An injured nerve is able to recover and reconnect to its target tissue as long as a couple of conditions exist.  The first condition requires that the outside or conduit of the nerve remains intact creating a tube-like pathway.  This allows the tip of the regenerating nerve fibers to enter the tube and travel to the tissue which it originally connected to.  When a nerve fiber makes its way to the original tissue, the likelihood for a return of function is greater.  If the physical nerve tube is interrupted due to degeneration or scarring, nerve fibers may be unable to reach their target tissue leading to poor recovery. 

Nerve fibers that connect with the wrong tissue can result in a complex set of symptoms. 

Nerve fibers that are unable to cross a region may become intertwined in a ball of nerve tissue referred to as neuroma.  Neuromas can be very painful and are often characterized by pain or numbness.

The second condition that is required for nerve axons to regrow is that the entry level of the nerve tube or conduit be sufficient in size and be open to allow the growing nerve fiber to enter and reach to its target tissue.  The conduit can become filled with scar tissue, which can be relatively impenetrable for a nerve ending attempting to regrow to its target tissue.  The size of the tube can shrink with time.

How Long Does it Take for a Nerve to Heal?

Nerve fiber (axon) re-growth is generally slow and occurs at a rate of about 1-2 mm per day in an ideal setting.  Axons regeneration from the level of the spine to a peripheral muscle in the arm or leg could take many months. Additionally different types of nerves grow at slower rates and thus recovery would be slower.