SENSORIMOTOR MODULATION OF HUMAN CORTICAL SWALLOWING PATHWAYS

1. Transcranial magnetic stimulation over motor areas of cerebral cort ex in man can activate short latency bilateral cortical projections to the pharynx and oesophagus. In the present paper we investigate the i nteraction between pathways from each hemisphere and explore how activ ity in these pathways is modulated by afferent feedback from the face, pharynx and oesophagus. 2. Comparison of unilateral and bilateral sti mulation (using interstimulus intervals (ISIs) of 1, 5 or 10 ms betwee n shocks) showed spatial summation of responses from each hemisphere a t an ISI of 1 ms, indicating that cortical efferents project onto a sh ared population of target neurones. Such summation was not evident at ISIs of 5 or 10 ms. There was little evidence for transcallosal inhibi tion of responses from each hemisphere, as described for limb muscles. 3. Single stimuli applied to the vagus nerve in the neck or the supra orbital nerve, which alone produce intermediate (onset 20-30 ms) and l ong (50-70 ms) latency reflex responses in the pharynx and oesophagus, were used to condition the cortical responses. Compared with rest, re sponses evoked by cortical stimulation were facilitated when they were timed to coincide with the late part of the reflex. The onset latency was reduced during both parts of the reflex response. No facilitation was observed with subthreshold reflex stimuli. 4. Single electrical s timuli applied to the pharynx or oesophagus had no effect on the respo nse to cortical stimulation. However, trains of stimuli at frequencies varying from 0.2 to 10 Hz decreased the latency of the cortically evo ked responses without consistently influencing their amplitudes. The e ffect was site specific: pharyngeal stimulation shortened both pharyng eal and oesophageal response latencies, whereas oesophageal stimulatio n shortened only the oesophageal response latencies. 5. Cortical swall owing motor pathways from each hemisphere interact and their excitabil ity is modulated in a site-specific manner by sensory input. The latte r may produce a mixture of excitation and inhibition at both brainstem and cortical levels.