Sympathectomy alleviates mechanical allodynia in an experimental animal model for neuropathy in the rat

We attempted to determine the effects of surgical sympathectomy on an animal model for neuropathic pain. The L5 and L6 spinal nerves on one side were tightly ligated in anesthetized rats. Mechanical sensitivity of the affected hind paw was significantly elevated from the first day after the surgery as evidenced by the increased occurrence of foot withdrawal to innocuous mechanical stimulation applied with von Frey filaments to the hind paw. The increased mechanical sensitivity continued for three weeks, at which time surgical sympathectomy was performed by removing the L2-L6 sympathetic chain. The sympathectomy produced an immediate and almost complete reversal of the increased mechanical sensitivity, whereas sham sympathectomy had no effect. The data suggest that sympathectomy alleviates mechanical allodynia in this experimental animal model.

Neurogenic hyperalgesia: central neural correlates in responses of spinothalamic tract neurons

1. The contribution of activity in spinothalamic tract (STT) neurons to the pain and neurogenic hyperalgesia produced by an intradermal injection of 100 micrograms of capsaicin was investigated. Electrophysiological responses of identified STT neurons recorded in anesthetized monkeys were compared with psychophysical measurements of pain and hyperalgesia obtained in humans using identical stimuli. 2. Magnitude estimates of pain in humans were obtained after an injection of capsaicin or the vehicle. Capsaicin produced immediate burning pain that was most intense within 15 s after injection and then declined over the next 10-30 min. The vehicle produced no pain. 3. Cutaneous hyperalgesia to gentle stroking (allodynia) and also hyperalgesia to punctate stimulation developed in a wide area surrounding the capsaicin injection. Within this area, magnitude estimates of pain produced by a punctate stimulus (von Frey type with force of 225 mN) increased over preinjection values by an average of sixfold at test sites, 1, 2, and 3 cm away from the injection site. At the capsaicin injection site, magnitude estimates of pain in response to punctate simulation typically remained the same or were decreased. 4. After capsaicin, but not vehicle, the mean heat pain thresholds were lowered from approximately 45 degrees C before injection to 34 degrees C after, but only in the immediate vicinity of the injection site. At a site located 2 cm away, the thresholds were not significantly altered. Similarly, magnitude estimates of pain produced by suprathreshold heat stimuli were increased after capsaicin only at the injection site. 5. STT neurons were classified as high-threshold (HT) or wide-dynamic-range (WDR) cells according to responses evoked by graded cutaneous mechanical stimulation. An intradermal injection of capsaicin excited 4 of 7 HT cells and 10 of 12 WDR cells. The discharge rates of STT neurons correlated in time course with the magnitude estimates of pain in humans. The correlation was considerably better for WDR than for HT neurons, suggesting a predominant contribution of WDR neurons to the pain from capsaicin. 6. Capsaicin significantly increased the responses of HT neurons (9-fold) and the responses of WDR neurons (2-fold) to stroking the skin within the receptive field. Similar increases in responses to a standard punctate stimulus were observed at test sites, 1, 2, and 3 cm away from the injection site. After injection of vehicle, the responses to punctate stimulation increased by a mean of only 1.2- and 1.4-fold for HT and WDR neurons, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for invasion of regenerated ventral root afferents into the spinal cord of the rat subjected to sciatic neurectomy during the neonatal period

Sectioning the sciatic nerve of experimental animals at the neonatal stage triggers growth of afferent fibers in the ventral root. The present study examined the possibility that the regenerating fiber terminals grow into the spinal cord. The sciatic nerve on one side was cut in neonatal rats. After the rats were fully grown, either an electrophysiological or a histochemical study was performed. The results of electrophysiological experiments showed that stimulation of certain loci in the L5 spinal cord evoked antidromic potentials in the L5 ventral root with a long latency. Various evidence suggests that the long latency potentials are due to activation of C fibers. These C-fiber potentials were on average bigger and were elicited from more numerous loci on the side ipsilateral to the sciatic nerve lesion than on the contralateral side. Furthermore, stimulation of the spinal cord of unoperated normal rats rarely evoked such potentials. For the histochemical study, horseradish peroxidase (HRP) was injected into the L5 spinal cord after cutting the L4-L6 dorsal roots. A lot more cells in the L5 dorsal root ganglion (DRG) on the side ipsilateral to the sciatic nerve lesion were labeled with HRP transported retrogradely through the L5 ventral root than on the contralateral side. Control experiments showed that few DRG cells are labeled with HRP in normal unoperated rats. The combined results of the electrophysiological and histochemical studies suggest invasion of ventral root afferents into the spinal cord, given enough postoperative time. It is not known whether or not these terminals make functional synaptic contacts in the spinal cord.

Descending adrenergic input to the primate spinal cord and its possible role in modulation of spinothalamic cells

The present study focuses on 3 different aspects of the descending adrenergic system in the primate: (1) the distribution of adrenergic fibers and terminals in the spinal cord, (2) the source of this input and (3) the possible physiological effects of this system on spinal nociceptive processing. Antibodies to the enzyme phenylethanolamine-N-methyltransferase (PNMT) were employed to map the distribution of epinephrine-containing axonal profiles in the primate spinal cord. Smooth longitudinally oriented fibers were localized to the outer edge of the lateral funiculus. PNMT-containing axonal enlargements were distributed to the superficial dorsal horn, intermediate gray matter and the region surrounding the central canal at all spinal cord levels. PNMT-immunostained profiles were also observed in the intermediolateral cell column. A double labeling study employing retrograde transport of HRP from the spinal cord and PNMT immunohistochemistry identified a small population of HRP-PNMT-labeled neurons in the 'C1' region at the levels of the medulla and ponto-medullary junction. Thus, these cells are a probable source of adrenergic input to the spinal cord. Electrophysiological studies demonstrated that iontophoresis of epinephrine onto identified primate spinothalamic tract neurons in the lumbar dorsal horn resulted in inhibition of the glutamate-induced firing of these cells. The data from these studies support the hypothesis that adrenergic (PNMT-containing) cells in the caudal brainstem project to all levels of the cord and may contribute to descending modulation of nociceptive processing at these levels.

Prolonged ongoing discharges of sensory nerves as recorded in isolated nerves in the rat

Whether or not injury to a mammalian sensory nerve produces prolonged discharges is a controversial issue. Because of this controversy and its potential ramifications in both experimental and clinical conditions, we examined discharges in sectioned sensory nerves of the rat. In anesthetized rats, either a dorsal root or a saphenous nerve was isolated by sectioning both proximally and distally. Multi-unit recordings from these isolated nerves showed low levels of prolonged ongoing discharges often lasting for at least 2 hr. Furthermore, results from short-term (1-2 days) survival surgeries indicated that prolonged ongoing discharges could last for days. Sectioning the ventral root produced discharges for only a short period. Various pieces of evidence suggested that the sources of impulse generation are multiple, occurring in the middle of an uninjured axon at a site away from the injury, as well as at the injured site. There is circumstantial evidence which suggests that these prolonged discharges are produced in physiological conditions or at least under normal experimental conditions.

Increased number of unmyelinated fibers in the ventral root after peripheral neurectomy in adult rat

We examined the possibility that peripheral nerve injury in the adult rat triggers sprouting of unmyelinated ventral root afferent fibers. Three to 5 months after the sciatic nerve was sectioned on one side in the adult rat, myelinated and unmyelinated fibers were counted at 3 sites along the length of the ventral root. A sciatic nerve lesion resulted in about a 3-fold increase in the number of unmyelinated fibers in the L5 ventral root. Our data suggest that a peripheral nerve lesion in the adult rat triggers sprouting of unmyelinated afferent fibers in the ventral root. No evidence was found that dorsal rhizotomy triggers sprouting of afferent fibers.

Fiber counts at multiple sites along the rat ventral root after neonatal peripheral neurectomy or dorsal rhizotomy

We hypothesized that the afferent fibers in the ventral root of the rat are the third branches of dorsal root ganglion cells; these afferent processes in the ventral root are of varying length and end bluntly along the length of the root. In the case of an injury at either the central or the peripheral processes of the dorsal root ganglion cells in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We cut either the sciatic nerve or the dorsal root on one side in neonatal rats. After the rats were fully grown, the number of both myelinated and unmyelinated fibers was counted in electron photomicrographs at multiple sites along the length of the ventral root. We observed a greatly increased number of unmyelinated fibers in the ventral root after the sciatic nerve had been cut at the neonatal stage. The magnitude of increase was more at the distal than at the proximal portion of the ventral root, suggesting that added fibers originated from the distal side. Neonatal dorsal rhizotomy, however, did not produce the same result. These results are consistent with our hypothesis that peripheral nerve injury at the neonatal stage triggers sprouting of the third branches of the dorsal root ganglion cells which end bluntly along the length of the ventral root in the normal animal.

Electrophysiological evidence for an increase in the number of ventral root afferent fibers after neonatal peripheral neurectomy in the rat

Our recent study has shown that many afferent fibers in the ventral root are third branches of dorsal root ganglion cells in addition to their processes in the peripheral nerve and the dorsal root. From results of this study, we hypothesized that most of the afferent fibers in the normal ventral root are extra processes of certain dorsal root ganglion cells. To accommodate experimental findings by others, we formulated several working hypotheses in the present study as an extension of our previous hypothesis: these afferent processes in the ventral root are of varying length; they end bluntly along the length of the root; and in an event such as peripheral neurectomy in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We tested the above hypotheses using electrophysiological methods. The sciatic nerve on one side in neonatal rats was cut. After the rat was fully grown, volleys of neural activity were recorded along the length of the ventral root while stimulating the dorsal root of the same segment. There was a great increase in the size of compound action potentials in the ventral root on the sciatic nerve-lesioned side. Various lines of evidence suggest that this enhancement of the evoked potentials is likely to be due to an increase in the number of afferent fibers in the ventral root in response to neonatal peripheral nerve injury. The results are consistent with our hypotheses.

Electrophysiological evidence for the presence of looping myelinated afferent fibers in the rat ventral root

We obtained neurophysiological evidence for the existence of looping fibers in the rat ventral root. In anesthetized rats, simultaneous recordings of spontaneous activity were made from the lumbar ventral root with two pairs of bipolar recording electrodes. In 6 ventral roots, single unit activity appeared as pairs of spikes with opposite polarity in both the distal and proximal recording electrodes. The timing and polarity of the spikes can only be logically explained by supposing that they are simultaneous recordings from two pairs of electrodes of two action potentials traveling opposite directions in a single myelinated fiber. This explanation was reinforced by recording the activity after sectioning various parts of the ventral root. These data suggest that there are spontaneously active ventral root myelinated fibers that enter the root and then loop back out toward the dorsal root ganglion.

Different classes of cat spinal neurons display differential sensitivity to sodium pentobarbital

The effect of graded doses of systemically injected sodium pentobarbital on several classes of spinal neurons was studied using spinal cats. Classes of spinal neurons included unidentified dorsal horn cells, ascending tract dorsal horn cells, and motoneurons. Single unit activity of spinal neurons was evoked by electrically stimulating a peripheral nerve with an intensity strong enough to excite both A and C fibers. The A- and C-fiber evoked activity was compared before and after intravenous injections of small incremental doses of sodium pentobarbital. The activity of different classes of spinal neurons showed different sensitivities to graded doses of systemically injected pentobarbital. The reflex activity of motoneurons elicited by stimulation of peripheral nerve was much more sensitive to pentobarbital than that of dorsal horn cells. In general, activity evoked by peripheral unmyelinated fibers was more susceptible to pentobarbital than was that evoked by myelinated fibers. However, intravenous injections of pentobarbital produced nondifferential suppression of dorsal horn cell activity evoked by noxious and innocuous mechanical stimuli applied to the peripheral receptive fields.

Proportion and location of spinal neurons receiving ventral root afferent inputs in the cat

Spinal neurons receiving ventral root afferent inputs were investigated in anesthetized and paralyzed cats. We were concerned with the afferent fibers in the ventral root that travel distally and then enter the spinal cord through the dorsal root. The questions to be answered included the proportion and distribution of spinal neurons receiving ventral root afferent inputs and their peripheral input characteristics. The 1.7 ventral root was cut near the spinal cord and the distal stump was stimulated while making a systematic search for neurons in the entire gray matter of the ipsilateral spinal cord that responded to the stimulation. The following conclusions were made: (i) the afferent fibers in the cat ventral root enter the spinal cord through the dorsal root and evoke a variety of responses (excitation, inhibition, or mixed) in a large proportion of spinal neurons (about 20%): (ii) these responses seem to be mediated largely by spinal mechanisms: (iii) spinal neurons receiving ventral root afferent inputs are situated in a wide region of the ventral spinal cord: (iv) ventral root fibers in a single root enter the spinal cord and exert their responses over a large region of the spinal cord (at least two spinal segments rostrally and caudally): (v) some of the spinal neurons that responded to ventral root stimulation were found to be ascending tract cells, suggesting that ventral root afferent inputs can reach supraspinal structures: (vi) ventral root afferent fibers converge onto spinal neurons that have a variety of peripheral receptive field characteristics: and (vii) with some exceptions, most neurons receiving ventral root inputs were excited best by mechanical and/or thermal noxious stimuli applied to the periphery.

Differential inhibition produced by peripheral conditioning stimulation on noxious mechanical and thermal responses of different classes of spinal neurons in the cat

The effect of conditioning stimulation of a peripheral nerve on responses of spinal neurons (dorsal horn cells and motoneurons) was studied in 16 decerebrate-spinal cats. The activity of dorsal horn cells was recorded with a microelectrode at the lumbosacral spinal cord and the single-unit activity of motoneurons was recorded from a filament of ventral rootlet divided from either the L7 or S1 ventral root. The responses of spinal neurons were evoked by noxious and innocuous mechanical stimuli and by noxious thermal stimuli applied to the receptive fields. The peripheral conditioning stimulation was applied to the tibial nerve with repetitive electrical pulses (2 Hz) at an intensity either suprathreshold for A delta or C fibers for 5 min. Applying conditioning stimulation to a peripheral nerve produced a powerful inhibition of the responses elicited by noxious stimuli, suggesting this inhibition is an antinociceptive effect. The inhibition produced by peripheral conditioning stimulation was differentially greater on the responses to noxious than to innocuous stimuli. Based on the results obtained from conditioning stimulation with graded strengths, afferent inputs from both myelinated and unmyelinated fibers seem to contribute to the production of the antinociceptive effect. The magnitude of the antinociceptive effect is bigger for the responses to noxious thermal than to mechanical stimuli. Furthermore, the reflex activity recorded in motor axons seemed to be more sensitive than in dorsal horn cells to the antinociceptive effect.

Many ventral root afferent fibers in the cat are third branches of dorsal root ganglion cells

The arrangement of the ventral root afferent fibers was investigated in anesthetized and paralyzed cats. Single unit activity was recorded from a fascicle of the distal stump of the cut S1 dorsal root. Activity was elicited by stimulating the distal stump of the cut S1 ventral root. Attempts were then made to collide this activity with that elicited by stimulation of the S1 spinal nerve. Single unit activity elicited by ventral root stimulation was recorded from a total of 33 axons. In 17 of these, the activity collided with that elicited by peripheral stimulation. These results indicate that more than half the sampled population of ventral root afferent fibers are branches of dorsal root ganglion cells that have at least 3 processes: one in the dorsal root, one in the ventral root and one in a peripheral nerve. In 10 of these units, the conduction velocity of each of 3 processes was determined using the collision technique. The conduction velocities differed in the processes of a given ganglion cell, with conduction in the ventral root process generally being the slowest. The change in conduction velocity along the length of the ventral root was examined by comparing latency differences for the unit activity elicited by ventral root stimulation at different sites in the same root separated by known distances. The conduction velocity was found not to be uniform along the course of the ventral root. In many cases, the conduction velocity slowed down as the fiber approached the spinal cord. We conclude from the present study that many ventral root afferent fibers are the third branches of dorsal root ganglion cells that also have processes in the dorsal root and in a peripheral nerve. The sizes of each of these 3 processes of the dorsal root ganglion cell may differ; the ventral root process tends to be the smallest and is usually unmyelinated. Furthermore, many of the ventral root afferent fibers may taper as they approach the spinal cord.

Immunological studies in patients with HBsAg-positive chronic active hepatitis--spontaneous lymphocyte transformation and natural killer cell activity

The present study was to address the nature of cells which are responsible for enhanced spontaneous lymphocyte transformation (SLT) observed in patients with HBsAg-positive chronic active hepatitis (CAH). The subjects consisted of 34 cases with HBsAg-positive CAH (group I), 31 HBsAg carrier (group II), and 27 normal persons (group III) who had no serological evidence of hepatitis B virus (HBV) infection. SLT values and the number of cells bearing HLA-DR antigens in group I (1021.45±276.40 cpm, 36.94±4.90%) were significantly (p<0.01) elevated as compared to group II (103.74±30.44 cpm, 13.26± 4.72%) and III (118.92 ± 30.84 cpm, 14.93±5.10%), but there was no difference of the number of the IL-2 receptor-bearing cells among each groups. Though natural killer (NK) cell activity in both group I (65.42±15.77%) and II (59.14±14.89%) were significantly enhanced as compared to group III (46.25±20.20%), there was no difference in between group I and II. These findings indicate that the cells bearing HLA-DR antigen, but not NK cells, are responsible for the enhanced SLT in patients with CAH.

A clinical and chromosomal study on those exposed to the atomic bomb and their offspring

Presented in this paper is the data from clinical laboratory examination of 50 Korean atomic-bomb survivors (Hiroshima and Nagasaki, Japan, 1945). Of them, 15 survivors were karyotyped from their lymphocyte culture for both “stable” and “unstable” types of chromosomal aberrations. Eight of their offspring were also tested for the chromosomal changes and SCE as well.

The results are as follows: All survivors were found to have suffered from various diseases, particularly from respiratory diseases. Two had malignancies, viz., one case having squamous cell carcinoma of uterine cervix and another, adenocarcinoma of stomach. Serum total protein and serum albumin levels were found to have decreased in 18% and 27% of the survivors, respectively. Alkaline phosphatase, SGOT, IgG and IgM increased in 22%, 6.3%, 36.7% and 13.6% of the survivors, respectively. IgA was within normal limit in all cases. Stable type of chromosomal aberrations was discovered in none of the survivors and the offspring examined. However, approximtely 50% of the survivors showed a significantly higher incidence of chromatid breakage than normal controls. None of the offspring showed the chromatid breakage, but 75% of them showed significantly higher rate of SCE when compared to controls.

Spinal entry route for ventral root afferent fibers in the cat

Twelve anesthetized and paralyzed cats were used to study the spinal entry routes of ventral root afferent fibers. In all animals, the spinal cord was transected at two different levels, L5 and S2. The L5 through S2 dorsal roots were cut bilaterally, making spinal cord segments L5-S2 neurally isolated from the body except for the L5-S2 ventral roots. From this preparation, a powerful excitation of the discharge rate of motor neurons and dorsal horn cells within the isolated spinal segments was observed after intraarterial injection of bradykinin (50 micrograms in 0.5 ml saline). This excitation of the spinal neurons can be considered the most convincing evidence of the potential physiologic role of the ventral root afferent fibers entering the spinal cord directly through the ventral root, because the apparent route of neuronal input from the periphery is through the ventral roots. However, additional control experiments conducted in the present study showed that the excitation persisted even after cutting all ventral roots within the isolated spinal segments, indicating that excitation was not mediated by the ventral roots. Furthermore, direct application of bradykinin on the dorsal surface of the spinal cord also increased the motoneuronal discharge rate, suggesting that excitation of spinal neurons produced by intraarterial injection of bradykinin is due to a direct action of bradykinin on the spinal cord. Thus, we provided an alternate explanation for the most convincing evidence indicating that physiologically important ventral root afferent fibers enter the spinal cord directly through the ventral root. Based on existing experimental evidence, it is likely that the majority of physiologically active ventral root afferent fibers travel distally toward the dorsal root ganglion and then enter the spinal cord through the dorsal root.

The long-latency component of cerebral evoked potentials in anesthetized cats

A late component of the cortical evoked potential elicited by somatosensory afferent input was studied in cats anesthetized with alpha-chloralose. Cortical evoked potentials were recorded from the somatosensory-motor cortex during stimulation of the sural nerve with graded intensities. The stimulus intensity was adjusted to activate A alpha beta fibers only, then both A alpha beta and A delta fibers, and both A and C fibers, as judged by afferent volleys monitored from the sural nerve proximal to the stimulating site. In addition to early components reported previously, a very late component was identified at a latency of 400 to 600 msec following stimulation of the sural nerve with intensities above threshold for A delta fibers. A further increase in stimulation intensity to include activation of C fibers did not reveal any more components. This late component was depressed by a systemic intravenous injection of morphine (2 mg/kg), and intravenous naloxone (0.1 mg/kg) reversed the effect of morphine. The late component of the evoked potential could also be recorded from subcortical tissue after decortication of the sensorimotor cortex. From these results, it appears that a very late component of the cortical evoked potential can be recorded from cats anesthetized with alpha-chloralose. The late component is evoked by activation of peripheral A delta fibers. Furthermore, its morphine sensitivity suggests that this component may be elicited by nociceptive afferent fibers. If further investigations prove this, the late component, which is analogous to human long-latency potentials, could be used in an experimental model for pain research.

Actions of opioids on primate spinothalamic tract neurons

The effects of iontophoretically applied opiates were tested on 24 spinothalamic tract cells in 12 anesthetized monkeys. The drugs used were chosen because of their agonist actions on different classes of opiate receptors (mu, morphine; kappa, dynorphin; delta, methionine enkephalinamide; sigma, N-allylnormetazocine or SKF 10047 and phencyclidine). The actions of the opiate drugs were generally inhibitory, although excitatory or mixed effects were sometimes seen, especially with morphine and dynorphin. Drug effects could change, depending on the position of the iontophoretic electrode array or on the current employed. Naloxone sometimes antagonized the action of the opiate drugs used, but naloxone did not seem to be a drug suited for iontophoretic application. A number of explanations are discussed to explain the variable actions of the opiate drugs.

Classification of primate spinothalamic and somatosensory thalamic neurons based on cluster analysis

Data analyzed in this study were derived from the responses of 128 spinothalamic tract (STT) cells and 110 thalamic neurons recorded in 75 anesthetized monkeys. A k-means cluster analysis, a nonhierarchical clustering technique, was performed using the relative magnitudes of responses to a graded series of innocuous and noxious mechanical stimuli applied to the receptive field. For comparison, a parallel analysis was performed based on definitions of low-threshold (LT), wide dynamic range (WDR), and high-threshold (HT) cells used by our laboratory. For 128 STT cells, a classification scheme with three clusters was found statistically to be the best. This yielded groups of 22, 57, and 49 cells in clusters 1, 2, and 3, respectively. Cluster 1 cells were activated best by low-intensity mechanical stimuli, whereas cluster 3 cells were activated primarily by nociceptive stimuli. Cluster 2 cells had intermediate characteristics. When the classification scheme based on the cluster analysis was compared with the classification of the same neurons as LT, WDR, and HT cells, cluster 1 cells were divided into LT and WDR cells, whereas cluster 2 and 3 cells included WDR and HT cells. For 110 thalamic neurons, a classification scheme with five clusters was found statistically to be the best. Clusters 1-5 contained 25, 34, 17, 10, and 24 cells, respectively. Response characteristics of cells in each group indicated a gradual change in sensitivity to higher intensities of peripheral input from cluster 1 to 5. When this classification scheme was compared with the classification scheme previously used by our laboratory, cluster 1 cells belonged to the LT group, clusters 2 and 3 split into LT and WDR cells, and clusters 4 and 5 included WDR and HT cells. It is concluded that a classification scheme based on a cluster analysis of the responses of neurons to standardized stimuli may provide an objective and functionally meaningful way to categorize somatosensory neurons.

Response characteristics of neurons in the ventral posterior lateral nucleus of the monkey thalamus

The activity of 132 neurons in the caudal part of the ventral posterior lateral nucleus (VPLc) of the thalamus was recorded from 23 anesthetized monkeys. All single thalamic units that could be excited by electrical search stimuli applied to the contralateral sciatic nerve were investigated. Responses of these cells to mechanical, thermal, and electrical stimuli applied in the periphery indicated that at least half of the sampled cells were nociceptive. Based on responses to graded mechanical stimuli applied to the periphery, 110 of the sampled cells that received a predominant input from cutaneous receptive fields were classified. There were 56 low-threshold, 39 wide dynamic range, and 15 high-threshold cells. The same neurons were also classified into five mechanical types based on a cluster analysis: types 1-5 contained 25, 34, 17, 10, and 24 cells, respectively. The fact that about half the population of cells belonged to either the wide dynamic or the high threshold group (or mechanical types 3-5) suggested that a large population of VPLc neurons respond to mechanical nociceptive stimuli either exclusively or preferentially. Responses of 63 thalamic neurons were tested to noxious heat pulses applied to their cutaneous receptive fields with a contact thermostimulator. Of these, 47 cells were excited, whereas only 16 cells did not respond. The peripheral nerve that innervated the receptive field of each of 82 thalamic neurons was stimulated with graded strengths to activate A fibers only or both A and C fibers. All tested cells responded to peripheral A fiber volleys. In addition, 42 of these cells responded to peripheral C fiber volleys. The C fiber responses could be either short lasting (a few hundreds of milliseconds) or long lasting (up to several seconds). The recording sites of 80 cells were reconstructed. Of these, 78 were in the VPLc nucleus and the remaining two were in the reticular nucleus of the thalamus. No obvious relationship between the response characteristics and the locations of the cells within the VPLc nucleus was found. Sampled thalamic units had a variety of sources of input from the periphery, including both cutaneous and/or deep tissue receptive fields. The majority of the cells, however, had exclusively cutaneous receptive fields. The sizes of the cutaneous receptive fields were often very small, so that nearly half (41%) of the receptive fields of cells sampled occupied an area of skin smaller than half the foot.(ABSTRACT TRUNCATED AT 400 WORDS)

Ascending spinal pathway for arterial pressor response elicited by ventral root afferent inputs in the cat

In an attempt to localize the spinal ascending pathway for ventral root afferent inputs, changes in ventral root stimulation-evoked arterial blood pressure responses were observed in anesthetized cats after selective spinal lesions. The results of these experiments indicate that the ascending spinal pathways responsible for the pressor response lie in the dorsal-most part of the lateral funiculus. The pathway was found to be bilateral, and it is likely to occupy the same area throughout the entire length of the spinal cord.

Inhibition and excitation of the nociceptive flexion reflex by conditioning stimulation of a peripheral nerve in the cat

A previous study in our laboratory showed a long-lasting, naloxone-reversible inhibition of the flexion reflex after prolonged repetitive stimulation of a peripheral nerve in the spinal cat. The present study employed a special pattern of conditioning stimulation for a shorter period (200 s) to determine the time course of the inhibition and the afferent fibers responsible for the inhibition. We stimulated the common peroneal nerve in 10 decerebrated and spinalized cats to elicit the flexion reflex, which we recorded as single-unit activity from filaments of the L7 ventral root. The C fiber-evoked late component of the flexion reflex was compared before, during, and after conditioning electrical stimulation applied to the tibial nerve. Stimulating the tibial nerve at an intensity that excited only A alpha beta fibers produced weak inhibition of the flexion reflex; increasing intensity above the threshold for A delta fibers produced much greater inhibition. Inhibition began during the first 10 s of conditioning stimulation and was maximum at about 100 s. Stimulation at a suprathreshold intensity for C fibers, however, produced an initial transient excitation, lasting 10 to 20 s, followed by inhibition. Intravenous injection of naloxone (0.05 mg/kg) produced no observable changes in this inhibition and excitation. These results suggest that conditioning stimulation of a peripheral nerve inhibits the flexion reflex. This inhibition has a short latency; the afferent fibers seem to be A delta fibers. In addition, input from afferent C fibers may trigger a mechanism that produces facilitation of the reflex. The differences in recovery time course and in sensitivity to naloxone suggest that two different mechanisms may be responsible for the fast-onset inhibition and the previously observed long-lasting inhibition produced after prolonged conditioning stimulation.

Correlation of cell body size, axon size, and signal conduction velocity for individually labelled dorsal root ganglion cells in the cat

Measurements of cell body and peripheral and central axon sizes were made for primary sensory neurons outlined by the intracellular injection of HRP. Conduction velocities were also measured on the outlined processes. The sensory neurons were then subdivided into A and C cells on the basis of the conduction velocity of the impulses carried by the processes of these cells. Central processes of both A and C cells are smaller than the peripheral processes, but the size differential is greater for the C cells. For A cells there is a linear relation between the size of the peripheral axon and the conduction velocity of the impulses carried by these axons, but the confidence limits are wide. For C cells there is a linear relation between the size of the central process and conduction velocity of the impulses carried by the processes, but for the peripheral processes two aberrant processes resulted in no correlation between process size and conduction velocity. For A cells, the size of the central and peripheral processes and the conduction velocity of the impulses carried by the peripheral processes are linearly correlated with cell body size. By contrast no such correlations can be demonstrated for C cells. This presumably implies an important difference in that the size of the cell body is correlated with axon size and impulse conduction velocity for A cells but not for C cells. A widely accepted generalization is that large sensory cells give rise to myelinated axons and small sensory cells to unmyelinated axons. In this study, myelinated and unmyelinated are defined on the basis of impulse conduction velocity. For those cells that are clearly large (greater than 50 microns in diameter), the conduction velocity of the impulses carried by their processes is always greater than 2.5 m/s, and for those cells that are clearly small (less than 35 microns in diameter), the conduction velocity is always less than 2.5 m/s. Thus for these cells the above generalization holds. For the intermediate-sized cells (35-50 microns), however, the size of the cell body bears no predictable relation to the conduction velocity of the impulses carried by those processes, and thus to whether the axons are myelinated or unmyelinated. Thus the above generalization does not hold for this intermediate group of cells, and since there are many cells in this size range, we feel that the generalization that large cells give rise to myelinated axons and small cells to unmyelinated axons is an oversimplification.

Blood pressure response evoked by ventral root afferent fibres in the cat

Systemic arterial blood pressure changes in response to stimulation of the distal stump of the cut spinal ventral root were investigated in anaesthetized, vagotomized, and carotid sinus-denervated cats. Low intensity electrical stimulation (less than 20 T, where T is threshold intensity) of the ventral root caused a rise in blood pressure. This elevation was abolished by paralysing the muscles with gallamine. This pressor response has been reported previously, and it is likely to be evoked by afferents excited by the contracting muscle. High intensity electrical stimulation (500 T) of the ventral root caused a second and marked pressor response. This was not affected by muscular paralysis or by cutting the sciatic nerve, but it was abolished by cutting the dorsal root. Threshold intensity for the second component of the pressor response was within the same range as the intensity needed for activation of C fibres in the ventral root, ranging between 200 T and 300 T. This response was graded with increasing stimulus intensity, and it showed both spatial and temporal summation. From the above results, we conclude that non-myelinated fibres in feline spinal ventral root course distally to the dorsal root ganglion and then enter the spinal cord via the dorsal root. Activation of these fibres results in a marked elevation of the systemic arterial blood pressure as in other somato-sympathetic reflexes induced by peripheral C fibre activation.