Guy Bewick, Aberdeen University, Wellcome Images.
When you clicked this link, your brain ordered your fingers to do its bidding. The brain pulls off this magic trick thanks to neuromuscular junctions, the interface between the nervous system and the body’s muscles.
These adult rat foot muscles (seen here in red) get their marching orders from motor neurons, whose axons extend to the muscle fibers. When the electric signal telling the muscle to jump arrives, the ends of the motor neuron release a burst of molecules as a chemical message that triggers the fiber to contract, before it can even ask, “how high?”