Age-Related Functional and Morphological Changes in the Rat Phrenic Motoneuron

Abstract

PURPOSE: There is a general correspondence between the sizes of motoneurons, motor units, and muscle fibers that has particular functional importance in motor control. This study aimed to investigate age-related morphplogical changes of phrenic motoneurons including dendritic airchtecture in the rat METHODS: By using an intracellular recording technique, the electrophysiological membrane properties of the motoneurons were measured in young adult (less than 1-year) and old (2-year) rats. Under anesthesia and artificially conditions, the recorded motoneurons were divided into recruited (spike discharge) and non-recruited (depolarization only) types. Electrophysiological membrane properties and mean interval of spike discharge were measured in the motoneurons. In another set of animals (5 rats from each age group), the phrenic motoneurons were retrogradely labeled with cholera toxin subunit B (CTB), and then the spinal cord was processed with immunohistocemical staining (1st antibody to CTBr and 2nd antibody with Cy3) and observed by confocal imaging techniques to examine dendrites architecture. RESULTS: In the recruited motoneurons of the old rat, the intervals of action potentials were markedly irregular, and the mean interval of the discharge was significantly longer than the values in the young adult rat (79 ms vs 112 ms). Compared to the young adult rat, there were significant decreases in the somal volume (15252 vs 19140 um3), initial diameter of primarily dendrite (6.3 vs 7.5 um), but not in the number of dendrite (7.7 vs 8.2) in the old rat. CONCLUSION:Motoneuron excitability is determined by both intrinsic electrophysiological properties and extrinsic factors such as synaptic input. For a given somal surface area and synaptic input density, a smaller dendritic surface area would imply lesser excitability due to lower input. The smaller primary dendrite might be a reason for unstable firing frequency observed in the old phrenic motoneuron.