Supraspinal and segmental signals can be transmitted through separate spinal cord pathways to enhance locomotor...

The fine control of locomotion results from a complex interaction between descending signals from supraspinal structures and sensory feedback from the limbs. In this report, we studied the interaction between vestibulospinal volleys descending from Deiters' nucleus and group I afferent input from extensor muscles. It has been shown that both pathways can exert powerful control over the amplitude and the timing of muscle bursting activity in the different phases of the step cycle. The effects of stimulating these pathways on the fictive locomotor rhythm were compared in decerebrate, partially spinal cats (ipsilateral ventral quadrant intact) injected with nialamide and L-dopa. As reported before, stimulation of both Deiters' nucleus and group I fibres from ankle extensor muscles, when given during the flexor phase, stopped the flexor activity and initiated activity in extensors. When applied during the extensor phase, the same stimulation prolonged the extensor activity and therefore delayed the onset of flexor activity. This similarity suggests that the two pathways might converge on common spinal interneurones. This possibility was tested with the spatial facilitation technique in lumbosacral motoneurones. Deiters' nucleus and group I fibres from extensor muscles were stimulated with different intensities and with several different coupling intervals. Motoneurones showing clear di- and/or polysynaptic excitation from both pathways were retained for analysis. Surprisingly, in all cases, there were no signs of spatial facilitation, but a simple algebraic sum of the two excitatory postsynaptic potentials. This result indicates that each input acts on the rhythm generator through separate interneuronal pathways.