RESPONSES OF SINGLE DORSAL CORD CELLS TO PERIPHERAL CUTANEOUS UNMYELINATED FIBRES

The thresholds of the jaw-opening reflex and trigeminal brainstem neurons to tooth-pulp stimulation in acutely and chronically prepared cats

Electrical stimuli were applied to tooth-pulp in cats and the thresholds of the jaw-opening reflex and of neurons in the trigeminal sensory nuclei were determined. The effects of the method of preparation of the animal for stereotaxic recording were determined by making observations on animals set up in one of three ways: acutely in the usual manner; chronically, three to five days before recording; and acutely with precautions to minimize nociceptive input to the central nervous system. The threshold of the jaw-opening reflex increased progressively during the setting up of the normal, acute preparations and at the time brainstem recording began was significantly higher in these than in either the chronic or low-trauma acute preparations. Previous studies have shown that the increase in threshold is maintained for several hours and is not due to the effects of the anaesthetic. In normal acute preparations, few units (27/154) were found that had thresholds below 50 microA, 0.1 ms, whereas many units were encountered that responded to such a stimulus in chronic (147/152) and low-trauma acute (99/127) animals. In the chronic and in low-trauma acute preparations, there was no significant difference between the thresholds of the units in the main sensory trigeminal nucleus and spinal subnucleus oralis compared with those in subnucleus caudalis. Thus the preparation of an animal for stereotaxic recording can cause a severe and long-lasting depression in the excitability of neurons in the trigeminal sensory nuclei and an increase in the threshold of the jaw-opening reflex. This effect will have influenced the results of previous studies on the responses evoked in central neurons by stimulation of tooth-pulp, and may have similarly affected recordings from other regions.

Long-lasting neuronal activity in rat dorsal horn evoked by impulses in cutaneous C fibres during noxious mechanical stimulation

The responses of 44 nociceptive neurones in the lumbar dorsal horn evoked by controlled mechanical stimulation of the skin, with or without conduction block in myelinated afferent fibres, were studied in the halothane-anaesthetized rat, in order to evaluate the effects of impulses in cutaneous nociceptive C fibres on dorsal horn neurones. Continuous non-noxious pinch of the skin evoked a short-latency discharge (mean latency 15 ms) in all the 13 class 2 neurones (i.e. neurones responding to both non-noxious and noxious stimulation of the skin) tested. The short-latency discharge was followed by weak prolonged activity in 6 neurones. Following noxious pinch of the skin a prominent late discharge (peak latency 150 ms-2 s) was evoked, which in all but two class 2 neurones outlasted the stimulation period (5-10 s). The discharge evoked by noxious pinch in class 3 neurones (i.e. neurones responding to noxious stimulation only) did not usually outlast the stimulation period. In all but two nociceptive neurones tested (n = 26) the late activity evoked by noxious pinch remained, albeit at a lower frequency in some neurones, during a conduction block in A fibres2,3. Hence this late discharge is probably mainly generated by impulses in nociceptive C fibers. It is concluded that nociceptive C fibres have an important role in sustaining long-lasting activation of class 2 neurones during noxious stimulation of the skin and that long-lasting discharges in these neurones indicates tissue damage to their receptive fields.

A psychophysical analysis of morphine analgesia

Intravenous administration of 0.04-0.08 mg/kg morphine sulfate reduced both sensory intensity and unpleasantness visual analogue scale (VAS) responses to graded 5 sec nociceptive temperature stimuli (45-51 degrees C) in a dose-dependent manner. The lower doses of morphine (0.04 and 0.06 mg/kg) resulted in statistically reliable reductions in affective but not sensory intensity VAS responses, possibly reflecting supraspinal effects on brain regions involved in affect and motivation. However, the highest dose of morphine tested (0.08 mg/kg) reduced both sensory and affective VAS responses to graded nociceptive stimuli as well as VAS sensory responses to first and second pain evoked by brief heat pulses. Morphine also had an especially potent inhibitory effect on temporal summation of second pain that is known to occur when intense nociceptive stimuli occur at rates greater than 0.3/sec. The results support current hypotheses about neural mechanisms of narcotic analgesia and further clarify the relative effects of morphine on sensory and affective dimensions of experimental pain. The derived morphine dose-analgesic response functions also provide a reference standard for quantitatively comparing magnitudes of different CNS-mediated forms of analgesia.

Nociceptor activation and pain

This paper reviews advances in our knowledge on the physiological properties of human nociceptors and their capacity to signal pain. Conventional microneurography was used in combination with intraneural microstimulation in subjects who estimated the magnitude of pain from nociceptor stimulation. The experimental evidence favours the notion that C polymodal nociceptors can provide a peripheral neuronal basis for determination of heat pain threshold and also an essential peripheral code for suprathreshold magnitude judgments of heat pain. Furthermore, sensitized C polymodal nociceptors can contribute to hyperalgesia after a mild heat injury to hairy skin. Temporal summation is documented for dull, delayed C fibre pain, which is different in quality and less accurately projected than the fast, sharp pain from high-threshold A delta nociceptors. A segmental organization is shown for projected and referred pain from deep structures. Examples are given of central inhibition of pain by a prostaglandin synthetase inhibitor, and by physical manoeuvres such as vibration and cooling. Recent reports on microneurographic findings after nerve injury indicate that the technique may be useful for future studies on pathophysiological pain mechanisms.

Do convergent neurones in the spinal dorsal horn discriminate nociceptive from non-nociceptive information?

Thirty convergent neurones responding to both noxious and non-noxious cutaneous stimuli were recorded at the lumbar level in either anaesthetized 'intact' rats or unanaesthetized 'spinal' rats. Their responses to radiant heat application and to repetitive innocuous mechanical stimulation of the centre of their receptive fields were analysed, both in terms of the maximal and the mean firing rates. These neurones increased their discharge rates in relation to the temperature applied to their receptive fields with the highest levels being produced by noxious intensities. This finding confirms earlier reports suggesting the capacity of these neurones to encode nociceptive information of thermal origin. However, a very high level of firing could also be evoked by repetitively applied innocuous mechanical stimuli. This was a consistent finding, observed both in intact and spinal animals, which was true for the two subgroups into which we divided the convergent neurones (warming/noxious heat units and noxious heat units). These results are discussed in terms of the role of convergent neurones in nociception. It is suggested that a single channelled signal emanating from these neurones could not be the basis of a clear nociceptive message to the brain; two alternative hypotheses involving multichannelled organizations of impulses are proposed for discussion.

Spinal cord conduction pathway for cerebellar evoked potentials from C-fibers

Evoked responses from saphenous nerve C fiber stimulation were identified on the ipsilateral intermediate cerebellar cortex. This late potential has a latency between 180 and 250 msec, duration 50-70 msec, and an amplitude 10-20 micro V. The potential is conducted in the ipsilateral dorsal quadrant as determined by differential section of the spinal cord. The probable pathway is assigned to the dorsal spinocerebellar tract.

Excitation of marginal and substantia gelatinosa neurons in the primate spinal cord: indications of their place in dorsal horn functional organization

Electrophysiological recordings were made from superficial parts of the spinal dorsal horn in monkeys, using dye-filled micropipette electrodes to permit iontophoretic marking of the recording sites for subsequent histological recovery. Focal field potentials and unitary activity evoked by dorsal root volleys including slowly-conducting components (both myelinated and unmyelinated) were found in the posteromarginal zone and the substantia gelatinosa (SG). Unitary potentials identified as being of the type recorded from cellular regions were separated into categories according to which group of slowly-conducting fibers and which kinds of cutaneous stimulation evoked the discharge. Recording locations for units excited by volleys in myelinated fibers conducting under 35 m/sec, by the types of skin stimulation activating either high-threshold mechanoreceptors (nociceptors) or cooling thermoreceptors, and giving no evidence of suprathreshold C-fiber excitation were centered on the posteromarginal zone. In contrast, recording loci for units exhibiting a strong C-fiber excitation and responses to cutaneous stimulation known to effectively excite C-fiber polymodal nociceptors or C-mechanoreceptors were centered in the SG. There appeared varying degrees of convergence of primary afferent input to the neuronal units, although most showed substantial specificity in their afferent excitation. On the bases of these results and consideration of existing morphological data, it is proposed that the marginal zone is a major synaptic termination region for the afferent fibers from high-threshold mechanoreceptors, cooling thermoreceptors, and perhaps other receptors with fine myelinated peripheral fibers. The SG, on the other hand, is suggested to be the terminal region for all types of unmyelinated primary afferent sensory neurons, and to have the complex role of integrating and distributing this input.

Peripheral suppression of first pain and central summation of second pain evoked by noxious heat pulses

Psychophysical experiments were carried out on 6 huma subjects to determine how first and second pain are influenced by peripheral receptor mechanisms and by central nervous system inhibitory and facilitatory mechanisms. For these experiments, brief natural painful stimuli delivered to the hand were a train of 4-8 constant waveform heat pulses generated by a contact thermode (peak temp. = 51-5% C). The magnitude of first and second pain sensations was estimated using cross-modality matching procedures and reaction times were determined. The latter confirmed the relationship between first and second pain and impulse conduction in Adelta and C noxious heat afferents, respectively. The intensity of first pain decreased with each successive heat pulse when the interpulse interval was 80 sec or less. This decrease was most likely the result of heat induced suppression of Adelta heat nociceptors since it did not occur if the probe location changed between successive heat pulses. In contrast, second pain increased in intensity with each successive heat pulse if the interval was 3 sec or less. This summation was most likely due to central nervous system summation mechanisms since it also occurred after blockage of first pain by ulnar nerve compression and when the location of the thermode changed between heat pulses. These observations and their interpretations are supported by our recording of responses of singlt Adelta heat nociceptive afferents, C polymodal nociceptive afferents, and "warm" afferents of rhsus monkeys to similar trains of noxious heat pulses. Their responses to these heat pulses show a progressive suppression. Furthermore, previous studies have shown that wide dynamic range dorsal horn neurons show summated responses to repeated volleys in C fibers (greater than 1/3 sec). These spinal cord summation mechanisms could account for the summation of second pain.

Long-term and short-term plasticity in the CA1, CA3, and dentate regions of the rat hippocampal slice

Subregions of the rat hippocampal slice were investigated in relation to (a) the presence of long-term potentiation and (b) responsiveness to low-frequency stimulation. Long-term potentiation was observed in CA1, CA3 and dentate. The effect occasionally lasted up to 6 h, developed gradually, and depended upon repeated low-frequency tetani for maximal effect. To low-frequency monosynaptic stimulation, areas CA3 and CA1 exhibit response facilitation whereas the dentate gyrus exhibits response depression. Reponsiveness in all areas was influenced by stimulus frequency. Recovery was rapid in all areas.

Effects of indomethacin and the other anti-inflammatory agents on activation of dorsal horn cell in the spinal cord induced by intra-arterial injection of bradykinin

Bradykinin (1-2 mug) injected into the femoral artery of the rabbit induced a marked increase in the firing rates of the dorsal horn cells (lamina V cells). This response was markedly inhibited by intravenous administration of indomethacin (1.0 mg/kg), acetylsalicyclic acid (20 mg/kg), aminopyrine (50 mg/kg) and oxyphenbutazone (20 mg/kg) in both intact and spinal preparations. Moreover, the intra-arterial injection of these agents depressed the response in lower doses. With these same low doses, no significant suppression occurred when the intravenous route was used. These results indicate that indomethacin and other anti-inflammatory agents tested suppress the bradykinin-induced response of the dorsal horn cell at the periphery, probably at the paravascular sensory nerve.

Effects of activity in non-myelinated afferent fibres on the spinocervical tract

Microelectrode recordings were made from axons of the spinocervical tract (SCT) in unanaesthetized decerebrate-spinal cats. Pure volleys in non-myelinated (C) cutaneous nerve fibres were obtained by DC-polarization block of conduction in the myelinated (A) fibres, and SCT cells were classified according to whether or not they were excited by C fibres. The effects of conditioning C responses in the SCT with A and C fibre input were examined, as were the effects of conditioning A fibre responses in the tract with C fibre inputs. The effects of stimulating descending systems in the cervical spinal cord on the C fibre-evoked responses of SCT cells were determined. SCT units with a slowly-adapting pressure-sensitive component in their receptive fields responded to cutaneous C fibres in addition to A fibres. Units which responded only to hair movement when the receptive field was stimulated mechanically were not excited by C fibres. Responses in SCT neurones produced by C fibres were profoundly inhibited by conditioning volleys in A fibres of both ipsilateral and contralateral cutaneous nerves and by activity in several descending systems. Conditioning with C fibres failed to affect the responses of SCT cells to either A or C fibres. It is concluded that cutaneous C fibres excite some SCT cells but have no other effect on transmission through this system. It is suggested that there are common inhibitory interneurones in the paths from descending systems and cutaneous A and C fibres to the excitatory input to SCT cells.

Patterned response of lamina V cells: cutaneous and dorsal funicular stimulation

Single unit activity was recorded in spinal lamina V (LV) in unansthetized cats. Poststimulus-time histogram (PSTH) analysis of responses to intracutaneous stimulation demonstrated similar firing patterns for all cells. In addition to a direct relationship between stimulus intensity and the duration of cellular response, modulation of firing frequency and distinct delayed PSTH peaks in 77 percent of the units were observed. The pattern of delayed peaks was associated with the same high intensity intracutaneous stimulation reported as painful when applied to human observers. A background of dorsal funicular stimulation selectively inhibited the delayed response of LV cells to high intensity stimulation. Thus, LV cells, by their duration of firing, trasmit information related to stimulus intensity, and by their patterned responses, have the potential to relay other information, possibly related to their nociceptive role. As delayed LV activity is associated with a high proportion of small fibers in the afferent input, presynaptic inhibition of A-delta and C fibers might account for the selective inhibition associated with dorsal funicular stimulation.

Characteristics of spinal neurones responding to cutaneous myelinated and unmyelinated fibres

1. Spike discharges were recorded from neurones in the lumbar spinal cord in cats anaesthetized by barbiturate.2. The neurones were examined systematically for various physiological parameters and for their location. Especially the neurones situated in the dorsal horn were classified for the following parameters: mono- or polysynaptic linkage to myelinated afferents; type of natural stimuli which excited the neurones; depth from the cord surface; number of impulses discharged upon a cutaneous A fibre stimulus; steady-state discharge in the absence of intentional stimulation.3. All neurones were also tested as to whether or not they responded to volleys in cutaneous C fibres. Of 111 units which were activated by the A fibres in nerves from the hairy skin, 57 (= 51%) responded to C volleys in those nerves too.4. By blocking conduction in the A fibres using polarizing currents it was shown that the responses to C fibre volleys were partially or totally suppressed by a preceding discharge of the neurone in response to an A volley. Using search stimuli which were suprathreshold for C fibres one cell out of 36 could be found which responded only to afferent volleys in C fibres.5. About half of all neurones were shown to be connected monosynaptically to cutaneous A fibres, as was judged from the synaptic delay. The other half were polysynaptically linked to the A fibres. Both mono- and polysynaptic neurones were found in all layers of the dorsal horn. About 15% of the cells had additional input from muscle Group II and/or III fibres via polysynaptic pathways.6. Subdividing the A and A+C responsive neurones according to their mono- (M) or polysynaptic (P) connexions yielded the following sub-samples: MC, 39%; PC, 15%; MA, 13%; PA, 33%. Most MC neurones had, and most PA units had not, a spontaneous discharge. About half of the PA cells could not be driven by natural skin stimulation. The majority of MC units responded specifically to movement of hairs.7. A model was proposed hypothesizing two pathways in the dorsal horn, one showing convergence of A and C fibres and the other not. Some relations concerning other observations on C fibre effects were discussed.

Shock-elicited pain and its reduction by concurrent tactile stimulation

Human affective reactions to nociceptive electrical stimulation were attenuated by application of a tactile stimulus to the shocked site. No alteration was perceived when the same tactile stimulus was applied to a similar contralateral site. These results and a lack of alteration at sensation threshold demonstrate the effect to be more than simple masking and support the Melzack-Wall theory.

Presynaptic depolarization of myelinated afferent fibres evoked by stimulation of cutaneous C fibres

1. The origin of the dorsal root potential observed following a volley in cutaneous unmyelinated or C fibres (C-DRP) has been investigated by several assays.2. Measurements of the spatial decrement of the C-DRP along the dorsal roots indicate that mainly the thick myelinated primary afferent fibres contribute to this C-DRP.3. The spatial configuration of the intraspinal potential field during the course of the C-DRP is similar to that of the field correlated to the A-DRP, i.e. the DRP evoked by a volley in the A fibres of the same cutaneous nerve. It is concluded therefrom that both A- and C-DRPs signal presynaptic depolarizations of afferent fibres in the same regions of the dorsal horn.4. Intrafibre recordings reveal that C input produces primary afferent depolarization (C-PAD) mainly in cutaneous and articular afferents, but not in muscle Group Ia afferents.5. These findings and direct measurements of the interactions between A- and C-DRPs indicate that the pathways involved in A- and C-PAD exhibit convergence.6. C-DRPs were also recorded from contralateral rootlets.7. Some functional aspects of the PAD produced by an afferent volley in C fibres are discussed.

C-fiber responses in the ventrolateral column of the cat spinal cord

Saphenous nerve C-fiber volleys generate, in the ventrolateral column of the cat spinal cord, a conducted postsynaptic response revealed by averaging the integrated temporally dispersed axonal discharges. This finding is compatible with the hypothesis that the feline ventrolateral column includes a major pathway related to nociception.

Skin sensory afterglows

Brief stimulation of the upper lip with the tip of a nylon thread frequently produces an " afterglow" sensation that may persist as long as several minutes after cessation of stimulation. The duration of afterglow is increased by strong suggestion, and decreased by vibration of the skin prior to stimulation. The afterglow may be followed by a period of acute "awareness" of the area, distincty different from the stimulus and afterglow sensations. The afterglows produced by a warm stimulator applied to the back often well up into a painful sting.