Bulbospinal control of spinal cord pathways generating locomotor extensor activities in the cat

1 Intracellular recording of lumbosacral motoneurones in the decerebrate and partially spinalized cat injected with nialamide and L‐dihydroxyphenylalanine (l‐DOPA) was used to investigate the interneuronal convergence of two bulbospinal pathways and of the segmental pathways involved with the generation of extensor activities during locomotion. 2 Deiter's nucleus (DN) or the medial longitudinal fasciculus (MLF) was stimulated in alternation with, and in combination with, stimulation of group I afferents from extensor muscles or of contralateral flexor reflex afferents (coFRA). The evoked polysynaptic EPSPs were recorded in extensor motoneurones when long‐latency, long‐lasting discharges were evoked by the stimulation of coFRA and when the group I autogenetic inhibition in extensors was reversed to polysynaptic excitation. Spatial facilitation was inferred when the amplitude of the EPSPs evoked by the combined stimuli was notably larger than the algebraic sum of the EPSPs evoked by individual stimulation. 3 Both DN (16 motoneurones) and MLF inputs (8 motoneurones) showed spatial facilitation when preceded by coFRA stimuli and both could reset the rhythm of fictive stepping by triggering a precocious extensor phase. MLF showed spatial facilitation with extensor group I inputs in 69 % of trials but DN failed to show spatial facilitation in any cells. 4 These results indicate that DN and MLF project to the coFRA pathways of the extensor half‐centre for locomotion and MLF, but not DN, converge on segmental interneurones of the extensor group I pathways. The implications of such convergence patterns on the functional organization of the extensor half‐centre are discussed.