THE ROLE OF SENSORY INFORMATION IN THE CONTROL OF RHYTHMIC OPEN-CLOSEMOVEMENTS IN HUMANS

Chewing requires a low level of muscle activity for jaw movement. Addi tional muscle activity is required to overcome the resistance of the f ood. The additional muscle activity consists of two contributions, an anticipating contribution before food contact and a peripherally induc ed contribution, about 23 ms after food contact. The amounts of both c ontributions depend on the information about food resistance obtained in preceding chewing cycles. It is not known whether this information is preserved if the resistance is absent during only a limited number of chewing cycles. Our aim was to investigate the extent to which info rmation about food resistance obtained during chewing is used during s ubsequent cycles to generate anticipating and peripherally induced mus cle activity. Subjects made rhythmic open-close movements at their nat ural chewing frequency, controlled by a metronome. Food resistance was simulated by an external force acting on the jaw in a downward direct ion during part of the closing movement. Taw movement and surface EMG of the masseter and suprahyoid muscles were recorded during experiment s in which sequences of at least 20 cycles with the force were alterna ted with a small, random number (from 1 to 10) of cycles without the f orce. The amount of anticipating muscle activity as well as the periph erally induced muscle activity in the first cycle with the force gradu ally decreased as a function of the number of preceding forceless cycl es. About 30% of the additional muscle activity had an anticipatory or igin, whereas the rest of the activity was evoked by the force regardl ess of the number of preceding forceless cycles.