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Effects of single intracortical microstimuli in motor cortex on activity of identified forearm motor units in
Abstract:
We examined the magnitude and extent of output effects elicited from focal cortical sites on the activity of individual motor units (MUs) by delivering single-pulse intracortical microstimuli (S-ICMS) (5-15 microA) during isometric wrist activity. Stimulation sites in the precentral gyrus (area 4) were chosen for study if stimulus-triggered averages (stimulus-TAs) of multiunit electromyograms (EMGs) revealed poststimulus facilitation (PStimF) of EMG activity in any of the coactivated wrist muscles. Single MUs were then isolated in the facilitated muscles with a remotely controlled tripolar microelectrode. MUs were identified by their signatures in their parent muscles (from MU-triggered averages of EMGs) and by their firing pattern during ramp-and-hold wrist responses. One objective was to quantify the magnitude and time course of the effects on single MUs by compiling peristimulus histograms of MU firing. The cross-correlation histograms between S-ICMS and MU action potentials showed peaks with onset latencies of 8.8 +/- 1.7 ms (mean +/- SD, n = 64) and durations of 1.8 +/- 1.2 ms (n = 104). The cumulative sums of the correlogram peaks resembled the rising phase of corticomotoneuronal excitatory postsynaptic potentials previously recorded in forelimb motoneurons. Comparison of correlogram peaks with stimulus-TAs of MU potentials suggests that the duration of PStimF of multiunit EMG can be accounted for, in approximately equal proportions, by l) the variation in firing time of single MUs (i.e., the width of the MU correlogram peaks), 2) the width of single MU potentials, and 3) the contribution of different MUs at different latencies. The sizes of the correlogram peaks relative to base line were larger than the PStimF of multiunit EMGs, and increased more rapidly with stimulus intensity, indicating appreciable cancellation in the multiunit records. A second objective was to determine whether S-ICMS affected all the MUs of a facilitated muscle, or only a particular subset. Of 104 MUs sampled in facilitated muscles, 99 (95%) were found to be individually facilitated (P less than 0.05). MU firing patterns during isometric ramp-and-hold torque responses were characterized as phasic, phasic-tonic, tonic, or decrementing; stimulation at a given cortical site was found to facilitate all four types of MUs. When more than one muscle showed PStimF from a site, MUs belonging to each of the facilitated muscles were facilitated individually by S-ICMS at that site.(ABSTRACT TRUNCATED AT 400 WORDS)
Insights
Single-pulse intracortical microstimulation (S-ICMS) effectively activates individual motor units (MUs) in the wrist muscles. This study reveals S-ICMS influences nearly all motor units, regardless of their firing pattern.
Area of Science:
- Neuroscience
- Motor Control
- Electrophysiology
Background:
- The precise control of voluntary movements relies on the coordinated activation of motor units (MUs) by the central nervous system.
- Understanding how cortical stimulation influences MU activity is crucial for developing targeted neuromodulation strategies.
Purpose of the Study:
- To investigate the effects of single-pulse intracortical microstimulation (S-ICMS) on the activity of individual MUs during isometric wrist contractions.
- To quantify the magnitude, time course, and specificity of S-ICMS-induced motor unit facilitation.
Main Methods:
- Focal cortical sites in the precentral gyrus were stimulated with S-ICMS (5-15 microA) during isometric wrist activity.
- Individual MUs in facilitated muscles were isolated and recorded using a microelectrode.
- Peristimulus histograms and cross-correlation histograms were used to analyze MU firing patterns and responses to S-ICMS.
Main Results:
- S-ICMS elicited facilitation in individual MUs with onset latencies of approximately 8.8 ms and durations of 1.8 ms.
- The duration of poststimulus facilitation (PStimF) in multiunit EMG was influenced by MU firing variability, MU potential width, and recruitment latency.
- Nearly all (95%) sampled MUs were individually facilitated by S-ICMS, irrespective of their firing pattern (phasic, tonic, etc.).
- S-ICMS facilitated MUs in all coactivated wrist muscles from a single cortical site.
Conclusions:
- S-ICMS is a potent tool for activating individual MUs in the motor cortex.
- Cortical stimulation affects a broad population of motor units, suggesting widespread cortical influence on motoneuron pools.
- These findings provide insights into the neural mechanisms underlying motor control and potential applications for neurorehabilitation.