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.
- presynaptic terminal
- synaptic vesicles
- Acetylcholine receptors
Acetylcholine Uploading into vesicles -- Magnesium (Mg)
Acetylcholine Release from vesicles -- Calcium (Ca)
Excitation-contraction coupling of vertebrate skeletal muscle :
- 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.
- This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron's cell membrane through SNARE proteins.
- 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.
- Acetylcholine diffuses into the synaptic cleft and can bind to the nicotinic acetylcholine receptors on the motor end plate.
- 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.
- 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).
- This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.
- 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).
- 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.