Inputs to group II-activated midlumbar interneurones from descending motor pathways in the cat.

1. Connections from descending motor pathways to group II-activated interneurones in the midlumbar segments of the spinal cord have been examined by intracellular recording. Interneurones, many of which had axonal projections to the hindlimb motor nuclei, were tested for inputs from rubro-, reticulo-, vestibulo- or corticospinal fibres. 2. Of 138 cells, 113 were monosynaptically excited by electrical stimulation of at least one of the descending motor pathways. Monosynaptic excitation from reticulo-, vestibulo- and rubrospinal pathways was common. Monosynaptic corticospinal EPSPs were identified in fewer neurones. 3. Convergent monosynaptic inputs from pathways which descend in the ventrolateral and ventral funiculi were common. Although few neurones with monosynaptic input from the corticospinal tract were identified, most also had monosynaptic rubrospinal input. In contrast, few neurones (4.3%) had convergent monosynaptic input both from pathways in the dorsolateral funiculus and from fibres in the ventral/ventrolateral funiculi. 4. The patterns of convergence from the different descending motor pathways differ from the patterns expected if the descending connections were distributed independently. Thus there is a significant segregation between rubrospinal and reticulo- or vestibulospinal inputs, and a significant association of reticulo- and vestibulospinal inputs. 5. Since descending motor pathways make monosynaptic connections with most group II-activated midlumbar neurones, many of which project to the hindlimb motor nuclei, some of these neurones provide a disynaptic pathway for the supraspinal control of hindlimb movements. The distribution of descending connections is consistent with the hypothesis that pathways descending in the dorsal part of the lateral funiculus and those descending in the ventrolateral or ventral funiculi contact different sets of interneurones.