D3 Motor Neurophysiology

Topic

Motor neurophysiology measures and analyzes the electrical activity generated by motor neurons and muscle fibers during movement planning and execution. The electrical signal recorded on the skin overlying the muscle represents the summation of individual electrical potentials from all motor units active beneath the electrode at that moment—a signal that is amplified to the millivolt range for recording purposes. The mean amplitude of this signal—calculated in a way that prevents positive and negative values ​​from canceling each other out—is proportional to the muscle's activation level and, under controlled conditions, to the force the muscle is generating.

This same electrical signal recorded from the muscle can also be analyzed based on its constituent frequencies: the frequency dividing the signal's power spectrum into two equal energy halves progressively decreases as muscle fatigue sets in, because the accumulation of metabolites within the fiber slows electrical conduction along the muscle membrane. By tracking this frequency over time—analyzing it window by window—one can observe how the signal's frequency content evolves as the muscle fatigues.

The activity of the motor neurons generating the muscle's electrical signal can also be studied "upstream," at the nerve itself: the nerve is electrically stimulated at a specific point, and the resulting compound muscle response is recorded in the muscle innervated by that nerve. Nerve conduction velocity is calculated by dividing the distance between two stimulation points by the difference in latency times recorded at each point: the greater the distance covered for a given latency difference, the higher the conduction velocity; conversely, the greater the latency difference for a given distance, the lower the velocity. The individual electrical potentials of each motor unit—which appear intermingled in the surface signal—can be recorded separately by inserting a needle electrode directly into the muscle; the amplitude, duration, and shape of each individual potential allow for the distinction between two types of disease. When the nerve innervating the motor unit is damaged, the recorded potential exhibits increased amplitude and prolonged duration; conversely, when the muscle fiber itself is diseased, the potential shows reduced amplitude and short duration.

In a healthy state, the electrical activity recorded by motor neurophysiology during movement follows an orderly pattern: motor units are activated sequentially, starting with the smallest and progressively recruiting larger units as the muscle needs to generate more force, with firing rates ranging from approximately eight to thirty cycles per second. This orderly activation is accompanied by synchronization of electrical activity within an intermediate frequency band—roughly fifteen to thirty-five cycles per second—reflecting the coupling between the cerebral cortex and the spinal cord and associated with the fine control of the force ultimately produced by the muscle.

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