Muscular System Notes Students 2024

Essentials of Anatomy & Physiology

  • Author: Martini/Bartholomew

  • Summary prepared by: Alan Magid, Duke University

  • Focus: Muscular System

  • Edition: 4th Edition

  • Publisher: Pearson Education, Inc. (2007)

Overview of Muscular System

Types of Muscle Tissue

  • Voluntary Control:

    • Skeletal Muscles: Attach directly or indirectly to bones, responsible for voluntary movements.

  • Involuntary Control:

    • Cardiac Muscle: Forms the heart wall.

    • Smooth Muscle: Found in visceral organs.

Functions of Skeletal Muscles

  1. Produce movement of skeleton.

  2. Maintain posture and body position.

  3. Support soft tissues.

  4. Guard entrances and exits.

  5. Maintain body temperature.

Anatomy of Skeletal Muscles

Gross Anatomy

  • Connective Tissue Organization:

    • Epimysium: Surrounds entire muscle.

    • Perimysium: Encloses fascicles (bundles of muscle fibers), contains blood vessels and nerves.

    • Endomysium: Covers individual muscle fibers (cells).

    • Tendons/Aponeurosis: Connect muscles to bones.

Microanatomy of a Muscle Fiber

  • Sarcolemma: Muscle cell membrane.

  • Sarcoplasm: Muscle cell cytoplasm.

  • Sarcoplasmic Reticulum (SR): Stores calcium, similar to smooth ER.

  • Transverse Tubules (T Tubules): Passages through muscle fibers.

  • Myofibrils: Organelle responsible for contraction.

  • Sarcomeres: Functional unit of muscle tissue.

Sarcomere Structure

  • Components:

    • Myofilaments:

      • Thin filaments (mainly actin)

      • Thick filaments (mainly myosin)

    • Z lines: Anchor for thin filaments.

    • M line: Middle proteins of thick filaments.

    • A band: Area with thick filaments.

    • I band: Region between A bands.

    • H zone: Area between ends of thin filaments.

Contraction Process

  • Mechanism of Skeletal Muscle Contraction:

    • Actin active sites and myosin cross-bridges interact, thin filaments slide past thick filaments.

    • Cross-bridge cycle: attach, pivot, detach, return.

    • Troponin-tropomyosin complex controls interaction, prevents interaction at rest.

Neuromuscular Junction (NMJ)

Components

  1. Synaptic Terminal: End of nerve where Acetylcholine (ACh) is released.

  2. Synaptic Cleft: Space between neuron and sarcolemma.

  3. Motor End Plate: Binding area for ACh, contains receptors.

  4. Acetylcholinesterase (AChE): Enzyme that removes ACh from the synaptic cleft.

Steps in Neuromuscular Transmission

  1. Arrival of action potential at synaptic terminal.

  2. Release of ACh into synaptic cleft.

  3. ACh binding at the motor end plate.

  4. Action potential appearance in sarcolemma, which spreads through T-tubules.

  5. Trigger calcium release from sarcoplasmic reticulum.

Muscle Mechanics

Muscle Definitions

  • Muscle Tension: The force generated during contraction.

  • Muscle Twitch: A brief contraction-relaxation response to a single action potential.

Types of Muscle Contractions

  • Isotonic Contraction: Muscle length changes, tension remains constant (e.g., lifting an object).

  • Isometric Contraction: Muscle length remains unchanged during contraction (e.g., holding an object).

Fatigue and Recovery

  • Muscle Fatigue: Loss of ability to contract, often due to low pH (lactic acid) or ATP levels.

  • Recovery Period: Time needed to restore conditions after muscle activity, oxygen debt refers to extra oxygen needed during recovery.

Types of Skeletal Muscle Fibers

  • Fast Fibers: Large, produce powerful contractions, rely on anaerobic metabolic pathways.

  • Slow Fibers: Small diameter, rich in mitochondria, provide steady contractions, supported by aerobic metabolism.

Cardiac and Smooth Muscle

Cardiac Muscle Characteristics

  • Small, single nucleus per cell.

  • Intercalated discs facilitate communication.

  • Automaticity allows self-excitation and contraction without prior nervous stimulation.

Smooth Muscle Characteristics

  • Non-striated, involuntary control, often under hormonal regulation.

  • Wide variety of operating lengths for flexibility in different organs.

Aging and the Muscular System

Age-Related Changes

  • Reduction in muscle size, elasticity, strength.

  • Decreased exercise tolerance and longer recovery time from injuries.