Lecture on Muscle Contraction and Neuromuscular Junction

Key Concepts

• Muscle Contraction

  • Skeletal muscles contract when stimulated by nerve impulses from motor neurons.
  • Without innervation, muscles become paralyzed and atrophied.

• Motor Units

  • A motor neuron axon branches to supply multiple muscle fibers, forming a motor unit.
  • Motor units vary in size:
    • Small Motor Units: Few muscle fibers, control subtle movements (e.g., eye muscles).
    • Large Motor Units: Many fibers, control strong movements (e.g., arms, legs).
  • Muscle fibers of a motor unit are scattered, not clustered.
  • Muscle contraction strength depends on the number of active motor units.
  • Muscle Tonus: Partial contraction state maintained by activation of small motor units.

Neuromuscular Junction

• Structure and Function

  • Connection between motor neuron and muscle fiber.
  • Involves a chemical synapse between the nerve terminal and motor end-plate.

• Action Potential and Neurotransmitter

  • Action potential at nerve terminal releases acetylcholine into the synaptic space.
  • Acetylcholine binds to nicotinic receptors (ligand-gated ion channels) on the end-plate.
  • Sodium entry depolarizes the cell membrane producing an end-plate potential.
  • Action potential is generated when end-plate potential reaches threshold.
  • Further depolarization by voltage-gated sodium channels, and repolarization by potassium channels.
  • Action potential spreads through muscle fiber and T-tubules, activating calcium channels.

• Calcium Release and Muscle Contraction

  • Calcium release from sarcoplasmic reticulum triggers muscle contraction via "sliding filament mechanism".

Role of Acetylcholinesterase

• Enzyme that removes acetylcholine to terminate synaptic activation.
• Prevents continuous contraction and muscle spasms.

Effects of Toxins and Drugs

• Botulinum Toxin: Prevents acetylcholine release, causing paralysis.
• Receptor Blockers: Block acetylcholine binding, preventing channel opening.
• Channel Blockers: Prevent sodium passage, causing flaccid paralysis.
• Pesticides: Inhibit acetylcholinesterase, causing muscle spasms and spastic paralysis.

These notes cover the functions and mechanisms of muscle contraction, the role of motor units and neuromuscular junctions, and how certain substances affect these processes. Understanding these basics is crucial for comprehending muscle physiology and the impact of various toxins on muscle function.