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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the neuromuscular junction.
The term neuromuscular junction refers to the synapse between a motor neuron and a skeletal
muscle fiber.
Activity at the neuromuscular junction is essential for the contraction of skeletal
muscle to occur, and even just to keep muscles from atrophying.
It is also the site where synaptic transmission was first studied and thus is the best understood
example of chemical signaling in the nervous system.
When a motor neuron approaches a muscle, it branches out into several extensions that
end in areas called synaptic boutons, which can release neurotransmitters.
These synaptic boutons are situated over a specialized region of muscle called the end-plate.
The synaptic boutons are separated from the end plate by a space called the synaptic cleft.
The end-plate beneath each synaptic bouton contains several deep indentations called
junctional folds.
These junctional folds contain high numbers of ligand-gated ion channel receptors for
the neurotransmitter acetylcholine.
When an action potential travels down the motor neuron, it causes the release of acetylcholine
into the synaptic cleft.
Acetylcholine binds to acetylcholine receptors in the junctional folds of the muscle membrane,
which causes ion channels to open to allow positive sodium ions to flow into the postsynaptic
cell.
This produces a depolarization of that cell called an excitatory post-synaptic potential,
also known as the end-plate potential when it occurs at the neuromuscular junction.
This depolarization leads to the opening of voltage gated sodium channels, which causes
the end-plate potential to lead to an action potential.
This action potential travels along the muscle fiber and causes contraction of the muscle.
The enzyme acetylcholinesterase is also present at the neuromuscular junction; it breaks down
acetylcholine and in the process terminates its effects on the muscle fiber, thus ending
the communication between the motor neuron and the muscle fiber.