Electromyography of the muscle spindle

Juhani V Partanen,Jukka Vanhanen,Sara K Liljander

doi:10.1038/s41598-022-08239-4

Abstract

In needle electromyography, there are two spontaneous waveforms, miniature end plate potentials and “end plate spikes”, appearing usually together. Miniature end plate potentials are local, non-propagating postsynaptic waves, caused by spontaneous exocytosis of acetylcholine in the neuromuscular junction. The prevailing hypothesis states that “end plate spikes” are propagated postsynaptic action potentials of muscle fibers, caused by presynaptic irritation of the motor nerve or nerve terminal. Using several small concentric needle electrodes in parallel with the muscle fibers, most “end plate spikes” are strictly local or propagating for 2–4 mm. At the end plate zone, there are miniature end plate potentials without “end plate spikes”. Local “end plate spikes” are junctional potentials of intrafusal gamma neuromuscular junctions of the nuclear bag fibers, and propagated “end plate spikes” are potentials of nuclear chain muscle fibers of muscle spindles. Miniature end plate potentials without “end plate spikes” at the end plate zone derive from alpha neuromuscular junctions. These findings contrast with the prevailing hypothesis. The history of observations and different hypotheses of the origin of end plate spikes are described.

Highlights

In needle electromyography, there are two spontaneous waveforms, miniature end plate potentials and “end plate spikes”, appearing usually together
A quiescent active site may at first show only miniature end plate potentials, while end plate spikes are activated after a long interval of up to 250 ms
It may be conjectured that the needle electrode stimulates nerve terminals directly, causing a release of acetylcholine vesicles and generating an end plate potential, which may reach the threshold for an action potential in the postsynaptic muscle fiber[6] (Fig. 3)

Summary

Introduction

There are two spontaneous waveforms, miniature end plate potentials and “end plate spikes”, appearing usually together. The nature of firing, activations, and waveforms was very accurately described[3] This “distinctive type of electric activity” was studied experimentally and found that it has two components, first a lowamplitude component characterized as “the sound of a seashell”, and a second component, which consists of a variable number of spike potentials, usually with an irregular rhythm[4]. This activity was observed in 5 to 10% of routine needle insertions in a normal muscle[4,5] Another hypothesis stated that these spike potentials, “spontaneous diphasic potentials”, originated in the muscle fibers when several synchronous miniature end plate potentials attained an amplitude sufficient to elicit a propagated response[6]. Motor junctional or action potentials are observed in muscle spindles[11] These potentials may be seen as local and propagated end plate spikes in needle EMG10. The different hypotheses of the origin of end plate spikes are described

Methods

Results

Conclusion