David S. Campion, M.D.

Denervation

As this term indicates part or all of a muscle has lost it’s nerve supply. In fact only in severe trauma does total denervation occur, and the more common scenario is the loss of a small percentage of the hundreds of nerve fibers supplying a muscle. (see MOTOR UNITS). This occurs for example when a bulging disc presses on the nerve root – radiculopathy – causing pain from the compression of the sensory fibers and weakness-sometimes imperceptible to the patient – in the limb. Unlike the damage to the nerve sheath, demyelination, (see DAMAGED NERVE) when there is a conduction block, here we are considering the more severe problems that follow when the central core of the nerve – the axon – is damaged. To understand denervation we need to review what happens during development of the nerve/muscle system. Denervation is a return to the embryonic state.

The muscles and nerves develop independently, but as the limb grows, nerves stretch out from the spinal cord and finally reach the muscle. Before the nerve arrives the muscle fiber, which has an electrical potential between the inside and outside of about one tenth of a volt, has been sensitive along its whole length to the neuromuscular transmitter acetylcholine (AcCh). The AcCh causes a voltage drop – depolarization – of the muscle membrane and a very small electrical signal, randomly generated, moves along the muscle fiber. As development proceeds and the nerve reaches the muscle- only one nerve branch innervates each muscle fiber- the sensitivity to AcCh becomes localized to the neuromuscular junction and now the only time the muscle fiber generates its signal is in response to the nerve’s command. This command comes from the motor nerve cell in the spinal cord and the transmitting of that signal down the nerve. (see EDX and NORMAL MUSCLE).

With denervation and loss of the nerve control, the muscle reverts back to the embryonic state and once again randomly generates a small electrical signal – the denervation potentials referred to as positive sharp waves and fibrillations. These are the signals shown diagrammatically. This reversion to the embryonic state takes about three weeks from the time of nerve damage so while examination before that interval may be negative, repeat examination can in fact pinpoint the time of injury.

Repair of the denervation can occur if the pressure is taken off the nerve and the nerve axons grow back into contact with the muscle. In many instances, the insulating myelin sheath has remained intact (it can stretch whereas the axon cannot) so the axons grow back down to the same muscle and function is restored. The alternative way function is restored, is for the intact remaining axons in the muscle to spread out and join up with – reinnervate – the "orphaned" muscle fibers that have been denervated. (see REINNERVATION)

FEATURED TOPICS

• Electromyogram (EMG)
• Denervation
• Reinnervation
• Myopathy

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