Crossed actions on group II-activated interneurones in the midlumbar segments of the cat spinal cord.

1. Evidence has been sought for crossed actions on midlumbar propriospinal neurones activated by ipsilateral group II muscle afferents, with particular emphasis on those neurones with projections to the ipsilateral hindlimb motor nuclei. 2. A large majority of group II-activated midlumbar neurones were influenced by stimulation of contralateral group II afferents. The most frequent and most powerful actions were from those nerves which most effectively influenced ipsilateral midlumbar neurones. Crossed actions from group I afferents were rare. 3. In the great majority of neurones the pattern of actions was similar from both limbs, the neurones being bilaterally excited, bilaterally inhibited or had both EPSPs and IPSPs from both sides. 4. The latencies of crossed actions suggest that the earliest crossed EPSPs from group II afferents were evoked disynaptically (i.e. via a single commissural neurone) and that the crossed IPSPs were evoked trisynaptically. 5. The pattern of crossed actions suggests a strong bilateral interaction between midlumbar neurones. The possible role of these neurones in postural control and the production of co-ordinated movements of the hindlimbs is discussed.