D5 Electromyography (EMG) and nerve conduction velocity

Topic

The motor unit—comprising a motor neuron and all the muscle fibers it innervates—is the entity directly evaluated by electromyography and nerve conduction studies to determine the functional integrity of the peripheral nervous system and the neuromuscular junction. These techniques extend the models of axonal propagation and motor unit loss—already utilized in other models within this catalog—into the diagnostic realm.

The compound muscle action potential results from the synchronous summation of action potentials from individual motor units activated simultaneously. The amplitude of this compound muscle action potential is proportional to the number of functional motor axons: the more functional motor axons contributing to the synchronous summation, the greater the amplitude of the compound muscle action potential.

The nerve conduction velocity of functional motor axons is calculated directly from the latency difference between two distinct stimulation points along the peripheral nerve; this is a direct clinical application of the propagation velocity concept previously used to describe axonal conduction in another model. The greater the latency difference between these two stimulation points for a given distance, the lower the calculated nerve conduction velocity.

In addition to nerve conduction velocity between two peripheral points, the F-wave specifically assesses the proximal conduction of functional motor axons toward the spinal cord. When comparing the compound muscle action potential amplitude obtained via distal stimulation with that obtained via proximal stimulation, the drop in amplitude between the two measurements quantifies the degree of conduction block in the functional motor axons, applying the same conduction block criteria previously used to describe multiple sclerosis. In needle electromyography, two findings provide direct information on the degree of motor unit denervation: the motor unit recruitment pattern—which shows how many motor units are progressively activated as contraction force increases—and the presence of pathological spontaneous activity known as fibrillations, which occurs when a muscle fiber completely loses innervation from its motor neuron. The greater the degree of motor unit denervation, the more altered the recruitment pattern becomes and the more fibrillations appear, directly reflecting the progressive motor neuron death described in the amyotrophic lateral sclerosis model.

ID:4301