Locations of Spinothalamic Tract Axons in Cervical and Thoracic Spinal Cord White Matter in Monkeys

Xijing Zhang, Heather N. Wenk, Christopher N. Honda, and Glenn J. Giesler, Jr.

Department of Neuroscience, Graduate Program in Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Zhang, Xijing, Heather N. Wenk, Christopher N. Honda, and Glenn J. Giesler Jr.. Locations of Spinothalamic Tract Axons in Cervical and Thoracic Spinal Cord White Matter in Monkeys. J. Neurophysiol. 83: 2869-2880, 2000. The spinothalamic tract (STT) is the primary pathway carrying nociceptive information from the spinal cord to the brain in humans. The aim of this study was to understand better the organization of STT axons within the spinal cord white matter of monkeys. The location of STT axons was determined using method of antidromic activation. Twenty-six lumbar STT cells were isolated. Nineteen were classified as wide dynamic range neurons and seven as high-threshold cells. Fifteen STT neurons were recorded in the deep dorsal horn (DDH) and 11 in superficial dorsal horn (SDH). The axons of 26 STT neurons were located at 73 low-threshold points (<30 µA) within the lateral funiculus from T9 to C6. STT neurons in the SDH were activated from 33 low-threshold points, neurons in the DDH from 40 low-threshold points. In lower thoracic segments, SDH neurons were antidromically activated from low-threshold points at the dorsal-ventral level of the denticulate ligament. Neurons in the DDH were activated from points located slightly ventral, within the ventral lateral funiculus. At higher segmental levels, axons from SDH neurons continued in a position dorsal to those of neurons in the DDH. However, axons from neurons in both areas of the gray matter were activated from points located in more ventral positions within the lateral funiculus. Unlike the suggestions in several previous reports, the present findings indicate that STT axons originating in the lumbar cord shift into increasingly ventral positions as they ascend the length of the spinal cord.


    INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Many lines of evidence extending back more than 100 yr indicate that the spinothalamic tract (STT) is the primary pathway for the transmission of nociceptive information from the spinal cord in primates, including humans (reviewed in Willis 1985). Until relatively recently, it was widely believed that virtually all STT axons ascend within the contralateral ventral and ventral lateral funiculi (VLF). This belief was based primarily on a large number of studies in which restricted lesions in the ventral spinal cord white matter, and not in other areas of the spinal cord, caused degeneration of axons in the ipsilateral thalamus (Boivie 1971; Bowsher 1961; Kerr and Lippman 1974; Mehler et al. 1960; Mott 1895). Such lesions also markedly reduce pain and temperature sensations on the contralateral side of the body in monkeys (Vierck et al. 1971) and humans (Spiller and Martin 1912; White and Sweet 1969).

Several recent anatomic studies have indicated that STT axons are not restricted to the ventral half of the cord. Apkarian and Hodge (1989) combined retrograde tract tracing methods with lesions in midthoracic segments to examine the location of STT axons in monkeys. They found that lesions of the dorsal lateral funiculus (DLF) prevented retrograde labeling of almost all STT neurons in the contralateral marginal zone below the lesion, and that lesions of the VLF prevented labeling of STT neurons in lamina V and other areas of the deep dorsal horn. These results suggested that the axons of STT neurons in the marginal zone ascend in the DLF, not in the VLF, as STT axons had been traditionally believed to do. These and other results led the authors to conclude that there are two separate spinothalamic pathways in cats and monkeys: a dorsal STT within the DLF made up primarily of the axons of lamina I cells, and a ventral STT within the VLF made up of the axons of lamina V-VIII neurons (Jones et al. 1985, 1987; Stevens et al. 1989, 1991).

Other workers (Craig 1991; Ralston and Ralston 1992) suggested that axons of STT neurons in the marginal zone are located at approximately the dorsal-ventral middle of the lateral funiculus. Using injections of the anterograde tracer phaseolus vulgaris leucoagglutinin (PHA-L) into the superficial dorsal horn (SDH) of cats, Craig (1991) labeled axons in an intermediate position in the contralateral white matter, at about the level of the denticulate ligament. The idea that STT axons are concentrated in this area of the lateral funiculus was supported by the findings of Ralston and Ralston (1992), who found that relatively small lesions within this area in monkeys greatly reduced anterograde labeling of STT axons in the posterior thalamus.

For several reasons, we have reexamined the organization of STT within the spinal cord. One reason is that a better understanding of the positions of the axons of STT neurons in different areas of the gray matter might provide insights into their physiological roles. For example, the question has been asked (e.g., Ralston and Ralston 1992), if the axons of marginal zone neurons in fact ascend within the DLF, why do lesions of the VLF eliminate pain? Does this mean that STT axons from marginal zone neurons are not sufficient for producing normal pain sensation? Does this also mean that STT axons from deep dorsal horn (DDH) neurons are necessary for pain sensation because cutting them within the VLF eliminates pain? A second reason is that a more conclusive description of the organization of the STT might be of value in better understanding the effects of damage to the human spinal cord produced by trauma or diseases. Finally, such information might be of value to neurosurgeons because cordotomies continue to be performed on patients suffering from several types of severe chronic pain (Garcia-Larrea et al. 1993; Jackson et al. 1999; Lahuerta et al. 1994; Nagaro et al. 1994; Sanders and Zurmond 1995).

We have used the technique of antidromic activation to examine the locations of STT axons for several reasons. 1) STT axons can be unambiguously identified using this technique. Previously used anterograde tracing methods, such as degeneration or PHA-L techniques, do not allow STT axons to be distinguished with certainty from other labeled axons in the spinal cord. 2) In contrast to methods in which lesions of spinal cord white matter have been used to locate STT axons, the antidromic method can determine the locations of individual axons with considerable accuracy (Dado et al. 1994a,c; Ranck 1975; Zhang et al. 1995). 3) The method of antidromic activation can be used to locate the same STT axon unambiguously at multiple levels of the cord, thereby revealing whether individual STT axons remain within one region of the cord or shift positions as they ascend. This method has revealed that individual STT and spinohypothalamic tract axons in rats often dramatically change their positions within the cord as they ascend the length of the spinal cord (Dado et al. 1994c). Individual axons cannot be identified at multiple segmental levels using other methods. 4) The use of antidromic activation allows the response characteristics of the examined axons to be directly determined. 5) In contrast to degeneration techniques or large injections of anterograde tracers, the locations of the examined STT cell bodies can be determined using antidromic activation.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

All procedures followed the guidelines of the International Association for the Study Pain and were approved by the institutional animal care and use committee.

Monkeys (Macaca fascicularis or mulatta) were anesthetized initially with ketamine (100 mg/kg im), followed by alpha -chloralose (60 mg/kg iv) and maintained with a continuous infusion of nembutal (2-4 mg · kg-1 · h-1 iv). Animals were placed in a stereotaxic frame, paralyzed with Flaxedil (10-20 mg · kg-1 · h-1 iv), and artificially ventilated. Body temperature, end-tidal CO2, and blood pressure were monitored and kept within physiological limits. Pneumothoraces were performed to improve mechanical stability. Laminectomies were made over lumbar, thoracic, and cervical segments. Craniotomies were made over the thalamus. Multiunit recordings of somatosensory responses were used to locate the ventral posterior lateral nucleus (VPL). The search stimulus consisted of cathodal pulses (200 µs, 500 µA) delivered within contralateral VPL. Single-unit recordings were made in the lumbar enlargement using stainless steel microelectrodes (5-10 MOmega ). After isolating an antidromically identified unit, the stimulating electrode was moved dorsal-ventrally, and antidromic thresholds were determined at 400-µm intervals. This procedure was repeated within multiple tracks in the one or more anterior-posterior planes until a point was located at which the antidromic threshold was <= 30 µA (referred to as low-threshold points). Current pulses <= 30 µA have been shown to activate spinohypothalamic tract axons in rats at a distance of <= 400 µm from the stimulating electrode (Burstein et al. 1991; Dado et al. 1994a). Criteria for antidromic activation included the following: response at a relatively constant latency (<0.2 ms variability), ability to follow high-frequency stimulation (>= 333 Hz), and collision of putative antidromic action potentials with orthodromic action potentials (Lipski 1981). All units included in this study met these criteria for antidromic activation.

Once antidromic activation from the contralateral thalamus was demonstrated, a second stainless steel stimulating electrode was inserted into the cervical or thoracic white matter contralateral to the recording site. Initially, large current pulses (500 µA, 200 µs) were delivered as the electrode was lowered through the cord until the same neuron was antidromically activated. The threshold for antidromic activation was determined at 200- to 400-µm intervals in each track. The electrode was lowered through a series of tracks, each separated by 500 µm, across the medial-lateral extent of the cord. At the lowest threshold point for antidromic activation, each neuron was activated using current pulses <= 30 µA. In each case, action potentials at the lowest threshold point in the cord collided with antidromic action potentials produced at the lowest threshold point in the thalamus. This procedure established that the action potentials elicited at both stimulation sites traveled in the same axon.

Cutaneous receptive fields and response characteristics of examined neurons were determined using innocuous and noxious mechanical stimulation. Responses to such stimuli were used to classify the examined neurons as low-threshold (LT), wide dynamic range (WDR) or high-threshold (HT) cells (see Dado et al. 1994b for details).

In almost all experiments, the location of more than one STT axon was examined on each side of the cord in the same monkey. In an effort to avoid cutting STT axons that were yet to be examined, lesions (25 µA for 40 s) were often made either 1.0 or 1.5 mm medial to the low-threshold point (at the same anterior-posterior plane). One lesion was made at the same dorsal-ventral position as the low-threshold point. A second lesion was made either 1.0 mm ventral or dorsal to the first. The distance between these two lesions in histological material was used as an indication of shrinkage or expansion of the tissue during processing. The distance between the two lesions, divided by 1.0, provided a ratio that was used in reconstructing the location of the low-threshold point. We attempted to test the accuracy of this method in two cases by making lesions medial to the low-threshold points and also at the actual low-threshold points. The results were similar in both cases. Figure 1 illustrates the results of one of these experiments. Lesions A and B were made 1.5 mm medial to the low-threshold point. Lesion B was made at the same dorsal-ventral position as the low-threshold point. Lesion A was made 1.0 mm dorsal to B. The distance between lesions A and B in histological material was 1.12 mm. The calculated position of the low-threshold point is indicated by the plus sign in Fig. 1; it falls within the lesion (C) made at the low-threshold point. Thus these findings indicate that this method can be used to reconstruct lesion sites accurately.



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Fig. 1. Illustration of the method used to determine the location of low-threshold points using medially placed lesions. After determining a low-threshold point for antidromic activation of a spinothalamic tract (STT) neuron and marking it with a lesion (C), the stimulating electrode was moved medially 1.5 mm from the low-threshold point in the same anterior-posterior plane. The electrode was lowered to the same dorsal-ventral position as the low-threshold point and a lesion was made (B). The electrode was then raised 1.0 mm, and another lesion was made (A). The distance between these 2 lesions in histological material was 1.12 mm. This distance divided by 1.0 mm provided a ratio that was used in calculating the lateral distance from the lesion to the low-threshold point. Note that the calculated position of the low-threshold point (+) is within the lesion made at the original low-threshold point. The method of marking medial to the low-threshold point appears to be accurate. LTP, low-threshold point.

In cases in which only one axon was examined on a side of the cord, lesions were made directly at each low-threshold point. Lesions were also made at each low-threshold point for the final case examined on each side of the cord. At the end of each experiment, electrolytic lesions were made at the tip of the stimulating electrode at each of the low-threshold points in the thalamus (100 µA for 40 s). Each recording side was marked (25 µA for 15 s).

Monkeys were perfused with 0.9% saline followed by 10% Formalin containing 1% potassium ferrocyanide (Prussian blue reaction). The areas of the thalamus and spinal cord containing lesions were cut transversely at 100 and 50 µm using a freezing microtome. Sections were counterstained with neutral red and microscopically examined. The locations of lesions were reconstructed with the aid of an attached camera lucida drawing tube.

In two monkeys, thoracic and cervical segments were removed with the dura intact. The denticulate ligaments were identified under a dissecting microscope in C6, T2, and T6. A needle was inserted into the bases of the ligaments to mark their positions on the surface of the lateral funiculus. The marked areas were sectioned on a freezing microtome. A line was drawn in the reconstructions from the mark on the lateral edge of the cord to the midline. In each case, this line passed in close proximity to the central canal. Therefore a line from the center of the central canal (at 90° to the midline) extending to the lateral edge of the cord was used as the border between the DLF and VLF. This line was used for statistical comparisons of the dorsal-ventral locations of low-threshold points.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Antidromic activation from the contralateral thalamus

Twenty-six neurons in lumbar enlargement were antidromically activated from low-threshold points (<= 30 µA) in the contralateral thalamus. A photomicrograph of a lesion at a low-threshold point in the contralateral thalamus is presented in Fig. 2A. Low-threshold points were concentrated in the ventral and lateral parts of VPL (Fig. 3). There was no apparent segregation of the locations of the low-threshold points for antidromic activation of axons of neurons recorded in the SDH or DDH.



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Fig. 2. Photomicrographs of lesions at low-threshold points and recording sites. A-C: lesions marking low-threshold points for antidromic activation in the contralateral thalamus (A), dorsal lateral funiculus (DLF; B), and ventral lateral funiculus (VLF; C) of the thoracic cord. D and E: lesions marking recording sites in superficial dorsal horn (SDH) and deep dorsal horn (DDH) in lumbar enlargement. Aq, aqueduct; CM, centromedian nucleus; IC, internal capsule; MD, medial dorsal thalamic nucleus; RT, reticular thalamic nucleus; SN substantia nigra; VPL, ventral posterior lateral thalamic nucleus; VPM, ventral posterior medial thalamic nucleus.



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Fig. 3. Locations of low-threshold points in or near the contralateral VPL for antidromic activation of 26 STT neurons. Note that the distributions of low-threshold points from which SDH and DDH neurons were antidromically activated overlap extensively. LG, lateral geniculate nucleus; LP, lateral posterior thalamic nucleus; Put, putamen; III, oculomotor nucleus.

Recording sites and physiological characteristics

The locations of lesions marking 26 recording sites are shown in Fig. 4. Two units were recorded in L5, 11 in L6, and 13 in L7. Eleven recording sites were located in the SDH. Fifteen were in the DDH. Within the DDH, eight recording sites were located in nucleus proprius and seven in the lateral reticulated area. These recording sites were concentrated laterally in nucleus proprius and the dorsal half of the lateral reticulated area (Fig. 4). Photomicrographs of lesions at recording sites in the SDH and DDH are shown in Fig. 2, D and E, respectively.



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Fig. 4. Locations of lesions marking recording sites of 26 STT neurons in the lumbar enlargement. Eleven STT neurons were recorded in the superficial dorsal horn, 15 were located in the deep dorsal horn. Seven high-threshold (HT) neurons and 19 wide dynamic range (WDR) neurons were recorded.

Seven STT neurons were classified as HT neurons, and 19 as WDR neurons. One HT neuron was recorded in the SDH, six in DDH, including five in nucleus proprius. Ten WDR neurons were recorded in the SDH, nine in DDH (3 in nucleus proprius, 6 in lateral reticular area).

All 26 STT neurons had excitatory receptive fields on the ipsilateral hindpaw or hindlimb. Most STT neurons (9/11) recorded in the SDH had receptive fields that covered an area of less than half of the hindpaw. Neurons recorded in the nucleus proprius tended to have slightly larger receptive fields. All neurons recorded in the lateral reticular area had large receptive fields that covered more than half of the hindlimb.

The estimated mean conduction velocity from the recording site to the lowest threshold point for antidromic activation in the contralateral thalamus for the examined neurons was 49.6 ± 3.0 (SE) m/s (Fig. 5). The mean conduction velocity from the recording site to the contralateral thalamus for 11 neurons recorded in the SDH was 45.5 ± 3.2 m/s (Table 1), and that for 15 neurons recorded in the DDH was 52.7 ± 4.5 m/s (no significant difference).



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Fig. 5. Histograms illustrating the conduction velocities between the recording sites in the lumbosacral enlargement and the low-threshold points in the contralateral thalamus for all examined axons.


                              
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Table 1. Mean conduction velocities for all STT neurons

Antidromic activation of STT axons in the contralateral spinal cord white matter

The positions of the axons of 26 STT neurons were determined at a total of 74 low-threshold points (<= 30 µA) in the contralateral lateral funiculus from T9 to C6. The mean threshold in the low-threshold point for antidromically activating STT neurons was 11.9 ± 0.7. Lesions were made directly at the low-threshold points in 48 cases. One of these lesions was not recovered and not included in the results. Lesions were made medial to 26 low-threshold points. The locations of axons of 2 neurons were determined at one segmental level, 5 axons at 2 levels, 14 axons at 3 levels, and 5 axons at 4 levels. At each segmental level, only one low-threshold point was determined for each axon.

Figure 6 illustrates an example of an STT neuron recorded in the SDH (Fig. 6C) that was antidromically activated from low-threshold points within the contralateral DLF. The neuron was initially antidromically activated from a low-threshold point in contralateral VPL (Fig. 6A, a1-a3) at a latency of 5.2 ms (a1). Within the spinal cord, the position of its axon was determined at three levels (Fig. 6B). In T4, a series of 10 electrode penetrations was made across the medial-lateral extent of the cord. The neuron could not be activated (using 500-µA pulses) from any of the sites within six tracks made throughout the ipsilateral side of the cord. The neuron was activated from a single lowest threshold point (15 µA) in the contralateral DLF (circled in Fig. 6B) at a latency of 2.6 ms (Fig. 6B, b1). Antidromic action potentials elicited in the thoracic cord and the thalamus collided (Fig. 6B, b2 and b3), indicating that the action potentials evoked from the two locations traveled in the same axon. The lowest threshold point was surrounded medially, laterally, dorsally, and ventrally by points in which higher currents were required for antidromic activation. In T6, five electrode penetrations were made; the lowest threshold point for antidromic activation (20 µA) was also located in the contralateral DLF (latency = 2.4 ms, Fig. 6B, b4). In T7, six electrode penetrations were made, and a single lowest threshold point (8 µA) was again located in the contralateral DLF (latency = 2.1 ms, Fig. 6B, b7). The neuron had a small receptive field on the ipsilateral foot (Fig. 6D). It responded to innocuous mechanical stimuli but responded at higher frequencies to noxious mechanical stimuli and was classified as a WDR neuron (Fig. 6E).



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Fig. 6. Example of an STT neuron recorded in the SDH that was antidromically activated from low-threshold points within the DLF of the contralateral upper thoracic cord. A: the low-threshold point in the contralateral thalamus at which the neuron was initially antidromically activated. The putative antidromic response had a stable latency (3 overlapping traces in a1), followed high-frequency trains of pulses (a2), and collided with orthodromic action potentials (a3). Amplitude and timing of stimulus pulses are indicated below digitized oscillographic traces. B: this STT neuron was also antidromically activated in 3 different levels of the contralateral thoracic cord (b1, b4, and b7). Spikes elicited from the spinal cord and thalamus collided (b2 and b3, b5 and b6, b8 and b9). C: recording site. D: receptive field. E: histograms of the responses to innocuous and noxious mechanical stimulation applied to the ipsilateral hindpaw. Note that each low-threshold point in the thoracic cord was located within the DLF.

Figure 7 shows an example of an STT neuron that was recorded in the DDH (Fig. 7C). In contrast to the example illustrated in Fig. 6, this neuron was antidromically activated from a low-threshold point within the contralateral VLF of each of the three examined thoracic segments (Fig. 7B). The neuron had a cutaneous receptive field on the ipsilateral hindpaw (Fig. 7D). It was classified as an HT neuron (Fig. 7E).



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Fig. 7. Example of an STT neuron recorded in the DDH with an axon that ascended in VLF of the contralateral thoracic cord. A: low-threshold point for antidromic activation in the contralateral thalamus. The response had a stable latency (a1, 3 overlapping traces), followed high-frequency pulses (a2), and collided with orthodromic spikes (a3). B: this STT neuron was also antidromically activated in 3 different levels of the contralateral thoracic cord (b1, b4, and b7). Spikes elicited from the spinal cord and thalamus collided (b2 and b3, b5 and b6, b8 and b9). C: recording site. D: receptive field. E: histograms of the responses to innocuous and noxious mechanical stimulation applied to the ipsilateral hindpaw. Note that each low-threshold point in the thoracic cord was located in the VLF.

The locations of 73 low-threshold points in the lower cervical and upper thoracic cord white matter (T9-C6) are illustrated in Fig. 8. Eleven STT neurons in the SDH were activated from 33 low-threshold points; 15 neurons in the DDH were activated from 40 low-threshold points. All low-threshold points were located in the lateral funiculus. Within segments T4-T9, low-threshold points for axons of neurons in SDH were located largely in the ventral part of the DLF, near the dorsal-ventral level of the denticulate ligament (Figs. 2B, 6B, and 8). Low-threshold points for axons of neurons in DDH were located farther ventrally, primarily within the VLF (Figs. 2C, 7B, and 8). Within upper thoracic segments and the cervical enlargement, low-threshold points for axons of neurons in SDH continued to be generally located dorsal to those of neurons in the DDH. However, low-threshold points for activation of both types of neurons were located farther ventrally. Indeed, within segments C6-C8, all low-threshold points, including those for activation of SDH neurons, were located in the VLF (Fig. 8). Axons of SDH neurons were significantly dorsal to the axons of DDH neurons in C6-C8, T1-T3, T4-T6, and T7-T9 (unpaired t-test, P < 0.002). The dorsal-ventral locations of low-threshold points for all neurons within each segmental grouping were analyzed using an ANOVA. Low-threshold points were found to be located farther ventrally in rostral segments (P < 0.05). Post hoc analysis revealed that low-threshold points within C6-C8 were significantly ventral to those in T7-C9 and T4-T6 (Duncan's multiple range test, P < 0.05).



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Fig. 8. Locations of lesions in the lower cervical and upper thoracic cord marking 73 low-threshold points for antidromic activation of 26 STT neurons. Note that at all examined levels, axons of STT neurons in the SDH were generally dorsal to those of DDH neurons. Note also that STT axons originating from neurons in both locations shifted ventrally as they ascended through the examined segmental levels.

In addition to shifting ventrally, STT axons also appeared to shift laterally as they ascended through the cord. In segment T4-T8, low-threshold points were concentrated in the approximate medial-lateral center of the lateral funiculus, and only one was found within 300 µm of the lateral edge of the cord. Within segments C6-C8, low-threshold points were concentrated farther laterally, and eight were located within 300 µm of the lateral edge of the cord (Fig. 8). The mean distance to the lateral edge of the cord was 740 ± 83 µm in T7-T9, 751 ± 69 µm in T4-T6, 566 ± 61 µm in T1-T3, and 474 ± 90 µm in C6-C8. Within segments T7-T9, none of the examined STT axons were located within 300 µm of the lateral edge of the cord. In contrast, in C6-C8, 8 of 15 (53%) STT axons were located within 300 µm of the edge of the cord. Low-threshold points were found to be located significantly farther laterally within rostral segments (ANOVA, P < 0.05). Post hoc analysis revealed that low-threshold points within C6-C8 were significantly lateral to those in T7-T9 and T4-T6 (Duncan's multiple range test, P < 0.05). There were no significant differences in the mean distance to the edge of the cord between axons of SDH and DDH neurons at any segmental level.

Sixteen low-threshold points for activation of axons of 7 HT neurons (1 SDH neuron, 6 DDH neurons) and 57 low-threshold points for activation of axons of 19 WDR neurons (10 SDH neurons, 9 DDH neurons) are plotted in Fig. 9. Low-threshold points for activation of axons of HT neurons were primarily found in the VLF; low-threshold points for activation of axons of WDR neurons were located within the DLF and VLF.



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Fig. 9. Locations of lesions at low-threshold points for activation of 7 axons of HT neurons and 19 axons of WDR neurons.

Estimated current spread in the spinal cord white matter

The distance at which current pulses activate STT and spinohypothalamic tract axons was examined previously in rats (Burstein et al. 1991; Dado et al. 1994a). Because primate STT neurons have average conduction velocities that are more than twice that of rat STT axons, it was necessary to determine the distance at which current pulses were effective for the presently examined axons. The effective spread of the stimulating current pulses in the cervical and thoracic cord white matter was estimated from plots of antidromic thresholds versus distances from lowest threshold points. Threshold readings from 73 electrode penetrations containing the lowest threshold are plotted in Fig. 10. The mean threshold for antidromic activation at the lowest threshold points in the spinal cord was 11.9 ± 0.7 µA. Four hundred micrometers dorsal or ventral to the lowest threshold point in each track, the mean current required for antidromic activation increased to 35.6 ± 2.6 µA (Fig. 10C). This difference of 23.7 µA over a distance of 400 µm indicates that between the lowest threshold point and points located 400 µm from the lowest threshold point, the effective spread of current was ~17 µm/µA. Thus the mean stimulus currents of 12 µA at the lowest threshold points probably spread effectively ~200 µm from the tip of the stimulating electrodes.



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Fig. 10. A: plots of antidromic thresholds vs. distances from low-threshold points (indicated by 0 on the abscissa) in the upper thoracic and lower cervical spinal cord. B: same data as in A plotted to show spread of currents of <= 30 µA. C: plot of the mean antidromic thresholds (±SE) as a function of distance from the lowest threshold points in the cervical and thoracic cord. Note that for thresholds <= 30 µA, the distance from the lowest threshold point was <400 µm.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, we have determined the location within the spinal cord of the axons of 26 antidromically identified STT neurons in monkeys. Eleven STT neurons were recorded in the SDH, and the remaining 15 were recorded throughout the DDH. In this sample, the distribution of the recording points and virtually all of the other physiological characteristics resembles those found in many previous studies of lumbar STT neurons in monkeys (e.g., Giesler et al. 1981; Willis et al. 1974). One feature of the neurons in this study does appear to differ from the results of several previous studies. In this study, 10 of the 11 of examined STT neurons in the SDH were classified as WDR. In many previous studies, the percentage of WDR neurons recorded in the SDH was considerably lower (e.g., see Giesler et al. 1981; Willis et al. 1974; however, cf. Ferrington et al. 1987 and Owens et al. 1992, who found high percentages of WDR-STT neurons in the SDH).

In an attempt to gather more accurate information on the position of primate STT axons within the thalamus, the stimulating electrode was lowered through several tracks until the examined neurons could be antidromically activated using current pulses of <= 30 µA (see also Applebaum et al. 1979). Because current pulses of these amplitudes do not spread effectively more than 400 µm (Fig. 10) (Dado et al. 1994a; Ranck 1975), it is highly unlikely that current spread effectively beyond the borders of VPL. We found that the areas in VPL from which SDH and DDH neurons could be antidromically activated overlapped almost completely. Although considerable attention has been paid in recent years to projections of STT neurons in the SDH to areas other than VPL, our findings are consistent with the idea that VPL is a prominent target for such projections (Applebaum et al. 1979; Willis et al. 1979).

The principle findings of this study are illustrated schematically in Fig. 11. At all examined levels of the cord, almost all of the axons of STT neurons in the SDH were located dorsal to those of neurons in the DDH. In mid- and lower thoracic segments, SDH axons were most often located at the level of, or even slightly dorsal to, the denticulate ligament; several were clearly located within the DLF. Within segments T1-T6, the axons of SDH neurons continued to be located dorsal to those of neurons in the DDH. However, at these levels SDH axons were found with increasing frequency ventral to the denticulate ligament, within the VLF, and less frequently within the DLF. This shifting ventrally of SDH axons continued within the cervical enlargement. At this level, all of the examined SDH axons were found within the VLF.



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Fig. 11. Organization of STT axons within the lateral funiculus. Note that the axons of neurons in the SDH ascend at all segmental levels in a position dorsal to those of neurons in the DDH. Note also that axons from both areas migrate ventrally and laterally as they ascend and that within the cervical enlargement all STT axons are located within the lateral part of the VLF.

The bulk of axons of examined neurons in the DDH were located within the VLF within all examined segments. However, as was the case with axons of SDH neurons, the axons of DDH neurons also migrated ventrally as they ascended. In mid- and lower thoracic segments, several axons of DDH neurons were located at the level of the denticulate ligament. Within the cervical enlargement, however, the axons of DDH neurons were all located within the VLF at some distance ventral to the denticulate ligament.

In addition to shifting ventrally, ascending STT axons migrated laterally. In mid- and lower thoracic segments, STT axons were distributed fairly widely across the medial-lateral extent of the cord. None were found in proximity to the lateral edge of the cord within T7-T9. However, at the level of the cervical enlargement, more than half of the STT axons were located within 300 µm of the lateral edge of the cord. These findings confirm early degeneration studies in monkeys and humans (Walker 1940; Weaver and Walker 1941; White 1954) in which it was noted that degenerating sacral and lumbar STT axons were located on the "extreme periphery" of the cervical spinal cord. Additional support for this idea was provided in a study in which antidromically identified STT axons were recorded in the upper lumbar VLF in monkeys (Applebaum et al. 1975). These workers found that STT axons within the lateral funiculus with sacral receptive fields were often located ventral and lateral to those having lumbar receptive fields.

At each examined segmental level, a stimulating electrode was lowered in a series of tracks spanning the entire side of the cord contralateral to the recording site. In every case, a single low-threshold point for antidromic activation was located within the lateral funiculus; none were found in the ventral funiculus. In early anatomic studies (Hyndman and Van Epps 1939; Kerr 1975; Morin et al. 1951; Walker 1940), it was noted that a small component of the STT was located within the ventral funiculus. This component of the STT was assigned the role of carrying information regarding "crude touch" or "pressure" (Foerster and Gagel 1932). Kerr (1975) reported that lesions restricted to the ventral funiculus of cervical segments in monkeys caused degeneration of STT fibers in a number of areas including VPL. We found no evidence to confirm an STT projection ascending through the ventral funiculus to VPL. There are several possible reasons for the conflicting findings that lesions of the ventral funiculus in upper cervical segments produced degeneration in VPL, and we did not find any STT axons within the ventral funiculus. One is that cell bodies in segments above the lumbar enlargement, the level sampled in the present study, might give rise to STT axons that ascend in the ventral funiculus. Another is that lesions in the ventral funiculus might cut the fibers of the large number of STT neurons in upper cervical segments as they cross through the ventral funiculus en route to the VLF and eventually to VPL. Clinically effective cordotomies often do not cut the ventral funiculus (White and Sweet 1969). Thus if an STT projection exists in the ventral funiculus, it is likely that the projection is not sufficient for production of normal pain sensation in humans.

In several early studies, it was concluded that STT axons shift dorsally, not ventrally, as they ascend the length of the spinal cord (Foerster and Gagel 1932; Hyndman and Van Epps 1939; Kahn and Rand 1952; Walker 1940; White 1954). This conclusion was based on two principle findings. 1) The authors concluded that it was necessary to cut farther dorsally in the lateral funiculus in the rostral spinal cord than it was in lower thoracic segments to produce analgesia in lower dermatomes. 2) Degenerating fibers were found farther dorsally within the lateral funiculus in rostral segments following surgically effective cordotomies. The conclusion that ascending STT axons migrate dorsally was also supported in studies in monkeys; degenerating axons were stained farther dorsally in the lateral funiculus in rostral segments following spinal cord lesions (Morin et al. 1951; Weaver and Walker 1941). It is difficult to explain fully how this conclusion, the opposite of that made here, was reached in these studies. Regarding the surgical result, it is possible that important nociceptive projections to thalamic areas other than VPL or to the brain stem ascend farther dorsally within the DLF. Regarding the anatomic results, it is possible that changes in the organization of STT axons were masked in these studies by labeling of several other types of degenerating ascending axons within the lateral funiculus. Our results indicate that STT axons consistently migrate ventrally as they ascend through upper thoracic segments and the cervical enlargement. In addition, in recent preliminary studies using the same antidromic stimulation techniques in monkeys, we (Zhang, Honda, and Giesler, unpublished observations) have found that STT axons are located ventrally within upper cervical segments.

Apkarian and Hodge (1989) reported that lesions of the DLF in squirrel monkeys disrupt the retrograde labeling of the majority of STT neurons in the contralateral marginal zone after injections of retrograde tracers into the thalamus. In addition, they found that lesions of the VLF block the retrograde labeling of STT neurons in lamina V and other areas of the deep dorsal horn. These findings led to the proposal of the existence of two spinothalamic tracts: one in the DLF, the DSTT, composed of the axons of marginal zone neurons and another in the VLF, the VSTT, composed of the axons of neurons in the DDH and ventral horn. It is important to note that all of the lesions in this study were made in midthoracic segments. The results of our studies using antidromic activation techniques in these segments confirm those of Apkarian and Hodge (1989). At these levels, we found that the majority of the axons of SDH neurons were located within the DLF and almost all axons of DDH neurons ascended within the VLF. However, our results suggest that lesions of the DLF and VLF at rostral levels would produce very different effects on retrograde labeling of STT neurons. Our data indicate that lesions at the level of the denticulate ligament and dorsal to it in the cervical enlargement would have little, if any, effect on retrograde labeling of STT neurons in either the SDH or DDH. Our data also indicate that lesions of the VLF in the cervical enlargement would have similar effects on the retrograde labeling of STT neurons in both areas of the gray matter; the labeling of both types of cells would likely be prevented.

Ralston and Ralston (1992) found that relatively small lesions of the lateral funiculus at the level of the denticulate ligament in lower thoracic and upper lumbar segments disrupted the anterograde labeling of many lumbar STT axons. They suggested that the labeled STT axons in this area of the cord emanated in large part from neurons in SDH. Our data support the idea that their lesions at the level of the denticulate ligament likely cut many STT axons of neurons in the SDH. Our data also suggest that had such lesions been made in the cervical enlargement, they would have cut far fewer lumbar STT axons.

Since the publication of the first papers (Apkarian et al. 1985) suggesting that the axons of marginal zone STT neurons ascend within the DLF and not the VLF, there has been considerable discussion of the implications of these findings. One implication is that, if cutting the VLF alone during a cordotomy produces analgesia and yet spares the axons of marginal zone neurons, then in these patients the unimpaired outflow of STT axons from SDH neurons is not sufficient to maintain normal nociception. The present findings bear on this controversy. Many cordotomies have been performed within rostral thoracic or cervical levels (Sweet et al. 1994). Our data suggest that at these segmental levels, the STT axons of both SDH and DDH neurons ascend within the VLF. Therefore our data would predict that a cordotomy restricted to the VLF at rostral levels of the cord would cut STT axons from both regions of the gray matter. Thus the results from cordotomies at these levels do not appear to provide any insight into the roles of the SDH versus DDH in nociceptive processing.

On the other hand, our findings confirm the suggestion by others that within mid- and lower thoracic segments the axons of SDH neurons are located farther dorsally within, or near, the DLF (Apkarian and Hodge 1989), at the level of the denticulate ligament (Craig 1991; Ralston and Ralston 1992). It is clear that many histologically confirmed cordotomies extended dorsally to at least the level of the denticulate ligament (Foerster and Gagel 1932; Gardner and Cuneo 1945; Kahn and Rand 1952; Nathan and Smith 1979; Sweet 1976; White and Sweet 1969). Thus in such cases, these cordotomies would have also probably cut STT axons from both DDH and SDH neurons.

There are examples in the literature of clinically effective, histologically confirmed small cordotomies that were restricted to the lateral funiculus at roughly the level of the denticulate ligament (Fig. 99, A and B, in White and Sweet 1969). These lesions would likely have cut SDH axons, while sparing most DDH fibers. Thus these results suggest that, in these cases, the STT projection originating in the DDH was not sufficient to maintain normal pain sensation. Surprisingly, we are unable to find in the literature examples of histologically confirmed cordotomies in thoracic and lower cervical segments that were sufficiently restricted to the VLF such that they would have been unlikely to cut the axons of SDH neurons. Therefore there appears to be little, if any, evidence to suggest that the axons of SDH neurons are not sufficient for production of the sensation of pain.

The results of this study indicate that the optimum strategy for cutting the lumbar component of the STT is dependent on the segmental level of the cord in which the cordotomy is performed. In midthoracic segments, lumbar STT axons are located in a relatively widespread distribution, extending dorsally into the DLF and medially to an area near the gray matter. Thus our results suggest that lesions intended to cut the entire lumbar STT at this level would have to extend a considerable distance medially and well into the DLF, into an area very near the corticospinal and rubrospinal tracts. Therefore in midthoracic segments, lesions large enough to cut the entire STT would be likely to produce ipsilateral motor impairments. However, our results also indicate that smaller, more circumscribed lesions would be sufficient to cut the lumbar STT if performed within the cervical enlargement. At this level, lumbar STT axons appear to be confined to an area closer to the lateral edge of the cord and, perhaps more importantly, restricted entirely to the VLF. Thus the present findings suggest that surgical lesions of the entire lumbar STT at the level of the cervical enlargement could be smaller and that they need not extend into the proximity of the corticospinal and rubrospinal tracts. Therefore the altered organization of ascending STT axons suggests that lesions of the lumbar STT in the cervical enlargement offer important advantages over lesions at lower levels of the cord.


    ACKNOWLEDGMENTS

We thank H. Truong for valuable technical assistance and Dr. Donald Simone for critically reading an early version of this manuscript.

This work was supported by National Institutes of Health Grants NS-25932 to G. J. Giesler, DA-09641 to C. N. Honda, and Training Grant DE-07288 to H. N. Wenk.


    FOOTNOTES

Address for reprint requests: G. Giesler, Dept. of Neuroscience, 6-145 Jackson Hall, University of Minnesota, Minneapolis, MN 55455.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 21 December 1999; accepted in final form 7 February 2000.


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