Principles & Practice of Pain Medicine > Part I. Pain: Biology, Anatomy, and Physiology > Chapter 1. Molecular Biology of Pain Tony L. Yaksh

Dynamic Aspects of Encoding of Injury-Generated Input

"The preceding section emphasized that tissue injury yielded activity in small primary afferents and that small afferent input resulted in monosynaptic and polysynaptic excitation of dorsal horn neurons that projected to the brainstem and higher centers. Importantly, the pathway appears to preserve several properties of the stimulus, the anatomic sign (localization), and intensity. Thus, input from an area of skin might be expected to activate a given population of spinal neurons that received afferent input from that part of the body surface, and the intensity of the stimulus was mirrored either by the specific neuronal population activated (e.g., nociceptive specific cells) or by the frequency of the discharge (as with the WDR neurons), or both. This linkage, even in its simplest form, would be described as the "pain pathway," as it reflects the connectivity by which afferent traffic generated by tissue injury reaches higher centers and the conscious state. This afferent substrate, in fact, represents only one component of the system that is essential to the processing of nociceptive input. The excitation of dorsal horn neurons evoked by small afferent input is subject to modulation by a number of receptor systems within the spinal cord. Technically, this modulation may be thought of in terms of those systems that increase or decrease the efficacy of synaptic connections of the afferent pathway.Acute activation of small afferents by high-intensity mechanical or thermal stimuli will result in a clearly defined pain behavior in..."