This is really the million dollar question for human kind, but in the case of nerves, I believe we have some ideas. I was recently asked why nerves are buried into muscle and what happens to them once they are in that location. Conventional wisdom states that by burying a transected nerve end into muscle, a neuroma will not form. This concept is based in large part on a paper written in 1985 which demonstrated in a non-human primate model that when a nerve is buried into muscle, the structure of the regenerating nerve fibers is different than a severed nerve left in the subcutaneous tissues. It is thought that this structural difference accounts for the relative paucity of symptoms post-operatively when a nerve is buried into muscle. However, as noted by some people, muscle burial isn’t always effective. Why is that? One possibility, of course is that the original theory is incorrect. Another is that the buried nerve has come out of the muscle which is why it is important to bury a good length of nerve into the muscle to minimize the chance of this occurrence. Another is that the amount of muscle covering the nerve is small and there is therefore still pain as a result of cutaneous pressure over the region.
A corollary to these questions is, “Why does it take a long period of time for the buried nerves to stop causing pain?” The simple answer to this question is that no one really knows however there are several plausible explanations. Keep in mind that a transected nerve is still attached to the spinal cord and the brain (we don’t rip them out of there) and therefore nerve impulses from the brain through the spinal cord to the nerve end continue to be generated. One reason that the patient might have discomfort following implantation is that the muscle into which the nerve is buried is still functioning which could irritate the nerve end thus sending messages back to the brain telling the patient that they have nerve irritation and hence pain. The original theory noted above would also presumably postulate that once the buried nerve has had a chance to regenerate in its non-neuromatous manner, the nerve would “calm down” and the pain would eventually decrease significantly or go away altogether, but this process doesn’t happen overnight. I personally believe that there is another potential explanation which comes from our recent experience with targeted re-innervation patients.
Re-innervation surgery literally involves rewiring the body’s peripheral nervous system, usually in an extremity, to alleviate the pain often caused by neuromas that form at an amputation stump. There are other goals of re-innervation, of course, but this goal is a primary focus. Among the steps performed in such operations is neuroma excision and coaptation (i.e. connection) of a sensory nerve to a motor nerve end within a muscle which is no longer relevant because of the amputation. An example would be a functional gastrocnemius (calf) muscle in a person who has a below-knee amputation. The calf muscle can still fire, but its purpose is to plantar flex the foot at the ankle (i.e. “step on the gas”); but the foot no longer exists, thereby making this muscle function irrelevant. My experience and the limited literature on this topic suggest that when a sensory nerve is connected to a motor nerve heading into a muscle (after removal of the injured portion, i.e. neuroma) the pain relief can be dramatic. While the muscle may not function as well (it is an irrelevant muscle as noted above), the signals from the sensory nerve attached to the motor nerve don’t match up; however a neuroma doesn’t form because the regenerating sensory nerve fibers have been given something to do, namely hook up with the motor nerve fibers downstream. Therefore, my feeling is that by burying a nerve end into muscle a neuroma doesn’t form because the nerve ends eventually make connections with motor nerves heading into that muscle. As above, however, such connections can take time to form which is why the patient may experience discomfort for several weeks-to-months after burial of a sensory nerve into a muscle.