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This study focuses on the structure and function of the primary sensory neurons that innervate vibrissal follicles in the rat. Both the peripheral and central terminations, as well as their firing properties were identified using intracellular labelling and recording in trigeminal ganglia in vivo. Fifty-one labelled neurons terminating peripherally, as club-like, Merkel, lanceolate, reticular or spiny endings were identified by their morphology. All neurons responded robustly to air puff stimulation applied to the vibrissal skin. Neurons with club-like endings responded with the highest firing rates; their peripheral processes rarely branched between the cell body and their terminal tips. The central branches of these neurons displayed abundant collaterals terminating within all trigeminal nuclei. Analyses of three-dimensional reconstructions reveal a palisade arrangement of club-like endings bound to the ringwulst by collagen fibers. Our morphological findings suggest that neurons with club-like endings sense mechanical aspects related to the movement of the ringwulst and convey this information to all trigeminal nuclei in the brainstem.

Ion channels at the nodes of Ranvier (NRs) are believed to play essential roles in intrinsic electrophysiological properties and saltatory conduction of action potentials (AP) at the NRs of myelinated nerves. While we have recently shown that two-pore domain potassium (K2P) channels play a key role at the NRs of Ab-afferent nerves, K 1 channels and their functions at the NRs of mammalian motor nerves remain elusive. Here we addressed this issue by using ex vivo preparations of lumbar spinal ventral nerves from both male and female rats and the pressure-patch-clamp recordings at their NRs. We found that depolarizing voltages evoked large noninactivating outward currents at NRs. The outward currents could be partially inhibited by voltage-gated K 1 channel blockers, largely inhibited by K2P blockers and cooling temperatures. Inhibition of the outward currents by voltage-gated K 1 channel blockers, K2P blockers, or cooling temperatures significantly altered electrophysiological properties measured at the NRs, including resting membrane potential, input resistance, AP width, AP amplitude, AP threshold, and AP rheobase. Furthermore, K2P blockers and cooling temperatures significantly reduced saltatory conduction velocity and success rates of APs in response to high-frequency stimulation. Voltage-gated K 1 channel blockers reduced AP success rates at high-frequency stimulation without significantly affecting saltatory conduction velocity. Collectively, both K2P and voltage-gated K 1 channels play significant roles in intrinsic electrophysiological properties and saltatory conduction at NRs of motor nerve fibers of rats. The effects of cooling temperatures on saltatory conduction are at least partially mediated by K2P channels at the NRs.

Hairs are known as a sensory apparatus for touch. Their follicles are innervated predominantly by palisade endings composed of longitudinal and circumferential lanceolate endings. However, little is known as to how their original primary neurons make up a part of the ending. In this study, innervation of the palisade endings was investigated in the auricular skin of thy1-YFP transgenic mouse. Major observations were 1) Only a small portion of PGP9.5-immunopositive axons showed YFP-positivity, 2) All of thy1-YFP-positive sensory axons were thick and myelinated, 3) Individual thy1-YFP-positive trunk axons innervated 4–54 hair follicles, 4) Most palisade endings had a gap of lanceolate ending arrangement, 5) PGP9.5-immunopositive 10–32 longitudinal lanceolate endings were closely arranged. Only a part of them were thy1-YFP-positive axons that originated from 1–3 afferents, and 6) Single nerve bundles of the dermal nerve network included both bidirectional afferents. Palisade endings innervated by multiple sensory neurons might be highly sensitive to hair movement.

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