Defference in physiological and mophological properties between jaw-closing and opening motoneurons

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Whole cell patch clamp recordings were made from masseter motoneurons (MaMNs) and digastric motoneurons (DiMNs) in neonatal rat brainstem slice preparations to study firing properties of jaw-closing and opening motoneurons. Input resistance and membrane time constant of MaMNs were smaller than those of DiMNs. Amplitude of the post-spike medium-duration afterhyperpolarization (mAHP) in MaMNs was smaller than that of DiMNs and duration of the mAHP in NaMNs was shorter than that of DiMNs. Decay time constant of the mAHP tail current in MaMNs was shorter than that in DiMNs. Ca^<2+> influx producing the mAHP through activation of Ca^<2+>-activated K^+ channel was monitored using Ca^<2+> imaging, in the presence of apamin ? -agatoxin IVA, ? -conotoxin CV1A and Ni^<2+>. The slope of the relationship between firing frequency (f) and membrane depolarization (V) induced by injection of 1-s current pulses was steeper in MaMNs than the slope in DiMNs. The slope of the f-V relationship were correlated with amplitude and duration of the mAHP and the difference in the slope of the f-V relationship between MaMNs and DiMNs was abolished by apamin. We conclude that smaller and shorter mAHP in MaMNs results in steeper slope of the the f-V relationship, compared with DiMNs. Such firing properties of jaw-closing and opening motoneurons might contribute to transduction of masticatory drive signal from central pattern generator of mastication to appropriate activities of jaw-closing and opening muscles.