Neuromuscular junction

Neuromuscular junction

• Neuromuscular junction Definition:
  • The junction between the terminal branch of the nerve fiber and muscle fiber is called neuromuscular junction.
• Neuromuscular junction Structure:
  1. Muscle fiber.
     • It contains small thickened part in the midpoint called motor end plate.
     • This shows a small depression called synaptic trough or synaptic gutter.
  2. Nerve fiber.
     • Its each terminal branches called axon terminal.
     • As it approaches close to muscle fiber it loses myelin sheath,  expands and form synaptic knob.

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• Neuromuscular junction Junction:
  • Synaptic knob fits into the synaptic gutter.
  • The membrane of the nerve ending is called the presynaptic membrane.
  • the membrane of the muscle fiber is called postsynaptic membrane.
  • Space present between these two is called synaptic cleft.
  • Synaptic cleft contains basal lamina to which large quantity of acetylcholinesterase is attached.
  • The postsynaptic membrane is thrown into numerous folds called subneural clefts.
  • The axon terminal contains.
    1. Synaptic vesicles.
       • It contains acetylecholine.
    2. Mitochondria.
       • It contains ATP, a source of energy.
       • Acetylcholine is synthesis by it.
  • The postsynaptic membrane contains nicotinic acetylcholine receptors.

Mechanism of transmission of nerve impulse:

Transmission Of Nerve Impulse Series of events:

• Nerve impulse/action potential reaches the presynaptic nerve ending.
• This increases the permeability of presynaptic membrane for calcium ions by opening voltage gated calcium channels.
• Calcium enters axon terminal and causes rupture of vesicles.
• Thus acetylcholine is release from these vesicles by exocytosis and enters synaptic cleft.
• This acetylcholine now attached to the nicotinic acetylcholine receptors present in post synaptic membrane.
• It opens the ligand gated channels for sodium and thus sodium ions enters the neuromuscular junction.
• Increased permeability of Na+ causes depolarization of the post synaptic membrane and generates end plate potential.
• When more and more acetycholine is released continuously, the miniature end plate potentials are added together.
• When endplate potential reaches a threshold of 3040 mV, it depolarizes the surface membrane of the muscle and results in generation of action potential.
• Once this action potential reaches inside the muscle fiber then the muscle gives mechanical response by contraction.
• The acetylcholine that is released is destroyed very quickly by the enzyme acetylcholinesterase.