Abstract
Electrophysiological properties of jaw-closing motoneurons (JCMNs) in developing rats aged 4-27 days were investigated in slice preparations using intracellular recording techniques. The input resistance of JCMNs from 4-9-day-old rats was larger than that of JCMNs from older rats. The action potential half-duration progressively decreased with age, whereas the incidence of post-spike afterdepolarization (ADP) increased during postnatal development. The amplitude and half-duration of the medium-duration afterhyperpolarization decreased with age. Firing frequency adaptation was observed during the first four spikes in a spike train. The subsequent firing pattern was classified into two groups, constant firing (type I) and rapid acceleration (type II). Percentage of type I neurons increased with age. In more than 50% of the neurons of each age group, an extra spike was produced from the ADP of the 1st spike in a spike train, leading to a high firing rate for the first inter-spike interval (1st ISI). Incidence of an extra spike also increased with age. The firing rate for the steady state was also high in JCMNs even at 4-9 days of age. About 50% of the neurons of this age fired at higher than 50 Hz. These findings demonstrate that JCMNs can fire at a high rate even in the early period after birth and firing rate for the 1st ISI further increases with age. [Jpn J Physiol 55 Suppl:S153 (2005)]
