Dual Convergence of Facial Nerve Branches Innervating Whisker Pad in Rats.

The precise anatomy of the facial nerve branches innervating rat whisker pad and the distribution of their corresponding motor neurons in facial nucleus area were investigated. The extratemporal facial nerves of 6 rats were anatomically observed under a surgical microscope, and then the nerve specimens of facial nerve branches at 7 anatomical sites were taken and examined for the axons and myelin sheath using Luxol fast blue staining. The distribution of facial motor neurons innervating the facial branches was observed in 12 rats by retrograde labelling. The distal pes, a fusing architecture of the buccal and marginal mandibular branches, was found to furcate into superior, middle and inferior branches to innervate whisker pad. Histologically, the myelin sheath of each branch was morphologically consistent, and the nerve fiber bundles of facial nerve branches became increasingly thinner and scattered, particularly after crossing the distal pes site and innervating the whisker pad. The facial motor neurons innervating the buccal and marginal mandibular branches were clearly distributed in similar regions in facial nucleus. This study confirmed the highly spatial synergy between the buccal and marginal mandibular branches innervating the whisker pad from extratemporal anatomy and distribution of facial motor neurons.

3D reconstruction and standardization of the rat facial nucleus for precise mapping of vibrissal motor networks.

The rodent facial nucleus (FN) comprises motoneurons (MNs) that control the facial musculature. In the lateral part of the FN, populations of vibrissal motoneurons (vMNs) innervate two groups of muscles that generate movements of the whiskers. Vibrissal MNs thus represent the terminal point of the neuronal networks that generate rhythmic whisking during exploratory behaviors and that modify whisker movements based on sensory-motor feedback during tactile-based perception. Here, we combined retrograde tracer injections into whisker-specific muscles, with large-scale immunohistochemistry and digital reconstructions to generate an average model of the rat FN. The model incorporates measurements of the FN geometry, its cellular organization and a whisker row-specific map formed by vMNs. Furthermore, the model provides a digital 3D reference frame that allows registering structural data - obtained across scales and animals - into a common coordinate system with a precision of ∼60 µm. We illustrate the registration method by injecting replication competent rabies virus into the muscle of a single whisker. Retrograde transport of the virus to vMNs enabled reconstruction of their dendrites. Subsequent trans-synaptic transport enabled mapping the presynaptic neurons of the reconstructed vMNs. Registration of these data to the FN reference frame provides a first account of the morphological and synaptic input variability within a population of vMNs that innervate the same muscle.