A note on motor units Recall that a motor unit consists of an anterior horn cell, its axon and branches, and all the muscle fibres which it innervates. A large limb muscle like the quadriceps contains several hundred motor units, whereas small muscles like the extraocular muscles contain only a few dozen The needle electrode is placed in the muscle, and electrical activity is assessed both with the muscle in a relaxed state and with the patient voluntarily contracting the muscle. In normal muscle innervated by normal nerve, there is no electrical activity at rest. In diseases in which the normal connection between a muscle fibre and its motor axon is broken, the muscle fibre membrane will spontaneously and regularly depolarize and repolarize. The detection of fibrillation potentials is a very sensitive means of determining that there are problems in the peripheral motor system.
Normal muscle shows no electrical activity at rest.
The action potentials of denervated muscle are known as fibrillation potentials. Voluntary contraction of a muscle is accompanied by electrical activity in all the contracting muscle fibres. If the motor unit potentials are abnormal, the specific pattern allows one to judge whether the problem lies in the anterior horn cell, the motor axon, or the muscle itself. In general, in diseases of muscle fibres, motor unit potentials become small, whereas disease of the anterior horn cell or the motor axons result in enlargement of motor unit potentials.