Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle

Abstract

Dideriksen JL, Holobar A, Falla D. Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle. J Neurophysiol 116: 611-618, 2016. First published May 25, 2016 doi:10.1152/jn.01117.2015Pain is associated with changes in the neural drive to muscles. For the upper trapezius muscle, surface electromyography (EMG) recordings have indicated that acute noxious stimulation in either the cranial or the caudal region of the muscle leads to a relative decrease in muscle activity in the cranial region. It is, however, not known if this adaption reflects different recruitment thresholds of the upper trapezius motor units in the cranial and caudal region or a nonuniform nociceptive input to the motor units of both regions. This study investigated these potential mechanisms by direct motor unit identification. Motor unit activity was investigated with high-density surface EMG signals recorded from the upper trapezius muscle of 12 healthy volunteers during baseline, control (intramuscular injection of isotonic saline), and painful (hypertonic saline) conditions. The EMG was decomposed into individual motor unit spike trains. Motor unit discharge rates decreased significantly from control to pain conditions by 4.0 Ϯ 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 Ϯ 2.8 pps; not significant). These changes were compatible with variations in the synaptic input to the motoneurons of the two regions. These adjustments were observed, irrespective of the location of noxious stimulation. These results strongly indicate that the nociceptive synaptic input is distributed in a nonuniform way across regions of the upper trapezius muscle.

Location-specific responses to nociceptive input support the purposeful nature of motor adaptation to pain

“…Interestingly, similar changes in the amplitude distribution within the VM were observed when VL was injected with hypertonic saline, although to a lesser extent and less consistently across participants. This stereotypical redistribution of activation appears to be, in part, consistent with that observed when noxious stimuli were applied to different regions of the upper trapezius [6,7]. The absence of changes in spatial distribution when VMP was injected may be related to the different function of the distal (patellar stabilization) vs. proximal (knee extension) regions of the vasti, although further interpretation of this proposal is beyond the scope of the present investigation.…”

Quantitative Response of Healthy Muscle Following the Induction of Capsaicin: An Exploratory Randomized Controlled Trial

“…From this hypothesis, it would be expected to observe uncoordinated motor unit activity and activation given that muscle ber damage has occurred and motor units may be differentially potentiated or sensitized. The ndings from studies inducing acute nociception in the muscle support this hypothesis [22,45].…”

“…The Delsys Trigno system, which includes sensors, the Trigno base station, and the EMGworks software will be used to decompose the acquired EMG signals into individual motor units. The following parameters will be analyzed: the ring times and frequency of individual motor units, motor unit action potential amplitude and shapes [40][41][42], the root mean square value (RMS) of each channel and of the entire signal [22], the coe cient of variation for force steadiness [43], and the centroid of the EMG signal in the cranial-caudal and the medial-lateral directions [22]. This will be repeated for each contraction.…”

“…Many other factors are likely involved in the chronic pain patient including disordered ascending and descending signals within the central nervous system, termed nocioplastic pain by the International Association for the Study of Pain (IASP) [19][20][21]. Dideriksen et al [22] and Falla et al [23] demonstrated that motor unit potentials reduce in amplitude following the injection of nociceptive hypertonic saline into the upper trapezius muscle. This is in line with the EMG characteristics of MTrPs cited earlier [10,11].…”

Effect of standing posture on inhibitory postsynaptic potentials in gastrocnemius motoneurons

“…MU firing rate (Holobar et al 2010) and reflex inhibition/facilitation (Yavuz et al 2015) estimated with this method were shown to be valid when compared to gold-standard intramuscular recordings. To identify the MUs that were active in both conditions, epochs from the recordings during each condition were spliced together and decomposed as a single recording and a procedure based on the spatial representation of the MU action potential (Dideriksen et al 2016) was used to verify the correct matching of MUs. With spike-triggered averaging, the spatial representation of the action potential of each MU was obtained for Free Standing and Lying Supine conditions separately.…”