The three components of electrodiagnosis useful in evaluation of the peripheral nervous system and spinal cord include electromyography (EMG), electroneurography (nerve conduction studies), and somatosensory evoked potentials. EMG examination involves introduction of a special recording needle into a muscle belly. Electrical potentials located within a few millimeters of the needle are picked up by an electrode and are transmitted from the muscle to amplifiers that filter and display results visually for the electromyographer. Three types of spontaneous activity in electrical potentials are of the greatest relevance: positive sharp waves, fibrillation potentials, and fasciculations (fasciculation potentials on the EMG result from irregular firing of motor units). Electromyography can help assess the status of nerve fibers indirectly, but the integrity of large myelinated sensory and motor neurons can be evaluated directly by nerve conduction studies (NCS), also known as electroneurography. NCS can assess motor neurons, sensory neurons, or mixed nerve trunks. Sensory nerve conduction velocity can be studied in a manner analogous to motor conduction velocity: sensory fibers can be directly stimulated, and the evoked response can be measured at the wrist and elbow. Somatosensory evoked potentials occasionally are useful as an adjunct to EMG and NCS in the diagnosis of peripheral nervous system pathology. These tests also are useful when it is unclear whether an individual has a true radiculopathy.

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