Muscles and Tendons

Skeletal Muscle

Gross Structure

  • Basic unit of a muscle: Sarcomere
  • Smallest functional unit: Myofibril (Many sarcomeres lined up together)
  • Muscle fibres grouped together form Fasicles (Visible by eye)
  • Muscle organization:
    • Sarcomere > Myofibril > Fibre > Fasicle > Muscle
    • Membranes:
      • Sarcolemma: Surrounds sarcomere
      • Sarcoplasm: Cytoplasm of the sarcomere
    • Layers:
      • Endomycium: Surrounds individual muscle fibres
      • Perimycium: Surrounds fasicles (groups of muscle fibres)
      • Epimycium: Surrounds whole muscle (grouped fasicles)

Ultra Structure

  • Myofibrils composed of:
    • Actin (small fibres) and Myosin (big fibres)
  • Sarcomeres lined up longitudinally forming a myofibril (2-2.5 microns in length, visible under electron microscope)
  • Bands contributing to contractility:
    • A Bands: Overlapping actin & myosin filaments
    • H Bands: Myosin filaments - no overlap with actin
    • I Bands: Actin filaments - no overlap with myosin
    • M Line: Central band - connections between myosins
    • Z Disc: Between adjacent sarcomeres
  • Arrangement: Hexagonal helix (6 actin to 1 myosin filament)
  • Supporting proteins:
    • Dystrophin (absent in Duchenne Muscular Dystrophy)
    • Troponin and Tropomyosin on actin

Muscle Cell Membrane

  • Components: Sarcoplasmic reticulum and T-tubules

Sarcoplasmic Reticulum

  • Surrounds each myofibril, stores calcium essential for contraction

T Tubules

  • Invaginations of the cell membrane extending into the cell
  • Function: Relays nerve signal deep into the cell, facilitating calcium entry via the T tubule system

Blood Supply

  • One main artery supplies each muscle, branching rapidly to form highly vascular arcades running with Perimycium
  • Terminal capillaries lie within the Endomycium

Function

Sliding Filament Mechanism

  • Contraction:
    • Myosin binds to actin, facilitated by calcium binding to troponin and moving tropomyosin.
    • ATP-dependent mechanism
  • Force generation:
    • Proportional to muscle cross-sectional diameter
    • Velocity of contraction is related to muscle length
    • Power: Function of force and velocity

Innervation

  • Motor neuron supplies multiple muscle fibres.
  • Muscle Spindles: Provide proprioceptive feedback.

Neuromuscular Junction (Motor End Plate)

  • Pre-synaptic membrane contains vesicles of acetylcholine.
  • Action potential triggers acetylcholine release.
  • Calcium release by sarcoplasmic reticulum allows for myosin-actin cross-bridging.
  • Acetylcholinesterase clears residual acetylcholine.
  • Conditions:
    • Botox blocks acetylcholine release, preventing spasticity.
    • Myasthenia Gravis: Reduced acetylcholine receptors.

Myotendinous Junction

  • The weakest part of the muscle unit, prone to injury.
  • Features to improve strength:
    1. Short sarcomere length
    2. Tissue interdigitation
    3. High synthetic ability of cells
    4. Folded membranes increasing surface area

Types of Muscle Fibre

  • Type 1 (Slow red ox): Slow-twitch, red, oxidative, fatigue-resistant
  • Type 2a: Fast-twitch, oxidative and glycolytic, relatively white, fatigable
  • Type 2b: Fast-twitch, glycolytic, white, highly fatigable

Muscle Metabolism

System Description
ATP Dependent Intense short duration activities, carbohydrate-stored, no oxygen or lactate required
Lactic Anaerobic Short-middle duration, produces lactic acid
Aerobic Long duration, reliant on oxygen, replenishes ATP

Types of Muscle Contraction

  • Isotonic: Constant muscle tension, change in length (e.g., biceps curl)
  • Isometric: Constant length, changing tension (e.g., pushing against a wall)
  • Isokinetic: Constant velocity, varying length and tension
  • Concentric: Muscle shortens
  • Eccentric: Muscle lengthens while generating force (e.g., lowering a weight)
  • Plyometric: Stretch followed by rapid contraction

Muscle Injury

  • Caused by crush, laceration, ischemia, infection, denervation, etc.
  • Leads to rising intracellular calcium, activating proteolytic enzymes, damaging muscle until stimulus is removed.

Muscle Repair

  • Requires vascularity and innervation.
  • Healing involves macrophage infiltration and scar tissue formation, resulting in around 50% power recovery.

EMG (Electromyography)

  • Shows denervation signs from muscle readings at 3 weeks (e.g., sporadic firing, sharp waves).

Tendons & Ligaments

Feature Ligament Tendon
Elastin Content Greater Less
Strength Weaker Stronger
Collagen Arrangement Layered, multidirectional Longitudinal, high tensile stress

Function

  • Ligaments: Stabilize joints, provide proprioceptive feedback.
  • Tendons: Transmit tensile loads, act as springs, place muscle optimally to joints.

Mechanical Properties

  • Stress-Strain Curve: Shows response to load; small plastic region for ligaments, failure occurs after slight elongation.

Injury & Repair

  • Tendons and ligaments are prone to breakage rather than bending, with limited plastic deformation.
Back to top