Biomedical subjects

J M Chung

Publications and source records attributed to J M Chung.

At least 73 records · Page 4Linked to original sources

PubMed1993

Chronic effects of topical application of capsaicin to the sciatic nerve on responses of primate spinothalamic neurons.

The responses of 144 spinothalamic tract (STT) cells were recorded in 15 anesthetized macaque monkeys (Macaca fascicularis). Three to 4 weeks prior to the acute experiment, the sciatic nerve was surgically exposed on one or both sides so that capsaicin or vehicle could be applied. Responses of STT cells recorded in 3 experimental groups were compared: untreated (21 cells), vehicle-treated (40 cells), and capsaicin-treated (83 cells). The background activity of cells in the vehicle- and capsaicin-treated groups was the same as in the untreated group (that is, cells on the side contralateral to surgery). Responses to innocuous (BRUSH) and noxious (PINCH) mechanical stimuli were unchanged by vehicle or by capsaicin treatment. However, responses to other noxious (PRESSURE and SQUEEZE) mechanical stimuli were significantly increased in the vehicle-treated group. Compared with a large reference population, all experimental groups showed a significant increase in overall responsiveness to mechanical stimuli (as determined by cluster analysis), greatest in the vehicle-treated group. Responses to noxious heat stimuli were significantly reduced in the capsaicin-treated group for 45 degrees C and 47 degrees C stimuli. Volleys in A fibers, probably A delta fibers, evoked prolonged responses in many STT cells of all treatment groups. Electron microscopic counts of axons in the sciatic nerves of animals treated with capsaicin showed a reduced number of C fibers but no appreciable loss of myelinated axons. This loss of unmyelinated sensory fibers was presumably responsible for the reduction in the responses of the STT cells to noxious heat stimuli. Increased responses to some noxious mechanical stimuli and to A fiber volleys may have been the consequence of several factors, including surgical manipulation, a chemical action of vehicle and a contralateral action of capsaicin treatment.

Administration, Topical↗

PubMed1993

Cutaneous sensory receptors in the rat foot.

1. A total of 574 cutaneous afferent units in the sural and plantar nerves supplying the skin of the rat foot was examined: 399 A beta-units, 55 A delta-units, and 120 C-units. Their receptive-field (RF) properties were similar to those described in other mammals. However, the receptor type composition of units was different between the two nerves. 2. The sural A beta-fiber sample (n = 160) consisted of G-hair (41%), field (11%), rapidly adapting (RA; 6%), slowly adapting type I (SA-I; 7%), and type II (SA-II; 35%) mechanoreceptors. The plantar A beta-fiber sample (n = 239) was composed of G-hair (3%), RA (35%), SA-I (30%), SA-II (24%), and Pacinian corpuscle (PC; 8%) mechanoreceptors. 3. The RFs of SA-II units were located on both hairy and glabrous skin overlying the foot joints. Many of the SA-II units responded to movement of the foot joints. The RFs of both SA-I and RA units were small in size and located in high density on the toe tips and footpads. PC units were very sensitive to vibration and had extremely large RFs as in other species, although they were rare and found only in the plantar nerve. Field units were similar to SA-II units in response properties and RF distribution. 4. The sural A delta-fiber sample (n = 44) included nociceptors (68%), D-hair (27%), and cold (5%) receptors. All sampled plantar A delta-fibers (n = 11) were nociceptors. Of A delta-nociceptor units, A delta-mechanical nociceptors (73%) were dominant. 5. The sural C-fiber sample (n = 85) included nociceptors (44%), C-mechanoreceptors (33%), and cold receptors (21%). The plantar C-fiber sample (n = 35) included nociceptors (77%) and cold receptors (23%). No warm units were found among either the sural or plantar nerve fibers. Of C-nociceptors, C-mechanoheat nociceptors (80%) were dominant. 6. The results indicate that all well-known types of cutaneous receptors, except warm receptors, exist in the foot skin of the rat. On the basis of the fact that RFs of RA and SA-I units are in high density on the toe tips and footpads, it is suggested that those regions may have a spatial discriminating capacity. It is also suggested that SA-II receptors may play a role in proprioception, because they have RFs on the skin over foot joints and respond to joint movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1993

Transient enhancement of low-threshold calcium current in thalamic relay neurons after corticectomy.

1. The alterations of voltage-sensitive calcium currents produced in thalamic cells by injury were investigated under voltage clamp using patch-clamp recordings in the whole-cell configuration. 2. One day after unilateral cortical ablation in immature rats (postnatal day 7), low-threshold transient calcium (T) currents in acutely isolated thalamic relay neurons (RNs) were increased by 68% compared with contralateral controls (P < 0.001). Three days after the operation, T currents in injured neurons were at 44% of control levels (P < 0.001). On the other hand, high-threshold (L) calcium currents in RNs did not change over the same interval. 3. To investigate the mechanism for the increase of T current, both kinetics and voltage dependency of activation and inactivation were examined. At a test voltage of -40 mV, the activation time constant decreased from 4.1 to 3.2 ms (P < 0.05); however, this small change was insufficient to explain the large increase in T current. Time constants for both fast and slow inactivation did not change significantly, nor did voltage dependence of activation or inactivation of thalamic T currents. 4. Methyl-phenyl-succinimide (MPS, 1 mM), a compound known to block T currents, was used to examine possible alterations in the pharmacological properties of T channels after injury. MPS was more effective in reducing T currents in normal versus injured RNs (24 and 20% reductions, respectively; P < 0.05), suggesting that pharmacological properties of T channels in the injured RNs may be different from those of the normal RNs.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗

PubMed1993

Abnormalities of mechanoreceptors in a rat model of neuropathic pain: possible involvement in mediating mechanical allodynia.

1. Peripheral nerve injury sometimes leads to the development of neuropathic pain. One of the symptoms of such neuropathic pain is mechanical allodynia, pain in response to normally innocuous mechanical stimuli. We hypothesized that sympathetically driven dysfunction of cutaneous mechanoreceptors is responsible for signaling mechanical allodynia. The present study was undertaken to identify the types of sensory receptors that potentially mediate mechanical allodynia, with the use of a rat neuropathic pain model we have developed. 2. One week to 10 days after tight ligations of the L5 and L6 spinal nerves on one side, the rats fully developed behavioral signs of mechanical allodynia on the affected hindlimb. Various cutaneous mechanoreceptors originating from the neuropathic foot were examined by single-fiber recordings from the L4 dorsal root. 3. Although no particular abnormalities were found in other types of cutaneous mechanoreceptors, an unusual type of mechanoreceptor was found to be innervating the neuropathic foot. The response characteristics of this type of receptor resemble those of rapidly adapting mechanoreceptors (RAs), but with low and irregular static discharges during a maintained mechanical stimulus. We termed this unusual type as a "modified rapidly adapting" mechanoreceptor (MRA). 4. The response characteristics of MRAs change to those of typical RAs after a systemic injection of phentolamine, an alpha-adrenergic receptor blocker. 5. We conclude that many RAs become abnormal under the influence of sympathetic efferents in neuropathic pain, so that their response patterns change to those of MRAs. We propose that this abnormality is responsible for signaling the mechanical allodynia that can be seen in neuropathic pain states such as causalgia.

Afferent Pathways↗

PubMed1993

Differential activation and classification of cutaneous afferents in the rat.

1. A total of 312 cutaneous afferent units identified in the rat foot as belonging to one of nine major types of sensory receptors were included in the present study. A natural stimulus set was defined to differentiate optimally among those receptor types according to the distinguishing response patterns that it produced. It included air puffs, 30- and 300-Hz sinusoids, 200-mN force indentation of the skin, 1.2- and 6-N compressions of a skin fold, cooling the skin by 5 and 20 degrees C, warming by 5 degrees C, and heating by 15 degrees C. 2. The responses to predefined stimuli of 188 units were subjected to multivariate statistical analyses. The responses of an individual unit were measured as the number of impulses evoked by 10 stimuli, each lasting 10 s. Additionally, the number of impulses occurring for 5 s after withdrawal of a 200-mN indentation (1 of the 10 stimuli) was counted. 3. In discriminant analysis, the 11 stimulus variables predicted fairly correctly the grouping of afferent units into nine predetermined receptor categories (175 of 188, 93.1%), indicating a powerful ability to discriminate among different receptor types. Using hierarchical cluster analysis, afferent unit data described by 11 variables were divided into clusters that well represented prior receptor categories (170 of 188, 90.4%), suggesting the reliable application of this procedure to the classification of newly recorded cutaneous sensory receptors. 4. Eleven variables were then reduced to 7 on the basis of the results of factor analysis (95% of variance accounted for). The seven variables corresponded to 1.2-N compression, heating the skin by 15 degrees C, cooling the skin by 20 degrees C, 30- and 300-Hz sinusoids, withdrawal of a 200-mN indentation, and air puffs. 5. The seven selected variables correctly assigned afferent units into five modality-based categories in the discriminant solution (177 of 188, 94.1%). In the cluster solution, afferent units described by the seven selected variables were divided into clusters, most of whose members were modality specific (176 of 188, 93.6%). 6. The results indicate that cutaneous receptors can be divided into modality-specific groups according to similarities in their responses to seven stimulus variables. It is proposed that the stimulus set developed here and multivariate statistical methods can be used as powerful tools for the functional classification of central somatosensory neurons.

Afferent Pathways↗

PubMed1993

Cisapride in chronic dyspepsia: results of a double-blind, placebo-controlled trial.

The efficacy and tolerability of cisapride in chronic dyspepsia was evaluated in a randomized, double-blind, placebo-controlled study. After 4 weeks' treatment with oral cisapride 10 mg three times daily (n = 14), bloating and epigastric discomfort were significantly reduced compared with placebo (n = 15; p < 0.05). Moreover, the global response to treatment was excellent or good in 71.4% of patients in the cisapride group versus 20.0% with placebo. No significant side effects were observed. It is concluded that cisapride is an effective and well-tolerated treatment for chronic dyspepsia.

Adolescent↗

PubMed1992

Somatic afferent fibers which continuously discharge after being isolated from their receptors.

A previous study in our laboratory has shown that some afferent axons produce prolonged discharges after the axons have been completely isolated. We have attempted to identify the type of afferent fibers displaying such activity. Single unit activity was recorded from a filament dissected from the distal stump of the cut sural or plantar nerve of an anesthetized rat. After thorough identification of the receptor type, the nerve was cut at a site between the recording electrode and the receptive field, completely isolating the fiber being recorded. Unit activity was recorded up to 1 h after sectioning the nerve. Upon sectioning the nerve, most units showed brief injury discharges lasting only a few seconds. However, 21 of 70 units exhibited prolonged discharges lasting at least 30 min after having been isolated from their receptors. These 21 units included 8 slowly adapting type II cutaneous mechanoreceptors, 3 Pacinian corpuscles and 10 muscle spindle afferent units. These results suggest that prolonged injury discharges can be produced in the axons of the slowly adapting type II cutaneous mechanoreceptors, Pacinian corpuscles, and muscle spindle afferents. This phenomenon may have important clinical and experimental consequences.

Afferent Pathways↗

PubMed1992

Responses of spinothalamic tract neurons to mechanical and thermal stimuli in an experimental model of peripheral neuropathy in primates.

1. An experimental peripheral neuropathy (EPN) was induced in three monkeys (Macaca fascicularis) by ligation of spinal nerve L7. Behavioral responses to innocuous mechanical stimuli were tested before and after the surgery. Two weeks after the nerve ligation, the activity of spinothalamic tract (STT) neurons was recorded on both sides of the spinal cord with the animal under general anesthesia. Responses of the STT neurons to the following stimuli applied to the skin were recorded: graded mechanical stimuli (brush, press, pinch and squeeze), von Frey filaments of different bending forces (0.077-19.05 g), 5-s heat stimuli ranging from 39 to 53 degrees C, and 15 s cold stimuli (32-8 degrees C). 2. Innocuous mechanical stimulation of the foot did not evoke hindlimb withdrawal in the animals before surgery. Within 24-48 h after nerve ligation, the animals showed hindlimb withdrawal to the same innocuous stimuli. This behavior was more pronounced on the side of the ligation than on the sham-operated side and more frequent during the second week after the surgery. 3. Responses of 51 STT neurons recorded on the side of the ligation (EPN all group) were compared with responses of 33 STT cells recorded on the sham-operated side (control group) and with records from STT neurons in unoperated animals obtained earlier (reference group). Neurons from the EPN all group were divided into two sets according to their rostrocaudal location (EPN R, rostral to L6/7 border, n = 40; EPN C, caudal to L6/7 border, n = 11). 4. Neurons from the EPN all and EPN R groups had significantly higher background frequencies than those from the control and reference groups. Innocuous brush stimuli evoked mean discharge frequencies of approximately 35 Hz in EPN R neurons and only approximately 15 Hz in both control and reference groups. Increased responsiveness of EPN R neurons to innocuous stimuli was also demonstrated by lower thresholds and higher discharge frequencies to von Frey filament stimulation and by discriminative analysis of the responses evoked by graded mechanical stimuli. 5. The responses of the EPN R neurons to heat stimulation of the skin showed decreased thresholds and increased responses to suprathreshold stimuli, resulting in a significant leftward shift of the stimulus-response curve compared with both reference and control groups. The neurons from the control group showed responses comparable to reference group values. 6. Neurons from the reference group tested with the cooling stimuli showed no evoked response above background.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

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.

Animals↗

PubMed1991

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.

Action Potentials↗

PubMed1991

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.

Animals↗

PubMed1991

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)

Animals↗

PubMed1990

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.

Animals↗

PubMed1990

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.

Anesthetics, Local↗

PubMed1989

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.

Animals↗

PubMed1989

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.

Action Potentials↗

PubMed1989

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.

Action Potentials↗

PubMed1989

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.

Action Potentials↗