Abstract

Chronic smoking has been shown to have deleterious effects on brain function and is an important risk factor for ischemic stroke. Reduced cortical excitability has been shown among chronic smokers compared with non-smokers to have a long-term effect and so far no study has assessed the effect of smoking on short-term motor learning. Paired associative stimulation (PAS) is a commonly used method for inducing changes in excitability of the motor cortex (M1) in a way that simulates short-term motor learning. This study employed PAS to investigate the effect of chronic cigarette smoking on plasticity of M1. Stimulator output required to elicit a motor-evoked potential (MEP) of approximately 1 mV was similar between the groups prior to PAS. MEP response to single pulse stimuli increased in the control group and remained above baseline level for at least 30 min after the intervention, but not in the smokers who showed no significant increase in MEP size. The silent period was similar between groups at all time points of the experiment. This study suggests that chronic smoking may have a negative effect on the response to PAS and infers that chronic smoking may have a deleterious effect on the adaptability of M1.

Highlights

  • The human sensorimotor cortex is described as being “plastic” which refers to its ability to adapt by reorganizing neural pathways in response to changes or injuries

  • The present study found that the stimulus intensity required to elicit an motor-evoked potential (MEP) of 1 mV was similar between the groups, and in line with the study by Lang et al [22]

  • The level of magnetic stimulator output (%MSO) required for eliciting a 1 mV MEP response, slightly higher for the smokers, was statistically similar between the groups. This demonstrates comparable baseline excitability between the experimental and control groups while the muscle is performing a light isometric contraction, previous research has found a reduction in cortical excitability at rest in smokers compared with non-smokers [22]

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Summary

Introduction

The human sensorimotor cortex is described as being “plastic” which refers to its ability to adapt by reorganizing neural pathways in response to changes or injuries. Reorganization within the motor cortex can be stimulated by repetition of a simple task [1] and skill acquisition [2]. Excitability of the horizontal connections within the motor cortex can be increased, known as long-term potentiation (LTP) [3]. LTP refers to an enhancement of the amplitude of synaptic potentials, which can last for an extended period following repetitive activation of a specific neural circuit [4,5,6]. Long-term depression (LTD) describes the opposite effect, reducing the size of synaptic potentials [7]. Transcranial magnetic stimulation (TMS) activates intracortical fibres, which are oriented parallel to the scalp [8]

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