Evidence that the human jaw stretch reflex increases the resistance of the mandible to small displacements.

Abstract

Small step or sinusoidal displacements were imposed on the mandible while human subjects maintained an average biting force of 10 N. Phase-related changes in the force resisting sinusoidal displacement were used to determine the mechanical stiffness of the human mandibular system as a function of the frequency of stretching. Jaw-muscle electromyographic (emg) responses to step stretches were of 8 ms latency and generated a very substantial force response. The mechanical stiffness of the human mandible was relatively constant as a function of the frequency of stretching. The force resisting displacement was phase-advanced at all frequencies. Modulation of jaw muscle electrical activity evoked by sinusoidal stretches increased in amplitude as a function of increasing stretch frequency. The jaw stretch reflex is apparently dependent on jaw muscle spindle afferent fibers exciting jaw-closing motoneurones by relatively direct (but not necessarily monosynaptic) connexions. The relationship between jaw-muscle activity and voluntary fluctuations of isometric biting force suggests the human jaw muscles can probably be modelled as a 2nd-order linear filter. Human jaw muscle stiffness is apparently similar to the intrinsic stiffness of the gastrocnemius of the cat. The stretch reflex of the human mandible contributes functionally to its postural stability. Reflex stiffness is greater in the monkey mandible relative to muscle stiffness than in the human mandible. The difference is argued to be a manifestation of the difference in jaw muscle contraction speed between the 2 species. The mandibular stretch reflex is apparently stronger than the stretch reflex of the limbs of intact animals and humans. The special anatomical and functional features of the mandible are discussed.