Siegesmund, K.A., Sances Jr, A. and Larson, S.J. The effects of electrical currents on synaptic vesicles in monkey cortex. In Wageneder, F.M. and St. Schuy (Eds.), Electrotherapeutic Sleep and Electroanaesthesia. International Congress Series No. 136. Excerpta Medica Foundation, Amsterdam, Pp. 31-33, 1967.

Biased rectangular current pulses of 2.5 mS duration, 75 Hz, and 5 mA were delivered through electrodes on the nasion and inion of 4 squirrel monkeys. Electron photomicrographs were made of neural tissue biopsies prior to, during, and post stimulation and the number of synaptic vesicles in close apposition to the presynaptic membranes were counted for at least 200 synapses in each group. They found that in the control specimen 65% of the presynaptic terminals contained 14 ±2 vesicles near the cleft. Shortly after maximum stimulation was reached, there was an increase of terminals with less than 9 vesicles and corresponding decrease in those with 14. After 5 minutes at an applied current of 5 mA, a further decrease in terminals with 14 vesicles and a corresponding increase in terminals with more than 24 vesicles were found. Shortly after discontinuation of stimulation, the number of terminals with 24 or more vesicles returned to control levels while the number with 9 vesicles or less remained at a higher level than in the controls.