Respiratory fluctuations in pupil size

Regular fluctuations in pupil size of the cat were measured and the properties, nervous pathways, and origin of these oscillations were investigated. The rhythm of pupil movements under control conditions appeared to be either locked to the central respiratory cycle or to the artificial ventilatory cycle. These movements were only seen in lightly anesthetized or tranquilized cats, but not in alert or deeply anesthetized cats (ether, halothane or pentobarbital). The fluctuations proved to be independent of sympathetic innervation but related to variations in parasympathetic outflow. At least two sources for pupil oscillations appeared to be involved: central respiratory activity and respiratory blood pressure fluctuations that modulated pupil width via sinoaortic baroreceptors. Lung movements per se, as a third possible factor, did not modulate pupil width, whereas electrical stimulation of the afferent lung vagi did; therefore the role of this mechanical factor is not clear. A review of the pertinent literature shows that in the organism there are many phenomena exhibiting respiratory oscillations. It seems likely that these oscillations have the same origin as the respiratory pupil fluctuations.