PROPERTIES OF INDIVIDUAL EMBRYONIC PRIMARY AFFERENTS AND THEIR SPINALPROJECTIONS IN THE RAT

Embryonic (E19-E20) and early postnatal (P2) spinal cords with intact saphenous and sciatic nerves were isolated and placed in aerated artif icial cerebral spinal fluid (CSF). Intracellular recordings were made from cells in the L-2-L-6 dorsal root ganglia using microelectrodes fi lled with 3 M potassium acetate or 5% neurobiotin (NB) in 1 M potassiu m acetate. Several physiological properties of adequately impaled cell s were measured, including peripheral conduction velocity, action pote ntial (AP) amplitude and duration, duration of afterhyperpolarization (AHP), input impedance, rheobase, presence of inward rectifying curren t, and maximum somal firing frequency. The extent to which these prope rties are correlated also was determined. One cell per ganglion was in jected with NE. Stained somata and their central projections in the sp inal cord were visualized in serial 50 mu m sections. Cell size was de termined and the central morphology of the central projections examine d. Although some fibers were in the process of growing into the spinal cord, others had established projections over several millimeters in the dorsal columns. Although most of these fibers supported projection s in the gray matter, 22% only maintained fibers in the dorsal columns . Fibers with projections in the dorsal horn exhibited three types of morphology: projections confined to the superficial dorsal horn (lamin ae I, II); terminals confined to laminae III-V; and projections spanni ng laminae II-V. In addition, some embryonic fibers maintained project ions to the dorsal horn that extended over five lumbar segments. Somal APs could be divided into two soups: broad spikes with inflections on their falling phase and narrow spikes without inflections. On average , cells with broad spikes (BS) had the following characteristics: slow er peripheral conduction velocity, larger amplitude, higher rheobase a nd input impedance, longer AHP duration, and lower maximum firing freq uency. There were significant correlations between conduction velocity and several of the physiological properties. Conduction velocity was negatively correlated with AP duration, rheobase, and input impedance and positively correlated with maximum firing frequency. Comparisons b etween spike shape and central morphology revealed that cells lacking collaterals in the gray matter and those with projections in the super ficial dorsal horn always had broad somal spikes with inflections. Tho se with projections confined to laminae m-V always had narrow somal sp ikes (NS).