First observed in neonatal muscles in 1917. Starting early in prenatal life, one spinal motoneuron synaptically innervates more than one fiber in skeletal muscle (polyinnervation). Subsequently, supernumerary or collateral axons are retracted so that one motoneuron innervates one muscle fiber (monoinnervation), a process that takes place mainly prenatally, but also after birth depending on the muscle. The details of the mechanisms responsible for synapse elimination, axon retraction and the loss of polyinnervation are still not completely understood, but are thought to involve competition for the uptake of neurotrophic substances in the target sites (i.e., muscle fibers). Like apoptosis, the process is activity dependent (i.e., it is dependent on epigenetic factors such as functional activity, a version of Lamarck’s‚aa ‘use it or lose it’ principle), which reduces the genetic information required to specify connections. Given that muscle fibbers are small and weak during early development, polyinnervation may ensure sufficient muscle strength to produce movements, but ones that are not very precise. With monoinnervation, motor control of force generation and coordination may become more precise as it allows a single muscle fiber to be controlled more accurately.
See Apoptosis (or cell death), Lamarckism, Muscle fiber, Musculoskeletal system, Nerve growth factor (NGF), Quantitative and qualitative change, Striated (or stripe or voluntary) muscle