Comparison of Responses of Primate Spinothalamic Tract Neurons to Pruritic and Algogenic Stimuli

2004 ◽  
Vol 91 (1) ◽  
pp. 213-222 ◽  
Author(s):  
Donald A. Simone ◽  
Xijing Zhang ◽  
Jun Li ◽  
Jun-Ming Zhang ◽  
Christopher N. Honda ◽  
...  
Keyword(s):  
Dynamic Range ◽  
Intradermal Injection ◽  
High Threshold ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Wide Dynamic Range ◽  
Antidromic Activation ◽  
Lateral Nucleus ◽  
Maximal Time

We investigated the role of mechanosensitive spinothalamic tract (STT) neurons in mediating 1) the itch evoked by intradermal injection of histamine, 2) the enhanced sense of itch evoked by innocuous stroking (alloknesis), and 3) the enhanced pain evoked by punctate stimulation (hyperalgesia) of the skin surrounding the injection site. Responses to intradermal injections of histamine and capsaicin were compared in STT neurons recorded in either the superficial or the deep dorsal horn of the anesthetized monkey. Each neuron was identified by antidromic activation from the ventral posterior lateral nucleus of thalamus and classified by its initial responses to mechanical stimuli as wide dynamic range (WDR) or high-threshold (HT). Approximately half of the WDRs and one of the HTs responded weakly to histamine, some with a duration > 5 min, the maximal time allotted. WDRs but not HTs exhibited a significant increase in response to punctate stimulation after histamine consistent with their possible role in mediating histamine-induced hyperalgesia. Neither type of neuron exhibited significant changes in response to stroking, consistent with their unlikely role in mediating alloknesis. Furthermore, nearly all STT neurons exhibited vigorous and persistent responses to capsaicin, after which they became sensitized to stroking and to punctate stimulation. We conclude that the STT neurons in our sample are more likely to contribute to pain, allodynia, and hyperalgesia than to itch and alloknesis.

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.

Nitric Oxide–Mediated Spinal Disinhibition Contributes to the Sensitization of Primate Spinothalamic Tract Neurons

1999 ◽  
Vol 81 (3) ◽  
pp. 1086-1094 ◽  
Author(s):  
Qing Lin ◽  
Jing Wu ◽  
Yuan Bo Peng ◽  
Minglei Cui ◽  
William D. Willis
Keyword(s):  
Nitric Oxide ◽  
Dorsal Horn ◽  
Dynamic Range ◽  
Gaba Release ◽  
Intradermal Injection ◽  
High Threshold ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
No Donor ◽  
Spinal Inhibition

Nitric oxide–mediated spinal disinhibition contributes to the sensitization of primate spinothalamic tract neurons. This study concentrated on whether an increase in spinal nitric oxide (NO) diminishes inhibition of spinothalamic tract (STT) cells induced by activating the periaqueductal gray (PAG) or spinal glycinergic and GABAergic receptors, thus contributing to the sensitization of STT neurons. A reduction in inhibition of the responses to cutaneous mechanical stimuli induced by PAG stimulation was seen in wide dynamic range (WDR) STT cells located in the deep layers of the dorsal horn when these neurons were sensitized during administration of a NO donor, 3-morpholinosydnonimine (SIN-1), into the dorsal horn by microdialysis. In contrast, PAG-induced inhibition of the responses of high-threshold (HT) and superficial WDR STT cells was not significantly changed by spinal infusion of SIN-1. A reduction in PAG inhibition when STT cells were sensitized after intradermal injection of capsaicin could be nearly completely blocked by pretreatment of the dorsal horn with a NO synthase inhibitor, 7-nitroindazole. Moreover, spinal inhibition of nociceptive activity of deep WDR STT neurons elicited by iontophoretic release of glycine and GABA agonists was attenuated by administration of SIN-1. This change paralleled the change in PAG-induced inhibition. However, the inhibition of HT and superficial WDR cells induced by glycine and GABA release did not show a significant change when SIN-1 was administered spinally. Combined with our recent results, these data show that the effectiveness of spinal inhibition can be reduced by the NO/cGMP pathway. Thus disinhibition may constitute one mechanism underlying central sensitization.

Grouping of somatosensory neurons in the spinal cord and the gracile nucleus of the rat by cluster analysis

1994 ◽  
Vol 72 (6) ◽  
pp. 2590-2597 ◽  
Author(s):  
J. W. Leem ◽  
B. H. Lee ◽  
W. D. Willis ◽  
J. M. Chung
Keyword(s):  
Cluster Analysis ◽  
Dorsal Horn ◽  
Dynamic Range ◽  
High Threshold ◽  
Spinothalamic Tract ◽  
Wide Dynamic Range ◽  
Gracile Nucleus ◽  
Noxious Stimuli ◽  
Thermal Stimuli ◽  
Wide Dynamic Range Neurons

1. A set of 11 cutaneous stimuli defined previously to differentiate among different types of cutaneous sensory receptors in the rat hindpaw was also effective in differentially activating second-order sensory neurons in the dorsal horn and the gracile nucleus of rats. 2. All sampled units were responsive to more than 1 of the 11 stimuli. However, none responded to innocuous warming or cooling stimuli. Therefore further analysis was restricted to responses to nine of the selected stimuli. 3. Cluster analysis of the responses to nine selected innocuous and noxious mechanical stimuli and noxious thermal stimuli yielded seven classes that seemed functionally distinct from each other: a class of high-threshold neurons, three classes of convergent (wide dynamic range) neurons, a class of a mixture of poorly responsive neurons and neurons receiving Pacinian inputs, and two classes of low-threshold neurons. 4. High-threshold neurons responded predominantly to noxious mechanical and thermal stimuli and presumably received an input from both mechanically and thermally sensitive nociceptors. These cells were located in the dorsal horn, and some were spinothalamic tract cells. Wide dynamic range neurons were excited by innocuous and noxious stimuli, but better by noxious stimuli. These classes of cells were either in the dorsal horn (some were spinothalamic tract cells) or in the nucleus gracilis.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatosensory response properties of contralaterally projecting spinothalamic and nonspinothalamic neurons in the second cervical segment of the cat

1991 ◽  
Vol 66 (1) ◽  
pp. 83-102 ◽  
Author(s):  
M. V. Smith ◽  
A. V. Apkarian ◽  
C. J. Hodge
Keyword(s):  
Dynamic Range ◽  
Cervical Spinal Cord ◽  
Average Distance ◽  
Whole Body ◽  
High Threshold ◽  
Average Depth ◽  
Spinothalamic Tract ◽  
Antidromic Activation ◽  
Response Properties ◽  
Electrolytic Lesions

1. The upper cervical spinal cord contains over one-third of the cells of the spinothalamic tract (STT). This study investigated response properties of contralaterally projecting STT neurons in C2 of the cat by the use of single-unit, microelectrode recordings. Standard antidromic stimulation and collision techniques were used to identify STT units projecting to the contralateral thalamus. Once an STT unit was found, its receptive field (RF) and responses to cutaneous stimuli such as touch, pressure, deep muscle squeeze, tap, noxious pinch, and heat were characterized. C2 units that were not activated from the contralateral thalamus (non-STT units) were also characterized. The locations of thalamic stimulation electrodes and spinal recording sites were reconstructed from electrolytic lesions. 2. A total of 48 STT and 68 non-STT units were well characterized. RF sizes were classified as small, intermediate, large, or whole body. Each unit was also classified as having one of two possible response types: simple units were those with homogeneous responses within the RF and were classified as low threshold (LT), high threshold (HT), wide dynamic range (WDR), deep, or tap. Complex units were those that responded differently in different regions of the RF. 3. The average depth of non-STT units subdivided by RF size was 2.1 +/- 0.6 (SD) mm for cells with small RFs, 2.4 +/- 0.8 mm for cells with intermediate RFs, 2.8 +/- 0.3 mm for cells with large RFs, and 2.7 +/- 0.5 mm for cells with whole-body RFs. The average depth of non-STT units based on response type was 2.0 +/- 0.5 mm for LT, 2.3 +/- 0.7 mm for HT, 2.1 +/- 0.7 mm for WDR, 2.6 +/- 0.9 mm for deep, 2.6 +/- 0.5 mm for tap, and 2.4 +/- 0.2 mm for complex. 4. A somatotopic organization along the rostrocaudal length of C2 and upper C3 was observed for non-STT units with small- and intermediate-size RFs. The average distance of the recording sites from the rostralmost dorsal rootlet of C2 was 3.8 +/- 2.1 mm for units with RFs on the face, 7.1 +/- 4.3 mm for units with RFs on the neck, and 11.9 +/- 5.1 mm for units with RFs on the forelimb. 5. The average threshold for antidromic activation of STT units was 175 +/- 120 microA. Most C2 STT units were activated from the ventroposterior region of the thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Position of Spinothalamic Tract Axons in Upper Cervical Spinal Cord of Monkeys

2000 ◽  
Vol 84 (3) ◽  
pp. 1180-1185 ◽  
Author(s):  
Xijing Zhang ◽  
Christopher N. Honda ◽  
Glenn J. Giesler
Keyword(s):  
Spinal Cord ◽  
Dorsal Horn ◽  
Dynamic Range ◽  
Cervical Spinal Cord ◽  
High Threshold ◽  
Spinothalamic Tract ◽  
Antidromic Activation ◽  
Lateral Funiculus ◽  
Upper Cervical ◽  
Low Threshold

Percutaneous upper cervical cordotomy continues to be performed on patients suffering from several types of severe chronic pain. It is believed that the operation is effective because it cuts the spinothalamic tract (STT), a primary pathway carrying nociceptive information from the spinal cord to the brain in humans. In recent years, there has been controversy regarding the location of STT axons within the spinal cord. The aim of this study was to determine the locations of STT axons within the spinal cord white matter of C2 segment in monkeys using methods of antidromic activation. Twenty lumbar STT cells were isolated. Eleven were classified as wide dynamic range neurons, six as high-threshold cells, and three as low-threshold cells. Eleven STT neurons were recorded in the deep dorsal horn and nine in superficial dorsal horn. The axons of the examined neurons were located at antidromic low-threshold points (<30 μA) within the contralateral lateral funiculus of C2. All low-threshold points were located ventral to the denticulate ligament, within the lateral half of the ventral lateral funiculus (VLF). None were found in the dorsal half of the lateral funiculus. The present findings support our previous suggestion that STT axons migrate ventrally as they ascend the length of the spinal cord. Also, the present findings indicate that surgical cordotomies that interrupt the VLF in C2 likely disrupt the entire lumbar STT.

Spinothalamic and spinohypothalamic tract neurons in the cervical enlargement of rats. I. Locations of antidromically identified axons in the thalamus and hypothalamus

1994 ◽  
Vol 71 (3) ◽  
pp. 959-980 ◽  
Author(s):  
R. J. Dado ◽  
J. T. Katter ◽  
G. J. Giesler
Keyword(s):  
Dorsal Horn ◽  
Dynamic Range ◽  
Ventral Horn ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Noxious Heat ◽  
Antidromic Activation ◽  
Posterior Thalamus ◽  
Electrolytic Lesions ◽  
Nociceptive Input

1. Seventy-seven neurons in the cervical enlargement of rats anesthetized with urethan were initially antidromically activated using currents < or = 30 microA from the contralateral posterior thalamus. A goal of these experiments was to determine the course of physiologically characterized spinal axons within the diencephalon. Therefore, in 38 cases, additional antidromic mapping was done throughout the mediolateral extent of the diencephalon at multiple anterior-posterior planes. 2. Electrolytic lesions marking the recording sites were recovered for 71 neurons. Thirty-one were located in the superficial dorsal horn (SDH); 39 were in nucleus proprius or the lateral reticulated area of the deep dorsal horn (DDH), and one was in the ventral horn. 3. Eight of 38 (21%) neurons that were tested for more anterior projections could only be antidromically activated with currents < or = 30 microA from sites in the contralateral posterior thalamus. Such neurons are referred to as spinothalamic tract (STT) neurons. Lesions marking the lowest threshold points for antidromic activation were located in or near the posterior thalamic group (Po). At more anterior levels, considerably higher currents were required for antidromic activation or it was not possible to activate the neurons with currents up to 500 microA. Four of these neurons were physiologically characterized and each responded preferentially to noxious mechanical stimuli (wide dynamic range, WDR). Each of the three neurons that were tested responded to noxious heat stimuli. These findings confirm anatomic studies that have shown that a number of STT axons terminate in Po and suggest that such axons that originate in the cervical enlargement carry nociceptive input from the upper extremity. 4. In 15 cases, electrode penetrations were made systematically throughout much of the contralateral ventrobasal complex (VbC). In 17 cases, penetrations were made throughout the intralaminar nuclei contralaterally, including the central lateral nucleus (CL). Surprisingly, only one of the examined axons was antidromically activated with low currents from CL and one from VbC, although both of these nuclei are known to receive sizeable inputs from the STT. 5. Many of the axons (27 of the 38 tested, 71%) that were initially antidromically activated from the contralateral posterior thalamus could also be antidromically activated with low currents (< or = 30 microA) and at increased latencies from sites located anteriorly in the contralateral hypothalamus. Such neurons are referred to as spinothalamic tract/spinohypothalamic tract (STT/SHT) neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Termination Zones of Functionally Characterized Spinothalamic Tract Neurons Within the Primate Posterior Thalamus

2008 ◽  
Vol 100 (4) ◽  
pp. 2026-2037 ◽  
Author(s):  
Steve Davidson ◽  
Xijing Zhang ◽  
Sergey G. Khasabov ◽  
Donald A. Simone ◽  
Glenn J. Giesler
Keyword(s):  
Dynamic Range ◽  
Lumbar Spinal Cord ◽  
High Threshold ◽  
Chemical Information ◽  
Thalamic Nuclei ◽  
Spinothalamic Tract ◽  
Noxious Heat ◽  
Antidromic Activation ◽  
Posterior Thalamus ◽  
Medial Geniculate

The primate posterior thalamus has been proposed to contribute to pain sensation, but its precise role is unclear. This is in part because spinothalamic tract (STT) neurons that project to the posterior thalamus have received little attention. In this study, antidromic mapping was used to identify individual STT neurons with axons that projected specifically to the posterior thalamus in Macaca fascicularis. Each axon was located by antidromic activation at low stimulus amplitudes (<30 μA) and was then surrounded distally by a grid of stimulating points in which 500-μA stimuli were unable to activate the axon antidromically, thereby indicating the termination zone. Several nuclei within the posterior thalamus were targets of STT neurons: the posterior nucleus, suprageniculate nucleus, magnocellular part of the medial geniculate nucleus, and limitans nucleus. STT neurons projecting to the ventral posterior inferior nucleus were also studied. Twenty-five posterior thalamus-projecting STT neurons recorded in lumbar spinal cord were characterized by their responses to mechanical, thermal, and chemical stimuli. Sixteen of 25 neurons were recorded in the marginal zone and the balance was located within the deep dorsal horn. Thirteen neurons were classified as wide dynamic range and 12 as high threshold. One-third of STT neurons projecting to posterior thalamus responded to noxious heat (50°C). Two-thirds of those tested responded to cooling. Seventy-one percent responded to an intradermal injection of capsaicin. These data indicate that the primate STT transmits noxious and innocuous mechanical, thermal, and chemical information to multiple posterior thalamic nuclei.

The primate spinocervicothalamic pathway: responses of cells of the lateral cervical nucleus and spinocervical tract to innocuous and noxious stimuli

1988 ◽  
Vol 59 (3) ◽  
pp. 861-885 ◽  
Author(s):  
J. W. Downie ◽  
D. G. Ferrington ◽  
L. S. Sorkin ◽  
W. D. Willis
Keyword(s):  
Conduction Velocity ◽  
Dynamic Range ◽  
Receptive Fields ◽  
Glabrous Skin ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Sinusoidal Vibration ◽  
Antidromic Activation ◽  
Lateral Cervical Nucleus ◽  
Stimulation Of

1. The response properties of neurons of the spinocervicothalamic pathway were studied in anesthetized macaque monkeys. Graded innocuous and noxious mechanical stimuli, including sinusoidal vibration and thermal pulses, were applied to the cutaneous receptive fields. 2. Forty-nine cells in the lateral cervical nucleus (LCN) were identified by antidromic activation from the ventral posterior lateral (VPL) nucleus of the contralateral thalamus. Twelve spinocervical tract (SCT) cells in the lumbosacral enlargement of the spinal cord were identified by antidromic activation from stimulation of the ipsilateral dorsolateral funiculus below C3 but not above C1. 3. Latencies for antidromic activation of LCN neurons averaged 2.3 ms, corresponding to a mean conduction velocity of approximately 17 m/s. Mean latency for orthodromic activation of LCN neurons following electrical stimulation of peripheral nerves was 12.6 ms. Overall mean conduction velocity for the monkey spinocervicothalamic pathway was estimated to be 29 m/s. 4. Most LCN cells had receptive fields on hairy skin, but some had input from glabrous skin and a few had subcutaneous fields. The receptive fields of most SCT cells had a glabrous skin component. Receptive fields tended to be smaller for SCT than LCN cells even for fields on a comparable part of the distal hindlimb. 5. Based on their responses to a series of mechanical stimuli (brushing, pressure, pinch, and squeeze), LCN and SCT cells were classified as low-threshold (LT), wide dynamic range (WDR), or high-threshold (HT) neurons. Most of the cells were in the LT or WDR classes. Thus the spinocervicothalamic pathway in the monkey differs from the spinothalamic tract (STT), in that STT cells are generally of the WDR or HT classes. 6. With the use of discriminant analysis, LCN and SCT neurons were allocated to categories determined from a k-means cluster analysis of the responses of 318 STT cells. The LCN and SCT neurons were in different proportions in the various categories than were STT cells, suggesting differences in the signaling properties of the spinocervicothalamic and spinothalamic paths. 7. Innocuous steady indentation of the skin failed to excite any of the neurons tested. Thus no positive evidence was obtained for an input to LCN neurons from slowly adapting mechanoreceptors. 8. Sinusoidal vibratory stimuli were used to test the ability of LCN and SCT neurons to follow repeated innocuous mechanical stimuli. Vibration at 10 Hz and an amplitude of 100 micron resulted in repetitive discharges in most LCN neurons and half the SCT neurons tested; many LCN neurons had thresholds below 25 micron.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurokinin 1 and 2 antagonists attenuate the responses and NK1 antagonists prevent the sensitization of primate spinothalamic tract neurons after intradermal capsaicin

1994 ◽  
Vol 72 (4) ◽  
pp. 1464-1475 ◽  
Author(s):  
P. M. Dougherty ◽  
J. Palecek ◽  
V. Paleckova ◽  
W. D. Willis
Keyword(s):  
Dorsal Horn ◽  
Mechanical Stimulation ◽  
Intradermal Injection ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Sensory Stimuli ◽  
Neurokinin Receptors ◽  
Neurokinin 1 ◽  
Stimulation Of

1. Activation of neurokinin receptors contributes to the excitation of many dorsal horn neurons by cutaneous sensory stimuli, particularly noxious stimuli. In the present study we investigate the role of neurokinin receptors in the activation of primate spinothalamic tract (STT) neurons by cutaneous mechanical stimuli and by intradermal injection of capsaicin. This was done by testing the responses of these neurons to a battery of cutaneous stimuli before and during infusion by microdialysis of antagonists selective for NK1 and NK2 receptors. 2. The NK1 receptor antagonists cis-3-(2-methoxybenzyl-amino-2-benzhydrylquinuclidine (CP96345) and D-Pro9-[Spiro-y-lactam]-Leu10,Trp11)-Physalaemin(1-11) (GR82334) did not significantly reduce the responses of STT cells to mechanical stimulation of the skin. Both NK1 antagonists did, however, produce a significant reduction in the responses of STT neurons to an intradermal injection of capsaicin. Finally, despite having no effects on responses to mechanical stimuli, both NK1 antagonists prevented the sensitization of the responses to cutaneous stimuli that is usually observed after intradermal injections of capsaicin. 3. The NK2 selective antagonists PhCO-Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH2 (GR98400) and [Tyr5,D-Trp6,8,9,Lys10]-NKA (4–10) (MEN10376) had effects very similar to those of the NK1 antagonists, but with an important difference. Neither NK2 antagonist affected the responses of STT neurons to noxious or innocuous mechanical stimulation of the skin, but they did reduce the responses to intradermal capsaicin injections. These compounds failed to prevent capsaicin-induced sensitization. In fact, cells exposed to GR98400 or MEN10376 showed unusually sustained increases in the responses to mechanical stimuli after the first capsaicin injection, suggesting that these compounds actually induced sensitization. 4. These results support the contention that both neurokinin receptors participate in the processing of nociceptive information in the dorsal horn, especially responses to strong stimuli such as intradermal injection of capsaicin. NK1 receptors are also involved in the sensitization of STT neurons after peripheral injury. A clearer understanding of the role of NK2 receptors in sensitization requires further studies with improved antagonists.

Electrophysiological study of spinothalamic inputs to ventrolateral and adjacent thalamic nuclei of the cat

1991 ◽  
Vol 66 (3) ◽  
pp. 1033-1047 ◽  
Author(s):  
C. T. Yen ◽  
C. N. Honda ◽  
E. G. Jones
Keyword(s):  
Dynamic Range ◽  
Electrophysiological Study ◽  
The Other ◽  
Reticular Nucleus ◽  
High Threshold ◽  
Medial Lemniscus ◽  
Spinothalamic Tract ◽  
Conduction Velocities ◽  
Lateral Nucleus ◽  
Stimulation Of

1. Extracellular and intracellular methods were used to record from fibers and neurons in the ventral lateral (VL) and adjacent nuclei of the cat thalamus. The receptive fields of the recorded units were analyzed and the units tested for inputs from the medial lemniscus (ML) and spinothalamic tract (STT) by electrical stimulation of the dorsal columns (DC) and ventrolateral funiculus (VLF) at the C2-3 spinal level. 2. Thirty-eight STT fibers were isolated in the thalamus. Their conduction velocities ranged from 15 to 75 m/s (mode 36 m/s). Adequate stimuli were found for 23 of these fibers. Seventeen were low-threshold (LT), 3 were wide-dynamic-range (WDR), and 3 were high-threshold (HT) units. 3. Five STT fibers were intra-axonally injected. Three were sufficiently well filled for analysis of their terminal fields. An intermediate-velocity STT fiber (conduction velocity 38 m/s) had a 4.3-microns axon and a single large terminal field in the central lateral nucleus (CL). The other two STT fibers were smaller, with diameters of 2.5 and 2.3 microns, conduction velocities of 15 and 19 m/s, and terminal fields made up of a few small boutons at the borders of the ventral posterior lateral nucleus (VPL). 4. Of 319 neurons isolated, 14 out of 129 (10.8%) in VL, 14 out of 76 (18.4%) in the VPL or ventral posterior medial (VPM) nucleus, 27 out of 64 (42.2%) in the CL nucleus, and 5 out of 50 (10%) in the reticular nucleus (R) responded at latencies less than 50 ms to VLF stimuli. A train of three pulses was more effective in driving VLF-responding neurons in all these nuclei than a single pulse. VLF-responding cells were widely dispersed in VL, concentrated in a focus in CL, and distributed around the borders of VPL. Most of those in VL and a small number in CL could be antidromically activated by stimulation of motor cortex. 5. Latencies of presynaptic responses (STT fibers) to VLF stimulation were short and varied from 0.8 to 3.9 ms (mode 1.6 ms). Despite this, very few fast-responding neurons were found. These were six VPL neurons (2.5 to 4 ms), one VL neuron (3 ms), and four CL neurons (3-4 ms). The initial spike latencies of the majority of thalamic neurons responding to VLF stimulation appeared in two peaks, one between 6 and 8 ms and the other at 10-15 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

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