A massive pruning of surplus axons from muscle fibers occurs during the first four weeks of a mouse's life - and in young humans. So each fiber ends up with only one axon. But how this elimination occurs has been one of the intractable problems in neuroscience. In the 1980s, Lichtman and colleagues invented a way to look repeatedly at the same synapses in living mice. And you can watch the connections disappear over time.
Nerves activate muscles by releasing packets containing a chemical called acetylcholine at synapses.
Muscle fibers detect this message with membrane proteins called acetylcholine receptors, which cluster directly under nerve terminals. Receptor activation sparks a chain of events that makes the muscle contract. Studies in Lichtman's lab with a snake toxin that inactivates acetylcholine receptors led to a surprising finding, described in Nature in 1994. Before this time, scientists thought that two axons on the same fiber might act like battering rams, competing with each other for growth factors and space. So the muscle cell is not just a passive witness - it seems to know which synapses are going to be removed before elimination actually occurs.