Generation of masticatory rhythm in the brainstem.

Mastication is a typical rhythmical behavior in mammals. Like respiration, it is now generally accepted that the motor command for the basic pattern of rhythmical oral-facial movements is generated by a neuronal population in the brainstem (central pattern generator, CPG). The central pattern generation of rhythmical masticatory movements can be divided into three processes: (1) generation of the masticatory rhythm, (2) generation of a pattern of activities of the jaw, tongue and facial muscles, and (3) coordination of the activities of these muscles. There are several lines of evidence that the masticatory CPG is functionally subdivided into two neuronal groups: one for generation of the masticatory rhythm, giving the timing signal for rhythmical alternation of jaw closing and jaw opening (central rhythm generator, CRG), and the other for generation of the spatiotemporal pattern of activities of the jaw, tongue and facial muscles. This review will deal, first of all, with the localization of the CRG for rhythmical masticatory jaw movements, sources for its activation, and the premotor neurons mediating its output to the trigeminal motoneurons. Next, we will discuss the neurochemical basis for rhythmical trigeminal motoneurons activity as well as central masticatory rhythm generation. Finally, our recent attempt at induction of neural activities reflecting sucking movements (fictive sucking) in an in vitro preparation is presented.