Convergence in segmental reflex pathways from nociceptive and non-nociceptive afferents to alpha-motoneurones in the cat

1. Reflex interaction between nociceptive and non-nociceptive segmental afferents was investigated by testing for spatial facilitation of postsynaptic potentials (PSPs) in alpha-motoneurones recorded in anaemically decapitated, high spinal cats. Nociceptive segmental afferents were activated by applying noxious radiant heat to the skin. Non-nociceptive skin mechanoreceptors were activated by puffs of air. Non-nociceptive skin, joint and group I-III muscle afferents were stimulated by electrical pulses delivered to various nerves. 2. Conditioning by stimulation of nociceptive afferents facilitated transmission in various ipsilateral segmental pathways. Such spatial facilitation occurred in both excitatory and inhibitory pathways. Pathways that were facilitated included those activated by low to medium threshold cutaneous afferents, joint afferents, and group Ib and II muscle afferents. 3. In contrast, monosynaptic EPSPs evoked by stimulating ipsilateral group Ia muscle afferents did not show spatial facilitation but rather inhibition during conditioning stimulation of nociceptive afferents. Spatial facilitation of reciprocal group Ia IPSPs was rare and small if it occurred. 4. Pathways activated by cutaneous and group II muscle afferents were depressed by contralateral stimulation of nociceptive afferents. 5. We conclude that spatial facilitation observed between nociceptive and non-nociceptive afferents results from a convergence of inputs on common interneurones in the reflex pathways to alpha-motoneurones. Therefore nociceptive afferents have to be regarded as constituents of flexor reflex afferents (FRAs) and may add a specific nocifensive function to the FRA system.