Intracellular recordings were made from the cells in the interm ediolateral nucleus (IML) of neonatal rat spinal cord slices in vitro. Both resting and active electrophysiological properties of the cellular membrane were investigated. Resting membrane potential ranged from -46 to -70 mV. The means of input resistance, time constant and membrane capacitance were 108.3 +/- 67.9 M omega (mean +/- SD), 9.9 +/- 5.6 ms and 138.6 +/- 124.2 pF, respectively. Repetitive firing at high frequency (up to 150 Hz) could be produced in thirty five cells (85.4%) in response to intracellular injection of depolarizing current, while single spike firing produced in the remaining cells (15.6%). The amplitude and duration of action potentials evoked by intracellular stimulation were 63.4 +/- 9.0 mV and 2.4 +/- 0.6 ms, respectively. The threshold at the level of 18.7 +/- 6.2 mV was more depolarized than the resting potential. In most of cells, spikes induced by intracellular stimulation were followed by afterhyperpolarization, whose peak amplitude and duration were 5.1 +/- 2.7 mV and 90 +/- 31.8 ms, respectively. EPSP, orthodromic action potentials and, rarely, IPSP could be evoked by dorsal root stimulation, and antidromic action potentials were obtained by ventral root stimulation. Identification of sympathetic preganglionic neuron and functional significance of electrophysiological properties were discussed.
Animals Animals, Newborn