Myology: Anatomy and Functions of Muscles

Myology

• Study of muscles.
• Four key characteristics: excitability, contractibility, extensibility, and elasticity.

Main Muscle Functions

• Movement: Muscles contract to produce motion.
• Support: Muscles help maintain posture and stability.
• Body temperature maintenance: Muscle contraction generates heat.
• Guarding orifices: Muscles control openings in the body.

Muscle Types

• Smooth: Involuntary muscle found in internal organs.
• Cardiac: Involuntary muscle found in the heart.
• Skeletal: Voluntary muscle attached to bones, responsible for movement.

Gross Anatomy of Skeletal Muscle

• Three layers of connective tissue:
  • Epimysium: Surrounds the entire muscle.
  • Perimysium: Divides muscle into compartments (fascicles).
  • Endomysium: Surrounds individual muscle fibers.
• Satellite cells: Located between the endomysium and muscle fibers, responsible for muscle repair.
• Tendon: Connective tissue that attaches muscle to bone.
• Neuromuscular junction: Site where a motor neuron communicates with a muscle fiber.

Microanatomy of Muscle Fibers

• Sarcolemma: Plasma membrane of muscle fiber.
• Sarcoplasm: Cytoplasm of muscle fiber.
• Muscle fibers: Long, multinucleated cells formed by fusion of myoblasts.
• Transverse tubules (T-tubules): Invaginations of the sarcolemma that conduct electrical impulses.
• Myofibrils: Contractile units within muscle fibers, composed of myofilaments.
• Mitochondria: Abundant in muscle fibers to provide energy.
• Glycogen: Stored energy source in muscle fibers.

Structure of Myofibrils

• Composed of thin filaments and thick filaments.
• Thin filaments: Primarily made of actin.
• Thick filaments: Primarily made of myosin.
• Troponin and Tropomyosin: Regulatory proteins associated with actin.
• Titin: Large protein that helps maintain structure and elasticity.

Sarcomere: The Functional Unit of Muscle

• Z-disks/lines: Separate adjacent sarcomeres.
• A-band: Dark band, where thick and thin filaments overlap.
• M-line: Center of the A-band, where thick filaments connect.
• H-zone: Light zone in the center of the A-band, containing only thick filaments.
• I-band: Light band, containing only thin filaments.

Motor Neurons and Muscle Contraction

• Motor neurons: Stimulate muscle fibers.
• Synaptic vesicles: Contain acetylcholine, a neurotransmitter.
• Synaptic cleft: Space between the axon terminal and sarcolemma.

All-or-None Principle

• Muscle fibers within a motor unit contract maximally or not at all, depending on the strength of the stimulus.

Muscle Atrophy and Hypertrophy

• Disuse atrophy: Muscle wasting due to lack of use.
• Denervation atrophy: Muscle wasting due to nerve damage.
• Hypertrophy: Muscle growth due to increased workload.

Types of Skeletal Muscle Fibers

• Red muscle fibers (Type I): Slow-twitch, oxidative, fatigue-resistant. High myoglobin content, rich in mitochondria and blood supply.
• White muscle fibers (Type II): Fast-twitch, glycolytic, fatigable. Low myoglobin content, fewer mitochondria and blood supply.
• Type IIA fibers (Intermediate): Fast-twitch, oxidative and fatigue-resistant.

Abnormal Muscle Contractions

• Spasm: Involuntary contraction of a large group of muscles.
• Tremor: Rhythmic, involuntary contractions of opposing muscle groups.
• Fasciculation: Brief, involuntary twitch of a muscle visible under the skin.
• Fibrillation: Brief, involuntary twitch of a muscle not visible under the skin.

Organization of Muscle Fibers

• Four patterns of fascicle arrangement:
  • Parallel: Fibers run parallel to the long axis of the muscle.
  • Convergent: Fibers converge from a broad origin to a single insertion.
  • Pennate: One or more tendons run through the muscle.
    • Unipennate: Muscle fibers on one side of the tendon.
    • Bipennate: Muscle fibers on both sides of the tendon.
  • Circular or sphincter: Concentrically arranged around an opening.

Muscle Attachments, Actions, and Naming

• Origin: Fixed point of attachment.
• Insertion: Movable point of attachment.
• Action: Movement produced by muscle contraction.

Muscle Action Types

• Prime movers (agonists): Muscles primarily responsible for a specific movement.
• Synergists: Muscles that assist prime movers.
• Antagonists: Muscles that oppose the action of prime movers.
• Fixators: Muscles that stabilize the origin of prime movers.

Skeletal Muscle Naming Conventions

• Directional:
  • Rectus: Straight fibers.
  • Transversus: Fibers run across.
  • Obliques: Fibers run at an angle.
• Unusual structure features:
  • Biceps: Two tendons of origin.
  • Triceps: Three tendons of origin.
  • Quadriceps: Four tendons of origin.
• Shape:
  • Trapezius: Trapezoid.
  • Deltoid: Triangular.
  • Rhomboideus: Rhomboid.
  • Orbicularis: Circular.
  • Longus: Long.
  • Longissimus: Longest.
  • Teres: Long and round.
  • Brevis: Short.
  • Magnus: Big.
  • Major: Bigger.
  • Maximus: Biggest.
  • Minor: Small.
  • Minimus: Smallest.
• Location
  • Externus or superficial: Visible at body surface.
  • Internus or profundus: Internal.
  • Extrinsic: Muscles that position or stabilize an organ.
  • Intrinsic: Muscles that operate within an organ.
• Origin and Insertion:
  • Genioglossus: Originates at the chin (genio) and inserts on the tongue (glossus).
• Function:
  • Flexor: Muscle that decreases joint angle.
  • Extensor: Muscle that increases joint angle.
  • Retractor: Muscle that draws a structure backwards.
• General:
  • Most skeletal muscles have "muscle" in their names, except for the platysma and diaphragm.