Final answer:
The pupillary light reflex involves two cranial nerves. The afferent branch, which is the optic nerve (CN II), receives sensory input and sends signals to the pretectal nucleus in the midbrain. The efferent branch, involving the sympathetic nervous system, causes pupil dilation through action on the smooth muscles of the iris.
Step-by-step explanation:
The pupillary light reflex is a complex process involving two cranial nerves. The sensory input responsible for the pupillary light reflex comes through the afferent branch, which is the optic nerve (CN II). When light hits the retina, it activates retinal ganglion cells, which send signals along the optic nerve to the pretectal nucleus in the midbrain. From there, a neuron projects to the Edinger-Westphal nuclei in the oculomotor complex, which gives rise to preganglionic parasympathetic fibers. These fibers travel through the oculomotor nerve (CN III) to reach the ciliary ganglion, where they synapse with postganglionic parasympathetic fibers. These fibers then project to the iris and stimulate the circular fibers to constrict the pupil and reduce the amount of light hitting the retina.
On the other hand, the efferent branch responsible for the competing reflex of pupil dilation involves the sympathetic nervous system. In low light conditions, sympathetic activation leads to the dilation of the pupil. This response is mediated by sympathetic preganglionic fibers that project through the superior cervical ganglion and release norepinephrine onto the smooth muscles of the iris, causing dilation.