When a peripheral nerve is injured, signals to and from the brain can be disrupted and normal body functions may be lost. Peripheral nerves have the ability to regenerate and heal themselves depending on the severity of the damage. A crushed or compressed nerve may be able to heal on its own while a nerve that has been completely transected will need surgical intervention to help in reconnecting the axons to their targets.
When a peripheral nerve is cut, it is separated into two parts, the proximal part and the distal part. The proximal side of the nerve is closest to the spinal cord and is still in communication with the central nervous system (brain and spinal cord). The distal side of the nerve is farther away from the spinal cord and has lost communication with the central nervous system. Once severed, the axons in the distal nerve begin to break down and the body sends special cells to clear the resulting cellular debris. This process is called Wallerian degeneration and leaves relatively hollow tubes in the nerve where the axons used to be.
Following the injury, specialized cells called Schwann cells proliferate from both the proximal and distal nerve stumps to support regeneration. The proximal end of the nerve fiber begins to sprout toward the distal nerve. The nerve fiber advances at about 1mm per day and may eventually reach its target tissue where it can once again provide sensation or movement.
Surgery to repair severed nerves is done to help contain or guide the nerve sprouts to the distal end of the nerve. Without guidance, these sprouts can potentially form a neuroma or die back into the proximal nerve. You can find more information about treatment of severed nerves here.
Compressed or Crushed Nerves
When a nerve is crushed or compressed the signals that flow through the axon are disrupted and may not reach their target organ. This can lead to pain, numbness, or loss of functionality. Depending on the severity of the compressed or crushed nerve, it may be able to regain functionality without surgical intervention. The force that caused the crush or compression should be eliminated to relieve pressure and allow healing to begin. After nerves are crushed, they may become replaced with scar tissue which can hinder nerve recovery and can require surgery to remove the scarred part of the nerve. In less severe cases, where there is little or no scar formation within the nerve, the injury can potentially recover from the compression or crush and regain normal function without surgical intervention. You can find more information about treatment of crushed or compressed nerves here.