Nature of neck motor unit activities evoked by different trigeminal inputs in rats

Recent reports have suggested functional interactions between the masticatory and head motor systems. However, the neurophysiological mechanisms behind these phenomena have not been thoroughly investigated. In this study, the motor unit activity was recorded bilaterally from the dorsal neck splenius muscles (SPL) while the cell bodies of trigeminal afferent fibers were stimulated at either the trigeminal mesencephalic nucleus (Me5) or the Gasserian ganglion (5Gn) in 13 barbiturate-anesthetized Wistar rats. After Me5 electrical stimulation, SPL motor unit responses were recorded ipsilaterally (n = 24), with an onset latency distribution between 1.3 and 1.8 ms. After 5Gn stimulation, SPL responses were recorded bilaterally from 18 single units, with latencies ranging from 2.2 to 2.4 ms. Our results suggest that there are at least two distinct pathways from the trigeminal afferents to the neck muscles, with significant functional differences.