Yet another image from the Lichtman lab: Three mouse neuromuscular junctions. Neuron projections (axons) are in white; Schwann cells (which wrap around the axons) are in green; the postsynaptic membrane of the muscle fiber onto which the axon connects is in red.
It is thought that early neurological development is less a process of “accretion and building” than one of “blind variation and selective retention.” At the neuromuscular junction, several axons will innervate a single muscle fiber, and then sequentially retract as they are digested from the tip-back by Schwann cells and by intracellular digestive vesicles, leaving only one axon connected to each muscle fiber. It appears that it in this image one such retraction is taking place. Adjacent to the middle junction, an axon that is not innervating the muscle is surrounded by a Schwann cell.
