Motoneurons have different membrane resistance during fictive scratching and weight support

The passive membrane properties of motoneurons may be affected in a behavior-specific manner because of differences in synaptic drive during different motor behaviors. To explore this possibility, the changes in input resistance (R(in)) and membrane time constant (tau(m)) of single extensor motoneurons were compared during two different types of motor activities: fictive scratching and fictive weight support. These two activities were selected because the membrane potential of extensor motoneurons follows a very different trajectory during fictive scratching (multiphasic, mostly rhythmic trajectory) and fictive weight support (monophasic, tonic trajectory). The intracellular recordings were performed in vivo in the immobilized, decerebrate cat using QX-314-containing microelectrodes to block action potentials. The R(in) and tau(m) at rest (control) were reduced substantially during all phases of fictive scratching. In contrast, R(in) and tau(m) changed only little during fictive weight support. Such a differential effect on the membrane resistance was observed even in motoneurons in which the peak voltage of the rhythmic depolarization during scratching was similar to the peak voltage of the tonic depolarization during weight support. The differential effect was attributed mainly to a difference in synaptic drive and, in particular, to a larger amount of inhibitory synaptic activity during fictive scratching. The present study demonstrates how the same motoneuron can have a different membrane resistance while participating in two different behaviors. Such tuning of the membrane resistance may provide motoneurons with behavior-specific integrative capabilities that, in turn, could be used advantageously to increase motor performance.