Anatomy of the light reflexes

The direct and consensual light reflexes are both based around a similar neuronal pathway. Afferent signals pass sequentially: retina - optic nerve - optic chiasm - optic tract. A minority of the optic tract fibres pass to the pretectal nuclei between the rostral border of the mesencephalon and the caudal border of the epithalamus. From the pretectal nuclei, two efferent pathways arise:

• a link via the posterior commissure to the contralateral pretectal nucleus - this permits the bilaterality of the consensual response
• a forward pathway to the parasympathetic nuclei of Edinger Westphal

From the nucleus of Edinger Westphal, preganglionic fibres run with the oculomotor nerve to the ciliary ganglion and thence, as postganglionic fibres, to the ciliary body - broadens lens - and constrictor pupillae of the iris.