Anticipatory changes in human motoneuron discharge patterns during motor preparation

Abstract

The influence of motor preparation on human motoneuron activity was studied by combining single motor unit recording techniques with reaction-time (RT) methods. The tonic activity of wrist extensor motor units associated with voluntary isometric contractions was analysed during preparation for a ballistic wrist extensor muscle contraction, using a time preparation procedure. Two durations of the preparatory period elapsing between the warning signal and the response signal were used in separate blocks of trials: a short preparatory period (1 s) allowing optimum time preparation, and a longer, non-optimum one (3 s). Changes in motoneuron tonic discharge patterns not associated with any changes in the force output were observed during the preparatory period, which suggests that these changes were subtle enough to prevent any changes in muscle contraction from occurring before the forthcoming movement. The changes observed were a lengthening of the mean interspike interval (ISI) and a decrease in the ISI variability. These data confirm that inhibitory mechanisms are activated during motor preparation and suggest that spinal inhibitory mechanisms are involved in the preparatory processes. The mechanisms possibly involved, such as presynaptic inhibition, dysfacilitation processes or AHP conductance changes, are discussed. The fact that the preparation-induced effects on motoneuron activity were particularly prominent during the last part of the 3 s preparatory period suggests that they were probably related to the neural processes underlying temporal estimation. The anticipatory changes in motoneuron activity observed here during preparation for action provide evidence that central influences act on spinal motoneurons well before it is time to act.

Response specific temporal expectancy: Evidence from a variable foreperiod paradigm

“…In a TMS study with a variable-FP design, Van Elswijk, Kleine, Overeem, and Stegeman (2007) found that corticospinal excitability increased with temporal expectancy, and they speculated that expectation synchronizes the spinal motor signal (see also Duque & Ivry, 2009;Duque, Lew, Mazzocchio, Olivier, & Ivry, 2010). However, Duclos, Schmied, Burle, Burnet, and Rossi-Durand (2008b) found that in a constant-FP design, expectancy was associated with inhibition in spinal motor neurons (see also Duclos, Burnet, Schmied, & Rossi-Durand, 2008a; but see also Sinclair & Hammond, 2009, for an alternative interpretation of these findings).…”

Modulation of human motoneuron activity by a mental arithmetic task

“…It is hypothesized that a concurrent mental task may likely interact with the motor control of wrist muscles taking into account their involvement in the fine motor skill. In support of this assumption, it has been shown that wrist extensor motor unit activity is affected by a cognitive task related to the instruction to get ready to act during motor preparation (Duclos, Schmied, Burle, Burnet, & Rossi-Durand, 2008).…”

Approximate entropy of motoneuron firing patterns during a motor preparation task

“…This cannot be done with the more classical Friedman test, which does not take period order into account. The hypothesis that ApEn values decreased monotonically from the control to the end of the motor preparation task was selected since Duclos et al (2008) established that the variability of the motor unit discharge pattern, as assessed from CV values, decreased during the preparation for motor action. Page's test was conducted as follows.…”

“…The experimental set-up, data recordings and equipment used here have been previously described in detail (Duclos et al, 2008). …”

“…Recently, Duclos et al (2008) established that the variability of the motor unit discharge pattern, as assessed from the coefficient of variation (CV), decreased during the preparation for motor action. The aim of the present study was to establish whether the ApEn approach can be used to detect motor preparation-induced changes in the regularity of motor unit discharge pattern.…”