Sympathetic Influences on the PLR

It is generally agreed that the parasympathetic pathway discussed above is the primary route ofpupillary constriction associated with the PLR. However, there is some evidence that increases in retinal illumination may cause a reduction in the tone of the dilator muscle of the iris through the sympathetic pathway outlined in Figure 1 , and thus enhance the PLR. Studies in cats have shown a light-induced inhibition of postganglionic pupillodilation fibers at the level of the long ciliary nerves, and preganglionic, pupillodilation fibers at the level of the cervical sympathetic nerve. These studies found that the pupillo-dilation fibers were inhibited by light in an intensity-dependent manner, that is, a more intense light brought about a greater inhibition in firing rate. However, these findings have not been replicated in primates, in which the evidence suggests that the sympathetic system does not contribute to the dynamics of the PLR and only contributes to tonic modulation of pupil diameter.

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