The anatomical substrates subserving the pupillary light reflex in rats: origin of the consensual pupillary response.

While the olivary pretectal nucleus has been shown to be central to the pupillary constriction response in rats, it is not at all clear at which level the consensual response is generated. To examine this we have investigated the efferent projections of this nucleus, as well as the effect of unilateral lesions of the olivary pretectal nucleus, on the direct and consensual pupillary light reflexes. The results demonstrate that the olivary pretectal nucleus projects bilaterally to the Edinger-Westphal nucleus, as well as to the nucleus of the posterior commissure, which itself projects bilaterally to the Edinger-Westphal nucleus. The olivary pretectal nucleus also projects to the ipsilateral ventral lateral geniculate nucleus. Unilateral lesions of the olivary pretectal nucleus decrease, but do not abolish, the direct and consensual pupillary light reflexes by as much as 66%. Since some degree of consensual response remains, this is likely to be due to the bilateral projection from the olivary pretectal nucleus, either directly or indirectly through the nucleus of the posterior commissure, to the Edinger-Westphal nucleus. These results show that while the bilateral projection from the olivary pretectal nucleus to the Edinger-Westphal nucleus contributes to the consensual pupillary light reflex, the bilateral retinal projection to the olivary pretectal nucleus is the more determinant component of the pathway. In addition to providing insights into the anatomical substrates of the pupillary response, this work also provides a background to ongoing studies of the retinal graft-mediated pupillary light reflex, as well as serving as a model of a relatively simple reflex system.