What is the dark staining structure? (Hint: note the small nerve twig associated with the structure).
The neuromuscular junction (NMJ; AKA myoneural junction or motor end-plate) is the synapse between the nerve and muscle. It has three basic components: 1) presynaptic motor nerve terminal, 2) synaptic space (synaptic cleft), and 3) the postsynaptic surface of the skeletal muscle fiber. It is important to note that the nerve ending does not come into direct contact with the surface of the muscle fiber. Release of the neurotransmitter acetylcholine from the presynaptic nerve terminal into the synaptic space is triggered by the arrival of a nerve action potential that has traveled along the motor nerve.
Neuromuscular Junction Notes:
The released acetylcholine is bound by acetylcholine receptors embedded in the specialized postsynaptic cell membrane of the muscle fiber. Initial depolarization of the muscle fiber is triggered if enough acetylcholine is released and bound to acetylcholine receptors to pass the depolarization threshold. Successful initial depolarization triggers the generation of an action potential that propagates throughout the muscle fiber, releasing calcium from the sarcoplasmic reticulum that is needed for sarcomeres to contraction. The enzyme acetylcholinesterase located in the synaptic cleft rapidly breaks down acetylcholine, thereby preventing continued depolarization of the muscle fiber (tetany). Esterase enzyme histochemical stain highlights were acetylcholinesterase activity is present, and explains why the staining shown in image above is located outside of the muscle fiber and within the synaptic space.
Note: This illustration is provided courtesy of Dr. Wilson.
References:
Slater CR. The Structure of Human Neuromuscular Junctions: Some Unanswered Molecular Questions. Int J Mil Sci 2017:18:2183
PDB-101 Molecule of the Month: Acetylcholinesterase: https://pdb101.rcsb.org/motm/54
Li L, et al. Neuromuscular Junction Formation, Aging, and Disorders. Annu. Rev. Physiol 2018:80:159-188
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