Pain threshold responses to two different modes of sensory stimulation in patients with orofacial muscular pain: psychologic considerations.

This study focuses on the influence of trait anxiety and mood variables on changes in tooth pain threshold following two similar methods of somatic afferent stimulation, one familiar (manual acupuncture) and one unfamiliar (low-frequency transcutaneous electrical nerve stimulation [low-TENS]). Twenty-one acupuncture responders, treated for long-lasting orofacial muscular pain but naive to low-TENS, were selected for the study. In an experimental session, acupuncture and low-TENS were randomly given during two periods separated by a rest interval. Tooth pain thresholds (PT) were measured before and after stimulation with a computerized electrical pulp tester. Trait anxiety and depression were assessed with psychometric forms before the experimental session in all patients, whereas momentary mood was assessed in 10 randomly selected patients with visual analogue scales during and after the two types of stimulation. Following acupuncture, the group average PT increased significantly, whereas no significant change was observed following low-TENS. Higher scores on trait anxiety correlated significantly with a low PT increase following low-TENS, and higher ratings of stress correlated significantly with a low PT increase following acupuncture. This indicates that the magnitude of analgesia induced by these methods may be modified by psychologic factors like anxiety and stress.

Effects of repeated sensory stimulation sessions (electro-acupuncture) on skin temperature in chronic pain patients.

Changes in skin temperature and haemodynamics were studied during experimentally administered electro-acupuncture before and after a 4-week period of electro-acupuncture (EA) treatments. Subjective pain intensity was evaluated using a pain questionnaire. Twelve patients with long-lasting nociceptive pain were included. Before clinical treatment, skin temperature tended to decrease after 30 minutes' stimulation. In contrast, a significant increase was seen after the clinical treatment. No significant changes were seen for blood pressure, heart rate or pain intensity before and after the clinical treatment. The data indicate that an increased skin vasoconstrictor sympathetic activity may be responsible for the decreased skin temperature during the electro-acupuncture in the initial test sessions, whereas an inhibition of skin sympathetic activity and/or a release of vasodilatory substances may be responsible for the increase in temperature after completed clinical treatment. Despite a small number of subjects and correction for multiple inference, the difference in temperature effects before and up to 3 months after acupuncture treatment was significant.

Reduction of blood flow impedance in the uterine arteries of infertile women with electro-acupuncture.

In order to assess whether electro-acupuncture (EA) can reduce a high uterine artery blood flow impedance, 10 infertile but otherwise healthy women with a pulsatility index (PI) >=3.0 in the uterine arteries were treated with EA in a prospective, non-randomized study. Before inclusion in the study and throughout the entire study period, the women were down-regulated with a gonadotrophin-releasing hormone analogue (GnRHa) in order to exclude any fluctuating endogenous hormone effects on the PI. The baseline PI was measured when the serum oestradiol was <=0.1 nmol/l, and thereafter the women were given EA eight times, twice a week for 4 weeks. The PI was measured again closely after the eighth EA treatment, and once more 10-14 days after the EA period. Skin temperature on the forehead (STFH) and in the lumbrosacral area (STLS) was measured during the first, fifth and eighth EA treatments. Compared to the mean baseline PI, the mean PI was significantly reduced both shortly after the eighth EA treatment (P < 0.0001) and 10-14 days after the EA period (P < 0.0001). STFH increased significantly during the EA treatments. It is suggested that both of these effects are due to a central inhibition of the sympathetic activity.

Relations between experimentally induced tooth pain threshold changes, psychometrics and clinical pain relief following TENS. A retrospective study in patients with long-lasting pain.

The present study investigates the relationships between clinical pain relief, physiological and psychological parameters. Out of 50 patients with long-lasting musculoskeletal neck- and shoulder-pain treated with transcutaneous electrical nerve stimulation (TENS), 21 were selected and classified as responders (n = 13) or non-responders (n = 8). Tooth pain thresholds (PT) were measured before and after an experimental TENS treatment and the relative change in PT following the stimulation was calculated. Three psychometric self-inventories were administered: Zung Depression Scale, Spielberger's Trait Anxiety Scale and the Multidimensional Health Locus of Control Scale. Responders (R) and non-responders (NR) differed significantly from each other in the PT measurements as well as on the psychometric scales. NR exhibited higher levels of anxiety and depression, a more pronounced powerful other orientation and no change or a decrease in PT following TENS compared to R. These findings indicate relationships and interactions between physiological and psychological factors in patients with long-lasting pain.

Effects of naloxone on dental pain threshold following muscle exercise and low frequency transcutaneous nerve stimulation: a comparative study in man.

Previous studies have shown that muscle exercise and low frequency transcutaneous nerve stimulation (TNS) give rise to an analgesic effect in humans and animals. Endorphin has been proposed to mediate this analgesia. In this investigation, the effect of muscle exercise and low frequency TNS, on dental pain thresholds was studied and the possible involvement of endorphinergic mechanisms was investigated using naloxone as an antagonist. Dental pain thresholds were measured in 11 volunteers following leg or arm exercise and after low frequency TNS of the hands or face. After exercise (20 min) or stimulation (30 min) either 0.8 mg naloxone (2 ml) or saline (2 ml) was injected i.v. in a double-blind fashion. Pain thresholds were measured repetitively before and after exercise or stimulation. Both leg and arm exercise increased pain threshold. Stimulation of the hands also increased pain threshold, but less than arm exercise. A marked increase in pain threshold was seen after face stimulation. These changes in pain threshold were unaffected following injections of either naloxone or saline, except for an early and short-lasting reduction when naloxone was injected following arm exercise. The increases in pain threshold following muscle exercise and after low frequency TNS, showed similarities suggesting that a common mechanism might be involved. The pain threshold increase after arm exercise could only be partially mediated by endorphinergic mechanisms.

Evaluation of the pain suppressive effect of different frequencies of peripheral electrical stimulation in chronic pain conditions.

The pain suppressive effect of low (2/s) and high (50-100/s) frequency electrical stimulation was studied in 12 patients with severe chronic pain in the back and/or the legs. All patients were subjected to a thorough physical examination before and 2-3 weeks after a series of stimulation sessions. The stimulation was applied via surface electrodes to areas both segmentally related and segmentally unrelated to the regions of chronic pain. Low frequency stimulation induced a partial pain relief in only one patient whereas stimulation with high frequency gave a suppression of pain in seven patients. The effect was short-lasting in most cases and the pain started to increase usually within 30 min. It is concluded that the observed pain suppression is not due to psychological factors but to more basic neurophysiological mechanisms. Augmentation of the effect after repeated stimulation sessions was not observed, neither was there any alteration in the neuro-orthopedic status nor any lasting pain relief in any patient.

On acupuncture analgesia and the mechanism of pain.

The effect on the experimental tooth pain threshold of conditioning electrical stimulation via needles or surface electrodes applied to the hands and cheeks was studied in 34 dental students. Conditioning stimulation with 2/sec. gave a slowly increasing pain threshold followed by a slow return to the control level in the post-conditioning period. In each individual the amplitude of the threshold increase was reproduceable. It was concluded that these effects are not due to motivational but to more basic neurophysiological mechanisms. The pain threshold was increased mainly by segmental conditioning stimulation; segmentally unrelated stimulation gave usually only small effects. Conditioning stimulation with 100/sec. produced only a strict segmental short-lasting effect. Effects with characteristics of both 2/sec. and 100/sec. were obtained by conditioning at 10/sec. It is suggested that the transmission of impulses from the pain afferents to ascending pathways is controlled at the segmental level by (a) presynaptic inhibition within the group of afferents giving rise to the flexion reflex of which the pain afferents are assumed to be a part; (b) postsynaptic inhibition between alternate pathway excited by flexion reflex afferents; and (c) descending control from supraspinal systems which may utilize similar segmental mechanisms as the primary afferents.

Some factors involved in the thalamic control of spontaneous barbiturate spindles.

1. The origin of thalamic and cortical spontaneous spindles was studied in cats anaesthetized with sodium pentobarbital.2. Complete removal of all cortical grey matter left the thalamic rhythmic spindle activity unchanged.3. Removal of the entire thalamus or pronounced oedema in the thalamus abolished completely the spindle activity in the corresponding hemisphere.4. In a neuronally isolated cortical area, a fast, low voltage background activity appeared, interrupted by occasional irregular rapid potential changes (sharp waves) of high voltage. Regular spindle rhythms were seldom observed unless excited by a depolarizing drug. Spontaneous spindles did not invade the isolated cortex via a bridge of intact cortical tissue.5. With increasingly larger destruction of the thalamus in a rostro-caudal direction, the activity in the post-cruciate cortex did not change until the anterior third of the thalamus was encroached upon. A transverse section in front of the thalamus nearly eliminated the cortical spindles.6. Complete removal of the mid line and intralaminar nuclei left the spontaneous rhythmic activity of the lateral thalamic nuclei and of the frontal cortex principally unchanged.7. Removal of the laterally located thalamic nuclei, including the n. ventralis posterolateralis (VPL), abolished virtually all spontaneous spindle activity of the frontal cortex, including the post-cruciate area.8. Local cortical cooling reduced the amplitude but not the frequency of the cortical spindles.9. Cooling of the whole brain reduced both the amplitude and the frequency of the spindles. At low temperatures, all spindle activity in the cortex disappeared, and occasional sharp waves occurred, as with de-afferentation.10. It is concluded that the rhythm of the cortical spontaneous barbiturate spindles is generated exclusively by thalamic neurones. The electromotive force of the corticographic waves, however, has a cortical origin.

Nature of thalamo-cortical relations during spontaneous barbiturate spindle activity.

1. The relation between thalamic and cortical spontaneous spindles was investigated in cats anaesthetized with barbiturates.2. Simultaneous recordings with multiple electrodes in the thalamus and cortex revealed a high correlation between thalamic and cortical spindle activity, both with regard to the occurrence of the spindles as well as to their individual waves, provided a critical location of the electrodes was secured. The results indicate a point-to-point relation between a group of thalamic cells and a small cortical area to which these cells project. In the spontaneous rhythm, the cortical columns are probably individually controlled by a thalamic rhythmic entity. This point-to-point relation was found in all the major sensory projection systems and in one thalamic ;association' nucleus and its corresponding ;association' cortex.3. Cortical barbiturate spindles appeared either as local spindles in a restricted cortical area or as compound spindles in several areas. Spindles recorded from electrodes separated by 2 mm or more were clearly different with regard to intraspindle wave frequency, duration, and the time of start and stop of the spindle. These differences increased with increasing distance between the electrodes, and were most pronounced when the corticograms of the two hemispheres were compared.4. Spontaneous spindle activity interfered with orthodromic transmission through n. ventralis posterolateralis (VPL) and medial geniculate nucleus (MG), judged by depression of the thalamic and cortical responses to peripheral nerve volleys or clicks. Such inhibition required the afferent volley to be delivered at a particular time of the spontaneous oscillations. Further, orthodromic volleys reset the rhythmic spindle waves in the appropriate thalamic and cortical areas.5. These findings lead to a new concept of the thalamic pace-maker function. During barbiturate anaesthesia, small assemblages of thalamic neurones seem to have the ability to generate independent rhythmic discharges, and thereby control the rhythm of the particular cortical column to which this thalamic group projects.6. During barbiturate anaesthesia, many facultative pace-makers seem to be present in the thalamus. The total number may be as large as 25,000-35,000. Usually, many of these rhythmic thalamic units beat in synchrony or near synchrony.

The thalamic relay and cortical projection of group I muscle afferents from the forelimb of the cat.

1. Stimulation of Group I muscle afferents in contralateral forelimb nerves evoked a response in nucleus ventralis posterolateralis (VPL) in the thalamus of the cat. This response was located in the rostral two-thirds of VPL in a narrow zone near the dorsomedial border of the nucleus.2. Group I afferents in nerves from more than one of the muscles in the contralateral forelimb often excited the same thalamic relay cell. In addition these cells were often discharged by skin afferents from the contralateral forelimb. They were not affected by electrical stimulation of the dorsal column-lemniscal or the spino-cervico-lemniscal paths from the contralateral hind limb.3. In experiments with peripheral conduction paths of similar length, the latency of the thalamic focal potential evoked by stimulation of Group I muscle afferents in the nerve to m. extensor carpi radialis was 3.8 S.E. +/- 0.1 msec, and that of the focal potential evoked by skin afferents (n. radialis superficialis) in the centromedial part of VPL was 4.3 +/- 0.1 msec.4. The majority of the thalamic neurones discharged by Group I muscle afferents responded with a latency shorter than 1 msec to electrical stimulation of the cerebral cortex in the region of the post-cruciate dimple. A considerable number of the thalamic Group I relay cells were also discharged with a similar short latency from another cortical focus located on either side of the anterior suprasylvian sulcus near the S II hind limb areas. These responses were considered to be antidromic in nature, and the findings were interpreted as indicating two separate cortical projection areas for the Group I path. The second projection area was assumed to be located in the cortical fold formed by the anterior suprasylvian sulcus.5. Cortical stimulation also excited the thalamic Group I relay cells trans-synaptically. Trans-synaptic excitation with short (1-2 msec) and longer (2-7 msec) latency was observed. The cortical focus near the S II area was particularly potent in evoking trans-synaptic excitation in the thalamic region where the Group I relays were located.