What substance is primarily released at neuromuscular junctions?

At neuromuscular junctions, the primary substance released is acetylcholine. This neurotransmitter plays a crucial role in signal transmission from motor neurons to skeletal muscle fibers. When an action potential arrives at the neuromuscular junction, voltage-gated calcium channels open, allowing calcium ions to flow into the presynaptic terminal. This influx of calcium triggers synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft.

Once released, acetylcholine binds to nicotinic acetylcholine receptors on the postsynaptic membrane of the muscle fiber. This binding initiates a series of events that lead to muscle contraction. The rapid action and breakdown of acetylcholine by the enzyme acetylcholinesterase in the synaptic cleft are important for regulating muscle activity and ensuring that the signal does not persist indefinitely. Thus, acetylcholine is essential for neuromuscular transmission and muscle activation.