title: Correlation between discharge timings of pairs of motor units reveals the presence but not the proportion of common synaptic input to motor neurons.
abstract: We investigated whether correlation measures derived from pairs of motor unit (MU) spike trains are reliable indicators of the degree of common synaptic input to motor neurons. Several 50-s isometric contractions of the biceps brachii muscle were performed at different target forces ranging from 10 to 30% of the maximal voluntary contraction relying on force feedback. Forty-eight pairs of MUs were examined at various force levels. Motor unit synchrony was assessed by cross-correlation analysis using three indexes: the output correlation as the peak of the cross-histogram (ρ) and the number of synchronous spikes per second (CIS) and per trigger (E). Individual analysis of MU pairs revealed that ρ, CIS, and E were most often positively associated with discharge rate (87, 85, and 76% of the MU pairs, respectively) and negatively with interspike interval variability (69, 65, and 62% of the MU pairs, respectively). Moreover, the behavior of synchronization indexes with discharge rate (and interspike interval variability) varied greatly among the MU pairs. These results were consistent with theoretical predictions, which showed that the output correlation between pairs of spike trains depends on the statistics of the input current and motor neuron intrinsic properties that differ for different motor neuron pairs. In conclusion, the synchronization between MU firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.NEW & NOTEWORTHYThe strength of correlation between output spike trains is only poorly associated with the degree of common input to the population of motor neurons. The synchronization between motor unit firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.
authors.affiliation: Department of Electrical and Electronic Engineering, Public University of Navarra, Pamplona, Spain; javier.rodriguez.falces@gmail.com.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodriguez-Falces
authors.affiliation: Department of Clinical and Experimental Sciences, University of Brescia, Brescia, Italy; and.
authors.forename: Francesco
authors.initials: F
authors.lastname: Negro
authors.affiliation: Department of Bioengineering, Imperial College London, London, United Kingdom.
authors.forename: Dario
authors.initials: D
authors.lastname: Farina
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000328:Adult; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D008959:Models, Neurological; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D013223:Statistics as Topic; D013569:Synapses / Q000502:physiology*; D055815:Young Adult
keywords: common synaptic input; discharge rate; interspike interval; motor unit; short-term synchronization