Encoding of electrical, thermal, and mechanical noxious stimuli by subnucleus reticularis dorsalis neurons in the rat medulla

1989 ◽  
Vol 61 (2) ◽  
pp. 391-402 ◽  
Author(s):  
L. Villanueva ◽  
Z. Bing ◽  
D. Bouhassira ◽  
D. Le Bars
Keyword(s):  
Linear Relationship ◽  
The Body ◽  
Thermal Stimulation ◽  
Entire Body ◽  
Evoked Responses ◽  
Mechanical Stimuli ◽  
Noxious Heat ◽  
C Fibers ◽  
C Fiber ◽  
Stimulation Of

1. In anesthetized rats, recordings were made within the medullary subnucleus reticularis dorsalis (SRD) from neurons that exhibited convergence of nociceptive inputs from the entire body. Neurons with total nociceptive convergence (TNC) responded to suprathreshold percutaneous electrical stimuli (2-ms duration) with an early and a late peak due to activation of A delta- and C-fibers, respectively, no matter which part of the body was stimulated. Neurons with partial nociceptive convergence (PNC) responded to the same stimuli with an A delta-peak regardless of which part of the body was stimulated and with a C-peak of activation from some, mainly contralateral, parts of the body. The characteristics of the responses of these neurons to the application of graded intensities of electrical, thermal, and mechanical stimuli were analyzed. 2. All TNC neurons and 85% of PNC neurons responded to A delta- and C-fiber activation following percutaneous electrical stimulation of the contralateral hindpaw. With regard to A delta-fiber-evoked responses, a linear relationship between the logarithm of the applied current and the magnitude of the responses was found within the 0.25- to 6.0-mA and 0.5- to 24-mA ranges for TNC and PNC neurons, respectively; however, these curves were essentially similar. With regard to C-fiber-evoked responses, such a linear relationship was found within the 1.5- to 6.0-mA range for both types of SRD neurons, although the TNC neurons presented larger C-fiber-evoked responses than did the PNC neurons. 3. TNC and PNC neurons linearly increased their discharges during the application of noxious thermal stimuli to the contralateral hindpaw within the range 44-52 degrees C; the mean responses evoked by noxious heat from TNC neurons were of higher magnitude than those from PNC neurons. The majority of SRD neurons presented long-lasting afterdischarges, especially with the highest temperature employed (52 degrees C). 4. TNC neurons monotonically increased their discharges during graded mechanical or thermal stimulation of the tail. When mechanical stimuli were applied, a linear relationship was found between the logarithm of the strength of the mechanical stimulus and the neuronal discharges, in the 5.3- to 7.4-N/cm2 range. With thermal stimulation, TNC neurons linearly increased their discharges in the 44-52 degrees C range. When increasing amounts of the tail were immersed in a 50 degrees C waterbath, TNC neurons increased their discharges within a restricted range of tail surface areas (0.9-5.7 cm2); further increases in the stimulated surface size were not followed by increases in firing rate.(ABSTRACT TRUNCATED AT 400 WORDS)

Leukotriene B4 decreases the mechanical and thermal thresholds of C-fiber nociceptors in the hairy skin of the rat

1988 ◽  
Vol 60 (2) ◽  
pp. 438-445 ◽  
Author(s):  
H. A. Martin ◽  
A. I. Basbaum ◽  
E. J. Goetzl ◽  
J. D. Levine
Keyword(s):  
Acetic Acid ◽  
Glacial Acetic Acid ◽  
Leukotriene B4 ◽  
Thermal Stimulation ◽  
Mechanical Stimuli ◽  
Noxious Heat ◽  
C Fibers ◽  
Mechanical Threshold ◽  
C Fiber ◽  
Heat Threshold

1. We have recently shown that leukotriene B4 (LTB4), a product of the 5-lipoxygenase pathway of arachidonic acid metabolism, sensitizes nociceptors to mechanical stimuli. The present study examined whether LTB4 also induces a heat sensitization of cutaneous C-fiber nociceptors. The C-fiber nociceptors studied had von Frey hair thresholds greater than 5 g and were characterized according to their responses to noxious heat and chemical stimuli, including glacial acetic acid, bradykinin, and capsaicin. Thirty-four of the C-fibers that were activated by intense thermal stimulation were also activated by topical application of glacial acetic acid. They were classified as C-polymodal nociceptors (2, 28). Those that were activated by intense mechanical and thermal stimulation, but were unresponsive to acid, were classified as C-mechanoheat nociceptors (27). 2. Ninety-four percent of C-polymodal nociceptors and 60% of C-mechanoheat nociceptors were sensitized by LTB4. All C-fiber nociceptors that showed a decrease of their heat threshold also had a decrease of their mechanical threshold. LTB4 (75 ng) lowered the average heat threshold from 45 degrees C to 35 degrees C and produced an average decrease in the mechanical threshold of 86%. 3. The magnitude of the LTB4-evoked decrease in thermal threshold was similar to that produced by 75 ng of prostaglandin E2 (PGE2). These data demonstrate that LTB4 sensitizes C-mechanoheat nociceptors to both mechanical and thermal stimuli. 4. We conclude that LTB4 may contribute to the component of hyperalgesia that is resistant to nonsteroidal anti-inflammatory agents.

Responses of primate SI cortical neurons to noxious stimuli

1983 ◽  
Vol 50 (6) ◽  
pp. 1479-1496 ◽  
Author(s):  
D. R. Kenshalo ◽  
O. Isensee
Keyword(s):  
Receptive Field ◽  
Cortical Neurons ◽  
Receptive Fields ◽  
The Body ◽  
Thermal Stimulation ◽  
Discharge Frequency ◽  
Mechanical Stimuli ◽  
Noxious Heat ◽  
Nociceptive Neurons ◽  
Heat Pulses

Recordings were made from single SI cortical neurons in the anesthetized macaque monkey. Each isolated cortical neuron was tested for responses to a standard series of mechanical stimuli. The stimuli included brushing the skin, pressure, and pinch. The majority of cortical neurons responded with the greatest discharge frequency to brushing the receptive field, but neurons were found in areas 3b and 1 that responded maximally to pinching the receptive field. A total of 68 cortical nociceptive neurons were examined in 10 animals. Cortical neurons that responded maximally to pinching the skin were also tested for responses to graded noxious heat pulses (from 35 to 43, 45, 47, and 50 degrees C). If the neuron failed to respond or only responded to 50 degrees C, the receptive field was also heated to temperatures of 53 and 55 degrees C. Fifty-six of the total population of nociceptive neurons were tested for responses to the complete series of noxious heat pulses: 46 (80%) exhibited a progressive increase in the discharge frequency as a function of stimulus intensity, and the spontaneous activity of two (4%) was inhibited. One population of cortical nociceptive neurons possessed restricted, contralateral receptive fields. These cells encoded the intensity of noxious mechanical and thermal stimulation. Sensitization of primary afferent nociceptors was reflected in the responses of SI cortical nociceptive neurons when the ascending series of noxious thermal stimulation was repeated. The population of cortical nociceptive neurons with restricted receptive fields exhibited no adaptation in the response during noxious heat pulses of 47 and 50 degrees C. At higher temperatures the response often continued to increase during the stimulus. The other population of cortical nociceptive neurons was found to have restricted, low-threshold receptive fields on the contralateral hindlimb and, in addition, could be activated only by intense pinching or noxious thermal stimuli delivered on any portion of the body. The stimulus-response functions obtained from noxious thermal stimulation of the contralateral hindlimb were not different from cortical nociceptive neurons with small receptive fields. However, nociceptive neurons with large receptive fields exhibited a consistent adaptation during a noxious heat pulse of 47 and 50 degrees C. Based on the response characteristics of these two populations of cortical nociceptive neurons, we conclude that neurons with small receptive fields possess the ability to provide information about the localization, the intensity, and the temporal attributes of a noxious stimulus.4+.

Regulation of cutaneous C-fiber heat nociceptors by nerve growth factor in the developing rat

1994 ◽  
Vol 71 (3) ◽  
pp. 941-949 ◽  
Author(s):  
G. R. Lewin ◽  
L. M. Mendell
Keyword(s):  
Nerve Growth Factor ◽  
Growth Factor ◽  
Sural Nerve ◽  
Mechanical Stimuli ◽  
Noxious Heat ◽  
Nerve Growth ◽  
C Fibers ◽  
Mechanical Threshold ◽  
C Fiber ◽  
Developing Rat

1. Previous work has indicated that nerve growth factor (NGF) is an important factor for the development of nociceptive A-delta fibers. Here we asked if the availability of NGF during the first 2 wk of development can influence the phenotypic development of unmyelinated afferent C-fibers. 2. To do this, we treated newborn rats with antibodies to NGF from postnatal day (PND) 2-14, a treatment known not to lead to cell death, or with exogenous NGF (PND 0-14). Untreated litter mates served as controls. When the animals were mature (5-20 wk later), they were anesthetized (urethan, 1.25 g/kg ip) and single-unit recordings were made from sural nerve C-fibers in dorsal root filaments. To obtain an unbiased estimate of the physiological types present, an electrocutaneous search technique was employed to isolate the location of the putative receptive field of the C-fiber being studied within the sural nerve territory. This technique enabled us to acquire a sample of C-fibers that was unbiased by natural search stimuli which are often damaging to skin. 3. We found that the proportion of C-fibers that could be driven by noxious heat and mechanical stimuli (C-mechanoheat fibers, C-MH) was reduced from 28% (11/40 fibers) of the sample in controls to 10% (3/31 fibers) in anti-NGF treated animals. The C-MH fibers appeared to have been replaced by a novel type of pressure receptor with an unusually low mechanical threshold (mean 0.86 +/- 0.58 compared with 13.0 +/- 8.3 g for control mechanonociceptors).(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the brain structures involved in diffuse noxious inhibitory controls: the mesencephalon

1990 ◽  
Vol 64 (6) ◽  
pp. 1712-1723 ◽  
Author(s):  
D. Bouhassira ◽  
Z. Bing ◽  
D. Le Bars
Keyword(s):  
Receptive Fields ◽  
Conditioning Stimulus ◽  
Ibotenic Acid ◽  
Brain Structures ◽  
The Body ◽  
Evoked Responses ◽  
Diffuse Noxious Inhibitory Controls ◽  
C Fibers ◽  
C Fiber ◽  
Conditioning Stimuli

1. Diffuse noxious inhibitory controls (DNIC) were compared in control sham-operated rats and in rats with lesions of mesencephalic structures involved in the modulation of pain, namely the periaqueductal gray (PAG), cuneiformis nucleus (CNF), and parabrachial nucleus (PB). 2. Lesions were induced by ibotenic acid: 4 micrograms (0.2 microliter) injected bilaterally in the PAG or the CNF-PB area or 10 micrograms (0.5 microliter) injected unilaterally in the CNF or PB. Control animals were microinjected with the vehicle (artificial CSF) alone. Histological controls were performed at the end of each electrophysiological experiment. Only the animals in which the target structure (PAG, CNF, or PB) was completely destroyed in its entire rostrocaudal length were selected. With the exception of the cell bodies of the trigeminal mesencephalic nucleus, all neurons were destroyed in these regions. 3. At least 1 wk after the microinjection procedure, recordings were made from convergent neurons in both the right and left trigeminal nucleus caudalis. These neurons were activated by both noxious and nonnoxious stimuli applied to their excitatory receptive fields and gave responses due to activation of both A- and C-fibers after percutaneous electrical stimulation of their receptive fields. These types of response were inhibited by applying noxious conditioning stimuli to heterotopic areas of the body, namely immersing a paw in a 50 degrees C water bath. A virtually total block of the responses was observed during the application of the noxious conditioning stimulus, and this was followed by long-lasting poststimulus effects. 4. The general properties of neurons (sizes of receptive fields, spontaneous activity, thresholds to obtain C-fiber-evoked responses, responses to C-fiber activation) were all found to be similar in the control and the lesioned animals. The percentage inhibition of the C-fiber-evoked responses of the trigeminal convergent neurons elicited by the noxious conditioning stimuli were found to not be significantly different in any group of animals; in all the animals, inhibitions exceeded 85% during the immersion of either paw and were followed by long-lasting poststimulus effects. 5. We conclude that the PAG, CNF, and PB, three structures that are putatively involved in the modulation of pain, do not participate directly in the supraspinal part of the loop subserving DNIC. The involvement of other structure(s) and a possible indirect modulation of DNIC are discussed. It is also concluded that the PAG, CNF, and PB do not participate directly in the tonic descending inhibitory controls, which are presumed to modulate the activity of convergent neurons.

Nociceptive responses of neurons in medial thalamus and their relationship to spinothalamic pathways

1978 ◽  
Vol 41 (6) ◽  
pp. 1592-1613 ◽  
Author(s):  
W. K. Dong ◽  
H. Ryu ◽  
I. H. Wagman
Keyword(s):  
Mechanical Stimulation ◽  
The Body ◽  
Direct Stimulation ◽  
Spinothalamic Tract ◽  
Medial Thalamus ◽  
C Fibers ◽  
Nociceptive Responses ◽  
C Fiber ◽  
Noxious Mechanical Stimulation ◽  
Stimulation Of

1. An extracellular study of the cat medial thalamus has revealed four types of somatosensory neurons. These were located primarily in the n. parafascicularis, n. subparafascicularis, and n. centralis lateralis; none were found in the n. centrum medianum. There was no functional segregation of neurons within each nucleus or between nuclei. Each type of neuron had large and often bilateral receptive areas. No somatotopic organization of neurons was found within the medial thalamus. 2. Noxious (N) and noxious-tap (NT) neurons comprising 72% of the sample (78 of 109 total) were considered to be nociceptive. N cells responded exclusively to noxious mechanical stimulation of skin, muscle fascia, tendons, and joints, and to direct stimulation of A-delta- and C-fiber groups in cutaneous, articular, and muscle nerves. NT cells responded to noxious and tap stimulation in a differential manner and to stimulation of the entire spectrum of A- and C-fibers. N and NT cells accurately signaled the duration of noxious mechanical stimulation. Their nociceptive responses were also graded as a function of both noxious stimulus intensity and the number of activated A-delta- and C-fibers. Stimulation of A- and C-fibers evoked, respectively, an inital burst and a late burst of discharges. A brief period of inhibition intervened between the initial and late bursts of NT cells. Prolonged afterdischarge was often observed following noxious natural stimulation or stimulation of A-delta- and C-fibers. The phenomenon of discharge "windup" was observed during iterative stimulation of C-fibers. 3. Tap (T) neurons (10%) responded only to brisk but innocuous taps applied to skin or underlying tissue. These cells were driven only by activation of A-alpha- and A-beta-fibers. The response to such stimulation was seen as an initial burst of discharges followed by an inhibitory period. 4. Inhibited (I) neurons (18%) had resting discharges that were inhibited by noxious stimuli and stimulation of A-beta- and C-fiber groups. 5. The results obtained from monitoring the peripherally evoked responses of nociceptive N and NT neurons before and after selective lesions of the spinal cord strongly suggested that the spinothalamic tracts were the only spinal projections mediating A- and C-fiber input to these cells. Each spinothalamic tract apparently carried information originating from both sides of the body.

Excitation of primate spinothalamic neurons by cutaneous C-fiber volleys

1979 ◽  
Vol 42 (5) ◽  
pp. 1354-1369 ◽  
Author(s):  
J. M. Chung ◽  
D. R. Kenshalo ◽  
K. D. Gerhart ◽  
W. D. Willis
Keyword(s):  
Dynamic Range ◽  
High Threshold ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Noxious Heat ◽  
C Fibers ◽  
C Fiber ◽  
Excitatory Action ◽  
Low Threshold ◽  
Lateral Nucleus

1. The responses of spinothalamic tract cells in the lumbosacral spinal cords of anesthetized monkeys were examined following electrical stimulation of the sural nerve or the application of noxious thermal and mechanical stimuli to the skin on the lateral aspect of the foot. 2. The spinothalamic tract neurons were classified as wide dynamic range (WDR), high-threshold (HT), or low-threshold (LT) cells on the basis of their responses to mechanical stimuli. 3. All of the WDR and HT spinothalamic tract cells tested responded to volleys in A- and C-fibers. However, strong C-fiber responses were more common in HT than in WDR cells. 4. The responses atributed to C-fibers were graded with the size of the C-fiber volley. The latencies of the responses attributed to C-fibers indicated that the fastest afferents involved had a mean conduction velocity of 0.9 m/s. The responses remained after anodal blockade of conduction in A-fibers. 5. Temporal summation of the responses of spinothalamic tract cells was demonstrated both to brief trains of stimuli at 33 Hz and to single stimuli repeated at 1- to 2-s intervals. The latter phenomenon is often called "windup." 6. The responses of several spinothalamic tract cells to noxious heat pulses could still be elicited during anodal blockade of conduction in A-fibers. Similarly, it was possible to demonstrate an excitatory action of noxious mechanical stimuli despite interference with conduction in A-fibers by anodal current. 7. The cells investigated were located either in the marginal zone or in the layers of the dorsal horn equivalent to Rexed's laminae IV-VI in the cat. The cells were generally activated antidromically from the caudal part of the ventral posterior lateral nucleus of the thalamus.

scholarly journals Selective stimulation of the ferret abdominal vagus nerve with multi-contact nerve cuff electrodes

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Jonathan A. Shulgach ◽  
Dylan W. Beam ◽  
Ameya C. Nanivadekar ◽  
Derek M. Miller ◽  
Stephanie Fulton ◽  
...  
Keyword(s):  
Vagus Nerve ◽  
Nerve Fibers ◽  
The Body ◽  
Evoked Responses ◽  
Gi Tract ◽  
Selective Stimulation ◽  
Nerve Cuff ◽  
Cuff Electrodes ◽  
Nerve Cuff Electrodes ◽  
Stimulation Of

AbstractDysfunction and diseases of the gastrointestinal (GI) tract are a major driver of medical care. The vagus nerve innervates and controls multiple organs of the GI tract and vagus nerve stimulation (VNS) could provide a means for affecting GI function and treating disease. However, the vagus nerve also innervates many other organs throughout the body, and off-target effects of VNS could cause major side effects such as changes in blood pressure. In this study, we aimed to achieve selective stimulation of populations of vagal afferents using a multi-contact cuff electrode wrapped around the abdominal trunks of the vagus nerve. Four-contact nerve cuff electrodes were implanted around the dorsal (N = 3) or ventral (N = 3) abdominal vagus nerve in six ferrets, and the response to stimulation was measured via a 32-channel microelectrode array (MEA) inserted into the left or right nodose ganglion. Selectivity was characterized by the ability to evoke responses in MEA channels through one bipolar pair of cuff contacts but not through the other bipolar pair. We demonstrated that it was possible to selectively activate subpopulations of vagal neurons using abdominal VNS. Additionally, we quantified the conduction velocity of evoked responses to determine what types of nerve fibers (i.e., Aδ vs. C) responded to stimulation. We also quantified the spatial organization of evoked responses in the nodose MEA to determine if there is somatotopic organization of the neurons in that ganglion. Finally, we demonstrated in a separate set of three ferrets that stimulation of the abdominal vagus via a four-contact cuff could selectively alter gastric myoelectric activity, suggesting that abdominal VNS can potentially be used to control GI function.

Convergence of heterotopic nociceptive information onto neurons of caudal medullary reticular formation in monkey (Macaca fascicularis)

1990 ◽  
Vol 63 (5) ◽  
pp. 1118-1127 ◽  
Author(s):  
L. Villanueva ◽  
K. D. Cliffer ◽  
L. S. Sorkin ◽  
D. Le Bars ◽  
W. D. Willis
Keyword(s):  
Electrical Stimulation ◽  
Reticular Formation ◽  
Mechanical Stimulation ◽  
Background Activity ◽  
The Body ◽  
Thermal Stimulation ◽  
Medullary Reticular Formation ◽  
Nociceptive Information ◽  
Noxious Mechanical Stimulation ◽  
Stimulation Of

1. Recordings were made in anesthetized monkeys from neurons in the medullary reticular formation (MRF) caudal to the obex. Responses of 19 MRF neurons to mechanical, thermal, and/or electrical stimulation were examined. MRF neurons exhibited convergence of nociceptive cutaneous inputs from widespread areas of the body and face. 2. MRF neurons exhibited low levels of background activity. Background activity increased after periods of intense cutaneous mechanical or thermal stimulation. Nearly all MRF neurons tested failed to respond to heterosensory stimuli (flashes, whistle sounds), and none responded to joint movements. 3. MRF neurons were excited by and encoded the intensity of noxious mechanical stimulation. Responses to stimuli on contralateral limbs were greater than those to stimuli on ipsilateral limbs. Responses were greater to stimuli on the forelimbs than to stimuli on the hindlimbs. 4. MRF neurons responded to noxious thermal stimulation (51 degrees C) of widespread areas of the body. Mean responses from stimulation at different locations were generally parallel to those for noxious mechanical stimulation. Responses increased with intensity of noxious thermal stimulation (45-50 degrees C). 5. MRF neurons responded with one or two peaks of activation to percutaneous electrical stimulation applied to the limbs, the face, or the tail. The differences in latency of responses to stimulating two locations along the tail suggested that activity was elicited by activation of peripheral fibers with a mean conduction velocity in the A delta range. Stimulation of the contralateral hindlimb elicited greater responses, with lower thresholds and shorter latencies, than did stimulation of the ipsilateral hindlimb. 6. Electrophysiological properties of monkey MRF neurons resembled those of neurons in the medullary subnucleus reticularis dorsalis (SRD) in the rat. Neurons in the caudal medullary reticular formation could play a role in processing nociceptive information. Convergence of nociceptive cutaneous input from widespread areas of the body suggests that MRF neurons may contribute to autonomic, affective, attentional, and/or sensory-motor processes related to pain.

Endogenous BK stimulates ischemically sensitive abdominal visceral C fiber afferents through kinin B2 receptors

1994 ◽  
Vol 267 (6) ◽  
pp. H2398-H2406 ◽  
Author(s):  
H. L. Pan ◽  
G. L. Stahl ◽  
S. V. Rendig ◽  
O. A. Carretero ◽  
J. C. Longhurst
Keyword(s):  
Receptor Antagonist ◽  
Impulse Activity ◽  
Discharge Rate ◽  
Visceral Afferents ◽  
C Fibers ◽  
C Fiber ◽  
Hoe 140 ◽  
B2 Receptors ◽  
The Right ◽  
Stimulation Of

Abdominal ischemia and reperfusion reflexly activate the cardiovascular system. In the present study, we evaluated the role of endogenously produced bradykinin (BK) in the stimulation of ischemically sensitive visceral afferents. Single-unit activity of abdominal visceral C fiber afferents was recorded from the right thoracic sympathetic chain of anesthetized cats during 5 min of abdominal ischemia. Abdominal ischemia increased the portal venous plasma BK level from 49 +/- 10 to 188 +/- 66 pg/ml (P < 0.05). Injection of BK (1 microgram/kg ia) into the descending aorta significantly increased impulse activity (0.88 +/- 0.16 impulses/s) of 10 C fibers, whereas a kinin B1-receptor agonist, des-Arg9-BK (1 microgram/kg), did not alter the discharge rate. Inhibition of kininase II activity with captopril (4 mg/kg i.v.) potentiated impulse activity of 14 ischemically sensitive C fibers (0.44 +/- 0.09 vs. precaptopril, 0.33 +/- 0.08 impulses/s; P < 0.05). In addition, a kinin B2-receptor antagonist (NPC-17731; 40 micrograms/kg i.v.) attenuated activity of afferents during ischemia (0.39 +/- 0.08 vs. pre-NPC-17731, 0.72 +/- 0.13 impulses/s; P < 0.05) and eliminated the response of 10 C fibers to BK. Another kinin B2-receptor antagonist, Hoe-140 (30 micrograms/kg iv), had similar inhibitory effects on six other ischemically sensitive C fibers. In 15 separate cats treated with aspirin (50 mg/kg i.v.), Hoe-140 (30 micrograms/kg i.v.) attenuated impulse activity of only 3 of 16 ischemically sensitive C fibers. These data suggest that BK produced during abdominal ischemia contributes to the stimulation of ischemically sensitive visceral C fiber afferents through kinin B2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of spinothalamic tract cells to mechanical and thermal stimulation of skin in rats with experimental peripheral neuropathy

1992 ◽  
Vol 67 (6) ◽  
pp. 1562-1573 ◽  
Author(s):  
J. Palecek ◽  
V. Paleckova ◽  
P. M. Dougherty ◽  
S. M. Carlton ◽  
W. D. Willis
Keyword(s):  
Dynamic Range ◽  
Background Activity ◽  
Thermal Stimulation ◽  
Spontaneous Pain ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
High Background ◽  
Threshold Temperatures ◽  
Thermal Stimuli ◽  
Stimulation Of

1. Responses of spinothalamic tract (STT) neurons to mechanical and thermal stimulation of skin were recorded under urethane and pentobarbital anesthesia in 12 control rats and in 20 rats with experimental neuropathy. Activity of the STT cells in neuropathic rats was recorded 7, 14, and 28 days after inducing the neuropathy by placing four loose ligatures on the sciatic nerve. 2. All neuropathic animals showed guarding of the injured hindpaw and a shorter withdrawal latency from a radiant heat source of the neuropathic hindpaw than that of the sham-operated paw. 3. STT neurons in neuropathic animals showed the most profound changes 7 and 14 days after the nerve ligation. When compared with STT cells in unoperated animals, approximately half of the neurons had high background activity, responses to innocuous stimuli represented a larger percentage of the total evoked activity in wide dynamic range neurons, and the occurrence and magnitude of afterdischarges to mechanical and thermal stimuli were increased. 4. The mean threshold temperatures of heat-evoked responses of the STT cells in neuropathic animals were not different than those of cells from control animals. However, in neuropathic rats, cells reacting to small heat stimuli usually already had afterdischarges. 5. The increase in the background activity of STT cells is consistent with behavioral observations of spontaneous pain in this model of experimental neuropathy. Furthermore, the afterdischarges of STT cells may parallel the prolonged paw withdrawal in response to noxious stimuli that is seen in these animals and that is evidence for hyperalgesia. However, there was no indication of a lowered threshold for thermal stimuli as might be expected if the animals have thermal allodynia. Mechanical allodynia may have resulted from a relative increase in responsiveness to innocuous mechanical stimuli. However, responses to noxious mechanical stimuli were reduced compared with control, at least at 28 days after the ligation. Peripheral and central mechanisms responsible for the changes in responses of STT cells in neuropathic animals are suggested.

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