The Human Brain : From Neurone to Nervous System

CLINICAL NEUROPHYSIOLOGY : ElectroMyoGraphy (EMG)


  1. Electromyography is the study of the electrical activity of skeletal muscles and is commonly used to examine the extent of denervation or re-innervation of skeletal muscle

  2. The recording of electrical signals from skeletal muscle fibres can be accomplished using hypodermic needles that have a central electrode insulated from the outside of the needle. The potentials recorded from muscle fibres in contact with the central electrode at the end of the needle are measured relative to the tissue surrounding the exterior surface of the needle.

  3. Normal muscle fibres conduct action potentials that originate from the relase of transmitter at the nerve-muscle junction; denervation causes action potentials to disappear. However the muscle fibres begin to produce small local (non-conducted) potentials known as fibrillation potentials following denervation. Fibrillation potentials are easily distinguished from action potentials because they are smaller, shorter and irregular in time and shape.

  4. When alpha-motoneurones regenerate, fibrillation potentials disappear and are replaced by conducted action potentials that often have different characteristics - they are often larger and irregular in shape.

  5. Regenerating nerve endings do not usually make contact with their original muscle fibres, but tend to innerve closely packed, localised groups of muscle fibres. Action potentials in these groups appear large because potentials arising from each muscle fibre in the group occur almost simulataneously. These large irregular potentials represent large motor units, and are characteristic of reinnervated skeletal muscle.



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