MODULATION OF OLIGOSYNAPTIC CUTANEOUS AND MUSCLE AFFERENT REFLEX PATHWAYS DURING FICTIVE LOCOMOTION AND SCRATCHING IN THE CAT

Modulation of oligosynaptic cutaneous and muscle afferent reflex pathw ays during fictive locomotion and scratching in the cat. J. Neurophysi ol. 79: 447-463, 1998. We have compared state-dependent transmission t hrough oligosynaptic (minimally disynaptic) reflex pathways from low-t hreshold cutaneous and muscle afferents to some flexor and extensor lu mbosacral motoneurons during fictive locomotion and scratching in dece rebrate unanesthetized cats. As reported in earlier work, oligosynapti c cutaneous excitatory postsynaptic potentials (EPSPs) in flexor digit orum longus (FDL) and inhibitory postsynaptic potentials (IPSPs) in ex tensor digitorum (EDL) longus motoneurons were enhanced markedly durin g the early flexion phase of fictive locomotion. We show in this paper that, in contrast, these cutaneous reflex pathways were depressed mar kedly during ail phases of fictive scratching. On the other hand, disy naptic EPSPs produced by homonymous and synergist group I muscle affer ents in flexor (tibialis anterior and EDL) motoneurons were present an d strongly modulated during both fictive locomotion and scratching. Du ring both actions, these disynaptic group I EPSPs appeared or exhibite d the largest amplitude when the motoneuron membrane potential was mos t depolarized and the parent motor pool was active. There was an inter esting exception to the simple pattern of coincident group I EPSP enha ncement and motoneuron depolarization. During locomotion, disynaptic g roup I EPSPs in both FDL and flexor hallucis longus (FHL) motoneurons cells were facilitated during the extension phase, although FDL motone urons were relatively hyperpolarized whereas FHL cells were depolarize d The reverse situation was found during fictive scratching: group I E PSPs were facilitated in both FDL and FHL cells during the flexion pha se when FDL motoneurons were depolarized and FHL cells were relatively hyperpolarized. These observations suggest that the disynaptic EPSPs in these two motor nuclei are produced by common interneurons. Recipro cal disynaptic inhibitory pathways from group Ia muscle afferents to a ntagonist motoneurons were also active and subject to phase-dependent modulation during both fictive locomotion and scratching. In all but o ne cell tested, reciprocal disynaptic group Ia IPSPs were largest duri ng those phases in which the motoneuron membrane potential was relativ ely hyperpolarized and the parent motor pool was inactive. Oligosynapt ic PSPs in motoneurons produced by stimulation of the mesencephalic lo comotor region (MLR) were modulated strongly during fictive locomotion but were suppressed powerfully throughout fictive scratching. Large c ord dorsum potentials generated by MLR stimuli also were suppressed ma rkedly during fictive scratching. These results allow certain inferenc es about the organization of interneurons in the pathways examined. Th ey also suggest that the central pattern generators that produce ficti ve locomotion and scratching are organized differently.