Pupil pathways

The iris's movement and hence its image, the pupil of the eye, is controlled by the action of two antagonistic muscles, the sphincter and the dilator. Activation of the sphincter of the iris causes the pupil to constrict (i.e., miosis), This constriction is largely under parasympathetic control, while the dilator muscle receives mostly sympathetic innervation and causes the pupil to dilate (i.e., mydriasis). As with any aperture in a lens system, the size of the iris determines the amount of light that is captured by the system and, in the case of the eye, the level of retinal illuminance. In addition, the pupil controls the aberrations and the depth of field of the eye. The parasympathetic pathway is understood better and involves the Edinger-Westphal (EW) nucleus, the ciliary ganglion, and the short ciliary nerves (Kardon, 1998). The firing rate of pupilloconstrictor neurons in the EW nucleus is high, even in the absence of external influences (Sillito and Zbrozyna, 1973). In this respect, the pupilloconstrictor neurons in the EW nucleus act as a signal generator whose output is modulated by a number of inputs, the major one being the afferent projection from the retina via the olivary pretectal nucleus (OPN) of the midbrain (Gamlin et al., 1997; Pierson and Carpenter, 1974). The strength of these modulating inputs can, in turn, be determined by a number of other factors that may involve the cortical processing of specific stimulus attributes.

Strong inhibitory inputs to the EW nucleus from other areas of the brain have also been reported (Smith et al., 1970). The inhibition of the parasympathetic system may involve at least two pathways, a noradrenergic pathway from the nucleus coeruleus to the EW complex (Breen et al., 1983; Koss et al., 1984; Loewy et al., 1973) and a second indirect pathway from A1/A5 nuclei in the brainstem via the hypothalamus (Koss and Wang, 1972; Loewenfeld, 1958; Szabadi and Bradshaw, 1996). Signal changes in these inhibitory pathways can be used to explain a number of other pupil-related observations such as the constriction of the pupil during sleep or anesthesia (Larson et al., 1996), the gradual decrease in pupil size with age (Loewenfeld, 1972), and the slow oscillations of the pupil observed during periods of sleepiness (Lowenstein et al., 1963; Wilhelm et al., 1999).