Response characteristics of oral mechanosensory neurons in the mouse trigeminal ganglion

Mechanosensory neurons play a crucial role in determining the location of stimuli on the receptor surface, movement, as well as the identification and discrimination of textures. To date, little is known about mechanosensory neuron types that innervate the oral cavity. Here, we recorded from mechanosensitive neurons innervating the oral cavity, to examine their diversity and function mediating touch. We first recorded a rough topographical map to aid in locating mechanosensory neuron types innervating the oral cavity. Electrophysiological mapping indicated that neurons innervating the tongue were located within and between maxillary (V2) and mandibular (V3) receptive fields, resembling a "strip" similar to the anatomical position of the tongue. We found that both rapidly adapting and slowly adapting neurons innervate the anterior tongue and lips. Conduction velocity experiments showed that all the lip-innervating neurons were classified as C-fibers, whereas there was a broader range for the tongue-innervating neurons, ranging from C-fibers to A-fast. Interestingly, we found that rapidly adapting and slowly adapting neurons were present at all 3 conduction velocity classifications. The majority of pressure-sensitive neurons also responded to brush stimulation (91%); however, there was a small subset of neurons that only responded to pressure stimulation (9%). The majority of the A-fibers had receptive fields on the anterior tip of the tongue. Furthermore, we found that when a mechanically sensitive neuron innervated a fungiform papilla, its receptive field did not include the surrounding filiform papillae. Our findings provide important contributions to understanding some of the coding features of mechanosensory neurons that innervate the oral cavity.