Abstract
The purpose of this study is to investigate the relationship between the incisal opening and the amplitude of the jaw-jerk reflex in the human beings.
The findings were summerized as follows;
1. The amplitudes of jaw-jerk reflexes were decreased according to the increase of incisal opening.
2. In all subjects, jaw-jerk reflexes were not evoked at mostly maximum jaw opened.
3. The regression lines between the amplitudes of the reflex and the displacements of the incisor elicited by the chin tapping at the different jaw openings suggest two different ways of the decreases of the reflex amplitude. The first is the increase of the threshold of the jaw-jerk reflex circuit and the second is the decrease of the sensitivity of the reflex circuit. The considerable mechanism for these changes of the reflex circuit are as follows;
a. The membrane potential changes of α-motoneurons, that is depolarization or hyperpolarization.
b. The change of muscle spindles sensitivity which is controlled by γ-motoneuron activity.
c. Primary afferent depolarization could change the efficiency of the synaptic transmission from Ia afferents to α-motoneurons.
These findings suggest that the increase of occlusal vertical dimension by such as occlusal splints for temporomandibular joint disoders could lead the relaxation of jaw-closing muscles and susseedingly doctors are able to improve the occlusion of the patients easier.
