Estimated magnitude and interactions of cortico-motoneuronal and Ia afferent input to spinal motoneurones of the human hand.

Magnitude and interactions of cortico-motoneuronal (CM) and Ia afferent input to spinal alpha-motoneurones (MNs) of the human hand are largely unknown. This is, however, an important question, which bears on the cortical versus peripheral-segmental 'interest' in controlling alpha-MN excitation. Alpha-MN excitation can be quantified by estimating the amplitude of alpha-MN compound excitatory post-synaptic potentials (cEPSPs) from single motor unit (SMU) recordings, if certain assumptions about the membrane trajectory are made [Exp. Brain Res. 47 (1982) 33]. Here we recorded 29 SMUs from three different hand muscles (FDI, first dorsal interosseous; ADM, abductor digiti minimi; APB, abductor pollicis brevis) of healthy subjects. Each SMU was tested for CM input by transcranial magnetic stimulation of the contralateral motor cortex, for Ia afferent input by electrical peripheral nerve stimulation, and for the interaction between inputs by paired stimulation timed to arrive coincidently at the alpha-MN. Mean cEPSP amplitude elicited by CM input was larger in the alpha-MNs of the FDI than in those of the ADM or APB, whereas mean cEPSP amplitude elicited by Ia input was larger in the alpha-MNs of the APB than in those of the FDI. Generally, cEPSP amplitude evoked by paired input closely matched the arithmetic sum of the cEPSP amplitudes evoked by the single inputs. In conclusion, alpha-MNs of the human hand can be viewed as linear integrators of CM and Ia excitatory inputs. The weights of these inputs may relate to the specific functions of the different intrinsic hand muscles in skilled finger movements.