0 Neuromuscular junction (NMJ)

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Neuromuscular junction (NMJ):
The neuromuscular junction connects the nervous system to the muscular system via synapses between efferent nerve fibers and muscle fibers, also known as muscle cells.








Neuromuscular junction 
  1. presynaptic terminal
  2. sarcolemma
  3. synaptic vesicles
  4. Acetylcholine receptors
  5. mitchondrion




Points to remember : ( Mnemonic - MU CaR )

Acetylcholine Uploading into vesicles -- Magnesium (Mg)
Acetylcholine Release from vesicles -- Calcium (Ca)



Excitation-contraction coupling of vertebrate skeletal muscle :
  1. Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
  2. This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron's cell membrane through SNARE proteins.
  3. Fusion of the vesicular membrane with the presynaptic cell membrane results in the emptying of the vesicle's contents (acetylcholine) into the synaptic cleft, a process known as exocytosis.
  4. Acetylcholine diffuses into the synaptic cleft and can bind to the nicotinic acetylcholine receptors on the motor end plate.
  5. These receptors are ligand-gated ion channels, and when they bind acetylcholine, they open, allowing sodium ions to flow in and potassium ions to flow out of the muscle cell.
  6. Because of the differences in electrochemical gradients across the plasma membrane, more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end-plate potential (EPP).
  7. This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.
  8. The action of acetylcholine is terminated when ACh diffuses away from the synapse or the enzyme acetylcholinesterase degrades part of ACh (producing choline and an acetate group).
  9. The choline produced by the action of acetylcholinesterase is recycled — it is transported, through reuptake, back into the presynaptic terminal where it is used to synthesize new acetylcholine molecules.


ref: 
http://en.wikipedia.org/wiki/Neuromuscular_junction


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