Pupillary Light Reflex and the Accommodation Reflex

The deeper collicular layers are the source of several efferent projections. One group of fibers crosses the midline and runs caudally, sending terminals to the brainstem reticular formation and then continuing on to cervical and thoracic levels as the tectospinal tract; these fibers are probably involved in the orienting movements of the head and body. A second group of fibers projects to the posterior thalamus (pulvinar), which then projects to the cortical association areas. Fiber projections responsible for eye movements relay in the mesencephalic reticular formation below the superior colliculus (vertical eye movements), and in the paramedian pontine reticular formation (horizontal eye movements).

PUPILLARY LIGHT REFLEX AND THE ACCOMMODATION REFLEX

The pretectum, like the superior colliculus, receives visual information from optic nerve fibers not destined to synapse in the lateral geniculate bodies. This area is involved in the pupillary light reflex (which regulates the size of the pupil) and the accommodation reflex (which controls the degree of curvature of the lens). The former is a subcortical reflex and relays in the accessory oculomotor (Edinger-Westphal) nucleus, whereas the latter involves pathways through the cerebral cortex. In the pupillary light reflex, afferent pupillary fibers leave the optic tract before the lateral geniculate bodies, travel in the brachium of the superior colliculus, and synapse in the pretectal nuclei (explaining why lesions of the geniculate bodies, the optic radiations, or the visual cortex do not affect the pupillary reactivity, and why lesions of the brachium of the superior colliculus can cause a relative afferent pupillary defect without causing a visual field defect). Both pretectal nuclei receive input from both eyes, and each sends axons to both Edinger-Westphal nuclei. Parasympathetic fibers for pupillary constriction leave the Edinger-Westphal nucleus and travel along the ipsilateral third cranial nerve to the ipsilateral ciliary ganglion within the orbit. The postganglionic parasympathetic fibers innervate the pupillary constrictor muscle and the ciliary muscle for accommodation.

Only gold members can continue reading. Log In or Register to continue