Central distribution of masticatory and lingual proprioceptive afferents relative to jaw and tongue movements--Part I. Masticatory proprioceptive afferents of the mouse

Abstract

The central projection of the trigeminal mesencephalic tract neurons innervating the jawclosing muscle spindles was investigated on the mouse by injecting horseradish peroxidase (HRP) into the masseteric nerve. The HRP-labeled cells were ipsilaterally identified throughout the rostrocaudal extent of the trigeminal mesencephalic tract nucleus. The central processes of the mesencephalic tract neurons terminated ipsilaterally into the nuclei as follows: the trigeminal motor nucleus, trigeminal main sensory nucleus, supratrigeminal nucleus, trigeminal spinal tract nucleus, lateral parts of pontine and medullary reticular formations, vagal dorsal motor nucleus, hypoglossal nucleus and lamina V of the C1-C2 segments of the spinal cord. But, they were not identified in the facial nucleus, solitary nucleus and cerebellum. The localization of the trigeminal mesencephalic tract neurons innervating the jaw-closing muscle spindles was investigated by injecting fluorescent compounds as retrograde tracers into the jaw-closing muscles: masseter, temporal and medial pterygoid muscles. Fluorescent-labeled cells were found intermingled throughout the whole nucleus rostrocaudally. Double or triple fluorescent-labeled cells were not identified in the nucleus. It is suggested that there is no axonal branching from the muscle spindle afferent outside the jaw-closing muscle bellies.