What neurotransmitter is primarily responsible for muscle activation?

The neurotransmitter primarily responsible for muscle activation is acetylcholine. This chemical messenger plays a critical role in the transmission of signals from motor neurons to muscle fibers at the neuromuscular junction. When a motor neuron is stimulated, it releases acetylcholine into the synaptic cleft, the space between the neuron and the muscle fiber. Upon binding to specific receptors on the muscle cell's membrane, acetylcholine triggers an influx of sodium ions, leading to depolarization of the muscle cell and ultimately resulting in muscle contraction. This process is fundamental for all voluntary movements, as well as some involuntary movements, making acetylcholine essential for muscle function.