Hyperexcitable dendrites in motoneurons and their neuromodulatory control during motor behavior.

Dendrites contain voltage-sensitive conductances that, in vivo, can be influenced by neuromodulatory inputs. In spinal motoneurons, dendrites have voltage-activated persistent inward currents that are facilitated by neuromodulatory input from monoaminergic axons originating in the brainstem. The highest levels of monoamine input are likely to occur during 'fight or flight' behavioral situations. At these high levels, the persistent currents are so large that the dendrites of motoneurons become hyperexcitable, enhancing ionotropic inputs by fivefold or more and allowing the firing rates required for maximal activation of muscle fibers to be generated by surprisingly small inputs. Moderate dendritic excitability (twofold to threefold enhancement) is likely to be a standard component of many normal motor behaviors, such as locomotion. Thus, during normal motor behavior, synaptic integration might be dominated by active currents intrinsic to the dendritic tree rather than by the synaptic current entering via ionotropic channels.