Human jaw reflexes.

Although the jaw reflexes are analogous in many ways to the corresponding limb reflexes, important differences do exist. The myotatic reflex appears to contribute more stiffness to the jaw-closing muscles than to limb muscles. The jaw tends to swing up and down during locomotion, and, to maintain a stable position in relationship to the skull, it is necessary that the muscles be made stiff by tonic contraction and/or through a powerful servoreflex. The short conduction pathway and rapid contraction of jaw muscles allow reflex effects to act with little phase lag and to provide efficient compensation. If limb muscle reflexes were equally powerful, their effects could be of more nuisance than help in overcoming expected loads because they occur so late. Perhaps the lack of Renshaw cell inhibition of trigeminal MNs and the potentiation of the jaw jerk reflex by chin vibration are features designed to maintain the strength of the myotatic reflex during locomotion. The jaw-opening reflex (including exteroceptive suppression of jaw-closer muscle activity) is bilaterally symmetrical rather than bilaterally reciprocal, as are the analogous spinal flexor withdrawal reflexes. Bilateral braking is necessary to stop closure, because the mandible crosses the midline, whereas withdrawal of a limb often needs to be compensated for by extension of the other to maintain balance. It has recently been shown in animals that limb and jaw reflex responses are highly context dependent: the size and direction of limb reflexes depend on the phase of locomotion (Forssberg et al., 1977), and the gain of the jaw-opening reflex is increased during the closing phase of mastication (Lund et al., 1981).