Integration of synaptic and intrinsic dendritic currents in cat spinal motoneurons

The results of recent studies designed to reveal some of the 'rules' governing the integration of synaptic and intrinsic dendritic currents in spinal motoneurons are reviewed. When two or more sources of synaptic input are activated concurrently, their combined postsynaptic effects on cat spinal motoneurons with 'passive dendrites' are generally equal to or slightly less than those predicted from the linear sum of their individual effects. However, for experimental preparations in which active conductances on motoneuron dendrites are enabled, instances of greater-than- or less-than linear summation can occur. Further, these studies demonstrate that the persistent inward currents that are generated by motoneuron dendrites provide an intrinsic source of excitatory drive that is larger than those associated with any of the individual synaptic input systems studied to date. Since these intrinsic depolarizing currents can be rapidly inactivated by a hyperpolarizing input, they are ideally suited to providing a major source of the alternating 'drive' to motoneurons during locomotion.