Neuromuscular transmission occurs when a quantum of acetylcholine (Ach) from the nerve ending is released and binds to the nicotinic Ach receptors on the postjunctional muscle membrane. The nicotinic Ach receptors on the end plate respond by opening channels to the influx of sodium ions, and subsequent end-plate depolarization leads to muscle contraction. The Ach immediately detaches from the receptor and is hydrolyzed by the enzyme acetylcholinesterase in the synaptic cleft.

A reliable way of detecting blocking (see definition below) at the neuromuscular junctions in many muscle fibers is by recording, via surface electrodes over the end-plate zone of the entire muscle, while repetitively stimulating the appropriate nerve innervating the muscle. In a healthy muscle, each successive muscle response is the same. In myasthenia gravis (MG), at rest, the size of the initial compound muscle action potential (CMAP) recorded is normal or near normal.

With stimulation at a low frequency of 3–5 Hz, the first two to five stimuli show a decremental response over the weak muscle. This initial decrement is due to progressive blocking at the neuromuscular junction of hundreds of muscle fibers as Ach is depleted, causing an increasing number of end-plate potentials to become subthreshold. This initial fall in the CMAP then levels off, or even slowly increases, during subsequent stimuli. This increase is usually not more than 10–20 %. A reproducible CMAP decrement of 10 % or more between the first and the smallest of the first five responses is considered abnormal. This decrement in the amplitude is considered more characteristic of myasthenic muscles, but a progressive increase in latency may also occur in some of these muscles. To avoid false-negative results in patients with selective involvement of muscles, it is necessary to sample muscles both proximally and distally. Positive tests in the proximal or facial muscles are not uncommon, even when the distal limb muscles reveal no abnormalities.

Rapid neuromuscular stimulation can be achieved by stimulating the nerve at rates between 20 and 40 Hz (painful). The same effect can also be achieved by voluntary exercise of the muscle for several seconds (less painful). Immediately after such an exercise, the size and area of the evoked response may be larger, and the decrement is less. This is called post-activation potentiation (or post-activation facilitation). By assessing the area under the evoked response before and after exercise, post-activation potentiation can be differentiated from “pseudofacilitation.” The amplitude of the response is greater after exercise because of the increased synchronization of the components, but the area of the response remains the same.

A few minutes after exercise, the size of the evoked response in the myasthenic muscle may be smaller, and the decrement is greater than at rest. This is called post-activation decrement. This post-activation decrement is helpful in the diagnosis of mildly involved myasthenic muscles, as it may be the only abnormality to occur during repetitive testing. In Lambert-Eaton myasthenic syndromes (pre-synaptic transmission defect), the initial CMAP is low in amplitude, unlike the case in MG, where it is in the normal range. At a low rate of stimulation, a further decrement in amplitude.

In myasthenic syndromes (pre-synaptic transmission defect), the initial CMAP is usually low in amplitude, unlike the case in MG, where it is usually in the normal range. At a low rate of stimulation, a further decrement in amplitude occurs, resembling that in MG. At high rates of stimulation of over 10 Hz, a marked increase in amplitude occurs, and the amplitude can increase to more than 200 % of the initial value.

A symptomatic and clinically weak muscle is selected. This could be a facial, proximal, or distal muscle. The electrodes are placed as for a motor nerve conduction study. Movement of the body part is controlled.

NEE may show variation in the shape and amplitude of the action potential of a single motor unit during a weak voluntary contraction. This variation is a clue to the presence of a neuromuscular transmission defect.