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Human pain responsivity was defined as the subject's behavioral pain endurance time (PET) to the 1 ± 0.3°C cold-pressor test, a naturalistic and clinical analogue tonic pain model. Over the past 2 years, we have consistently observed a behavioral dichotomy of pain responsivity in each of our 6 studies (all at P < 0.000001 effect level), totaling 205 subjects. Overall, the pain-tolerant (PT) subjects could endure the whole 5 min (note that 3 min was the ceiling criterion in the last study) of cold-pressor test, while the pain-sensitive (PS) subjects could merely tolerate the test for an overall mean of 60 sec, 20% of PET in the PT group. No overlapping of distribution was observed between these 2 populations. Further, we observed that the percentage of subjects in each of these 2 groups varied substantially across studies. The mean pain perception (Visual Analogue Scale) of tonic pain ranged from 60–70 for both aversiveness and intensity scales. The characteristics of this tonic pain, assessed by the McGill Pain Questionnaire (MPQ), showed similar patterns across each study with a high degree of consistency. Although ratings of pain aversiveness did not differ in the PT vs. PS subjects, ratings of pain intensity did differ, with the PT subject reporting less pain. It was found that state anxiety correlated with MPQ scores for PS, but not PT, subjects. Additionally, psychological tests (Tellegen Absorbance Scale, Kleinknecht Fear, Spielberger Trait-Anxiety) were positively correlated with certain MPQ measures for PS, but not PT subjects. Multivariate regression analyses indicated, in the PS but not the PT group, that 36% of variance in pain score (MPQ-T) could be predicted by the psychological trait factors. The general level of fear contributed singularly as the major predictor variable in the pain-sensitive individuals. We consider this tonic pain model indeed offers a succinct empirical paradigm to study human pain responsivity in general. The psychological/physiological etiology of such drastic human pain responsivity requires intense systematic investigations. This report discusses the results in: 1. (a) individual differences in pain responsivity, 2. (b) characterization of the cold-pressor test as a model for tonic pain, 3. (c) contrast between PS and PT groups of pain perception and state anxiety, and 4. (d) psychological determinants of measures for cognitive, perceptual and affective domains. Discussion was also focused on the experimental tonic pain model and its generality for clinical pain, as well as the basic model of the cold-pressor test for human tonic pain responsivity.

Electron microscopy was used to study normal human extracted teeth in order to define the junctions between sensory nerve endings and other cells in external pulp and inner dentin at the crown tip. Two sets of associated cells were found: (1) Connective tissue cells. The pulpal fibroblast network made occasional desmosome junctions with the odontoblast network, and the cells of each network formed many gap junctions and desmosomes with one another. (2) Nerve endings. The terminal axons formed a succession of appositions with each other or with Schwann cells in the plexus of Raschkow and the cell-free zone, possibly with fibroblasts inthe cell-free zone and odontoblast layer, and with odontoblasts in the odontoblast layer, predentin and dentin. The appositions between nerve endings and their companion cells at all levels usually maintained a regular intercellular spacing of at least 15–20 nm. In predentin and dentin, axons could be easily identified by their distinctive vesicles and mitochondria, and they often occurred within clusters of adjacent dentinal tubules; in the odontoblast layer axon identification was much more difficult. Axo-axonic appositions were found in the plexus of Raschkow, the cell-free zone, predentin and dentin; in many cases, bare axons were separated from each other only by a 5–10 nm extracellular space. Dental sensory mechanisms are discussed in relation to these observations.

The effects of electrical acupunctural stimulation (2 Hz) on pain judgments and evoked potentials are reported for two experiments using dental dolorimetry. In the first experiment subjects received acupuncture at points located in the same neurologic segment as the test tooth. In the second experiment subjects received acupuncture at points on the hands located on acupuncture meridians. In both instances acupuncture resulted in a reduction in pain intensity and smaller evoked potential amplitudes, but naloxone neither reversed the analgesia nor did it affect the evoked potentials. A pilot study was carried out to determine whether manual rather than electrical stimulation would produce an analgesia reversible by naloxone, but it failed to do so. These findings contribute to the growing evidence that acupunctural stimulation significantly reduces pain sensibility in volunteers undergoing dolorimetric testing, but they do not support the hypothesis that endorphin release is a mechanism by which acupuncture exerts analgesia.

The effects of acupunctural stimulation on the perception of induced dental pain were compared with those of placebo acupuncture and transcutaneous electrical stimulation (TES) at an acupuncture site. Each of 4 groups of 15 subjects received one of the following treatments: acupuncture, placebo acupuncture, TES, or control conditions. Every subject was tested twice, once in a baseline session and on another day in a test session. Four levels of painful dental stimuli were delivered repeatedly and in random order to each subject in each session, who rated the perceived intensity of each stimulus on a pain category scale. All three treatment groups showed a significant reduction in magnitude of stimulus ratings after treatment. A Sensory Decision Theory analysis of the data was employed to assess the sensory sensitivity of each subject to each of 4 levels of dental stimulation and the willingness of the subject to label the strongest stimulus as painful. Acupuncture and TES groups showed a small but significant sensory analgesic response to treatment and a significant reduction in willingness to identify the strongest stimulus as painful when contrasted to controls, but placebo acupuncture subjects failed to show significant change on either of the response measures. The effects of acupuncture were most pronounced at the lowest level of stimulation, while TES affected the perception of all levels of dental stimuli. The observed effects appeared to be small, reliable, and dependent on the stimulation of a particular anatomical locus.

Individual differences in human pain responsivity were characterized by the 1 degree C cold-pressor test. Behaviorally, a pain-tolerant group (PT = 29 Ss) tolerated the entire 3-min test (means = 180 +/- 0 sec), while a pain-sensitive group (PS = 13 Ss) averaged only 50.31 +/- 20.81 sec of the cold-pressor test (t = 16.75, P less than 0.0001), replicating our earlier studies. Physiologically, the PT group exhibited no mean differences from the PS group in cortical power densities at the baseline stage. Under the noxious stress of the cold-pressor test, both groups exhibited markedly heightened delta and beta cortical power densities. However, the PS subjects showed significantly higher delta power, but not beta power, than the PT subjects. We conclude that heightened delta activity may reflect the stress component of human pain responsivity, and that beta activity reflects the vigilance scanning of pain processes.