Intrinsic electrical properties of spinal motoneurons vary with joint angle

The dendrites of spinal motoneurons amplify synaptic inputs to a marked degree through persistent inward currents (PICs). Dendritic amplification is subject to neuromodulatory control from the brainstem by axons releasing the monoamines serotonin and norepinephrine; however, the monoaminergic projection to the cord is diffusely organized and does not allow independent adjustment of amplification in different motor pools. Using in vivo voltage-clamp techniques, here we show that dendritic PICs in ankle extensor motoneurons in the cat are reduced about 50% by small rotations (±10°) of the ankle joint. This reduction is primarily due to reciprocal inhibition, a tightly focused input shared only among strict muscle antagonists. These results demonstrate how a specific change in limb position can regulate intrinsic cellular properties set by a background of diffuse descending neuromodulation.