SUPRASPINAL AND SEGMENTAL SIGNALS CAN BE TRANSMITTED THROUGH SEPARATESPINAL-CORD PATHWAYS TO ENHANCE LOCOMOTOR-ACTIVITY IN EXTENSOR MUSCLES IN THE CAT

The fine control of locomotion results from a complex interaction betw een descending signals from supraspinal structures and sensory feedbac k from the limbs. In this report, we studied the interaction between v estibulospinal volleys descending from Deiters' nucleus and group I af ferent input from extensor muscles. It has been shown that both pathwa ys can exert powerful control over the amplitude and the timing of mus cle bursting activity in the different phases of the step cycle. The e ffects of stimulating these pathways on the fictive locomotor rhythm w ere compared in decerebrate, partially spinal cats (ipsilateral ventra l quadrant intact) injected with nialamide and L-dopa. As reported bef ore, stimulation of both Deiters' nucleus and group I fibres from ankl e extensor muscles, when given dining the flexor phase, stopped the fl exor activity and initiated activity in extensors. When applied during the extensor phase, the same stimulation prolonged the extensor activ ity and therefore delayed the onset of flexor activity. This similarit y suggests that the two pathways might converge on common spinal inter neurones. This possibility was tested with the spatial facilitation te chnique in lumbosacral motoneurones. Deiters' nucleus and group I fibr es from extensor muscles were stimulated with different intensities an d with several different coupling intervals. Motoneurones showing clea r di- and/or polysynaptic excitation from both pathways were retained for analysis. Surprisingly, in all cases, there were no signs of spati al facilitation, but a simple algebraic sum of the two excitatory post synaptic potentials. This result indicates that each input acts on the rhythm generator through separate interneuronal pathways.