Trigeminal convergence from human teeth: influence of contralateral stimulation and stimulus frequency on the pain perception threshold

The arrangement of trigeminal nerve fibres and their secondary and subsequent neurones in the central nervous system is incompletely understood, but primary neurones from teeth pass to the trigeminal spinal nucleus on the same side. This means that stimulation of adjacent teeth on one side of the mouth could be associated with spatial summation in the trigeminal spinal nucleus, and this should reduce the intensity of stimulation required to reach the pain perception threshold. If the two teeth stimulated were on opposite sides of the mouth, no spatial summation would be possible at that level, so the pain perception threshold would be relatively higher. To test the hypothesis, electrical stimuli were applied to the teeth of human subjects who were able to indicate when the pain perception threshold was reached. This threshold was determined for single teeth and also for a series of teeth. The series started with an upper canine and extended tooth by tooth to the contralateral canine. The threshold value for two or more teeth was called the multiple value (M) and this was compared with the added individual values (A) for the same number of teeth. The $\text{M}\text{A}$ ratio thus obtained was not decisively affected by stimulation being extended to teeth across the midline.The investigation was made using two stimulation frequencies, 50 and 7 Hz. and the same result was obtained in each series. However, the peak current values were consistently higher when the lower frequency was used. This result had been predicted, and is explained by there being less opportunity for temporal summation at the synapses when using the lower frequency.