Motor Unit Discharge and EMG Observations

What can be observed from a single discharge of a motor unit in intramuscular EMG?

a) Individual muscle fiber contractions
b) Overall electrical activity
c) Nerve impulses
d) Muscle tone

Answer:

For any one discharge of a motor unit, it is not possible to observe the individual muscle fiber contractions. EMG captures the overall electrical activity, not the actions at the individual fiber level.

Explanation: For any one discharge of a motor unit, identified from the intramuscular EMG, it is not possible to observe the individual muscle fiber contractions of each fiber within that unit. EMG captures the summed electrical activity of a motor unit and thus cannot distinguish the fine details of actions at the individual fiber level.

The sequence of events that result in the contraction of an individual muscle fiber begins with a signal-the neurotransmitter, ACh-from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize, leading to the contraction. However, this process is not discernible in individual fibers during EMG.

During a motor exam, muscle tone is essential as it is an indicator of resting tension and slight contraction of fibers. Lack of muscle tone, or hypotonicity, may suggest that the lower motor neuron (LMN) isn't conducting action potentials properly. This can affect the basal level of acetylcholine in the neuromuscular junction.