Most neural growth occurs in the axon, the signal transmitting part of the neuron. During fetal development, axons need to navigate without the benefit of GPS, mostly because the average uterus only gets one bar of service. (Or serviced by one bar. Whichever.) Luckily, axons are tiny frat boys: they can be attracted or repelled by chemicals called chemotrophic factors, eventually stumbling their way home to pass out on their molecular sofas. Once in place, the axons are surrounded by support cells called glia and the whole thing is shipped out without any quality control whatsoever.
This sort of precise neural direction is required during development. Screw up any step and you'll end up with some variety of catastrophic neurological failure ranging from inability to feel painto being born with only one eye There are billions of neurons and they all need to get to where they're going and goddamn stay put. And this is where the glia work their dark magic.
Those glia are the In a healthy person,that keeps the axon from wandering. But after a spinal cord injury, the growing axon is doing the equivalent of backing down the off ramp because it took the wrong exit. The glia stop all growth while the axon frantically tries to explain that it's late for a synapse in the knee.