Fig. 4.

Pain pathways in the dorsal horn. A: basic scheme of Melzack ad Wall (1965) in which both the small (Aδ, C) and the large (Aβ) fibers are capable of exciting the projection neuron (P), but their nerve terminals are subject to tonic (presynaptic) inhibition from neuron 1 in the substantia gelatinosa. If the small nociceptor fibers are excited neuron 1 is itself inhibited (Wall and Melzack did not show the inhibitory neuron involved), so that P can now be excited more powerfully. In contrast, large fiber inputs stimulate neuron I and thereby reduce excitation of P by terminals of small (and large) fibers. B: current scheme of the pain pathway and is based on Guo and Hu (2014). Once again neuron 1 exerts presynaptic inhibition on the terminals of the large and small fibers, not only those ending directly on the projection cell (P), but also those innervating excitatory neurons (3, 4, 5) in polysynaptic pathways to P. In addition P receives postsynaptic inhibition via neuron 2.