ELECTROPHYSIOLOGICAL PROPERTIES AND RECRUITMENT ORDER OF RAT PHRENIC MOTONEURONS

951 Electrophysiological membrane properties and recruitment order of rat phrenic motoneurons were examined in adult rats that were anesthetized and artificially ventilated throughout the experiment. The after-hyperpolarization duration, input resistance and rheobase were measured by intracellular recordings with glass microelectrodes. To quantify the recruitment order, the relative onset times were calculated as the delay from the initiation of whole phrenic nerve discharge to the first spike divided by the inspiratory time and was expressed as a percentage. The intracellular recording of the phrenic motoneurons indicated a shorter after-hyperpolarization duration (62.2 ± 10.0 ms), a higher membrane input resistance (3.0 ± 1.1 Mohm), and a lower rheobase (3.5 ± 1.9 nA), compared to the motoneurons innervating the gastrocnemius muscle in the adult rat and cat. In addition, quiescent motoneuron which was not recruited during normal experimental conditions had a similar membrane input resistance and rheobase, compared to those of recruited motoneurons. There were no significant relationship between the relative onset times and the electrophysiological membrane properties of phrenic motoneurons. Based on these results, we proposed that the intrinsic factors such as the characteristics of electrophysiological membrane properties are not a main contributor to the recruitment order of phrenic motoneurons. Supported by a grant from Japanese Ministry of Education, Science and Culture (No. 10780024)