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

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

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.