Induction of cortical swallowing activity by transcranial magnetic stimulation in the anaesthetized cat

Transcranial magnetic stimulation (TMS) over human fronto-central areas of scalp can activate short latency responses in the muscles of the face, pharynx and oesophagus. However, the physiological relationship between this early activity and the swallowing activity programmed by the brainstem central pattern generator (CPG) remains unclear. The aim of this study was to explore the relationship between TMS-induced early muscle and late swallowing activities in the feline model. Twelve adult cats were studied under light anaesthesia. Mylohyoid and oesophageal EMG, together with pharyngeal, upper oesophageal sphincter (UOS) and upper oesophageal manometry, were recorded to single-pulse TMS of cat cortex. TMS at low stimulation intensities evoked consistent short latency EMG responses in the mylohyoid and oesophageal muscles (6.1 +/- 1.2 ms and 12.7 +/- 0.7 ms, respectively), and early contractile activity in the UOS (latency 31.8 +/- 3.6 ms). By contrast, TMS at high intensities induced swallowing activity as indicated by mylohyoid EMG, and UOS relaxation (latencies 1.1 +/- 0.4 s and 0.8 +/- 0.1 s, respectively). Both the early muscle and late swallowing activities were intensity-dependent, increasing stimulus strength producing a reduction in latency and greater number of swallows. The characteristics of the early response suggest an oligosynaptic projection from cortex to swallowing muscles. The induction of swallows at high intensities suggests a requisite for greater recruitment of cortical motoneurones, or associated swallowing regions.