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Anatomy
The human nervous system consists of four parts: the brain, spinal cord, autonomic nervous system, and peripheral nerves. Peripheral nerves are cord-like structures containing bundles of nerve fibers that carry information from regions of the body to the spinal cord (and vice versa). Constructed in a fashion similar to telephone cables, peripheral nerves have a tough outer layer of connective tissue that surrounds discrete groups of miniscule fibers, called “axons,” each originating from its own nerve cell.
Peripheral Nerve Function
The primary function of a peripheral nerve is to transmit signals from the spinal cord to the rest of the body, or to transmit sensory information from the rest of the body to the spinal cord. For example, to flex a muscle, the brain sends a signal to the spinal cord that is received by a motor neuron in the spinal cord. This neuron sends the signal down an axon, which may be up to 3 feet long. This axon terminates in a muscle, where the impulse to flex is received. Once the muscle receives this signal, it flexes. The whole process requires only a fraction of a second to occur. Some nerves in the body, such as the sciatic nerve, are quite large and can be over 1 centimeter in diameter. Other peripheral nerves are so small that they cannot be readily seen without magnification.
Peripheral Nerve Injury And Healing
The central nervous system includes the brain and spinal cord. Injuries in these locations have only a very limited capacity to heal, because nerve regeneration tends not to occur. In contrast, peripheral nerves have a striking capacity for regeneration. Even completely severed peripheral nerves, if repaired in a timely fashion, can regrow, allowing the patients to enjoy complete, or nearly complete recovery in many cases.
When a peripheral nerve is cut, the axon segments distal to the injury (furthest away from the spinal cord) die off in a process called “Wallerian degeneration.” When the nerve is repaired, the axons in the proximal segment (closest to the spinal cord) regrow into the distal, denervated segment. This growth occurs at a rate of about 1 mm per day. This translates roughly to 1 inch per month, or 1 foot per year. Once the axons regrow back into the denervated muscles, the muscles will begin to function again. During the time it takes for the axons to regrow into the muscles, a process that can take many months, or even years, the muscles will be paralyzed and will atrophy.
This is an important point when dealing with peripheral nerve injuries. The healing process almost invariably requires an extensive amount of time to occur. It is important for patients not to lose hope during this time. It is vital that they participate in their exercises, keeping the affected muscles and joints flexible and ready to be used once again when the axons regrow into them. It is not unusual for patients to undergo a lengthy, complex, peripheral nerve reconstruction procedure, only to see no evidence of recovery for a year or more. This can be immensely frustrating for the patient. Unfortunately, there is nothing in medical science currently that can make these axons grow any faster. Perhaps it is best to think of this delay all as part of the healing process, paving the road to getting better!
