Adjustable amplification of synaptic input in the dendrites of spinal motoneurons in vivo

The impact of neuromodulators on active dendritic conductances was investig ated by the use of intracellular recording techniques in spinal motoneurons in the adult cat. The well known lack of voltage control of dendritic regi ons during voltage clamp applied at the soma was used to estimate dendritic amplification of a steady monosynaptic input generated by muscle spindle I a afferents. In preparations deeply anesthetized with pentobarbital, Ia cur rent either decreased with depolarization or underwent a modest increase at membrane potentials above -40 mV. In unanesthetized decerebrate preparatio ns (which have tonic activity in axons originating in the brainstem and rel easing serotonin or norepinephrine), active dendritic currents caused stron g amplification of Ia input. In the range of -50 to -40 mV, peak Ia current was over four times as large as that in the pentobarbital-anesthetized pre parations. Exogenous administration of a noradrenergic agonist in addition to the tonic activity further enhanced amplification (sixfold increase). Am plification was not seen in preparations with spinal transections. Overall, the dendritic amplification with moderate or strong neuromodulatory drive was estimated to be large enough to allow the motoneurons innervating slow muscle fibers to be driven to their maximum force levels by remarkably smal l synaptic inputs. In these cells, the main role of synaptic input may be t o control the activation of a highly excitable dendritic tree. The neuromod ulatory control of synaptic amplification provides motor commands with the potential to adjust the level of amplification to suit the demands of diffe rent motor tasks.