Relationship between electrophysiological signature and defined sensory modality of trigeminal ganglion neurons in vivo.

The trigeminal ganglia (TG) innervate a heterogeneous set of highly sensitive and exposed tissues. Weak, innocuous stimuli can evoke pain as a normal response in some areas such as the cornea. This observation implies, however, the capability of low-threshold mechanoreceptors, inducing pain in the normal condition. To clarify this matter, the present study correlates the electrical signature (both fiber conduction velocity and somatic electrical properties) with receptor field, mechanical threshold, and temperature responsiveness of sensory afferents innervating tissues with dissimilar sensitivity (skin vs. cornea) in the trigeminal domain. Intracellular recordings were obtained in vivo from 148 neurons of the left TG of 62 mice. In 111 of these neurons, the peripheral receptor field was successfully localized: 96 of them innervated the hairy skin, while the remaining 15 innervated the cornea. The electrical signature was defined and peripheral responses correlated with tissue target. No high threshold neurons were found in the cornea. Moreover, the electrical signature of corneal afferents resembles nociceptive neurons in the skin. TG skin afferents showed similar membrane electrical signature and sensory modality as skin afferents from dorsal root ganglion, although TG afferents exhibited a shorter duration of afterhyperpolarization then those previously described in dorsal root ganglion. These data suggest than new or different ways to classify and study TG sensory neurons may be required.