A study of the parameters of electrical stimulation of unmyelinated fibres in the pituitary stalk.

1. The milk-ejection response in lactating rabbits has been used to study the effect of electrical stimuli of different types applied to the supraopticohypophysial tract in the pituitary stalk.2. Sine-wave alternating-current pulses were compared with balanced biphasic square-wave pulses of the same frequency and peak-to-peak current strength. At a pulse duration of 2-4 msec the square-wave stimulation was less effective than the sine wave, but at a pulse duration of 8 msec and over, more effective.3. Above threshold levels of 0.12 mA for the current strength, and of 0.5 msec for the pulse duration, the response increased with increasing current strengths to 2.4 mA and increasing pulse durations to 10 msec.4. With constant and effective strength and duration of the pulse a slight diminution of the response was seen as the frequency was diminished from 100/sec to 50/sec. Further diminution in the frequency revealed that at some point between 50 and 10 c/s a sudden abrupt diminution in the magnitude of the responses occurred over a small range of frequency. This was a reversible phenomenon and it is suggested that it may be related to neurosecretory events occurring in the nerve terminals.5. Stimuli with parameters within the ranges mentioned above are effective in eliciting an oxytocic response on the uterus in the rabbit and ovulation responses (when applied to the hypothalamus) in rats and rabbits. It appears likely that stimuli with such parameters are suitable for experiments concerned with stimulation of the hypothalamus.