The adrenergic mechanism in the nictitating membrane.

The contractions of the nictitating membrane in response to postganglionic nerve stimulation have been studied in experiments in which cats' heads were perfused. When eserine was added to the perfusion fluid there was a rapid increase in the size of the contractions. In the presence of eserine the contractions produced by injecting acetylcholine into the perfusion fluid were greatly increased. When atropine was injected into the perfusion fluid the contractions caused by postganglionic nerve stimulation returned to the size before eserine was added. In experiments with cats anaesthetized with chloralose, atropine or hyoscine was given first and the effect of eserine on the response to submaximal postganglionic nerve stimulation was determined. Eserine slowly increased the responses to stimulation without increasing the contraction produced by injecting noradrenaline. In other experiments in which maximal stimuli were used, the relation of stimulus frequency to height of contraction was determined. The optimal frequency was low, being 5 to 10 shocks/sec. In the presence of hyoscine, eserine or neostigmine increased the response to stimulation; this increase was greater at lower frequencies, and lessened as the frequency rose to the optimal value.