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Degeneration of the primary snout sensory afferents in the cervical spinal cords following the infraorbital nerve transection in some mammals.

To obtain the neuroanatomical information on the role of the snout sensory input in mastication, the present study was conducted on young and adult mice, young Wistar rats and adult Japanese shrew-moles. The animals were subjected to unilateral and bilateral infraorbital nerve transection. Transganglionic degeneration was studied by the Nauta method and electron microscopy including HRP application to the neck muscles. Transganglionic degeneration was found in every experimental case. 1. Transganglionic degeneration of the fibers was found not only in the main sensory nucleus and spinal tract nucleus of the trigeminal nerve but throughout the cervical and the upper part of the thoracic spinal cord. 2. These transganglionically degenerated fibers descended bilaterally through the cuneate nucleus and then caudally through the posterior funiculus at the obex level. They then entered the dorsal and ventral horns to make a synaptic contact with the degenerated synapses on the dorsal horn cells and with the multipolar cells in the ventral horns. This neuroanatomical information suggests: 1) that the trigemino-neck muscle reflex will be generated monosynaptically by the primary neurons arising from the snout sensory organs and 2) that these primary neurons may play a large role as a neuronal bridge in connecting the masticatory reflex system and the cranio-neck reflex system.