The postsynaptic potentials produced in cat genioglossus and styloglossus motoneurons (CG- and SG-Mns) during swallowing were studied. During swallowing elicited by placing wwater on the dorsum of the tongue, the GG-muscle discharged for 80-210 ms (mean +/- S.D. 123 +/- 31 ms, N = 59) and was abruptly suppressed, and the SG-muscle began discharging in synchrony with the GG-muscle and discharged for 200-360 ms (mean +/- S.D. 247 +/- 36 ms, N = 59). The GG- and the SG-Mns were identified if unitary muscle activity followed the induced spike of the motoneuron one-for-one. During swallowing, excitatory postsynaptic potentials (EPSPs) were evoked in the SG-Mns regardless of the respiratory drive on the SG-Mns, and inhibitory postsynaptic potential (IPSP) or EPSP-IPSP was evoked on the GG-Mns regardless of the respiratory drive on the GG-Mns. By increasing the intracellular concentration of chloride ions, the IPSP elicited in the GG-Mn during swallowing was turned into a depolarizing potential. In immobilized cats, a depolarizing potential and a depolarizing-hyperpolarizing potential sequence was evoked successsively on a tongue retractor motoneuron and a tongue protruder motoneuron by repetitive electrical stimulation of the superior laryngeal nerve.
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