Induction of long-term plasticity in human swallowing motor cortex following repetitive cortical stimulation

Objective: The excitability of corticobulbar projections to swallowing musculature undergoes remarkable long-term increases after short periods of pharyngeal stimulation. The aim of this study was to investigate the excitability of swallowing motor cortex following repetitive transcranial magnetic stimulation (rTMS). Methods: Twelve healthy subjects were given 100 rTMS pulses over motor cortex at frequencies of 1, 5 and 10 Hz at an intensity of 80% threshold for corticobulbar activation. The excitability of the corticobulbar projection was assessed before and after rTMS and compared both to sham stimulation and to the corticospinal projection. Results: Stimulation at 5 Hz, but not 1 Hz, 10 Hz or sham stimulation increased the excitability of the corticobulbar projection to the pharynx, reaching a peak 60 min after rTMS (Δ increase: 65%, P=0.016). Excitability in the projection from the opposite hemisphere also increased, suggesting the presence of inter-hemispheric interactions, whereas excitability in the projection to thenar muscles was unchanged. Conclusions: Corticobulbar and corticospinal projections may differ in response to rTMS, implying differences in relative thresholds of inhibitory and excitatory elements in hand versus swallowing cortex. Significance: This might be a useful approach in the motor rehabilitation of dysphagic stroke patients who have damage to sensory projections to the swallowing cortex.