The mesencephalic nucleus is involved with proprioception of the face, that is, the feeling of position of the muscles. Unlike many nuclei within the central nervous system (CNS), the mesencephalic nucleus contains no chemical synapses but are electrically coupled. Neurons of this nucleus are pseudounipolar cells that receive proprioceptive information from the mandible and send projections to the motor trigeminal nucleus to mediate monosynaptic jaw jerk reflexes. It is also the only structure in the CNS to contain the cell bodies of a primary afferent, which are usually contained within ganglia (like the trigeminal ganglion).
A lesion involving the trigeminal mesencephalic nucleus would cause ipsilateral motor deficits.