Non-invasive magnetic stimulation of the human cerebellum facilitates cortico-bulbar projections in the swallowing motor system

Background Animal and human brain imaging studies suggest that the cerebellum plays an important role in the control of swallowing. In this study, we probed the interaction between cerebellar and pharyngeal motor cortical activity with transcranial magnetic stimulation (TMS) to determine if the cerebellum can modulate cortical swallowing motor circuitry. Methods Healthy volunteers (n = 16, eight men, mean age = 32, range 19-57 years) underwent TMS measurements of pharyngeal electromyography (EMG) recorded from a swallowed intraluminal catheter to assess cortical and cerebellar excitability. Subjects then underwent a paired pulse paradigm, where active or sham TMS conditioning pulses over the cerebellum and control sites were followed by suprathreshold TMS over the cortical pharyngeal area. Paired pulses were delivered at varying inter-stimulus intervals (ISIs) with the cortical response amplitudes being assessed. Key Results Stimulation of the cerebellum over its midline or hemispheres evoked distinct pharyngeal EMG responses. There was no difference in EMG amplitudes following cerebellar hemispheric or midline stimulation (mean 55.5 +/- 6.9 vs 42.8 +/- 5.9 mu V, P = 0.08). In contrast, after cerebellar preconditioning, the cortically evoked responses underwent maximal facilitation at ISIs of 50-200 ms (P < 0.05), an effect not seen with sham or trigeminal nerve preconditioning. Conclusions & Inferences Posterior fossa stimulation excites the cerebellum and evokes direct motor responses within the pharynx. When conditioned with TMS, the cerebellum strongly facilitates the cortical swallowing motor pathways. This finding suggests that the cerebellum exerts a modulatory effect on human swallowing and raises the possibility that excitatory neurostimulation of the cerebellum may be therapeutically useful in promoting recovery of dysphagia after neural damage.