Sensorimotor modulation of human cortical swallowing pathways

Abstract

Transcranial magnetic stimulation over motor areas of cerebral cortex in man can activate short latency bilateral cortical projections to the pharynx and oesophagus. In the present paper we investigate the interaction between pathways from each hemisphere and explore how activity in these pathways is modulated by afferent feedback from the face, pharynx and oesophagus. Comparison of unilateral and bilateral stimulation (using interstimulus intervals (ISIs) of 1, 5 or 10 ms between shocks) showed spatial summation of responses from each hemisphere at an ISI of 1 ms, indicating that cortical efferents project onto a shared population of target neurones. Such summation was not evident at ISIs of 5 or 10 ms. There was little evidence for transcallosal inhibition of responses from each hemisphere, as described for limb muscles. Single stimuli applied to the vagus nerve in the neck or the supraorbital nerve, which alone produce intermediate (onset 20‐30 ms) and long (50‐70 ms) latency reflex responses in the pharynx and oesophagus, were used to condition the cortical responses. Compared with rest, responses 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. Single electrical stimuli applied to the pharynx or oesophagus had no effect on the response to cortical stimulation. However, trains of stimuli at frequencies varying from 0.2 to 10 Hz decreased the latency of the cortically evoked responses without consistently influencing their amplitudes. The effect was site specific: pharyngeal stimulation shortened both pharyngeal and oesophageal response latencies, whereas oesophageal stimulation shortened only the oesophageal response latencies. Cortical swallowing motor pathways from each hemisphere interact and their excitability is modulated in a site‐specific manner by sensory input. The latter may produce a mixture of excitation and inhibition at both brainstem and cortical levels.

Differential changes in human pharyngoesophageal motor excitability induced by swallowing, pharyngeal stimulation, and anesthesia

“…However, pharyngeal stimulation, being more direct, might only activate the corticopharyngeal pathway without altering excitability in other cortical swallowing networks and hence not affect the "reflex" response. Against this notion is the fact that following volitional swallowing (and with anesthesia) there were latency shifts in both pathways, which are more typically seen when motoneurons in the bulbar nuclei and/or muscle are depolarized (7). At present, our data cannot fully resolve the issue of the nature of the reflex pathway, and consequently this remains open to further study.…”

“…These afferent fibers are not only capable of influencing brain stem motoneuron and interneuron excitability but also that of higher circuitry in the cerebral cortex (12,18,19). Indeed, previous TMS studies (6,7) have shown that excitation of (afferent) pathways in cranial nerves V and X, for example from the face or neck, produces a "reflex" response in the human pharynx and esophagus that is likely to be generated via neurons within the brain stem and possibly through the central pattern generator itself. Stimulation of these craniobulbar responses, when combined with cortical input, produce short-term (100-200 ms) facilitation of the cortically evoked pharyngeal and esophageal re-sponses, suggesting that both pathways involve similar populations of bulbar neurons.…”

Effects of Somatosensory Stimulation on Use-Dependent Plasticity in Chronic Stroke

“…9 Stimulation of whiskers in rats enhances c-fos expression, 10 which has been postulated to upregulate the expression of neurotrophins that may be involved with dendritic and axonal sprouting. SS enhances excitability of the stimulated body part representation in the motor cortex in healthy subjects 6,11 and results in improvements in motor performance in stroke patients. 11, 12 Our results demonstrate that this intervention may enhance the beneficial effects of a single motor training session on UDP in stroke patients.…”

“…SS enhances excitability of the stimulated body part representation in the motor cortex in healthy subjects 6,11 and results in improvements in motor performance in stroke patients. 11, 12 Our results demonstrate that this intervention may enhance the beneficial effects of a single motor training session on UDP in stroke patients. Combination of SS with training protocols could conceivably be applied in the future to improve the effects of customary rehabilitative interventions.…”

Pharyngeal Stimulation in Head and Neck Cancer Patients with Dysphagia: Functional Outcomes and Transcranial Magnetic Stimulation Motor Evoked Potentials

“…The application of electrical stimulation, both externally to the submental region and internally to the pharynx has been explored [2][3][4].…”