Rank-ordered Recruitment of Masseter Motoneurons during Slow Closing Phase of Mastication Cycle Presumably by the Activity of Mesencephalic Trigeminal Neurons

It is well established that motor units are recruited depending on the order of the size or input resistance of motoneurons, which is known as the size principle. During the slow-closing phase of the mastication cycle, the isometric contraction of jaw-closing muscles is developed through the recruitment of jaw-closing motoneurons. Two-pore-domain acid-sensitive K+ (TASK1/3) channels were recently found to be the molecular correlates of input resistance, and were also found to be expressed in the masseter motoneurons. Here, we discuss the possibility that spindle Ia inputs can induce the orderly recruitment of masseter motoneurons, and the possible involvements of TASK channels in the orderly recruitment.