Lecture Notes on Neuromuscular Junction and Skeletal Muscle Contraction

Introduction to Neuromuscular Junction

• Definition: Junction where a neuron meets a muscle, specifically a skeletal muscle.
• Components:
  • Neuro: Related to neurons.
  • Muscular: Related to muscles.
  • Junction: Meeting point between neuron and muscle.

Importance of Neuromuscular Junction

• Skeletal muscles do not contract randomly; contraction is initiated by motor neurons.
• Motor Neuron Anatomy:
  • Large cell body, long axon.
  • Terminal branches with synaptic knobs.
  • Transmission of electrical impulses from cell body to terminal branches.

Process of Muscle Contraction

1. Action Potential in Motor Neuron:
   • Motor neuron sends an electrical impulse.
   • Action potential reaches neuromuscular junction.
2. Release of Neurotransmitter:
   • Acetylcholine (ACh) is released at the neuromuscular junction.
   • ACh binds to receptors on the sarcolemma (muscle cell membrane).
3. Calcium Ion Involvement:
   • Sarcoplasmic reticulum releases calcium ions upon stimulation.
   • Calcium ions bind to troponin on actin filaments, initiating contraction.

Structure of Skeletal Muscle Fiber

• Components:
  • Sarcolemma: Cell surface membrane.
  • T-tubule: Infoldings connected to sarcoplasmic reticulum.
  • Sarcoplasm: Equivalent to cytoplasm, contains calcium ions.
  • Myofibrils: Composed of light (actin) and dark (myosin) bands.

Detailed Process of Muscle Contraction

• Motor Neuron Stimulation:
  • Action potential causes opening of calcium ion channels.
  • Calcium influx causes vesicles to release ACh.
• Transmission Across Neuromuscular Junction:
  • ACh diffuses across junction, not a synapse, and binds to receptors.
  • Sodium ion channels open, depolarizing the sarcolemma.
• Depolarization and Calcium Release:
  • Depolarization travels via T-tubules.
  • Calcium channels in the sarcoplasmic reticulum open.
  • Calcium ions enter sarcoplasm, bind to troponin.

Sliding Filament Theory

• Actin and Myosin Interaction:
  • Calcium binds to troponin, allowing myosin to pull actin.
  • Myofibrils contract as actin and myosin slide past each other.
  • Sarcomere shortens, resulting in overall muscle contraction.

Exam Tips

• Differentiate between neuromuscular junction processes and sliding filament theory.
• Be prepared for questions combining both topics, though rare.

Conclusion

• Summary of muscle contraction mechanics.
• Overview of human coordination and response.