone

  • Motor Unit Definition: A neuron and the muscle fibers it innervates; smaller motor units allow for more precise movement.

  • Muscle Tone: Continuous partial contractions, maintaining muscle readiness.

Contraction Types

  • Twitch Contraction: Quick, brief contraction from a single stimulus.

    • Tetanus: Smooth, sustained muscle contractions through wave summation.

  • Isotonic Contraction:

    • Muscle length changes with constant tension.

    • Concentric: Muscle shortens.

    • Eccentric: Muscle lengthens while contracting.

  • Isometric Contraction: Muscle length remains unchanged while tension increases.

Cardiac Muscle

  • Function: Self-stimulating, involuntary muscle that does not fatigue quickly, ensures continuous blood circulation.

    • Cardiac muscle cells interconnect at intercalated discs for synchronized contraction.

Smooth Muscle Characteristics

  • Structure: Non-striated, composed of small, tapered cells with a single nucleus.

  • Mechanism of Contraction: Involves calmodulin, with calcium sourced externally instead of from the SR.

  • Types:

    • Single-unit: Functions as a whole (e.g., digestive tract).

    • Multiunit: Independent fibers responsive only to nervous stimulation.