Alterations in spontaneous activity of the isolated frog spinal cord in Calcium-free environment

Spontaneous electrical activity of the isolated frog spinal cord was examined in Ca2+-free environment. Spontaneous discharges from the ventral root altered in four distinguishable steps. The first step was an immediate increase in the rate of spontaneous discharges and the second was a gradual decrease. The third was the occurrence of rhythmical bursts, and the last, the appearance of continuous firing. The rhythmical bursts could be depressed by the addition of metabolic inhibitors (ouabain or dinitrophenol in a concentration of 5 x 10(-5) M) as well as of Ca2+-chelating agents (EDTA or EGTA in a concentration of 10(-3) M). Our results suggest that the occurrence of the rhythmical bursts requires a metabolic pumping process to redistribute Na+ and K+ across the membrane and a small amount of Ca2+ for transmitter secretion.