DEVELOPMENT OF HYPOGLOSSAL MOTONEURONS

Hypoglossal motoneurons (HMs) are brain stem motoneurons that innervat e tongue muscles. Their function is critical in the control of the upp er airway. Results from in vitro studies of rat HMs have shown that pr operties of HMs change during the postnatal period. For example, these studies have uncovered changes in HM morphology and electrical proper ties (both in ion channels and firing properties) as well as changes i n chemical synaptic transmission to HMs during the postnatal period. M orphologically, a marked reduction in complexity of the dendritic tree takes place over the first 2 wk postnatal. In terms of electrical pro perties, a substantial and progressive fall in motoneuronal input resi stance occurs during the first month of life, due to a decrease in spe cific membrane resistivity. This is primarily responsible for the prog ressive increase in rheobase and consequent reduction in cell excitabi lity. In addition, the densities of at least two types of membrane ion channels are altered in early postnatal life, contributing to changes in their electroresponsive properties. On the one hand, the depolariz ing mixed cationic current that is activated by membrane hyperpolariza tion was found to be similar to 10-fold larger in adult than in neonat al HMs. By contrast, neonatal HMs possess a transient low-voltage-acti vated T-type Ca2+ channel with a low single-channel conductance (simil ar to 7 pS), the density of which rapidly declines during the early po stnatal period. The functional relevance of these and other changes oc curring during the postnatal period is discussed.