Abstract
The effects on the pain threshold of teeth during conditioning stimulation with different frequencies were studied in volunteers. High intensity stimulation of the cheeks at 100/sec produced at onset a transient increase of the pain threshold essentially restricted to teeth of the upper jaw. No increase of the pain threshold was obtained by stimulation at 100/sec of the hands. Conditioning with 10/sec of the cheeks gave a rapid rise of the threshold followed by a gradual increase, but during prolonged conditioning the threshold declined. Stimulation with 2/sec produced a slow gradual increase of the threshold which remained at high level throughout a longlasting period. The after-effect was more pronounced at stimulation with 2/sec as compared to 10/sec. The segmental mechanisms of pain are discussed and it is suggested that the pain afferents should be considered as a subgroup of the flexor reflex afferents and that the segmental connexions of the pain afferents are subject to similar pre- and postsynaptic segmental and supraspinal inhibitory mechanisms as those known to exist with regard to the transmission from the flexor reflex afferents to the flexor motoneurones. The effect of low frequency, high intensity conditioning stimulation on the pain threshold and on acute pain is discussed in relation to an increased inhibition at the input stage by feedback systems via the brain stem. High frequency stimulation is suggested to influence the pain threshold and chronic pain mainly due to pre- and postsynaptic inhibition elicited by activity in primary afferents at the segmental level.
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