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2-Minute Neuroscience: Neuromuscular Junction

The term neuromuscular junction refers to a synapse between a motor neuron and muscle fiber; activity here is essential for muscle contraction and thus movement. At the neuromuscular junction, the synaptic boutons of a motor neuron are situated over a specialized region of muscle called the end plate. The synaptic boutons release acetycholine, which travels across the synaptic cleft and activates acetylcholine receptors on the muscle fiber. This causes excitation of the muscle cell, and muscle contraction. Excess acetylcoholine is removed from the synaptic cleft by the enzyme acetylcholinesterase. TRANSCRIPT: 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. REFERENCE: Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, McNamara JO, White LE. Neuroscience. 4th ed. Sunderland, MA. Sinauer Associates; 2008.