If any muscle cell has no nerve input for a period of time, the muscle generates a chemical signal for help. Any intact motor nerve in the immediate area will sprout and will send a new branch to this muscle cell. This budding process takes between ten to sixteen weeks. When the muscle cell is reconnected, this muscle cell can contract.
On the EMG, this appears as a more coarse contraction. Intact axons (nerve cells) can bud up to ten times. That is, a single nerve can pick up ten times the number of muscle cells it normally services.
If the nerve is so severely damaged, there may not be enough intact functioning nerve cells immediately around the surrounding muscle. These muscle cells will fibrose (die).
Nerve budding or sprouting also occurs in the situation where the nerve can still regenerate (see Wallerian Degeneration) but regeneration takes a longer period of time than the process of budding will take.
Another recovery function has to do not with muscle re-innervation but with muscle conditioning. The remaining muscle cells that continue to function can hypertrophy to compensate for the missing cells. I call this “Arnold Schwarzenegger compensation”. This is where physical therapy can help to condition and strengthen the remaining functioning muscle cells to compensate for the missing muscle cells.
Treatment of Nerve Injuries
The problem for the spinal clinician is that there is no way to know which injury has occurred to the nerve in the case of a nerve compression syndrome (herniated disc or spur). In the presence of nerve compression with weakness due to herniated disc or bone spur, it is my opinion that the nerve needs to be surgically decompressed as time of compression most likely makes a difference in recovery.
Once the nerve is decompressed, the only way to really know about the type of injury is with “tincture of time”. Recovery will occur based upon the timelines noted above. That is, functional compression injuries will immediately recover, myelin sheath injuries will take between four to twelve weeks to recover, nerve budding will take ten to sixteen weeks and full nerve injuries with the myelin sheath intact will take as long as the length of the nerve to the muscle (up to one year). If weakness has not recovered in one year, the nerve will generally not recover and the weakness will remain.
Therapy should be instituted for work on strengthening and to compensate for any weakness that might lead to further injury (ankle weakness can lead to ankle sprain/fracture or knee weakness that could lead to a fall). Braces or walking aids such as a cane or walker might be necessary to help with function and prevent injury.
If you would like additional information on how muscles recover from nerve injuries, or would like to learn more about treatment of nerve injuries, please contact the Vail, Aspen, Denver and Grand Junction, Colorado area office of back doctor and spine specialist Dr. Donald Corenman.