Receptive fields of rat lamina 1 projection cells move to incorporate a nearby region of injury

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Abstract
The effects of punctate burns to the skin on the response properties of a group of lamina 1 projection cells in the dorsal horn of the spinal cord has been studied in decerebrate rats. The burns were made 5-15 mm outside the boundaries of the receptive fields (RF) of the cells and they produced a slow increase in the excitability of the cells as seen by an enlargement of RF, towards the lesion, a decreased mechanical threshold and an increased responsiveness to transcutaneous electrical stimulation applied with needle electrodes both inside and outside the RF. These changes were first seen 10-15 min after burning and were not reversed during the time course of the investigation. Local anesthetic injected at the site of injury produced only a partial and slow reversal of the excitability increases. Since responses to transcutaneous electrical stimulation were increased, injury-induced changes were unlikely to be due directly to alterations in the properties of primary afferent fibers, but this was directly tested by attempting to mimic the afferent barrage produced by the burn by electrical stimulation of the central cut ends of either the sural nerve or a dorsal rootlet. In both cases a single 10 min period of stimulation (0.5 Hz supramaximal intensity) induced a slow and progressive increase in RF size, confirming the existence of a central component. The changes seen with peripheral burns were probably triggered at least in part by a discharge in primary afferent C fibers since units studied in capsaicin treated animals (which have an impairment of C fiber function) did not show equivalent changes. These results suggest that mechanisms of sensory processing, especially nociception, are subject to reorganization in the face of altered input. The slow time course of the changes suggests that the underlying mechanism may not be the rapid post-synaptic potentials of traditional neurophysiology.