Final answer:
The integration center for the pupillary light reflex is the brainstem.' is C) brainstem.
Step-by-step explanation:
The integration center for the pupillary light reflex is the brainstem. This autonomic reflex is a critical function of homeostasis, involving a sensory input through the optic nerve and a motor response through the oculomotor nerve. When light hits the retina, it is detected by photosensitive ganglion cells and a signal is sent through the optic nerve to the pretectal nucleus in the superior midbrain. From here, a neuron projects to the Edinger-Westphal nuclei in the oculomotor complex.
The neurons in the Edinger-Westphal nuclei give rise to the preganglionic parasympathetic fibers that travel through the oculomotor nerve to the ciliary ganglion, which then projects to the iris. As a result, acetylcholine is released onto circular fibers that contract the pupil. This process is regulated by the hypothalamus and is vital for controlling the amount of light entering the eye. The hypothalamus does influence this reflex, but it is not the primary integration center.
Therefore, the correct option for the question 'The integration center for the pupillary light reflex is the ________.' is C) brainstem.