Principles of Nerve Repair
The peripheral nervous system bridges the mind and external world. It is largely comprised of nanoscopic biologic wires called axons that relay movement commands from the brain and sensory information from the periphery in the form of electrical signals. Axons are extensions of nerve cells (‘neurons’) and are bundled together by the thousands into cables known as peripheral nerves that measure a few millimeters in width and are surgically accessible.
1) Peripheral nerves can regenerate
Unlike those of the brain, axons of peripheral neurons can regenerate after injury and form meaningful connections with distant muscles and sensory organs such as the skin, lip vermillion, and surface of the eye. This regenerative capacity of peripheral neurons is the basis for surgical intervention to restore lost movement or sensation. Given an appropriate environment, axons regenerate at a rate of approximately 1-2 mm/dy.
2) Nerves require support and guidance to regenerate
Axons of peripheral nerves require supportive Schwann cells to survive and regenerate. Schwann cells form a continuous wrap around each axon supplying nutrients and insulation enabling propagation of axonal signals to effect movement or relay sensory information. Schwann cells reside within microscopic conduits in peripheral nerves, guiding regenerating axons along the length of the nerve towards a distal target tissue. Missing nerve tissue can be reconstructed using grafts that provide this essential guidance.
3) Peripheral nerves are partly interchangeable and redundant
Sensory and motor nerves can be surgically rerouted so that a nerve that once served one area can instead provide feeling or movement to another. This partial interchangeability is the basis for nerve transfer procedures that restore critical sensation or motion. For example, in facial paralysis, part of the nerve that normally moves the tongue on the affected side can be connected to the paralyzed facial nerve to help restore facial tone and movement. Because many body areas and muscles have overlapping nerve supply, these transfers usually cause little noticeable loss of function at the “donor” site. A useful way to think about nerve transfers is as “robbing Peter to pay Paul,” except that, in most cases, Peter hardly notices.
4) “Time is muscle”
After losing nerve input, muscles have a limited window of receptivity to neurotization. Because axons grow slowly, timing of intervention is critical. There is no general time limit for when sensation can be surgically restored to a specific body area.