GENERATION OF MASTICATORY RHYTHM IN THE BRAIN-STEM

Mastication is a typical rhythmical behavior in mammals. Like respirat ion, it is now generally accepted that the motor command for the basic pattern of rhythmical oral-facial movements is generated by a neurona l population in the brainstem (central pattern generator, CPG). The ce ntral pattern generation of rhythmical masticatory movements can be di vided into three processes: (1) generation of the masticatory rhythm, (2) generation of a pattern of activities of the jaw, tongue and facia l muscles, and (3) coordination of the activities of these muscles. Th ere are several lines of evidence that the masticatory CPG is function ally subdivided into two neuronal groups: one for generation of the ma sticatory rhythm, giving the timing signal for rhythmical alternation of jaw closing and jaw opening (central rhythm generator, CRG), and th e other for generation of the spatiotemporal pattern of activities of the jaw, tongue and facial muscles. This review will deal, first of al l, with the localization of the CRG for rhythmical masticatory jaw mov ements, sources for its activation, and the premotor neurons mediating its output to the trigeminal motoneurons, Next, we will discuss the n eurochemical basis for rhythmical trigeminal motoneuron activity as we ll as central masticatory rhythm generation. Finally, our recent attem pt at induction of neural activities reflecting sucking movements (fic tive sucking) in an in vitro preparation is presented.