A simplified version of the ocular sensory apparatus. First order neuron (red solid line) with nerve ending in the cornea, cell body in the trigeminal ganglion, and synapse in the subnucleus caudalis. In actuality, however, there are multiple synapses for each nociceptor in the trigeminal subnucleus interpolaris/subnucleus caudalis (Vi/Vc) transition zone, and in the subnucleus caudalis/upper cervical transition zone (Vc/C1–2). Second order neurons (red dashed line) decussate and join the contralateral spinothalamic pathways and synapse in the thalamus. Third-order neurons (red dashed line) then relay information to the supra-spinal centers, including the somatosensory cortex. Reproduced with permission from Ocul Surf. 2012 Jan;10(1):2–14.