Electrophysiological characteristics of lumbar spinal cord neurons backfired from lateral reticular nucleus in the rat

D. Menetrey, J. de Pommery and J. M. Besson Spinal neurons antidromically activated from either the lateral reticular nucleus (LRN) or immediately adjacent areas were identified in the rat lumbar spinal cord. In agreement with previous anatomical work (60), these neurons were widely distributed in both the dorsal and ventral horns of the spinal cord and could be subdivided into three main groups according to their location: a) deep ventromedial (DVM) cells, which project more substantially to the LRN than to other supraspinal targets; b) cells of the median portion of the neck of the dorsal horn (mNDH), which project exclusively to the LRN; c) cells lying in other parts of the dorsal horn (superficial layers, nucleus proprius, reticular extension of the neck), by their location, they are indistinguishable from cells projecting to other supraspinal targets. The probability is high that the DVM and mNDH cells contribute exclusively, or at least preferentially, to the lateral component of the spinoreticular tract (lSRT), defined as the direct spinal pathway to the LRN. Although electrophysiological properties of cells were clearly related to their spinal location, several subpopulations could be recognized in each of the three main groups. The majority of DVM neurons were in lamina VII, with some in laminae VI, VIII, and X. With the exception of a few lamina X cells, the DVM neurons had high conduction velocities. Four subpopulations of these neurons were recognized. a) Innocuous proprioceptive cells responded to small changes in joint position, some showing convergence of nonnoxious cutaneous inputs. b) High-threshold cells (approximately 50% of DVM cells). Seventy-five percent of these cells were excited from bilateral receptive fields (mostly symmetric) with noxious cutaneous pinching that extended to subcutaneous tissues. Their evoked responses had long-lasting postdischarges that continued up to several minutes after cessation of the stimulus. c) Inhibited cells had no demonstrable excitatory receptive fields and a high ongoing activity that was tonically depressed by pressure or pinch; poststimulus effects of long duration were observed. d) Cells with no resting discharge and demonstrable excitatory peripheral receptive fields. mNDH cells had recording sites at the medial border of the internal portion of the reticular area of the neck of the dorsal horn.