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When motor neurons die, other cells can grow extensions, or axons, to compensate for the lost nerve cell. Called reinnervation, this process often occurs as the human body ages, because of neurological diseases, or after an injury. The reinnervation process is usually gradual, and is triggered when one or a few nerve cells die. There are also surgical techniques for artificial reinnervation that can reconnect nerves or graft them to restore lost body functions. Nerves can be transplanted for use with prosthetic limbs, and to restore function to parts of the body such as the vocal cords or bladder.
Peripheral nerves can regenerate by reinnervation. Various reactions typically occur in the cell and on a molecular scale. The cellular cytoskeleton often breaks down before parts of the cell, called organelles, flow to the axons to build new fragments. Calcium flowing through a cell typically helps to break down cellular material before it regenerates, and then white blood cells clean up cellular pieces left behind. In cases when multiple nerve cells are affected, columns of cells can form and direct axons to grow toward a new area.
Reinnervation usually happens over time after an injury. Within muscles, however, the process many times does not result in fully restored muscle function. Medical treatments can include drugs to trigger nerve growth or delicate surgery to graft whole nerves.
Artificial reinnervation can be a challenging operation, but is often successful. Many times there is little risk with the operation; however, recovery can extend for a long period of time. When a limb is amputated or lost to disease or injury, the remaining nerves can be removed from the stump and moved to the chest muscles. Electrodes attached to the area can be used to trigger signals that control a prosthetic hand or arm. A patient who has had this surgery might feel their hand when a chest muscle is touched, but he or she can move the prosthetic just by thinking about flexing the hand.
Often using the body’s ability to regenerate nerve cells, reinnervation by surgery sometimes involves rerouting nerve fibers and connecting them to others. The function of prosthetics is often improved this way. Nerve roots in the spinal cord can also be grafted and surgically crossed over. While the body functions that can be restored this way are limited, reinnervation of bladder nerves has sometimes been successful. Some patients in surgical studies regained bladder function after several months of recovery.