Neuromuscular Junction and Muscle Physiology

Neuromuscular Junction (NMJ)

  • NMJ: Special intercellular connection between nervous system and skeletal muscle fiber.
  • Controls calcium ion release into sarcoplasm.
  • Motor neurons stimulate skeletal muscle fibers via axons.
  • Each axon forms a NMJ with a single muscle fiber.

Events at the NMJ

  1. Action potential arrives at axon terminal of motor neuron.
  2. Voltage-gated Ca²⁺ channels open and Ca²⁺ enters the terminal.
  3. Ca²⁺ causes synaptic vesicles to release acetylcholine (ACh) through exocytosis.
  4. ACh diffuses across synaptic cleft; binds to receptors on sarcolemma.
  5. Binding opens ion channels allowing Na⁺ in and K⁺ out.
  6. ACh effects are terminated by acetylcholinesterase.

Membrane Potentials

  • Resting membrane potential: Approximately -70mV due to ion distributions (Na⁺, K⁺).
  • Action potential: Rapid depolarization (threshold is -60 to -55mV) followed by repolarization.
  • Refractory periods: Absolute (no response) and relative (requires a stronger stimulus).

Muscle Contraction Mechanism

  • Skeletal muscle contraction involves shortening of fibers when thin filaments slide between thick filaments.
  • Ca²⁺ is released from sarcoplasmic reticulum in response to ACh stimulation.
  • Relaxation occurs when Ca²⁺ levels fall and tropomyosin re-covers binding sites.

Muscle Fiber Types

  1. Fast Fibers: Quick contractions, fatigue rapidly.
  2. Slow Fibers: Slow contractions, high endurance, rich in myoglobin.
  3. Intermediate Fibers: Mid-sized, moderate fatigue resistance.

Muscle Fatigue and Recovery

  • Caused by depletion of reserves, damage to membranes, and low pH (lactic acid).
  • Recovery involves lactic acid recycling and oxygen debt.

Muscle Types

  • Skeletal Muscle: Voluntary, striated, multiple nuclei.
  • Cardiac Muscle: Involuntary, striated, single nucleus, intercalated discs.
  • Smooth Muscle: Involuntary, non-striated, single nucleus, controls organ activities.