Pathways and nerve centers that control pupil size and the pupillary light reflex in humans. Colored structures are the central nerve centers and pathways that modify the pupillary light reflex. Edinger Westphal (EW) Nucleus neurons are pacemaker cells that are modified by excitatory and inhibitory inputs. Opioids block the inhibition of the EW nucleus. Green asterisks (*) show locations where hypercarbia, hypoxia, and opioids might potentially interfere with the light reflex, although our analysis demonstrates that the effect of opioids on the light reflex is primarily a function of pupil size. * = locations where hypercarbia, hypoxia, and opioids might potentially interfere with the light reflex. IN = inhibitory neuron.