The activity characteristics of the preganglionic pupilloconstrictor neurones.

1. Recordings have been made of pupilloconstrictor unit activity within the small-celled component of the oculomotor nucleus of the cat. The identity of these neurones was determined by testing unit responses to a sequence of stimuli which produced dilation and constriction of the sympathectomized pupil.2. The frequency range of unit activity was from 0 to 28/sec. A maximal constriction of the pupil was maintained by unit activity in the region of 8/sec. The resting level of unit activity under chloralose anaesthesia was typically in the range 6-10/sec.3. Stimulation in the defence area of the hypothalamus produced a maximal pupillodilation associated with a complete inhibition of unit activity and a block of the light reflex response.4. Onset of retinal illumination produced a burst discharge followed by a transient silent period and then an over-all increase in the rate of activity.5. Cessation of retinal illumination produced a post-excitatory depression of unit activity lasting from 190 to 700 msec. It is suggested that this effect is produced by retinal afferents activated by light ;off' that exert an inhibitory influence on the pupilloconstrictor neurones.