Burst characteristics of daily jaw muscle activity in juvenile rabbits.

Muscle activation varies with different behaviors and can be quantified by the level and duration of activity bursts. Jaw muscles undergo large anatomical changes during maturation, which are presumably associated with changes in daily muscle function. Our aim was to examine the daily burst number, burst length distribution and duty time (fraction of the day during which a muscle was active) of the jaw muscles of juvenile male rabbits (Oryctolagus cuniculus). A radio-telemetric device was implanted to record muscle activity continuously from the digastric, superficial and deep masseter, medial pterygoid and temporalis during maturation week 9-14. Daily burst characteristics and duty times were determined for activations, including both powerful and non-powerful motor behavior. All muscles showed constant burst numbers, mean burst lengths and duty times during the recording period. Including all behavior, the temporalis showed significantly larger daily burst numbers (205,000) and duty times (18.2%) than the superficial and deep masseter (90,000; 7.5%). Burst numbers and duty times were similar for the digastric (120,000; 11.1%) and medial pterygoid (115,000; 10.4%). The temporalis and deep masseter showed many short low activity bursts (0.05 s), the digastric showed many long bursts (0.09 s). For activations during powerful behaviors the superficial masseter and medial pterygoid had the largest burst numbers and duty times. Both muscles showed similar burst characteristics for all activation levels. It was concluded that activation of the jaw muscles is differently controlled during powerful and non-powerful motor behaviors and the functional organization of motor control patterns does not vary from 9 to 14 weeks of age.