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Biomedical subjects

H E Torebjörk

Publications and source records attributed to H E Torebjörk.

At least 19 recordsLinked to original sources
PubMed2003

Active "itch fibers" in chronic pruritus.

An itch-specific neuronal pathway was recently discovered in healthy humans and animals. Here the authors report that activity in this specific pathway coincides with itch under pathophysiologic conditions in a patient with chronic pruritus. Microneurographic recordings from the symptomatic area revealed spontaneous activity in six single C-fiber afferents that had the characteristic features of "itch fibers." Itch may be caused by activity in a specific subpopulation of C-fiber afferents.

Chronic Disease↗
PubMed2002

Innervation territories of mechano-insensitive C nociceptors in human skin.

Microneurographic recordings were obtained in the peroneal nerve from 20 mechano-insensitive units (CMi) and six mechano-heat responsive C units (CMH) in healthy human subjects. Their innervation territories in the skin of the leg or foot were assessed by transcutaneous electrical stimulation with a pointed probe at intensities of 10 to 100 mA (0.2 ms) and, when applicable, by mechanical von Frey hair stimulation. Electro-receptive fields (eRFs) of CMH units had a median area of 1.95 cm(2) when mapped with 10 mA that coincided approximately with mechano-receptive fields (mRFs) as mapped with a 750-mN von Frey hair. Fifty-milliampere stimuli increased the eRFs to 3.08 cm(2) in a concentric manner. This was probably due to current spread since these units are known to have low electrical thresholds. Further increase of the stimulus strength to 70 or 100 mA increased the eRFs only marginally. Mechano-insensitive units had much smaller eRFs (median: 0.35 cm(2)) than CMH units when mapped with the same pointed probe at 10 mA (n = 13). The receptive territories consisted of one distinct spot or of several spots separated by distances of more than 1 cm. However, when mapping stimuli of 50 mA were applied, eRFs became continuous and grew to a median area of 5.34 cm(2), i.e., larger than those of CMHs. The borders of eRFs of CMi units were significantly more irregular compared with CMH units. A further increase of the stimulus intensity to a maximum of 100 mA only marginally enlarged the eRFs. The CMi units could be activated by heat or chemical substances applied inside the 50-mA eRF, indicating that receptive nerve endings were mapped. Responsiveness to these stimuli was inhomogeneous within the eRFs. It was concluded that innervation territories of CMi units in human skin exceed those of CMH units in size by a factor of approximately 3. The widely branched terminals underlying the large fields are consistent with a role of this nociceptor class in axon reflex flare and preclude a role in exact spatial discrimination of noxious stimuli.

Adult↗
PubMed2000

Time course of post-excitatory effects separates afferent human C fibre classes.

1. To study post-excitatory changes of conduction velocity, action potentials were recorded from 132 unmyelinated nerve fibres (C fibres) in cutaneous fascicles of the peroneal nerve using microneurography in healthy human subjects. The 'marking' technique was used to assess responsiveness to mechanical and heat stimuli or sympathetic reflex provocation. 2. C fibres were classified into three major classes: mechano-responsive afferent (n = 76), mechano-insensitive afferent (n = 48) and sympathetic efferent C fibres (n = 8). 3. During regular stimulation at 0.25 Hz, conditioning pulses were intermittently interposed. Changes of conduction velocity were assessed for different numbers of conditioning impulses and varying interstimulus intervals (ISIs). For all three fibre classes the latency shift following conditioning pulses at an ISI of 1000 ms increased linearly with their number (n = 1, 2 and 4). However, the absolute degree of conduction velocity slowing was much higher in the 32 mechano-insensitive fibres as compared with 56 mechano-responsive or 8 sympathetic fibres. 4. Single additional pulses were interposed at different ISIs from 20 to 2000 ms. For 20 mechano-responsive fibres conduction velocity slowing increased with decreasing ISI (subnormal phase). In contrast, for 16 mechano-insensitive C fibres the conduction velocity slowing decreased with shorter ISIs, and at values lower than 417 +/- 49 ms (mean +/- s.e.m.) the conduction velocity of the conditioned action potential was faster than before (conduction velocity speeding). This supernormal phase had its maximum at 69 +/- 10 ms. 5. In this study we provide, for the first time, direct evidence of relative supernormal conduction in human mechano-insensitive C fibres. The implications for temporal coding in different afferent C fibre classes are discussed.

Action Potentials↗
PubMed2000

Which nerve fibers mediate the axon reflex flare in human skin?

Axon reflex vasodilatation due to transcutaneous electrical stimulation in human skin was measured by laser Doppler imaging. Constant current pulses of 10 mA, 0.2 ms, delivered at 1 or 10 Hz for 2 min through a probe of 30 mm2 surface area did not induce a significant flare response, though this stimulus previously has been found supra-maximal for cutaneous polymodal (mechano-heat responsive) C-nociceptors in microneurography experiments. Pulses of the same strength from a pointed probe yielding a higher current density induced an extended and persistent flare. This type of stimulus previously has been proven to recruit mechano-insensitive C-units in microneurography experiments, in contrast to stimuli from the 30 mm2 probe. It is concluded that mechano-insensitive C-nociceptors and not polymodal C-units mediate the axon reflex flare in human skin.

Adult↗
PubMed2000

Mechano-insensitive nociceptors encode pain evoked by tonic pressure to human skin.

Single unmyelinated axons in the superficial branch of the peroneal nerve of human subjects were recorded (microneurography) and the response patterns during tonic pressure stimulation (14N at 30 mm(2); 120 s) were assessed using the previously described "marking technique". It was found that tonic pressure stimuli induced augmenting pain responses which were matched by the discharges of initially mechano-insensitive ("silent") C-units, whereas mechano- and heat-responsive "polymodal" C-nociceptors showed a response pattern incompatible with the stimulus-induced perceptions, namely strong initial excitation, followed by adaptation. Eighteen mechano- and heat-responsive "polymodal" C-fibers and 11 mechano-insensitive units were studied. The former had von Frey thresholds <160 mN, the latter were not excited by a von Frey filament of 750 mN (six of them responded to radiant heat stimulation). However, in the course of pressure stimulation, nine of the 11 mechano-insensitive units were activated after more than 20s. A second, identical pressure stimulus induced a stronger response in mechano-insensitive and a weaker response in mechano-responsive units. The stronger response, indicating sensitization, matched the more intense pain perception during the second pressure stimulus. It is concluded that mechano-insensitive C-nociceptors encode pressure-induced pain in human hairy skin and that they play an important role in static mechanical hyperalgesia.

Adult↗
PubMed2000

Encoding of burning pain from capsaicin-treated human skin in two categories of unmyelinated nerve fibres.

Burning pain was induced in healthy human subjects by intracutaneous injections of capsaicin (20 microl, 0.1%) in the innervation territory of the cutaneous branch of the peroneal nerve and the pain responses were compared with the activation patterns of afferent C-fibres recorded by microneurography. Responsiveness of single units to mechanical or heat stimuli or to sympathetic reflex provocation tests was determined by transient slowing of conduction velocity following activation (marking technique). Capsaicin activated each of 12 mechano-responsive and 17 of 20 mechano-insensitive C-units. However, the duration of the responses to capsaicin was significantly longer in mechano-insensitive C-units (median 170 s; quartiles 80-390) compared with mechano-responsive C-units (8 s; 4-10). The activation times of mechano-insensitive C-units closely matched the duration of capsaicin-induced pain responses, whereas activation of mechano-responsive C-units was too short to account for the duration of the burning pain. The latter generally were desensitized to mechanical stimulation at the injection site, whereas 8 of 17 of the originally mechano-insensitive C-units became responsive to mechanical probing at the injection site after capsaicin. Responses typically started several seconds after the onset of the mechanical stimulus in parallel with pain sensations. We did not observe sensitization to brushing or to punctate stimuli in uninjured parts of the innervation territory. Differential capsaicin sensitivity adds to the cumulating evidence for the existence of two categories of functionally different nociceptors in human skin, with a special role for mechano-insensitive fibres in sensitization and hyperalgesia. Possible structural differences between these two categories are discussed, including the role of tetrodotoxin-resistant sodium channels.

Adult↗
PubMed1999

Functional attributes discriminating mechano-insensitive and mechano-responsive C nociceptors in human skin.

Microneurography was used in healthy human subjects to record action potentials from unmyelinated nerve fibers (C units) in cutaneous fascicles of the peroneal nerve. Activity-dependent slowing (n = 96) and transcutaneous electrical thresholds (n = 67) were determined. Eight units were sympathetic efferents according to their responses to sympathetic reflex provocations. Mechano-heat-responsive C units (CMH) (n = 56) had thresholds to von Frey hair stimulation </=90 mN (6.5 bar). Mechano-insensitive C units (n = 32) were unresponsive to 750 mN (18 bar). Twenty-six mechano-insensitive units responded to heat (CH), and the remaining six units did not respond to physical stimuli but were proven to be afferent by their response to intracutaneous capsaicin (CM(i)H(i)). Mechano-insensitive units had significantly slower conduction velocity (0.81 +/- 0.03 m/sec), and CH units had higher heat thresholds (48.0 +/- 0.6 degrees C) compared with CMH units (1.01 +/- 0.01 m/sec; 40.7 +/- 0.4 degrees C). Transcutaneous electrical thresholds were <9 mA for CMH units and >35 mA for CH and CM(i)H(i) units. Activity-dependent slowing was much more pronounced in mechano-insensitive than in mechano-responsive units, without overlap. Sympathetic efferent C units showed intermediate slowing, significantly different from CMH, and completely separate from CH and CM(i)H(i) units. The activity-dependent slowing of conduction provides evidence for different membrane attributes of different classes of C fibers in humans.

Adult↗
PubMed1998

Innervation territories of single sympathetic C fibers in human skin.

Microneurography techniques were used to record action potentials from unmyelinated nerve fibers (C fibers) in the cutaneous fascicles of the peroneal nerve in healthy volunteers. C units were identified by their long latency responses to electrical stimulation of their terminals in the skin. Their responsiveness to mechanical or heat stimuli applied to the skin or to sympathetic reflex provocation tests was determined by transient slowing of conduction velocity following activation (marking technique). In a sample of 381 C units, 59 were unresponsive to mechanical and thermal stimulation of their endings, but responded to sympathetic reflex provocation tests, e.g., arousal or deep inspiration. They were classified as sympathetic efferent units. On average, conduction velocities of sympathetic units were lower (0.78 +/- 0.12 m/s, mean +/- SD) than those of mechano-heat (CMH) or mechanoresponsive (CM) afferent C units (0.91 +/- 0.14 m/s). Endings of most of the sympathetic units were located in the skin of toes or in the foot dorsum. Innervation territories of 16 sympathetic units were mapped by means of conditioning transcutaneous electrical stimuli. Twelve units had one continuous skin territory, whereas two units had two and two other units had three and five separate territories, respectively. The mean innervated area was 128 mm2 (range: 24-350 mm2). Innervation territories of sympathetic units were of approximately the same size in different skin regions on the lower leg, foot, or toes. Based on responses to whole body cooling and warming, two units were tentatively classified as vasoconstrictor and sudomotor units, respectively. Eleven units were tested for responsiveness to iontophoresis of acetylcholine in their innervation territories. In five of them, activity was induced that was not due to central reflex activity but instead due to antidromic activation from the peripheral terminals. Iontophoresis of saline or histamine was ineffective. These findings confirm the existence of excitatory cholinergic receptors in the terminal membrane of some sympathetic units, possibly sudomotors.

Acetylcholine↗
PubMed1997

Specific C-receptors for itch in human skin.

In microneurography experiments 56 unmyelinated nerve fibers were studied in the cutaneous branch of the peroneal nerve of healthy volunteers. Units were identified with the "marking" technique as mechanically and heat-responsive (CMH; n = 30), heat-responsive (CH; n = 13), or unresponsive to mechanical and heat stimulation (CMiHi; n = 13). None of the units showed spontaneous activity. These units were tested for responsiveness to iontophoresis of histamine (1 mA, 20 sec) from a small probe (diameter, 6 mm), which induced itch sensations lasting several minutes. Twenty-three units were unresponsive to histamine, and 25 units responded weakly with a few spike discharges after iontophoresis. Eight units, however, responded with sustained discharges to histamine, and their discharge patterns were matching the time course of the itch sensations. All C-units in this group were mechanically insensitive, and five of them were heat-responsive. They had very low conduction velocities of only 0.5 m/sec, on average, which is significantly lower than conduction velocities of the "polymodal" CMH units. This slow conduction velocities attributable to small axon diameters may be one reason why these units have not been encountered in previous studies. Histamine-sensitive C-units had very large innervation territories extending up to a diameter of 85 mm on the lower leg. We conclude that these C-fibers represent a new class of afferent nerve fibers with particularly thin axons but excessive terminal branching. This type of C-fiber probably represents the afferent units long searched for mediating itch sensations.

Adult↗
PubMed1997

Innervation territories of mechanically activated C nociceptor units in human skin.

Innervation territories of mechanically activated C nociceptor units in human skin. J. Neurophysiol. 78: 2641-2648, 1997. Innervation territories of single mechanically activated C nociceptors in the skin of the leg and foot were explored in normal human subjects. Microneurographic recordings were obtained in the peroneal nerve from 70 mechano-heat responsive (CMH) and 7 mechano-(but not heat) responsive (CM) units. Units were identified by their constant long-latency response to intracutaneous electrical stimulation of their terminals. Responsiveness to mechanical, heat, or transcutaneous electrical stimuli was verified by transient slowing of conduction velocity after activation by such stimuli. We determined their thresholds to mechanical stimuli (mean 33.7 mN, median 30 mN, range 3-750 mN) and heat (mean 42.5 degrees C, median 42.5 degrees C, range 37-49 degrees C). Most mechano-receptive fields (mRFs) were found on the foot dorsum (60 units) and some on the lower leg (14 units) and toes (3 units). Most units had one continuous mRF, but 10 units had more complex fields. Areas of mRFs mapped with a von Frey filament (750 mN) ranged from 10 to 363 mm2 (mean, 106 mm2). The mRFs were oval or irregularly shaped with greatest diameters ranging from 3 to 45 mm. Mean areas of mRFs were largest on the lower leg (198 mm2), smaller on the foot dorsum (88 mm2), and smallest on the toes (35 mm2). Forty-nine of the 77 units had identical mRFs and electro-receptive fields (eRFs). Twenty-six units had larger eRFs than mRFs, whereas the opposite was found for two units only. Areas of eRFs ranged from 16 to 511 mm2 (mean 121 mm2). An estimate of the innervation density based on the present data and the presumed number of C fibers in cutaneous fascicles of the peroneal nerve suggests a considerable overlap of nociceptive endings in the skin. Such overlapping nociceptor innervation in the skin allows for substantial spatial summation in response to punctate noxious stimuli, which may be a prerequisite for high accuracy in localizing painful events from a C-fiber input. The reduction in size of innervation territories distally allows for finer discrimination of spatial dimensions of noxious stimuli distally as compared with proximal regions of the extremities. Mean maximal diameters of the mechano-receptive fields of CMH and CM units on the lower leg (22.3 mm) and foot (15.3 mm) are of similar size as the radius of axon reflex flares evoked by noxious mechanical stimuli in these regions.

Adult↗
PubMed1996

Limitation of sensitization to injured parts of receptive fields in human skin C-nociceptors.

Unmyelinated cutaneous mechano-heat fibers (CMH) in the peroneal nerve of healthy human volunteers were studied by means of a "marking" technique which allows stable recordings from identified single units over extended periods. Mechanoreceptive field sizes were 105 +/- 13 mm2 in 25 units. These large receptive fields indicate extensive terminal branching of C fibers in the skin of foot and lower leg. Sensitization of CMHs was tested by assessment of thresholds for mechanical (von Frey hair) and heat stimuli before and after topical application of mustard oil (allyl isothiocyanate) and capsaicin (8-methyl-N-vanillyl-6-noneamide). While in a group of 14 CMHs the entire receptive field was treated with these irritant substances, in another group of 11 CMH units only parts of the receptive field were treated to check for signs of spreading sensitization through axon collaterals. Mustard oil application did not change mechanical thresholds, regardless of whether parts of or complete receptive fields were treated. However, mean heat thresholds dropped by 5.6 degrees C to 36.5 +/- 1.5 degrees C in completely treated receptive fields and by 5.7 degrees C to 37.3 +/- 3.4 degrees C in treated parts of receptive fields ("primary sensitization"). In contrast, heat thresholds in the nontreated parts did not change significantly (42.1 +/- 3.4 degrees C vs 41.2 +/- 3.9 degrees C), i.e. "secondary sensitization" to heat was lacking. The absence of primary sensitization to probing with von Frey hairs indicates that sensitization of insensitive C fibers and recruitment of insensitive axon collaterals may be more important for mechanical hyperalgesia than sensitization of conventional CMH units-apart from the contribution of central mechanisms. The lack of spread of sensitization to untreated parts of the receptive fields o CMHs ("secondary sensitization") indicates that this fiber group is probably not involved in any form of secondary hyperalgesia to heating.

Adult↗
PubMed1995

Delayed responses to electrical stimuli reflect C-fiber responsiveness in human microneurography.

The slowing of impulse conduction during the relative refractory period has often been used to assess activation of C-fibers, in particular, in human microneurography. This study aimed to evaluate the sensitivity of this method and the factors affecting it. Thirty cutaneous C-fibers were recorded from the peroneal nerves of healthy human subjects. Intracutaneous electrical stimulation in the receptive field at 4 s intervals, after some minutes of adaptation, induced spike discharges at constant latency. One or more conditioning stimulus pulses were interpolated at different intervals and the increase in latency after the subsequent regular pulse was assessed. The latency shift was found to depend on the number of interposed pulses, on the time interval between conditioning and conditioned stimulus, and on the conduction velocity of the C-unit. The increase in latency was larger with greater distance between stimulating and recording electrodes, indicating a contribution of the conductile membrane over its whole length. On the other hand, slowing was more pronounced, on average, in slower conducting C-units and conduction velocities were slower when recordings were performed more distally. These findings indicate that the slower terminal nerve branches contribute most to the latency increases. Even a single additional spike in between two regular pulses caused a reliable latency shift of 1.2 +/- 0.2 ms (mean +/- SEM) and additional pulses lead to an approximately linear latency increase (2 pulses: 2.3 +/- 0.3 ms; 4 pulses: 5.9 +/- 0.7 ms). In contrast to the number of interposed stimuli, different intervals between interposed and regular stimuli had only a minor impact on the latency shifts.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Sensitization of insensitive branches of C nociceptors in human skin.

1. Eighteen cutaneous mechanosensitive C nociceptors were recorded from the peroneal nerves of healthy human subjects. Their identity was continuously monitored by intracutaneous electrical stimulation, and their activation by mechanical or transcutaneous electrical stimulation was detected by slowing of conduction velocity during the relative refractory period. 2. Mechanoreceptive fields (mRFs) mapped with suprathreshold von Frey hair stimuli covered an area of 99 +/- 21 mm2 (mean +/- S.E.M.). Two of the units had separate mRFs, with borders about 0.5-1.5 cm apart from each other and the largest of these units had a maximal diameter of 4.5 cm. 3. Successive topical application of mustard oil and capsaicin induced expansions of mRFs by 57 +/- 14 mm2 in eight of fifteen units. 4. In twelve units transcutaneous electrical stimulation delivered through a pointed electrode was used for mapping the electroreceptive fields (eRFs). The borders of the eRFs and the mRFs were identical for two of twelve units only. In the other ten units additional mechano-insensitive areas (55 +/- 22 mm2) were detected from which transcutaneous electrical stimuli could activate the respective unit. 5. Application of mustard oil and capsaicin to these mechano-insensitive areas sensitized five of eight units to mechanical stimuli. In these cases the mRF after sensitization exactly corresponded to the eRF. 6. It is concluded that there are insensitive branches in human mechanosensitive cutaneous C nociceptors that can be detected by transcutaneous electrical stimulation and sensitized by topical application of chemical irritants. Activation of those branches in the course of inflammatory processes may contribute to spatial summation at central synapses and hence to hyperalgesia.

Adult↗
PubMed1994

Responsiveness of the somatosensory system after nerve injury and amputation in the human hand.

We studied the responsiveness of the somatosensory system in humans after prolonged deprivation of peripheral input. Eight patients with traumatic transection of the median or ulnar nerve and 6 patients with amputation of a finger or hand underwent microneurography and intraneural stimulation. Bundles of nerve fibers were electrically stimulated through a microelectrode placed in the affected nerve proximally to the site of damage or in the case of amputees, in a nerve fascicle supplying the stump. During intraneural stimulation the subjects with nerve injuries reported distinct percepts in the hypoesthetic skin. Their projections were usually confined to the territory of a single or two adjacent palmar digital nerves, similar to the fascicular territories of healthy nerves in control subjects, but there was much less microneurographically recordable afferent activity than in normal subjects. In amputees intraneural stimulation evoked sensations in a phantom digit or digits in over three fourths of the fascicles studied. We conclude that (1) the somatosensory system remains able to process information from a nerve fascicle that has lost its cutaneous territory, and (2) somatosensory localization remains accurate despite the presumed central reorganization that takes place after nerve division or amputation. This lack of functional adaptation has important implications with regard to our understanding of human central nervous system plasticity.

Adolescent↗
PubMed1994

Nociceptor modulated central sensitization causes mechanical hyperalgesia in acute chemogenic and chronic neuropathic pain.

Brush-evoked pain (mechanical allodynia, dynamic mechanical hyperalgesia) is a hallmark of neuropathic and inflammatory pain states. Here we have examined the neural mechanisms that induce and maintain this component of mechanical hyperalgesia. The principle finding of these experiments is that the severity of brush-evoked pain correlates with the intensity of background pain in patients suffering from chronic painful neuropathies and in normal subjects with acute experimental chemogenic pain. In experiments on nine normal subjects topical application of mustard oil for 5 min evoked strong burning pain and hyperalgesia to light mechanical stimuli. Differential nerve blocks (by compression of the superficial radial nerve) revealed that the brush-evoked pain was transmitted by A beta-fibres, which normally encode non-painful tactile sensations, while the burning pain was signalled by C-fibres. Psychophysical measurements showed that mustard oil treatment resulted in a pronounced sensitization of nociceptors to heat so that subsequent innocuous changes of skin temperature from 35 to 40 degrees C resulted in a proportional increase of burning background pain. Changes in the magnitude of ongoing burning pain were closely correlated (r = 0.81) to the intensity of brush-evoked pain. While conduction block of A-fibres eliminated only touch-evoked pain, blockade of C-fibre excitation instantaneously abolished both ongoing and touch-evoked pain. In nine patients with chronic neuralgia (15 years mean duration) ongoing and brush-evoked pain were examined. In six patients, differential block of A beta-fibres eliminated touch-evoked pain, but ongoing pain persisted when only C-fibres were conducting. Complete relief of both ongoing and stimulus-induced pain was obtained in two patients with intravenous regional guanethedine block and in two other individuals by local anaesthetic blocks of nerves supplying the symptomatic skin, indicating that input from primary afferents was necessary for the maintenance of the pains and that ongoing pain was not self-perpetuated by central mechanisms alone. Quantitative sensory tests revealed heat hyperalgesia in four patients. In those individuals, an increase of skin temperature produced a graded increase of their ongoing pain which was closely correlated (r = 0.94) with the level of brush-evoked pain. In the remaining five patients there was no heat hyperalgesia and consequently no aggravation of pain by increases of skin temperature. Nevertheless when the intensity of the background pain fluctuated spontaneously there were also parallel changes (r = 0.88) of the severity of brush-evoked pain.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute Disease↗
PubMed1993

The ability of humans to localise noxious stimuli.

We have investigated the ability of humans to localise noxious stimuli on the dorsum of the hand. Pin-prick (non-penetrating needle prick), noxious heat (round 1 cm2 copper probe heated to 50 degrees C), mustard oil (100% applied topically in a small cotton ball, diameter 5 mm) and histamine (iontophoresis of 20 mC delivered to an area of 75 mm2) were applied to skin with intact innervation and during a differential nerve compression block of the superficial radial nerve when only C-fibres were conducting. The mean mislocalisation (+/- S.E.M.; n = 8) of all stimuli was 9.5 +/- 0.8 mm with normal nerve conduction and 8.9 +/- 1.2 during the differential nerve block. There was no significant difference between the noxious submodalities. By contrast, when nerve conduction was intact, purely tactile stimulation (7 mN von Frey hair) was significantly better localised having a mean error of 5.5 +/- 0.4 mm. We conclude that focal stimuli evoking itch or pain can be localised with high precision which is only marginally worse than for tactile stimuli. This suggests the existence of a somatotopical representation for noxious inputs in the brain similar to that found for tactile stimuli.

Adult↗
PubMed1992

Intra-neural electrical stimulation of cutaneous nociceptive fibres in humans: effects of different pulse patterns on magnitude of pain.

A study was performed to elucidate how different impulse frequencies and impulse patterns in cutaneous nociceptive fibres influence the subjective magnitude of pain. Groups of nociceptive A delta and C fibres were co-activated by electrical intraneural stimulation at constant intensity in cutaneous fascicles of the peroneal nerve in healthy human subjects. The resulting pain sensations were rated on a modified visual analogue scale. Five-second trains were administered randomly at irregular intervals of at least 30 s. Five of the stimulus patterns had regular interpulse intervals, corresponding to frequencies of 1, 2, 4, 10 and 15 Hz, and three other patterns were constructed to mimic to some extent the initially phasic and subsequently slowly adapting discharge patterns which may be encountered in recordings from human nociceptors. The results from these experiments using stimulation frequencies within physiological discharge ranges for human nociceptors indicate that the subjective magnitude of pain increases monotonously as a function of stimulus frequency, and that patterns mimicking nociceptor discharges in response to natural stimuli give rise to greater peak magnitudes of pain than artificial regular patterns with a corresponding number of impulses.

Adult↗