A sensory-motor trigeminal-hypoglossal loop modulates the whisker pad extrinsic muscles activity in the rat

Neuroanatomical and electrophysiological findings recently showed in rats that the whisker pad extrinsic muscles are driven not only by the facial nerve but also by small hypoglossal neurons whose axons travel along the ION (Mameli et al. 2006, 2008). Other recent experiments demonstrated that: 1) the trigeminal mesencephalic (Me5) neurons are significantly modulated by spontaneous and artificial movements of the macrovibrissae and, 2) the Me5 neuron peripheral axons provide a sensory innervation of the macrovibrissae (Mameli et al. 2010, 2014). It has been concluded that the Me5 neurons could encode kinetic informations related to macrovibrissae movements. The present study investigated the possibility that the small hypoglossal neurons innervating the extrinsic pad muscles represent the output of a sensory-motor loop activated by stimulation of the Me5 macrovibrissae receptors. Two tracers were used in the same animal: Fluoro Gold was injected into the whisker pad to retrogradely label the hypoglossal-whisker pad projective neurons, while Dil was injected into the Me5 nucleus to label its projections to the XII nucleus. Results demonstrated that the hypoglossal neurons innervating the whisker pad extrinsic muscles are monosynaptically targeted by axonal terminals of the Me5 neurons. It has been therefore hypothesized the existence of a Me5-XII loop aimed to induce reflex responses in whisker pad extrinsic muscles by stimulation of the Me5 macrovibrissae receptors.