title: Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle.
abstract: Pain 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 ± 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 ± 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.
authors.affiliation: Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark;
authors.forename: Jakob L
authors.initials: JL
authors.lastname: Dideriksen
authors.affiliation: Faculty of Electrical Engineering and Computer Science, University of Maribor, Maribor, Slovenia; and.
authors.forename: Ales
authors.initials: A
authors.lastname: Holobar
authors.affiliation: School of Sport, Exercise and Rehabilitation Sciences, College of Life and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom d.falla@bham.ac.uk.
authors.forename: Deborah
authors.initials: D
authors.lastname: Falla
mesh_terms: D000328:Adult; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D063806:Myalgia / Q000139:chemically induced / Q000473:pathology*; D010147:Pain Measurement; D012462:Saline Solution, Hypertonic / Q000633:toxicity; D018709:Statistics, Nonparametric; D064171:Superficial Back Muscles / Q000473:pathology / Q000503:physiopathology*; D013997:Time Factors; D055815:Young Adult
chemical_list: D012462:Saline Solution, Hypertonic
keywords: coherence; experimental pain; high-density EMG; motor units; upper trapezius