Segregation of motor nerves on a segmental basis during synapse elimination in neonatal muscles.

The distribution of motor axons from segmental roots L4 and L5 entering the gluteus muscle of neonatal rats via the inferior gluteal nerve was determined by visual inspection of contractions when the muscle was pinned out in vitro. In both 0-2-day-old rats and in rats older than 11 days, there was overlap in the distribution. But in rats older than 11 days there was a clear tendency for the L4 root to avoid the muscle's posterior margin and for the L5 to avoid the anterior portion of the muscle innervated by the inferior gluteal nerve. This segregation was less sharp in 0-2-day-old rats. Hence synapse elimination does not occur randomly, but by a process which favours particular root distributions.