Letters and notes: Warm fiber activity in human skin nerves

Skin Innervation

All layers and appendages of the skin are densely innervated by afferent and efferent neurons providing sensory information and controlling skin perfusion and sweating. In mice, neuronal functions have been comprehensively linked to unique single-cell expression patterns and to characteristic arborization of nerve endings in skin and dorsal horn, whereas for humans, specific molecular markers for functional classes of afferent neurons are still lacking. Moreover, bidirectional communication between sensory neurons and local skin cells has become of particular interest, resulting in a broader physiological understanding of sensory function but also of trophic functions and immunomodulation in disease states.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

Interoception as independent cardiac, thermosensory, nociceptive, and affective touch perceptual submodalities

Interoception includes signals from inner organs and thin afferents in the skin, providing information about the body's physiological state. However, the functional relationships between interoceptive submodalities are unclear, and thermosensation as skin-based interoception has rarely been considered. We used five tasks to examine the relationships among cardiac awareness, thermosensation, affective touch, and nociception. Thermosensation was probed with a classic temperature detection task and the new dynamic thermal matching task, where participants matched perceived moving thermal stimuli in a range of colder/warmer stimuli around thermoneutrality. We also examined differences between hairy and non-hairy skin and found superior perception of dynamic temperature and static cooling on hairy skin. Notably, no significant correlations were observed across interoceptive submodality accuracies (except for cold and pain perception in the palm), which indicates that interoception at perceptual levels should be conceptualised as a set of relatively independent processes and abilities rather than a single construct. DATA AVAILABILITY STATEMENT: Data of this study are available as a supplementary file.

Microneurography as a tool to study the function of individual C-fiber afferents in humans: responses from nociceptors, thermoreceptors, and mechanoreceptors

The technique of microneurography-recording neural traffic from nerves in awake humans-has provided us with unrivaled insights into afferent and efferent processes in the peripheral nervous system for over 50 years. We review the use of microneurography to study single C-fiber afferents and provide an overview of the knowledge gained, with views to future investigations. C-fibers have slowly conducting, thin-diameter, unmyelinated axons and make up the majority of the fibers in peripheral nerves (~80%). With the use of microneurography in humans, C-fiber afferents have been differentiated into discrete subclasses that encode specific qualities of stimuli on the skin, and their functional roles have been investigated. Afferent somatosensory information provided by C-fibers underpins various positive and negative affective sensations from the periphery, including mechanical, thermal, and chemical pain (C-nociceptors), temperature (C-thermoreceptors), and positive affective aspects of touch (C-tactile afferents). Insights from microneurographic investigations have revealed the complexity of the C-fiber system, methods for delineating fundamental C-fiber populations in a translational manner, how C-fiber firing can be used to identify nerve deficits in pathological states, and how the responses from C-fibers may be modified to change sensory percepts, including decreasing pain. Understanding these processes may lead to future medical interventions to diagnose and treat C-fiber dysfunction. NEW & NOTEWORTHY The technique of microneurography allows us to directly investigate the functional roles of single C-fiber afferents in awake human beings. Here we outline and discuss the current field of C-fiber research on this heterogeneous population of afferents in healthy subjects, in pathological states, and from a translational perspective. We cover C-fibers encoding touch, temperature, and pain and provide perspectives on the future of C-fiber microneurography investigations in humans.

Thermosensory micromapping of warm and cold sensitivity across glabrous and hairy skin of male and female hands and feet

The ability of hands and feet to convey skin thermal sensations is an important contributor to our experience of the surrounding world. Surprisingly, the detailed topographical distribution of warm and cold thermosensitivity across hands and feet has not been mapped, although sensitivity maps exist for touch and pain. Using a recently developed quantitative sensory test, we mapped warm and cold thermosensitivity of 103 skin sites over glabrous and hairy skin of hands and feet in male (M; 30.2 ± 5.8 yr) and female (F; 27.7 ± 5.1 yr) adults matched for body surface area (M: 1.77 ± 0.2 m2; F: 1.64 ± 0.1 m2; P = 0.155). Findings indicated that warm and cold thermosensitivity varies by fivefold across glabrous and hairy skin of hands and feet and that hands (warm/cold sensitivity: 1.25/2.14 vote/°C) are twice as sensitive as the feet (warm/cold sensitivity: 0.51/0.99 vote/°C). Opposite to what is known for touch and pain sensitivity, we observed a characteristic distal-to-proximal increase in thermosensitivity over both hairy and glabrous skin (i.e., from fingers and toes to body of hands and feet), and found that hairy skin is more sensitive than glabrous. Finally, we show that body surface area-matched men and women presented small differences in thermosensitivity and that these differences are constrained to glabrous skin only. Our high-density thermosensory micromapping provides the most detailed thermosensitivity maps of hands and feet in young adults available to date. These maps offer a window into peripheral and central mechanisms of thermosensory integration in humans and will help guide future developments in smart skin and sensory neuroprostheses, in wearable, energy-efficient personal comfort systems, and in sport and protective clothing.

NEW & NOTEWORTHY We provide the most detailed thermosensitivity maps across glabrous and hairy skin of hands and feet in men and women available to date. Our maps show that thermosensitivity varies by fivefold across hands and feet, distal regions (e.g., fingers, toes) are less sensitive than proximal (e.g., palm, sole), hands are twice as sensitive as feet, and men and women present small thermosensitivity differences. These findings will help guide developments in sensory neuroprostheses, wearable comfort systems, and sport/protective clothing.

Molecular basis of peripheral innocuous warmth sensitivity

The perception of innocuous warmth is a sensory capability that facilitates thermoregulatory, social, hedonic, and even predatory functions. It has long been recognized that innocuous warmth perception is triggered by activation of a subpopulation of specially tuned peripheral thermosensory neurons. In addition, there is growing evidence that thermotransduction by nonneuronal cells, such as skin keratinocytes, might contribute to or modulate our thermosensory experience. Yet, the precise molecular mechanisms underlying warmth transduction are only now being uncovered. Recent molecular genetics approaches have led to the identification of multiple candidate warmth-transducing molecules that appear to confer thermosensitivity upon innocuous warmth afferents and/or neighboring cell types. Most, but not all, of these candidate transducers are members of the transient receptor potential (TRP) ion channel family. Among the latter, evidence supporting a function in innocuous warmth sensation is strongest for TRPV1 and TRPM2 in mammals and for TRPA1 in nonmammalian species.

Peripheral thermoreceptors in innocuous temperature detection

The mammalian skin is innervated by cold-sensitive afferent neurons. These neurons exhibit ongoing activity at temperatures between ~10 and 42°C, are activated by innocuous cold stimuli, inhibited by warm stimuli and are mechanoinsensitive. Their axons are small-diameter myelinated (Aδ-) fibers in primates and unmyelinated (C-) fibers in nonprimate mammals. The mammalian skin is innervated by warm-sensitive afferent neurons. The density of innervation by these neurons is lower than that by cold-sensitive afferents. They exhibit ongoing activity between ~38 and 48°C, are activated by warm stimuli, inhibited by cold stimuli, and are mechanoinsensitive. Their axons are unmyelinated (C-) fibers. Cold-sensitive unmyelinated afferent neurons exhibit prominent cold sensitivity of their axons (in rats). The discharge pattern of the cutaneous cold-sensitive afferent neurons is fully preserved after nerve injury. Ongoing impulse activity and cold-evoked impulses originate ectopically at the nerve injury site. Deep somatic tissues and viscera are innervated by thermosensitive afferent neurons. Most are warm-sensitive and mechanoinsensitive and have unmyelinated axons. These afferent neurons have only rarely and incompletely been studied, e.g., in the upper gastrointestinal tract, the liver (both vagal afferents), the dorsal abdominal wall, and the skeletal muscle. Spinal cord warm sensitivity may be mediated by cutaneous afferent neurons with unmyelinated axons that are excited by spinal cord warming.

Sex differences in thermal detection and thermal pain threshold and the thermal grill illusion: a psychophysical study in young volunteers

BACKGROUND

Sex-related differences in human thermal and pain sensitivity are the subject of controversial discussion. The goal of this study in a large number of subjects was to investigate sex differences in thermal and thermal pain perception and the thermal grill illusion (TGI) as a phenomenon reflecting crosstalk between the thermoreceptive and nociceptive systems. The thermal grill illusion is a sensation of strong, but not necessarily painful, heat often preceded by transient cold upon skin contact with spatially interlaced innocuous warm and cool stimuli.

METHODS

The TGI was studied in a group of 78 female and 58 male undergraduate students and was evoked by placing the palm of the right hand on the thermal grill (20/40 °C interleaved stimulus). Sex-related thermal perception was investigated by a retrospective analysis of thermal detection and thermal pain threshold data that had been measured in student laboratory courses over 5 years (776 female and 476 male undergraduate students) using the method of quantitative sensory testing (QST). To analyse correlations between thermal pain sensitivity and the TGI, thermal pain threshold and the TGI were determined in a group of 20 female and 20 male undergraduate students.

RESULTS

The TGI was more pronounced in females than males. Females were more sensitive with respect to thermal detection and thermal pain thresholds. Independent of sex, thermal detection thresholds were dependent on the baseline temperature with a specific progression of an optimum curve for cold detection threshold versus baseline temperature. The distribution of cold pain thresholds was multi-modal and sex-dependent. The more pronounced TGI in females correlated with higher cold sensitivity and cold pain sensitivity in females than in males.

CONCLUSIONS

Our finding that thermal detection threshold not only differs between the sexes but is also dependent on the baseline temperature reveals a complex processing of "cold" and "warm" inputs in thermal perception. The results of the TGI experiment support the assumption that sex differences in cold-related thermoreception are responsible for sex differences in the TGI.

The biology of skin wetness perception and its implications in manual function and for reproducing complex somatosensory signals in neuroprosthetics

Our perception of skin wetness is generated readily, yet humans have no known receptor (hygroreceptor) to signal this directly. It is easy to imagine the sensation of water running over our hands or the feel of rain on our skin. The synthetic sensation of wetness is thought to be produced from a combination of specific skin thermal and tactile inputs, registered through thermoreceptors and mechanoreceptors, respectively. The present review explores how thermal and tactile afference from the periphery can generate the percept of wetness centrally. We propose that the main signals include information about skin cooling, signaled primarily by thinly myelinated thermoreceptors, and rapid changes in touch, through fast-conducting, myelinated mechanoreceptors. Potential central sites for integration of these signals, and thus the perception of skin wetness, include the primary and secondary somatosensory cortices and the insula cortex. The interactions underlying these processes can also be modeled to aid in understanding and engineering the mechanisms. Furthermore, we discuss the role that sensing wetness could play in precision grip and the dexterous manipulation of objects. We expand on these lines of inquiry to the application of the knowledge in designing and creating skin sensory feedback in prosthetics. The addition of real-time, complex sensory signals would mark a significant advance in the use and incorporation of prosthetic body parts for amputees in everyday life.NEW & NOTEWORTHY Little is known about the underlying mechanisms that generate the perception of skin wetness. Humans have no specific hygroreceptor, and thus temperature and touch information combine to produce wetness sensations. The present review covers the potential mechanisms leading to the perception of wetness, both peripherally and centrally, along with their implications for manual function. These insights are relevant to inform the design of neuroengineering interfaces, such as sensory prostheses for amputees.

■ REVIEW : The Pathology of Pain

It has recently become recognized that neuropathic forms of chronic pain represent true neurologic disease. Current investigations are largely molecular, yet knowledge of the anatomy and cell biology of pain is also important for the development of more effective medications. Although acute pain is beneficial, neuropathic pain is pathological and creates devastating disability. It occurs when an abnormal somatosensory system chronically transmits pain signals despite the absence of acute injury. Any type of lesion anywhere in the peripheral or central spinothalamic pathway can cause it. The most common scenario involves interruption of peripheral sensory axons with distal Wallerian degeneration. Regenerating peripheral sensory axons can develop ongoing spontaneous action potentials or ectopic mechano- and chemosensitivity that contribute to pain. Axotomy also induces morphological and functional alterations proximally that can contribute to pain. Central axon terminals can degenerate or sprout aberrantly within the dorsal horn. Higher order sensory neurons within the CNS can experience trans-synaptic damage. Lesions wholly within the CNS, such as stroke and multiple sclerosis, can also produce neuropathic pain. This review of a nascent field is presented in hopes of stimulating further investigation into this common, under-recognized medical problem. NEURO SCIENTIST 5:302-310, 1999

FMRI of spinal and supra-spinal correlates of temporal pain summation in fibromyalgia patients

Fibromyalgia syndrome (FM) is a debilitating chronic pain condition, which afflicts primarily females. Although the etiology of this illness is not completely understood, FM pain is thought to rely on enhanced pain sensitivity maintained by central mechanisms. One of these mechanisms is central pain amplification, which is characterized by altered temporal summation of second pain (TSSP). Here we use a TSSP paradigm and functional MRI (fMRI) of the spinal cord, brainstem, and brain to noninvasively examine the central nervous system contributions to TSSP in FM patients and normal controls (NC). Functional MRI of pain-free female adults (N = 15) and FM patients (N = 14) was conducted while brief, repetitive heat pain stimuli (0.33 Hz) were applied to the thenar eminence of the hand (C6 dermatome). The stimulus intensity was adjusted to each participant's heat pain sensitivity to achieve moderate pain. Data were analyzed by means of a General Linear Model and region-of-interest analyses. All participants demonstrated significant pain summation in the TSSP condition. FM subjects, however, required significantly lower stimulus intensities than NC to achieve similar TSSP. fMRI analyses of perceptually equal TSSP identified similar brain activity in NC and FM subjects; however, multiple areas in the brainstem (rostral ventromedial medulla and periaqueductal grey region) and spinal cord (dorsal horn) exhibited greater activity in NC subjects. Finally, increased after-sensations and enhanced dorsal horn activity was demonstrated in FM patients. In conclusion, the spinal and brainstem BOLD responses to TSSP are different between NC and FM patients, which may indicate alterations to descending pain control mechanisms suggesting contributions of these mechanisms to central sensitization and pain of FM patients.

Different Physiological and Subjective Responses to the Hyperthermia Between Young and Older Adults: Basic Study for Thermal Therapy in Cardiovascular Diseases

BACKGROUND

Thermal therapy has been used as adjuvant therapy in patients with cardiovascular disease. However, little is known about responses to thermal stress in older adults. We examined the effects of thermal stress in younger and older healthy Japanese individuals.

METHODS

The study included 12 young (mean age, 22 years) and 12 older (mean age, 68 years) healthy adults and was performed under strict temperature and humidity control to minimize confounding. Participants lay supine throughout three consecutive 30-minute phases: Phase I (heating at 70°C in a dome-shaped sauna), Phase II (insulation in the sauna), and Phase III (cool down). Physiological parameters and subjective thermal sensations were compared within and between two age groups.

RESULTS

Mean skin temperature increased significantly in both age groups (Phase I) and after the first 10 minutes was higher among older adults (by 6.8°C vs 6.0°C among younger; p < .01). Mean rectal temperature increased by 0.6°C in both groups (Phase II). Mean heart rate increased significantly in both age groups (Phase II) and was higher among younger adults (by 21.4 vs 11.3 beats/min among older adults; p < .05). Both systolic (by 15.1 mmHg) and diastolic (by 10.5 mmHg) blood pressure dropped significantly among older adults (Phase I), returning to baseline in Phase III; no changes were noted among those younger. There was no between-group difference in fluid loss or thermal sensations.

CONCLUSIONS

Compared with younger adults, older adults are more likely to drop blood pressure in response to thermal stress but had similar fluid loss and subjective responses.

Subclinical peripheral neuropathy in patients with multiple myeloma before chemotherapy is correlated with decreased fingertip innervation density

PURPOSE

The goal in this study was to determine the incidence of subclinical neuropathy in treatment-naive patients with multiple myeloma (MM) with no history of peripheral neuropathy using quantitative sensory tests (QSTs) and its correlation with innervation density of the extremities using noninvasive laser reflectance confocal microscopy.

PATIENTS AND METHODS

QST results were collected for 27 patients with a diagnosis of MM and compared with data collected from 30 age- and sex-matched healthy volunteers. Skin temperature, sensorimotor function (grooved pegboard test), and detection thresholds for temperature, sharpness, and low-threshold mechanical stimuli (von Frey monofilaments and bumps detection test) were measured. Meissner's corpuscle (MC) density in the fingertips was assessed using in vivo laser reflectance confocal microscopy.

RESULTS

Patients showed a high incidence (> 80%) of ≥ one subclinical QST deficit. These included increased von Frey, bumps, and warmth detection thresholds as compared with healthy volunteers. Patients also showed increases in cold pain, sensorimotor deficits (grooved pegboard test), and higher overall neuropathy scores. MC density was significantly lower in patients than controls and showed significant inverse correlation with bumps detection threshold.

CONCLUSION

Patients with MM commonly present with sensory and sensorimotor deficits before undergoing treatment, and these deficits seem to result from disease-related decreases in peripheral innervation density.

Temperature and excitable cells: Testable predictions from a thermodynamic perspective

Temperature affects a host of biological processes, one of which is the conduction velocity of action potentials (AP). The velocity-temperature profile of APs has remained remarkably conserved across excitable animal and plant cells. Herein, we will not analyze this behavior in terms of temperature sensitivities of single molecules (e.g., ion channels), but rather we present a phenomenological thermodynamic interpretation. By assuming that APs are acoustic phenomena, one arrives at testable predictions about the temperature-dependence of the macroscopic material properties of the excitable cell membrane. These material properties set constraints on the excitability of a cell membrane and allow us to hypothesize about its typical relaxation timescales. The presented approach-by virtue of its thermodynamic nature-is by no means limited to temperature. It applies equally well to all thermodynamic variables (e.g., mechanical stretch, pH, ion concentrations, etc.) and to underline this argument we discuss some implications and predictions for sensory physiology.

A Study of Wearabilities of Softwarm Heating Warm Fiber Fabric

With the improvement of people’s living standard, the traditional warm clothing already cannot satisfy people’s diversified demands about clothes’ comfortableness, functionality and aesthetics. Hygroscopic exothermic fibers as a positive heat production type of warm fibers have attracted much attention. Softwarm heating warm fiber is a kind of new functional material and at the same time it is a representative of hygroscopic exothermic fiber. It can absorb the moisture by skin breathe and transfer it into heat energy through chemical reaction so as to achieve the purpose of warmth. In this paper, warmth property, tensile property, and drapability of softwarm fiber plain knitted fabric were tested, and comfort property, mechanical property and appearance performances of softwarm heating warm fiber fabric were investigated. Our study showed that the wearability of softwarm fiber fabric is fine, especially the warmth property which can meet our needs of warmth to knitted underwear.

Thermal sensitivity in the elderly: a review

Aging is associated with a progressive decrease in thermal perception, as revealed by increased thermal detection thresholds in the elderly. This reduction in thermosensitivity follows a distal-proximal pattern, with more pronounced decrements observed in the limbs and in the perception of warmth vs. cold. The main underlying causes of this seem to be aging of the skin and subsequent reductions in thermoreceptor density and superficial skin blood flow. However, the results from some animal studies also suggest that changes in the peripheral nerve system, particularly fiber loss and decreased conduction velocity, may also be involved. In this paper, we review age-related changes in the thermal sensitivity of humans, their underlying mechanisms, and the strengths and limitations of some of the methodologies used to assess these changes.

Threshold and rate sensitivity of low-threshold thermal nociception

Previous studies have shown that sensations of burning, stinging or pricking can be evoked by warming or cooling the skin to innocuous temperatures [low-threshold thermal nociception (LTN)] below the thresholds of cold- and heat-sensitive nociceptors. LTN implies that some primary afferent fibers classically defined as warm and cold fibers relay stimulation to the nociceptive system. We addressed this question in humans by determining if different adaptation temperatures (ATs) and rates of temperature change would affect thermal sensation and LTN similarly. In Experiment 1 subjects rated the intensity of warmth, cold and nociceptive sensations produced by increasing steps in temperature (+/-0.5 degrees C increments) from ATs of 35, 33 and 31 degrees C for cooling, and 30, 32 and 34 degrees C for heating. Depending upon the AT, thresholds for nociceptive and thermal sensations estimated from the rating data differed by as little as -1.0 degrees C for cooling and +1.5 degrees C for heating. Thresholds of thermal and nociceptive sensations shifted by similar amounts across the three ATs during cooling, whereas during heating the nociceptive threshold was significantly affected only between ATs of 32 and 34 degrees C. In Experiment 2, increasing the rate of temperature change from 0.5 to 4.0 degrees C/s increased the intensity of thermal and nociceptive sensations significantly but the effect was greatest for nociceptive sensations during heating. The results of both experiments are consistent with the mediation of LTN by low-threshold thermoreceptors, although LTN caused by heating may depend on a subset of fibers that express less sensitive TRP channels than those that serve sensations of warmth at the mildest temperatures.

Human cutaneous C fibres activated by cooling, heating and menthol

Differential A-fibre block of human peripheral nerves changes the sensation evoked by innocuous cooling (approximately 24 degrees C) of the skin from 'cold' to 'hot' or 'burning', and this has been attributed to activity in unidentified unmyelinated fibres that is normally masked or inhibited by activity in Adelta cold fibres. Application of the TRPM8 agonist menthol to the skin evokes 'burning/stinging' as well as 'cold', and the unpleasant sensations are also enhanced by A-fibre block. In this study we used microneurography to search for C fibres in human skin activated by cooling and menthol, which could be responsible for these phenomena. Afferent C fibres were classified by activity-dependent slowing as Type 1A (polymodal nociceptor), Type 1B (mechanically insensitive nociceptor) or Type 2 (cold sensitive), and their responses to heating and cooling ramps were measured before and after topical application of menthol preparations (2-50%). The only C fibres activated by menthol were the Type 2 fibres, which discharged vigorously with innocuous cooling and were strongly activated and sensitized to cooling by menthol. Unlike an Adelta cold fibre, they continued to discharge at skin temperatures down to 0 degrees C, and most (13/15) were also activated by heating. We propose that the Type 2 C fibres, although resembling Adelta cold fibres in their responses to innocuous cooling and menthol, have a more complex sensory function, colouring with a 'hot-burning' quality the perceptions of low and high temperatures. Their bimodal thermoreceptive properties may help account for several puzzling psychophysical phenomena, such as 'innocuous cold nociception', 'paradoxical heat' and the thermal grill illusion, and also for some neuropathic pains.

Individual differences in temperature perception: Evidence of common processing of sensation intensity of warmth and cold

The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29 degrees C) or heating (37 degrees C) local regions of the forearm. Stimuli were delivered via a 4 x 4 array of 8 mm x 8 mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. Stimulation of the full array showed that perception of warmth and cold were highly correlated (Pearson r = 0.83, p < 0.05). Ratings of nonpainful nociceptive sensations produced by the two temperatures were also correlated, but to a lesser degree (r = 0.44), and the associations between nociceptive and thermal sensations (r = 0.35 and 0.22 for 37 and 29 degrees C, respectively) were not significant after correction for multiple statistical tests. Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.

Microneurographic single-unit recordings to assess receptive properties of afferent human C-fibers

Action potentials in unmyelinated peripheral axons can be recorded in awake humans by microneurography with small electrodes placed in a peripheral nerve. This technique provides extracellular recordings of single C-fibers and thus enables characterization of their sensory and axonal properties. By using microneurographical basic properties of afferent C-fibers such as conduction velocities, innervation territories, sensory thresholds and chemical responsiveness were measured. Moreover, axonal excitability changes induced by repetitive activation were assessed. Sensory and axonal properties of the different fiber classes cluster. Based on those specific properties, unitary functional classes of nociceptors (such as polymodal nociceptors and mechano-insensitive nociceptors) and non-nociceptors (such as tactile afferents and warm fibers) were classified. With normal data available, sensitization and desensitization of afferent fibers have been found in pathophysiologic states as detected in chronic pain patients. As subjects and patients are awake during the recording, microneurography provides a unique tool to correlate the discharge behaviour of afferent nerve fibers with the sensation evoked by certain stimuli.

Reminiscences of a journeyman scientist: studies of thermoregulation in non-human primates and humans

After graduating from Mount Holyoke College in 1948 where I majored in experimental psychology I worked at the College for 2 years with the Johns Hopkins Thermophysiological Unit. My graduate work later at the University of Wisconsin, centering on sensory psychology, culminated in my 1955 PhD thesis on human dark adaptation. I continued work in sensory psychology later with Neal Miller at Yale and then moved to the John B. Pierce Foundation--a Yale affiliate--where I began the studies of thermoregulation that constitute the center of my scientific career. Those studies were largely--later wholly--conducted using microwave energy as a thermal load and were thus published in Bioelectromagnetics even as I played an active role in the Bioelectromagnetics Society. In the beginning this work was centered on the responses of Squirrel Monkeys to thermal loads. Later, serving as Senior Scientist at the Air Force Research Laboratory at San Antonio, I completed an extensive analysis of thermal regulation in humans. I consider this work of special note inasmuch as the extraordinary human thermoregulatory ability was surely among the attributes that were paramount in initially separating humans from the other anthropoid primates.

Quantitative Sensory Findings in Patients With Bortezomib-Induced Pain

Bortezomib (PS-341) is a newly developed proteosome inhibitor that shows extremely promising antineoplastic effects against a variety of neoplasias. Neuropathic pain is emerging as a major complication of bortezomib. Although clinical reports have appeared in the literature describing the general symptoms of bortezomib chemoneuropathy, specific quantitative sensory data that detail the sensory deficits that might yield insight to the primary afferent dysfunction contributing to this pain is lacking. In this report, it is shown that patients with bortezomib-induced neuropathic pain have significantly elevated touch detection threshold and slotted peg board time, impaired sharpness detection, and elevated thresholds for the detection of skin warming and heat pain. Patients also had increased reports of cold pain. These data indicate that bortezomib-induced neuropathy is associated with deficits in Abeta, Adelta, and C caliber primary afferent fibers.

PERSPECTIVE

This work demonstrates that pain induced by the chemotherapy drug bortezomib is accompanied by dysfunction in all fiber types in sensory nerves. Impaired Abeta and C sensory function also extends into areas of skin that are not perceived as affected by pain.

Thermophysiological responses of human volunteers to whole body RF exposure at 220 MHz

Since 1994, our research has demonstrated how thermophysiological responses are mobilized in human volunteers exposed to three radio frequencies, 100, 450, and 2450 MHz. A significant gap in this frequency range is now filled by the present study, conducted at 220 MHz. Thermoregulatory responses of heat loss and heat production were measured in six adult volunteers (five males, one female, aged 24-63 years) during 45 min whole body dorsal exposures to 220 MHz radio frequency (RF) energy. Three power densities (PD = 9, 12, and 15 mW/cm(2) [1 mW/cm(2) = 10 W/m(2)], whole body average normalized specific absorption rate [SAR] = 0.045 [W/kg]/[mW/cm(2)] = 0.0045 [W/kg]/[W/m(2)]) were tested at each of three ambient temperatures (T(a) = 24, 28, and 31 degrees C) plus T(a) controls (no RF). Measured responses included esophageal (T(esoph)) and seven skin temperatures (T(sk)), metabolic rate (M), local sweat rate, and local skin blood flow (SkBF). Derived measures included heart rate (HR), respiration rate, and total evaporative water loss (EWL). Finite difference-time domain (FDTD) modeling of a seated 70 kg human exposed to 220 MHz predicted six localized "hot spots" at which local temperatures were also measured. No changes in M occurred under any test condition, while T(esoph) showed small changes (< or =0.35 degrees C) but never exceeded 37.3 degrees C. As with similar exposures at 100 MHz, local T(sk) changed little and modest increases in SkBF were recorded. At 220 MHz, vigorous sweating occurred at PD = 12 and 15 mW/cm(2), with sweating levels higher than those observed for equivalent PD at 100 MHz. Predicted "hot spots" were confirmed by local temperature measurements. The FDTD model showed the local SAR in deep neural tissues that harbor temperature-sensitive neurons (e.g., brainstem, spinal cord) to be greater at 220 than at 100 MHz. Human exposure at both 220 and 100 MHz results in far less skin heating than occurs during exposure at 450 MHz. However, the exposed subjects thermoregulate efficiently because of increased heat loss responses, particularly sweating. It is clear that these responses are controlled by neural signals from thermosensors deep in the brainstem and spinal cord, rather than those in the skin.

Chemically and electrically induced sweating and flare reaction

Both thin afferent (nociceptors) and efferent (sympathetic sudomotor) nerve fibers can be activated electrically and chemically, resulting in neurogenic erythema and sweating. These reactions have been used before to assess the impairment of sympathetic and nociceptor fibers in humans. In this study, electrically induced sweating and erythema were assessed simultaneously in the foot dorsum and thigh, and were compared to chemically induced activation. Reproducible intensity-response relations (stimulation intensities 0-30 mA, 1 Hz) were obtained from 32 subjects. The steepest increase of the sweat response was induced at lower intensities as compared to that of the erythema (18.3 mA vs. 25.7 mA, p<0.01) and reached a plateau for intensities above 25 mA, suggesting lower electrical thresholds for sudomotor fibers. Maximum flare areas induced electrically with 30 mA were smaller than those evoked chemically (flare size: 4.5 cm2 vs. 10.6 cm2). In contrast, the electrically evoked sweating rate was higher than that evoked chemically (acetylcholine, or ACh; sweating rate 0.31 vs. 0.21 microl/cm2/min, p<0.01), which might be attributed to an increased effectiveness of synchronized discharge in sympathetic fibers upon electrical stimulation.

Taxol-induced sensory disturbance is characterized by preferential impairment of myelinated fiber function in cancer patients

Taxol produces neuropathic pain with three distinct zones of involvement in the extremities. Most distally is an area of on-going pain and proximal to this is a zone of sensory disturbance but not overt pain. These two areas were confined in all but one case to the glabrous skin of the hands and/or feet. More proximal is an area not recognized by the patients as involved with pain or sensory disturbance yet wherein quantitative sensory tests nevertheless reveal altered sensibility. Impairment of perception to light touch, normally conveyed by myelinated fibers, was dramatically altered in all three areas, being approximately 50-fold greater than normal in areas of pain and sensory disturbance as well as in areas of skin perceived by the patients as not affected. Impairment of perception to sharpness, normally conveyed by small myelinated fibers, was most pronounced in areas of on-going pain, intermediate in areas of sensory disturbance and near baseline in more proximal skin of chemotherapy patients. In contrast to mechanical sensibility, thermal thresholds for warm and heat pain detection were normal throughout. Finally, chemotherapy patients showed paradoxical burning pain to skin cooling that was most pronounced in proximal areas of skin thought to be unaffected by the patients, intermediate in the border zone of altered sensibility and least pronounced in areas of on-going pain. These data suggest that taxol produces a neuropathy characterized by pronounced impairment of function in A-beta myelinated fibers, intermediate impairment of A-delta myelinated fibers, and a relative sparing of C-fibers.

Physiological characterization of neuropathy in Fabry's disease

Fabry's disease is commonly associated with a painful, debilitating neuropathy. Characterization of the physiological abnormalities is an important step in evaluating response to specific therapies. Twenty-two patients with Fabry's disease, and with relatively preserved renal function, underwent conventional and near-nerve conduction studies, electromyography, sympathetic skin responses, and quantitative sensory testing (QST). Nerve conduction studies were mostly normal except for an increased frequency of median nerve entrapment at the wrist in 6 (27%) patients. Sympathetic skin responses were preserved in 19 of 20 (95%) of the patients. The QST showed increased or immeasurable cold and warm detection thresholds in patients, significantly different from controls (n = 28) in the hand (P < 0.001, P = 0.04, respectively) and foot (P < 0.001 for both). Cold thresholds were more often abnormal than were warm thresholds. Vibration thresholds were normal in the feet and, in some patients, elevated in the hand only, probably due to frequent median nerve entrapment at the wrist. Our findings suggest that the neuropathy of Fabry's disease is characterized by an increased prevalence of median nerve entrapment at the wrist and by thermal afferent fiber dysfunction in a length-dependent fashion, with greater impairment of cold than warm sensation.

Thermosensitivity of muscle: high-intensity thermal stimulation of muscle tissue induces muscle pain in humans

Small-calibre afferent units responding to thermal stimuli have previously been reported to exist in muscle. The question as to whether these receptors in humans mediate subjective thermal sensations from muscle remains unresolved. The aims of the present study were to determine in humans whether intramuscular injection of warm and cold isotonic saline elicits temperature sensations, muscle pain or any other sensations. In 15 subjects, no thermal sensations assessed on a temperature visual analogue scale (VAS) could be detected with intramuscular injections of isotonic saline (1.5 ml) into the anterior tibial muscle at temperatures ranging from 8 to 48 degrees C. The same subjects recorded strongly increasing scores on a temperature VAS when thermal stimuli in the same intensity range were applied to the skin overlying the muscle by a contact thermode. However, I.M. isotonic saline of 48 degrees C induced muscle pain with peak scores of 3.2 +/- 0.8 cm on a VAS scale ranging from 0 to 10 cm. Using the the McGill pain questionnaire a subgroup, of subjects qualitatively described the pain using the 'thermal hot' and 'dullness' word groups. Temperature measurements within the muscle during the stimulating injections showed that the time course of the pain sensation elicited by saline at 48 degrees C paralleled that of the intramuscular temperature and far outlasted the injection time. The present data show that high-intensity thermal stimulation of muscle is associated with muscle pain. High-threshold warm-sensitive receptors may mediate the pain following activation by temperatures of 48 degrees C or more. Taken together, the data indicate that thermosensation from a given volume of muscle is less potent than nociception.

Thermal thresholds in complex regional pain syndrome type I: sensitivity and repeatability of the methods of limits and levels

OBJECTIVES

To study whether the method of levels (MLE) or the method of limits (MLI) is preferable as a method of measuring thermal perception thresholds in patients with complex regional pain syndrome type I (CRPS I).

METHODS

Perception thresholds for warmth and cold were measured twice, with both MLE and MLI, at a 1 month interval, both at unaffected and affected wrists (n=33) or feet (n=20) of patients with CRPS I of one extremity.

RESULTS

(1) Sensitivity for pathology was equal for both methods. (2) The agreement between thresholds measured by both methods was low at all locations, except for the unaffected wrist. Since thresholds measured with the MLI always contain reaction time artefacts, this lack of agreement favours the MLE. (3) At both unaffected and affected wrists, the MLE showed significantly better coefficients of repeatability as compared to the MLI for both sensations. However, at both unaffected and affected feet, there was no preference for either method as far as threshold measurement repeatability was concerned.

CONCLUSIONS

Abnormal thermal perception thresholds occurred in 20% (foot) to 36% (wrist) of the CRPS I patients on the affected side and in 15% (foot, wrist) on the unaffected side. The MLE is considered to be the preferable method to assess thermal perception thresholds in CRPS I.

Quantitative sensory testing of thermal and vibratory perception in familial dysautonomia

Familial dysautonomia (FD) is an inherited disorder that is known to affect both sensory and autonomic functions as a result of incomplete neuronal development and progressive loss but the degree to which patients are affected differs greatly. To determine if quantitative vibration and thermal testing refined the assessment of severity, 23 familial dysautonomia patients were evaluated by clinical examination, measurements of median, peroneal and sural nerve conduction velocities (NCV), and assessment of vibration thresholds at two body sites and of warm and cold perception thresholds at 6 body sites using the method of limits. Data from 80 age-matched normal individuals provided control data for vibration and temperature thresholds. All familial dysautonomia patients had abnormal thermal thresholds. Vibration perception was abnormal in 20 patients. NCVs were slowed in 8 of 16 patients who agreed to be tested. Abnormalities in thermal thresholds are consistent with the reduction of small nerve fibers in familial dysautonomia Abnormal vibration thresholds might be due to disturbed conduction of vibratory impulse trains and reflect the degree to which the disorder is progressive. Vibration and thermal sensation testing were better accepted and provided more information than NCV regarding severity of disease.

A model of the detection of warmth and cold by cutaneous sensors through effects on voltage-gated membrane channels

Warmth and cold sensations are known to derive from separate warm and cold cutaneous thermoreceptors in the form of differentiated afferent nerves. The firing rate of warm-sensing nerves increases as the temperature increases; the firing rate of cold-sensing nerves increases if the temperature is reduced. I postulate that the primary sensitivity of the warm sensors derives from voltage-gated Ca(2+) membrane channels configured such that an increase in temperature opens channels and increases the ion influx while a reduction in temperature increases the ion influx through voltage-gated Na(+) channels in the cold sensory nerve ends. In either case, the initial cation influx causes a small cellular depolarization that further opens Ca(2+) channels, admitting more cations in a positive feedback process that leads to the depolarization of the membrane, thus initiating an action potential pulse. Monte Carlo calculations based on a well defined model of such processes, which include effects of noise, demonstrate quantitative agreement of the model with an extensive body of data.

Determination of nerve conduction velocity of C-fibres in humans from thermal thresholds to contact heat (thermode) and from evoked brain potentials to radiant heat (CO2 laser)

This study was designed to estimate and compare nerve conduction velocity (NCV) of cutaneous heat-sensitive C-fibres obtained using two methods. The first is a method based on reaction times to different rates of temperature change produced by a large contact thermode (Thermotest). The second is a novel method based on ultra-late-evoked brain potentials to CO2 laser stimuli with tiny beam sections (< 0.25 mm2), allowing selective and direct activation of very slow conducting afferents. Both methods were applied on three sites of the right leg (foot, knee and thigh) of ten healthy subjects. When based on the reaction times to contact heat, NCV estimations were 0.4 +/- 0.22 m/s for the proximal segment (knee-thigh) and 0.6 +/- 0.23 m/s for the distal segment (foot-knee). When based on the difference in latency of the ultra-late positivity of laser-evoked brain potentials, NCV estimations were respectively 1.4 +/- 0.77 m/s and 1.2 +/- 0.55 m/s. For both methods, the difference in NCV between proximal and distal limb segments was not significant. Although both methods give NCV estimations within the range of C-fibres, the systematic difference between NCV obtained from each method may result from the activation of subpopulations of C-fibres with different NCV depending on the method of stimulation (low-threshold thermal receptors by the thermode and thermal nociceptors by the CO2 laser). Considering the difficulty of investigating peripheral fibres with slow conduction velocities (C-fibres) in humans, the methods used in the present study may be useful tools in both experimental and clinical situations.

Sympathetic skin response following thermal, electrical, acoustic, and inspiratory gasp stimulation in familial dysautonomia patients and healthy persons

To determine whether sympathetic skin response (SSR) testing evaluates afferent small or efferent sympathetic nerve fiber dysfunction, we studied SSR in patients with familial dysautonomia (FD) in whom both afferent small and efferent sympathetic fibers are largely reduced. We analyzed whether the response pattern to a combination of stimuli specific for large or small fiber activation allows differentiation between afferent and efferent small fiber dysfunction. In 52 volunteers and 13 FD patients, SSR was studied at palms and soles after warm, cold and heat as well as electrical, acoustic, and inspiratory gasp stimulation. In addition, thermal thresholds were assessed at four body sites using a Thermotest device (Somedic; Stockholm, Sweden). In volunteers, any stimulus induced reproducible SSRs. Only cold failed to evoke SSR in two volunteers. In all FD patients, electrical SSR was present, but amplitudes were reduced. Five patients had no acoustic SSR, four had no inspiratory SSR. Thermal SSR was absent in 10 patients with abnormal thermal perception and present in one patient with preserved thermal sensation. In two patients, thermal SSR was present only when skin areas with preserved temperature perception were stimulated. In patients with FD, preserved electrical SSR demonstrated the overall integrity of the SSR reflex but amplitude reduction suggested impaired sudomotor activation. SSR responses were dependent on the perception of the stimulus. In the presence of preserved electrical SSR, absent thermal SSR reflects afferent small fiber dysfunction. A combination of SSR stimulus types allows differentiation between afferent small or efferent sympathetic nerve fiber dysfunction.

A comparison of two methods for measuring thermal thresholds in diabetic neuropathy

Thermal thresholds can be measured psychophysically using either the method of limits or a forced-choice method. We have compared the two methods in 367 diabetic patients, 128 with symptomatic neuropathy. The Sensortek method was chosen for the forced-choice device, the Somedic modification of the Marstock method for a method of limits. Cooling and heat pain thresholds were also measured using the Marstock method. Somedic thermal thresholds increase with age in normal subjects, but not to a clinically significant degree. In diabetics Marstock warm threshold increased by 0.8 degrees C/decade, Sensortek by 0.1 degrees C/decade. Both methods had a high coefficient of variation in normal subjects (Sensortek 29%, Marstock warm 14%, cool 42%). The prevalence of abnormal thresholds was similar for both methods (28-32%), though Marstock heat pain thresholds were less frequently abnormal (18%). Only 15-18% of patients had abnormal results in both tests. Sensortek thresholds were significantly lower on repeat testing, and all thresholds were higher in symptomatic patients. Both methods are suitable for clinical thermal testing, though the method of limits is quicker. In screening studies the choice of a suitable apparatus need not be determined by the psychophysical basis of the test.

The effect of topically applied anaesthetics (EMLA cream) on thresholds to thermode and argon laser stimulation

The cold and warmth thresholds to thermode stimulation and the sensory and pain thresholds to argon laser stimulation were determined before and after topical application of EMLA (Eutectic Mixture of Local Anaesthetics) cream. The sensory threshold to argon laser stimulation and warmth threshold to thermode stimulation are both described in terms of warmth or faint heat. The sensory threshold persisted for more than 80 min of EMLA application, whereas the warmth and cold thresholds were detectable after 105 min in half the volunteers. Pain evoked by strong laser pulses was abolished after 80 min of cream application. The analgetic effect of topically applied lidocaine/prilocaine, evaluated by the cutaneous thermal and pain threshold, is compatible with the idea that topical application of EMLA cream blocks free nerve endings rather than the nerve fibres, and induces a sequence of sensory loss which, in some respects, differs from that typically observed after perineural application of local anaesthetics. The effect of topically applied anaesthetics is influenced by a number of thermodynamical, anatomical, and physiological factors in the skin.

The sequential-interval state space: a means of displaying temporal information in neuron firing

Variability in neuronal firing exhibits sufficient uncertainty so that a simple average firing frequency code is probably inadequate for most nervous system signalling. Temporal patterns certainly play an important role in neuronal coding. We have used interval histogram and 3-dimensional sequential interval state space plots to investigate various common patterns of firing in neurons of the land snail, Helix aspersa. Typical firing patterns included random, highly regular, doublet, and burst firing. Individual neurons could be made to change their temporal firing pattern in response to changes in transmembrane currents, or temperature, or the application of convulsant drugs. In every instance, the sequential interval state space plot provided a more distinctive display of temporal pattern than did the more common interval histogram. State space plots were also investigated for evidence of a predicted chaotic attractor. In no instance was this type of state space plot observed.

The conduction velocities of peripheral nerve fibres conveying sensations of warming and cooling

With the current practice of measuring thresholds for warming and cooling separately, the question of the exact nature of afferents subserving these sensations assumes new importance. Experiments to measure reaction times to warming and cooling stimuli at two sites on the lower limb are described. The conduction velocity for each sensation was estimated from the conduction distance and conduction time in the limb. The estimated mean conduction velocity for warming was 0.5, SD 0.2 m/s and cooling 2.1, SD 0.8 m/s. These figures confirm that the sensation of warming is conveyed in unmyelinated and cooling in small myelinated peripheral nerve fibres.

Local thermal sensation and finger vasoconstriction in the locally heated hand

The effects of local heating on finger blood flow (BF) and local thermal sensation (Sensw) were studied. Finger BFs in both hands were measured simultaneously; one hand was immersed in water the temperature (Tw) of which was raised from 35 degrees C to 43 degrees C by steps of 2 degrees C every 10 min, while the other hand was kept at Tw 35 degrees C. Finger BF in the locally heated hand decreased at Tw 37 to 41 degrees C, while finger BF in the control hand did not alter. Sensw in the heated hand showed a dynamic response, initially increasing concomitantly with an increase in Tw, then gradually returning and adapting to a new level of Sensw. The dynamic response of Sensw was not perceived during mental calculation even when Tw was raised to 40 degrees C, and the reduction in finger blood flow was not observed. These results suggest that finger vasoconstriction caused by local heating closely relates to the dynamic response characteristic of local thermal sensation at Tw above core temperature, and that the perception of local thermal sensation in the central nervous system is involved in the mechanism of this vasoconstrictor response.

Methods of measurement of thermal thresholds

Thermal tests were performed in 117 healthy subjects on the face, wrist and leg; 32 were tested on the legs with different rates of cooling and warming. Additionally 2 groups of diabetics (37 patients) were tested. Thermotesting was most sensitive on the legs using a rate of temperature change of 2.5-2.8 oC/s. Warm and cold perception should be tested separately. Cold perception testing is most sensitive. Combined tests of warm and cold thresholds as well as the testing of cool pain and heat pain do not improve results. Abnormal cold perception may be an early indicator of diabetic small fibre polyneuropathy, leading to cold trauma and ulcers on the feet.

Selective reduction of second pain sensations by systemic morphine in humans

A variety of forms of painful stimulation were delivered to human subjects in order to determine whether therapeutic dosages of systemic morphine might produce significant attenuation of some forms of phasic pain that are tolerable for experimental usage. Consistent with previous reports, simple application of thermal or electrical energy to the skin (for 3 sec) produced sensations of pain that were not significantly reduced by prior administration of morphine. Similarly, subjects that were trained to focus their attention on the magnitude of the immediate (first) pain sensation evoked by brief electrical or mechanical stimulation did not report reduction by morphine of pain attributed to conduction in myelinated peripheral nociceptors. In contrast, the magnitude of late (second) pain sensations produced by brief pulses of electrical, thermal or mechanical stimuli to the same subjects was consistently reduced significantly by doses of 5 or 10 mg of morphine. The simplest interpretation of the effect on second pain intensity is that morphine preferentially attenuates input from unmyelinated nociceptors. This conclusion was reinforced by an experiment in which chemicals were applied to the skin. Morphine reduced pain produced by capsaicin (presumed to selectively excite unmyelinated peripheral afferents) but did not diminish pain elicited by bradykinin (presumed to excite A delta and C nociceptors). Comparing long duration pains from chemical stimulation (lasting in excess of 5 min) with briefer pains elicited by 50 msec to 3 sec of stimulation did not support the notion that morphine acts selectively on tonic pain. Also, after-sensations that could be discerned following second pain were not eliminated by morphine, and paired pulse facilitation of first pain sensations remained after administration of morphine, indicating that temporal summation is not preferentially reduced. Regardless of duration, frequency or latency, pain arising exclusively from unmyelinated nociceptors was attenuated substantially, but other elicited sensations were not reliably affected. For example, detection thresholds for warmth were unaffected by morphine, demonstrating that input from all unmyelinated afferents is not reduced.