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PubMed1982

Effects of glycogen depletion and pedaling speed on "anaerobic threshold".

Nine male subjects performed continuous incremental exercise on a bicycle ergometer pedaling at 50 and 90 rpm in a normal glycogen state (NG) and at 50 rpm in a glycogen-depleted state (GD) to determine if alterations in pedaling frequency and muscle glycogen content would affect their "anaerobic thresholds." Ventilatory [T(vent)] and lactate [T(lac)] thresholds were identified as the points after which expired minute volume and blood lactate began to increase nonlinearly as a function of work rate. The GD protocol elicited a significant divergence between the two thresholds shifting the T(vent) to a lesser and the T(lac) to a greater work rate relative to the NG state. When the pedaling frequency was increased to 90 rpm in the NG condition, the T(lac) was shifted to a lesser work rate relative to the 50-rpm NG condition. A correlation of only 0.71 was obtained between subjects' T(vent) and T(lac). In subjects of less than 70 kg body wt, the T(lac) came at a work rate 400 kg.m.min-1 less than in subjects of greater than 80 kg body wt despite equivalent O2 uptake. The observation that the T(vent) and T(lac) could be manipulated independently of each other reveals limitations in using the T(vent) to estimate the so-called anaerobic threshold.

3-Hydroxybutyric Acid↗
PubMed1997

Strength-duration relationship for human transvenous defibrillation.

BACKGROUND: One of the basic characteristics of electrical defibrillation is the strength-duration relationship, or the effect of pulse width on defibrillation efficacy. This relationship is important for understanding the mechanism of defibrillation and for the design of optimal waveforms. However, a detailed evaluation of the strength-duration relationship for human transvenous defibrillation has not been performed previously. METHODS AND RESULTS: This was a prospective study of 29 patients undergoing initial defibrillator implantation with a uniform dual coil, transvenous lead. In each patient defibrillation thresholds were measured for either short (2, 3, 4, 6 ms) or long (6, 12, 18 ms) pulse durations, with the order of testing randomized. The shock waveform was a truncated monophasic pulse from a capacitor of 150 microF. The leading edge voltage at defibrillation threshold was 566+/-100 V for 2-ms pulses. Voltages declined exponentially with increasing pulse width reaching an asymptote by 6 ms (451+/-68 V, P<.05). Defibrillation threshold voltage was insensitive to longer pulse widths. Stored energy at defibrillation threshold showed a similar relationship with pulse width. In contrast, mean current decreased monotonically over the full range of pulse durations evaluated, and there was no evidence of a rheobase. CONCLUSIONS: The shape of the strength-duration curve and the lack of rheobase current indicate a fundamental difference between cardiac stimulation and defibrillation. The relationship between pulse duration and defibrillation threshold voltage or stored energy is well modeled by a parallel capacitor resistor circuit with a time constant of 5.3 ms.

Aged↗
PubMed1992

Multiple types of high-threshold calcium channels in rabbit sensory neurons: high-affinity block of neuronal L-type by nimodipine.

Whole-cell and cell-attached patch recording have been used to characterize multiple types of voltage-dependent calcium channels in neurons freshly dispersed from rabbit dorsal root ganglia. In whole-cell patch recordings, high-threshold current, strongly resistant to inactivation by depolarized holding potentials (L-type; V1/2 = -27.2 mV), was potently inhibited by nimodipine. Assuming 1:1 binding, the dissociation constant for nimodipine binding to the inactivated state of the L-type calcium channel (KI) was 5.3 nM (n = 8). In contrast, a second type of high-threshold current less resistant to inactivation by depolarized holding potentials (N-type; V1/2 = -56.9 mV) was not blocked by nimodipine. Nimodipine-resistant N-type calcium current was inhibited by omega-conotoxin (5 microM). Cell-attached patch recordings of single calcium channel currents demonstrated the existence of three different unitary conductances; 7.4 pS, 13.1 pS, and 24.1 pS. The 24.1 pS high-threshold channel was enhanced by (-) BAY K 8644 and inhibited by nimodipine in a concentration- and voltage-dependent manner. Hyperpolarization reversed this block. These results demonstrate that, as in cardiac and smooth muscle, there is a component of neuronal high-threshold current corresponding to the L-type calcium channel that can be blocked with high affinity by nimodipine.

Animals↗
PubMed1990

Two-point discrimination test. A valuable part of hand surgical rehabilitation, e.g. in tetraplegia.

In assessing the prospects for surgical and other kinds of rehabilitation in tetraplegia and stroke patients the two-point discrimination test, correctly performed by experienced examiners, is of great value. Valid and repeatable results depend on exact technique and proper tools. There is a firm correlation between the thresholds of two-point discrimination on the pulps of the fingers and the accuracy of position sense at the metacarpophalangeal and interphalangeal joints. Microneurography has shown that cutaneous receptors have proprioceptive as well as exteroceptive functions. Thus the two-point discrimination test can be used as a measure of proprioceptive function. The results can be expressed numerically. A two-point threshold less than 10 mm on the pulp is a valid measure of useful finger proprioception. It also shows tactile gnosis, necessary for precision sensory grips.

Differential Threshold↗
PubMed1989

Retinal damage in macaque after white light exposures lasting ten minutes to twelve hours.

We induced photochemical damage in small parts of the retinas of anesthetized macaques after light exposures of varying intensity, lasting between 10 min and 12 hr. Damage was assessed both with funduscopy and densitometry at several periods after exposure. Damage was most extensive 2 days post-exposure, with similar thresholds for both methods. Reciprocity between exposure time and irradiance was found for all exposures at a threshold irradiant dose of 230 J/cm2. This is in good agreement with part of the literature data on monkeys, yet contradicts another report (Sykes et al) in which a much lower threshold dose was found. The latter data probably concern a different class of damage. It remains unclear what critical factors distinguish the two classes. Observations more than 70 days post-exposure show a divergence between funduscopic and densitometric thresholds. Although the appearance of funduscopic lesions had changed, the threshold dose remained 230 J/cm2. Densitometry showed full recovery of the amount of visual pigment for doses below 600 J/cm2.

Animals↗
PubMed1981

[Lateralization thresholds for two-tone complexes in man with various interaural differences in stimulation].

With the aid of dichotic stimulation, one-tone signals (1 and 2 kHz) and two--tone complexes (mixture of these frequencies) were presented to subjects with interaural differences simulating shift of the sound image for azimuth angle alpha = 45 and 60 degrees in respect to situation of this image at the midline (alpha = 0 degree). At alpha = 0 degree, the lateralization threshold of the two--tone complex for interaural delay of the low-frequency component tone (1 kHz) is lesser by half than the lateralization threshold for the one--tone signal of the same frequency, whereas the lateralization threshold of the two--tone complex for interaural differences of the higher (2 kHz) tone intensity is somewhat greater than the analogous lateralization threshold for the one--tone signal of the same frequency. At alpha = 45 degrees, the differences between respective lateralization thresholds of the two--tone complex and one--tone signals become less obvious, and quite insignificant at alpha = 60 degrees.

Auditory Perception↗
PubMed1983

Voltage threshold and excitability among variously sized cat hindlimb motoneurons.

Intracellular recording has been performed to examine whether any differences in apparent initial-segment voltage threshold exist between types F and S cat triceps surae motoneurons. Voltage threshold was estimated using orthodromic action potentials initiated by large, monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by dorsal root stimulation. No significant differences in voltage threshold could be detected between types F and S motoneurons. Further, voltage thresholds did not covary with motoneuron input resistance, afterhyperpolarization duration, or the twitch contraction time of functionally isolated muscle units. Significant positive correlations were observed between voltage threshold and the motoneuron resting potential. Utilizing a compartmental neuron model, a theoretical analysis has been performed that examines the influence of specific passive membrane properties on current threshold for action potentials initiated by large, monosynaptic EPSPs. This analysis indicates that total membrane capacitance will be the primary determinant of these thresholds. Further analysis of available data suggests that active membrane properties will play a minimal role in setting these thresholds. Since specific membrane capacitance is likely to be similar among cat motoneurons, it is concluded that only size or surface area-related current threshold differences will exist among these cells for activation with brief currents such as those underlying large EPSPs. For motoneurons thus activated, it is suggested that variations in the excitatory/inhibitory balance or density of synaptic input would be the major mechanisms for producing differential recruitment thresholds among the motoneuron population. Other available evidence is discussed that indicates that factors intrinsic to the motoneurons themselves will contribute to the setting of functional recruitment thresholds for activation with longer duration currents.

Action Potentials↗
PubMed1986

Acute ipsilateral hyperalgesia and chronic contralateral hypoalgesia after unilateral 6-hydroxydopamine lesions of the substantia nigra.

The effect of unilateral 6-hydroxydopamine lesions on flinch-jump thresholds to footshock was investigated during 3 months postoperative in rats. Preoperatively, the ipsilateral-contralateral thresholds were closely matched but postoperatively the flinch-jump thresholds to footshock applied to the body surface contralateral to the lesion hemisphere increased, whereas ipsilateral thresholds decreased. The ipsilateral decrease in threshold was maximal at postoperative day 1 but the contralateral increase persisted throughout testing. In control rats no ipsilateral-contralateral differences in the threshold were obtained. In both the control and 6-hydroxydopamine-treated groups there was an additional bilateral decline in jump threshold during the course of the experiment. These results suggest a role for dopaminergic and nondopaminergic participation in an animal's response to nociceptive stimulation.

Animals↗
PubMed2000

Effect of calmodulin antagonists on the compound action potential of the cochlea.

This study aimed to evaluate the effect of calmodulin antagonists on the threshold of the compound action potential (CAP) and the functional recovery of the cochlea after transient ischemia. When trifluoperazine and W-7 were administered to albino guinea pigs with perilymphatic perfusion, these drugs did not significantly affect the CAP thresholds. Transient cochlear ischemia of 30-min duration was obtained via a skull base approach. Although trifluoperazine significantly ameliorated the post-ischemic CAP threshold shifts 4 h after the onset of reperfusion, 1 to 50 microM W-7 did not affect the CAP threshold shifts. These results suggest that the action antagonizing calmodulin has no effect on the CAP threshold, while the role that calmodulin plays in cochlear ischemia-reperfusion injury still remains unclear.

Action Potentials↗
PubMed2006

Quantitative sensory testing in cluster headache: increased sensory thresholds.

To determine if recently reported changes in sensory thresholds during migraine attacks can also be seen in cluster headache (CH), we performed quantitative sensory testing (QST) in 10 healthy subjects and in 16 patients with CH. Eight of the patients had an episodic CH and the other eight a chronic CH. The tests were performed on the right and left cheeks and on the right and left side of the back of the hands to determine the subjects' perception and pain thresholds for thermal (use of a thermode) and mechanical (vibration, pressure pain thresholds, pin prick, von Frey hairs) stimuli. Six patients were examined in the attack-free period. Three were also willing to repeat the tests a second time during an acute headache attack, which was elicited with nitroglycerin. The healthy subjects performed the experiments in the morning and evening of the same day to determine if sensory thresholds are independent of the time of day. If they were, this would allow estimation of the influence of the endogenous cortisone concentration on these thresholds. The control group showed no influence of the time of day on the thresholds. There was a significant difference in pain sensitivity between the back of the hands and the cheeks (P<0.05): higher thresholds were found on the back of the hands. The thresholds generally exhibited little intersubject variability, indicating that QST is a reliable method. There was also a significant difference between the test areas in the patient group (P<0.001): the cheeks were also more sensitive than the back of the hands. In comparison with reference data of healthy volunteers, the detection thresholds were increased in the patients on both test areas. These were statistically significant for warmth, thermal sensory limen (TSL), heat and pressure on the back of the hands (P<0.04) and for the warmth and TSL thresholds on the cheeks (P<0.05). There were no differences in the thresholds regardless of whether the patients were examined in or outside of a cluster bout. Furthermore, we found no cutaneous allodynia in the three patients tested during an attack. The increased sensory thresholds on the cheeks as well as on the back of the hands are in agreement with an increased activation of the patients' antinociceptive system. The seasonal variation and the temporal regularity of single attacks as well as the findings in imaging studies indicate that the hypothalamus is involved in the pathophysiology of CH. In view of the strong connectivity between the hypothalamus and areas involved in the antinociceptive system in the brainstem, we hypothesize that this connection is the reason for the increased sensory thresholds in CH patients found in our study.

Adaptation, Physiological↗
PubMed1981

Vascular and sensory responses of human skin to mild injury after topical treatment with capsaicin.

1 Immediately after several topical applications of capsaicin at 2-hourly intervals, human forearm skin would no longer develop flare (vasodilation) around a small injury. At the same time heat pain thresholds were reduced on average by 3.5 degrees C. These results are consistent with block by capsaicin of the effector side of the axon reflex, perhaps by depleting nerve terminals of substance P. 2 Over a period from several days to several weeks after treatment, flare was diminished and heat pain thresholds were slightly elevated. These changes may be due to long-lasting damage of cutaneous nerve terminals by capsaicin.

Administration, Topical↗
PubMed1997

[Relation between relative afferent pupillary defect and suprathreshold automated perimetry].

UNLABELLED: The purpose of this study was to test for correlation between relative afferent pupillary defect (RAPD) and visual acuity, central differential light threshold and visual field defect measured by suprasthreshold automated grid perimetry. METHODS: In 98 patients with optic neuropathies of different origin (inflammatory, ischemic, or compressive) in addition to a standard ophthalmological examination, static perimetry of the 30 degrees visual field with a dense grid of test spots, including measurement of the central differential light sensitivity, was done by means of the Tübingen Automated Perimeter. RAPD was tested using the swinging flashlight test quantified with neutral density filters. RESULTS: Visual acuity and central differential light sensitivity correlated only weakly with the RAPD. The visual field showed a better correlation (coeff. of corr. for all patients 0.58, for patients with optic neuritis 0.64. for patients with AION 0.53 and for patients with compressive neuropathies 0.84). One test spot not seen in perimetry corresponded to an RAPD of about 0.01 log-units. CONCLUSIONS: The results confirm those from previous studies obtained by threshold perimetry. The RAPD is a good parameter for the severity of an optic nerve lesion, especially of compressive origin.

Adult↗
PubMed1988

Relation of the intraoperative defibrillation threshold to successful postoperative defibrillation with an automatic implantable cardioverter defibrillator.

To determine the relation between the intraoperative defibrillation threshold and successful postoperative termination of induced ventricular fibrillation (VF) with the automatic implantable cardioverter defibrillator (AICD), 33 patients who underwent AICD implantation were studied. The defibrillation threshold, determined after at least 10 seconds of VF, was 5 J in 2, 10 J in 6, 15 J in 10, 20 J in 10 and 25 J in 5 patients. The AICD energy rating on the first discharge was 28 +/- 1.8 J. Defibrillation of induced VF was demonstrated postoperatively in 29 of 33 (88%) patients. The AICD terminated VF postoperatively in all 18 patients with a defibrillation threshold less than or equal to 15 J. Only 11 of the 15 (73%) patients with a defibrillation threshold greater than or equal to 20 J (p less than 0.04) had VF terminated postoperatively. In all 4 patients in whom the AICD failed to terminate induced VF, the energy difference between the AICD rating and the defibrillation threshold was less than or equal to 10 J. Among the 14 patients with a difference of less than or equal to 10 J between the AICD energy rating and the defibrillation threshold, there were no significant differences between the 4 patients with and the 10 without successful VF termination with respect to the duration of VF induced postoperatively or the AICD lead system. In summary, failure to terminate VF with the AICD is not uncommon (27%) when the defibrillation threshold approaches the energy delivering capacity of the AICD.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

The role of multiple opioid receptors in the maintenance of stimulation-induced feeding.

Feeding induced by lateral hypothalamic electrical stimulation is sensitive to opioid antagonism and has previously been blocked by naloxone and antibodies to dynorphin A fragments. In the present study, high affinity receptor-selective antagonists were used to determine the particular opioid receptor type(s) that mediates stimulation-induced feeding (SIF). Separate groups of rats were used to conduct i.c.v. dose-response studies with TCTAP (mu), naltrindole (delta) and norbinaltorphimine (kappa). TCTAP, at the highest dose tested (i.e. 5.0 nmol) and norbinaltorphimine, at doses of 10.0 and 50.0 nmol, increased the brain stimulation frequency threshold for eliciting SIF. Naltrindole, at doses up to 50.0 nmol, had no effect. Results of another study, recently conducted in this laboratory, indicate that the present doses of TCTAP and norbinaltorphimine have no effect on thresholds for lateral hypothalamic self-stimulation. This suggests that mu and kappa opioid activity are associated with feeding, rather than the eliciting brain stimulation, and excludes non-specific performance deficits as an explanation of elevated SIF thresholds. In the SIF test, where 5 determinations of threshold are obtained in serial order, naloxone characteristically increases thresholds toward the end of a test while conventional appetite suppressants increase thresholds uniformly throughout a test. TCTAP and norbinaltorphimine produced a 'naloxone-like' pattern of threshold elevation, suggesting that mu and kappa receptors are involved in the process whereby endogenous opioid activity sustains feeding once initiated.

Amino Acid Sequence↗
PubMed1990

Finding the common bond: stereoacuity and the other hyperacuities.

Hyperacuity thresholds for disparity, motion displacement and relative width (widths less than 10 min arc) were measured as a function of the separation between test and reference targets for separations ranging from 18-288 min arc. All three thresholds were similar in magnitude, and showed a nearly identical rise with increasing separation. Nevertheless, eccentricity, not separation, is the variable limiting all of these thresholds. This point is underscored by the fact that relative width judgments, made by comparing the narrow widths separating two pairs of lines (test and reference widths), are equally good without the reference pair. The most precise foveal judgments of stereo and motion do require a visible reference target because the observer cannot otherwise distinguish between oculomotor "jitter" and target-driven changes in disparity or motion. At eccentricities greater than 2 deg, stereo and motion thresholds for a single unreferenced line (150 msec duration) were equal to the referenced thresholds, presumably because the oculomotor noise is less than the positional uncertainty associated with these peripheral loci.

Depth Perception↗
PubMed1994

Effect of stimulus force on perioral direction discrimination: clinical implications.

Clinicians have used the same instrument (viz, Semmes-Weinstein pressure aesthesiometers [Research Design, Inc, Houston, TX] or "von Frey hairs") for tests of both contact detection and direction discrimination. However, patients' ability to discriminate direction may be underestimated by barely detectable moving stimuli. To determine whether the aesthesiometers underestimate direction discrimination, we evaluated the capacity of 13 normal subjects to distinguish opposing directions provided by 10 different hairs. The hairs were selected to deliver forces below and above the contact-detection threshold. Each was stroked over 1.0 cm of perioral skin at the velocity at which the subject was predicted to best discriminate direction of motion. It was found that valid estimates of perioral direction discrimination can be obtained with appropriately selected aesthesiometers. Specifically, the least stiff hair whose handle displays a manufacturer's marking two units greater than that of the contact-detection "threshold hair" should be used to deliver the moving stimuli. The resultant force applied by this hair will exceed 10 times the subject's contact-detection threshold force. If a less-stiff hair is used, the capacity to distinguish direction may be underestimated.

Adolescent↗