Pain. A central inhibitory balance theory

Knowledge of the segmental neuronal circuitry for noxious and non-noxious input is a necessary prelude to understanding pain mechanisms. Significant gaps in our understanding of the synaptology of these pathways in the dorsal horn have hindered progress in this area. This report describes the excitatory and inhibitory circuits that supply the marginal neurons of the spinal cord, whose role in nociceptive mechanisms is well established. On the basis of these data, it is proposed that large primary afferents (non-noxious) provide heavy inhibitory input to marginal neurons via gelatinosa cells. Conversely, small primary afferents (nociceptive) provide excitatory input to marginal neurons but relatively little inhibitory feedback to these cells via gelatinosa neurons. The modulation of pain-producing input depends thus on the balance between large-fiber and small-fiber activity via a postsynaptic inhibitory mechanism acting on the nociceptive relay neurons. This theory accounts satisfactorily for the modulation of pain by counterirritation and by various methods of stimulating large-fiber input.