Abnormalities of neural excitatory-inhibitory balance in the motor cortex associated with chronic smoking in male participants

Objective: Chronic smoking is associated with differences in intracortical circuits within the hand area of the primary motor cortex. However, given that smoking involves fine hand-to-mouth coordination, its potential impact on the excitability of the oral motor cortex remains unexplored. This study compared people who smoke chronically with those who do not, to examine differences in motor cortex excitability. Methods: Paired-pulse transcranial magnetic stimulation (TMS) was used to quantify intracortical circuit excitability in the hand and oral motor areas. Single-pulse TMS assessed corticospinal and corticobulbar excitability. Generalized linear mixed models (GLMMs) were applied for statistical analysis, and correlation analyses were conducted to explore associations between electrophysiological measures and smoking-related parameters. Results: Compared with people who do not smoke, people who smoke showed greater short-interval intracortical inhibition (SICI) in both the hand (5 ms) and oral (3 ms, 5 ms) motor cortices, and reduced intracortical facilitation (ICF) in the hand area (10 ms). Corticospinal excitability did not differ between groups, whereas corticobulbar excitability was lower in people who smoke at higher stimulation intensities. Conclusion: Chronic smoking is associated with widespread inhibitory plasticity in motor cortical circuits, as indicated by multi-muscle motor-evoked potential (MEP) recordings. Significance: These findings contribute to understanding of inhibitory circuit function and descending pathway excitability differences linked to chronic nicotine exposure.