Our bodies are covered by skin. More than a protective cover, skin is our largest sensory organ. Through it we sense warmth and heat, cooling, the gentlest of touch, the pain from injury, and the rapid warning of potential dangers. These sensations occur through the activation of peripheral neurons that densely innervate skin. The afferent projections of these peripheral neurons create a sensor net that is activated by stimuli in the external world. Despite appearances that might seem otherwise, most mammals including us are largely covered with hairs. Each hair is innervated by functionally and morphologically diverse afferent endings. These endings are created by distinct classes of peripheral sensory neurons with cell bodies residing in the trigeminal and dorsal root ganglia. In the late 19th and early 20th century, anatomists such as Maximilian von Frey and physiologists such as Charles Scott Sherrington formulated the hypothesis that specific classes of sensory neurons detect distinct types of stimuli. Their work has led to a long line of research to establish links between sensory neuron function, neurochemistry, and the morphology of the terminals in skin. In this study, we’ve investigated the function of the subset of sensory neurons that express Calca, one of two genes encoding the neuropeptide calcitonin gene-related peptide, CGRP. We chose to focus on these neurons because CGRP has a long history in pain research, and it was previously shown that neurons expressing Calca are critical for thermal sensation but dispensable for mechanical sensation. We reasoned that since recent advances in genetics and functional imaging would allow us to better understand the function and anatomy of this important class of sensory neurons. In our experiments, we used knock-in mice expressing tamoxifen inducible Cre recombinants under the Calca promoter. By developing an imaging preparation to visualize the evoked responses in sensory ganglia in vivo we could assay the responses of hundreds of neurons expressing the genetically encoded calcium indicator GCaMP6 at the same time to skin stimulation while leaving the system intact. We found that neurons expressing Calca that project to the cheek are insensitive to gentle stimulation such as stroking in both directions, with and against the grain of the hairs. Instead, these neurons responded robustly to high-threshold mechanical stimulation, notably, pulling of the hairs. Furthermore, many Calca GCaMP6 neurons responded to temperatures, but this was limited to the noxious range. Notably, we found that the subset of heat -insensitive Calca neurons were resistant to ablation via a potent Trpv1 agonist RTX. These were the larger diameter Calca neurons, and they responded solely to high-threshold mechanical stimulation. Calca neurons make two kinds of afferent endings on the hairy skin. One is the free nerve endings that are known to be polymodal nociceptors and one is a circumferential ending which has no known functions. Interestingly, only the circumferential endings resist the treatment with RTX. Thus, Calca-positive neurons that make circumferential endings sense high-threshold mechanical stimulation, notably hair pull. Based on their function and morphology we call these neurons Circ-HTMRs. Later, we found that our imaging approach can be extended to image lumbar dorsal root ganglia which innervate the trunk and the limbs. This allowed us to combine GCaMP6 imaging with electrophysiology recording, and we found that with this approach we can resolve single electrically evoked spikes with high fidelity. That provides us with knowledge that Circ-HTMRs are fast -conducting A delta mechanonociceptors, and they adapt very slowly to prolonged presentation of sensory stimuli. They are sensitive to the pull of even a single hair and yet they have large receptive fields. So when you put this all together, our results show that circ-HTMRs are a unique type of mechanosensory neuron. They function like nociceptors yet they make specialized endings that were traditionally associated with touch neurons. Their unique properties make them ideally suited to enable the rapid and precise localized sensation of having a hair pulled, and they evoke the appropriate behavioral responses. Thus, our work explains a sensation that we all experience in our daily lives and furthers our knowledge about how pain is encoded by our somatosensory system. It’s our hope that such knowledge will ultimately help us identify new targets for alleviating mechanical pain that is common to many pathological conditions. AAAAH!