These combined actions usually arrest movement before tooth contact is made. The unloading reflex mechanism can be explained by the fact that muscle spindles normally supply part of the excitation of the jaw closing motor neurones, the effect of sudden uncompensated shortening of the muscle is a brief failure on the part of the muscle spindles to generate afferent impulses. The resultant fall in the excitation of the motor neurones leads to the period of silence in the EMG known as the unloading reflex. This reflex can be elicited by brief mechanical stimuli applied to the teeth or mucosa or by electrical stimuli applied to any of the sensory nerve fibres of the second and third divisions of the trigeminal nerve. Under normal conditions, the jaw-opening reflex is elicited by periodontal loading when the teeth come into sudden contact with each other or with hard food.
Silent periods are particularly evident when chewing brittle foods, such as hard biscuit.
Similar inhibitory responses have been reported following electrical stimulation of several intra-oral sites. Subsequently the motor neurones return to their normal asynchronous pattern of firing and produce the normal EMG interference pattern shown in the oral reflex demonstration. There is considerable controversy surrounding the possibility of a jaw closing reflex. I would suggest that the existence of this reflex is questionable and that its function in normal feeding unproved. The muscle spindle involvement Masseter and temporalis muscle spindles contribute to the jaw jerk and unloading reflexes.