Physiological changes accompanying anatomical remodeling of mammalian motoneurons during postnatal development.

The development of respiratory motoneurons provides unique data that may be generalized to other mammalian motoneuron populations. Like other motoneurons, respiratory motoneurons undergo developmental changes in the shape of the action potential and their repetitive firing. The unique observations concern the postnatal change in the recruitment pattern of cat phrenic motoneurons that is correlated with a halving of mean input resistance, a stasis of growth in the cell membrane and a reduction in the complexity of the dendritic tree. A similar pattern of change was observed for hypoglossal motoneurons studied in rat brainstem slices. Without an increase in total membrane surface area, the decreased resistance must result from a reduced specific membrane resistance. Two mechanisms are proposed to explain this decrease in resistance: proliferation and redistribution of either synaptic inputs and/or potassium channels. Although there was a significant contribution of synaptic input in determining input resistance throughout postnatal development, it was the density of cesium- or barium-sensitive potassium conductances that differentiated low resistance from high resistance motoneurons. Low resistance motoneurons had more cesium- and barium-sensitive channels than their high resistance counterparts. Based on the variations in the relative changes observed in input resistance versus membrane time constant with these two potassium channel blockers (cesium and barium), it is proposed that the distribution of these potassium channels change with age. Initially, their distribution is skewed toward the dendrites but as development progresses, the distribution becomes more uniform across the motoneuron membrane. During postnatal development, the rapid decrease in input resistance results from a proliferation of potassium channels in the membrane and of synaptic inputs converging onto developing respiratory motoneurons while the membrane is being spatially redistributed but not expanded.