Figure 2.

Model for neural control of lacrimation. Corneal primary afferent neurons express a range of membrane channels, which corresponds to their physiological characteristics. The primary afferent neurons innervating the cornea regulate secretion of basal tearing with a relay through the spinal trigeminal nucleus. It has been proposed that polymodal nociceptors express channels responding to noxious chemical, thermal, and mechanical stimulation, including TRPV1, TRPA1, TRPV4, and acid sensing ion channels (ASIC) channels. In contrast, cold receptors express TRPM8 channels, which are sensitive to innocuous cooling. The channels responsible for mechanical responses in mechanoreceptive neurons and hyperosmotic responses in cold receptors have yet to be identified. PPG: Pterygopalatine ganglion