Figure 1: Oculosympathetic Pathway for Pupillary Dilation

The oculosympathetic tract is an uncrossed pathway that begins in the hypothalamus, with fibers descending in the brainstem (1st order, commonly affected in a lateral medullary syndrome), synapsing in the lower cervical/upper thoracic spinal cord (interomediolateral cell columns of C8-T2, also referred to as the ciliospinal center of Budge) and continuing on as the 2nd order fibers (in proximity to the lung apex). The tract ascends and then synapses in the superior cervical ganglion. The 3rd order neuron leaves the ganglion, with sudomotor fibers following the external carotid artery (explanation for absence of anhidrosis with an internal carotid artery dissection), while the remaining fibers ascend with the internal carotid artery (explanation for dissection causing a painful Horner's). The 3rd order fibers innervate the eyelid (superior [Muller muscle] and inferior tarsal muscles) and pupillary dilator muscles to open the eyes and dilate the pupils, respectively. A lesion along the oculosympathetic tract causes a Horner's syndrome, with ptosis, miosis, and sometimes clinically apparent anhidrosis (with 1st or 2nd order, but not 3rd order).