Direct projections from the masseteric nerve to the mesencephalic nucleus

The caliber spectrum of the nerve to the masseter muscle of the cat was found to be unimodal, ranging from 1 to 16 μ, with the majority of the fibers in the 4‐8 μ range. The upper 25%, which includes the afferent propriococoptive fibers, averaged 10.4 μ. The nerve to the masseter was stimulated and evoked potentials were recorded from the ipsi‐ and contralateral mesencephalic nuclei of the trigeminus and from the contralateral nerve. The average latencies were 0.846, 0.935 and 1.46 msec respectively. The mesencephalic nucleus was stimulated and evoked potentials were recorded from the contrlateral nucleus with an average latency of 0.385 msec. Analysis of the latencies of the responses recorded from the ipsilateral nucleus suggested that about half of the fibers of the nerve to the masster projected directly and the other half projected by way of an intervening synapse. A similar distribution of the fibers to the contralateral nucleus was deduced from the recorded latencies. Potentials recorded in the nuclei as a result of stimulation of the contralateral nucleus demonstrated that there were direct internuclear pathways without intervening synapses. The latencies of potentials recorded in the nerve as a result of stimulation of the contralateral nerve indicated that there was always at least one synapse intervening. It was also shown that there were direct projections from the nerve of the masseter to the contralateral motor nucleus of V. The work confirms our earlier report of bilateral projections from a muscle of mastication to the mesencephalic nuclei and provides a pathway for bilteral simultaneous activation of the muscles of mastication resulting from stimulation of unilateral tension receptors.