Primate spinothalamic pathways: II. The cells of origin of the dorsolateral and ventral spinothalamic pathways

The functional and anatomical characterization of three spinal output pathways of the anterolateral tract

The nature of spinal output pathways that convey nociceptive information to the brain has been the subject of controversy. Here, we provide anatomical, molecular, and functional characterizations of two distinct anterolateral pathways: one, ascending in the lateral spinal cord, triggers nociceptive behaviors, and the other one, ascending in the ventral spinal cord, when inhibited, leads to sensorimotor deficits. Moreover, the lateral pathway consists of at least two subtypes. The first is a contralateral pathway that extends to the periaqueductal gray (PAG) and thalamus; the second is a bilateral pathway that projects to the bilateral parabrachial nucleus (PBN). Finally, we present evidence showing that activation of the contralateral pathway is sufficient for defensive behaviors such as running and freezing, whereas the bilateral pathway is sufficient for attending behaviors such as licking and guarding. This work offers insight into the complex organizational logic of the anterolateral system in the mouse.

Homologies of spinal ascending nociceptive pathways between rats and macaques: can we transpose to human? A review and analysis of the literature

PURPOSE

Due to the difficulty of using neural tracers in humans, knowledge of the nociceptive system's anatomy is mainly derived from studies in animals and mainly in rats. The aim of this study was to investigate the morphological differences of the ascending spinal nociceptive pathways between the rat and the macaque monkey; in order to evaluate the variability of this anatomy during phylogenesis, and thus to know if the anatomical description of these pathways can be transposed from the rat to the human.

METHODS

A review and analysis of the literature were performed. The criteria used for comparison were: origins, pathways, their terminations in target structures, and projections from target structures of ascending spinal nociceptive pathways. The monkey was used as an intermediate species for comparison because of the lack of data in humans. The hypothesis of transposition of anatomy between rat and human was considered rejected if differences were found between rat and monkey.

RESULTS

An anatomical difference in termination was found for the spino-annular or spino-periaqueductal grey (spino-PAG) pathway and transposition of its anatomy from rat to human was rejected. No difference was found in other pathways and the transposition of their anatomy from rat to human was therefore, not rejected.

CONCLUSION

This work highlights the conservation of most of the ascending spinal nociceptive pathways' anatomy between rat and monkey. Thus, the possibility for a transposition of their anatomy between rat and human is not rejected.

Thermosensory thalamus: parallel processing across model organisms

The thalamus acts as an interface between the periphery and the cortex, with nearly every sensory modality processing information in the thalamocortical circuit. Despite well-established thalamic nuclei for visual, auditory, and tactile modalities, the key thalamic nuclei responsible for innocuous thermosensation remains under debate. Thermosensory information is first transduced by thermoreceptors located in the skin and then processed in the spinal cord. Temperature information is then transmitted to the brain through multiple spinal projection pathways including the spinothalamic tract and the spinoparabrachial tract. While there are fundamental studies of thermal transduction via thermosensitive channels in primary sensory afferents, thermal representation in the spinal projection neurons, and encoding of temperature in the primary cortical targets, comparatively little is known about the intermediate stage of processing in the thalamus. Multiple thalamic nuclei have been implicated in thermal encoding, each with a corresponding cortical target, but without a consensus on the role of each pathway. Here, we review a combination of anatomy, physiology, and behavioral studies across multiple animal models to characterize the thalamic representation of temperature in two proposed thermosensory information streams.

Somatotopy and Organization of Spinothalamic Tracts in the Human Cervical Spinal Cord

BACKGROUND

Understanding spinothalamic tract anatomy may improve lesioning and outcomes in patients undergoing percutaneous cordotomy.

OBJECTIVE

To investigate somatotopy and anatomical organization of spinothalamic tracts in the human cervical spinal cord.

METHODS

Patients with intractable cancer pain undergoing cordotomy underwent preoperative and postoperative quantitative sensory testing for sharp pain and heat pain on day 1 and 7 after cordotomy. Intraoperative sensory stimulation was performed with computed tomography (CT) imaging to confirm the location of the radiofrequency electrode during cordotomy. Postoperative magnetic resonance (MR) imaging was performed to define the location of the lesion.

RESULTS

Twelve patients were studied, and intraoperative sensory stimulation combined with CT imaging revealed a somatotopy where fibers from the legs were posterolateral to fibers from the hand. Sharpness detection thresholds were significantly elevated in the area of maximum pain on postoperative day 1 (P = .01). Heat pain thresholds for all areas were not elevated significantly on postoperative day 1, or postoperative day 7. MR imaging confirmed that the cordotomy lesion was in the anterolateral quadrant, and in this location the lesion had a sustained effect on sharp pain but a transient impact on heat pain.

CONCLUSION

In the high cervical spinal cord, spinothalamic fibers mediating sharp pain for the arms are located ventromedial to fibers for the legs, and these fibers are spatially distinct from fibers that mediate heat pain.

Structural mapping with fiber tractography of the human cuneate fasciculus at microscopic resolution in cervical region

Human spinal white matter tract anatomy has been mapped using post mortem histological information with the help of molecular tracing studies in animal models. This study used 7 Tesla diffusion MR tractography on a human cadaver that was harvested 24 hours post mortem to evaluate cuneate fasciculus anatomy in cervical spinal cord. Based on this method, for the first time much more nuanced tractographic anatomy was used to investigate possible new routes for cuneate fasciculus in the posterior and lateral funiculus. Additionally, current molecular tracing studies were reviewed, and confirmatory data was presented along with our radiological results. Both studies confirm that upon entry to the spinal cord, upper cervical level tracts (C1-2-3) travel inside lateral funiculus and lower level tracts travel medially inside the posterior funiculus after entry at posterolateral sulcus which is different than traditional knowledge of having cuneate fasciculus tracts concentrated in the lateral part of posterior funiculus.

Post-mortem 11.7 Tesla Magnetic Resonance Imaging vs. Polarized Light Imaging Microscopy to Measure the Angle and Orientation of Dorsal Root Afferents in the Human Cervical Dorsal Root Entry Zone

Background: Destruction of the afferents by dorsal root entry zone (DREZ) surgery may be an effective treatment of intractable neuropathic pain, though it remains a high-risk surgical intervention. Potential complications due to the lesioning of structures within the cervical spinal cord other than the DREZ can be minimized by accurate knowledge of the optimal insertion angle [i.e., the angle between the DREZ and the posterior median sulcus (PMS)]. The employed insertion angle was based on measurements between the DREZ and the PMS on post-mortem transverse slices. However, new, more sophisticated imaging techniques are currently available and are thought to yield higher spatial resolution and more accurate images.

Obejctive: This article measures the angle between the DREZ and the PMS on 11.7T post-mortem magnetic resonance images and compares these findings with polarized light imaging (PLI) microscopy images of the same specimens in order to quantify fiber orientation within the DREZ.

Methods: To visualize the anatomy of the cervical DREZ, magnetic resonance imaging (MRI), diffusion-weighted MRI (dMRI), probabilistic tractography, and PLI were performed on three post-mortem human cervical spinal cords at level C5–C6. The MR data was used to measure the angle between the DREZ and the PMS. MR images were complemented by probabilistic tractography results. Then, the orientation of fibers within the DREZ was quantified by use of PLI microscopy.

Results: Median angle between the DREZ and the PMS, as measured on MR-images, was found to be 40.1° (ranging from 34.2° to 49.1°) and 39.8° (ranging from 31.1° to 47.8°) in the left and right hemicord, respectively. Median fiber orientation within the DREZ, as quantified by PLI, was 28.5° (ranging from 12.0° to 44.3°) and 27.7° (ranging from 8.5° to 38.1°) in the left and right hemicord, respectively.

Conclusion: Our study, which provides an improved understanding of the anatomy of the DREZ, the angle between the DREZ and the PMS and the median fiber orientation within the DREZ, could contribute to safer DREZ-lesioning surgery to treat chronic neuropathic pain in the future.

Posterior parietal cortex contains a command apparatus for hand movements

Mountcastle and colleagues proposed that the posterior parietal cortex contains a "command apparatus" for the operation of the hand in immediate extrapersonal space [Mountcastle et al. (1975) J Neurophysiol 38(4):871-908]. Here we provide three lines of converging evidence that a lateral region within area 5 has corticospinal neurons that are directly linked to the control of hand movements. First, electrical stimulation in a lateral region of area 5 evokes finger and wrist movements. Second, corticospinal neurons in the same region of area 5 terminate at spinal locations that contain last-order interneurons that innervate hand motoneurons. Third, this lateral region of area 5 contains many neurons that make disynaptic connections with hand motoneurons. The disynaptic input to motoneurons from this portion of area 5 is as direct and prominent as that from any of the premotor areas in the frontal lobe. Thus, our results establish that a region within area 5 contains a motor area with corticospinal neurons that could function as a command apparatus for operation of the hand.

Hoxb8 intersection defines a role for Lmx1b in excitatory dorsal horn neuron development, spinofugal connectivity, and nociception

Spinal cord neurons respond to peripheral noxious stimuli and relay this information to higher brain centers, but the molecules controlling the assembly of such pathways are poorly known. In this study, we use the intersection of Lmx1b and Hoxb8::Cre expression in the spinal cord to genetically define nociceptive circuits. Specifically, we show that Lmx1b, previously shown to be expressed in glutamatergic dorsal horn neurons and critical for dorsal horn development, is expressed in nociceptive dorsal horn neurons and that its deletion results in the specific loss of excitatory dorsal horn neurons by apoptosis, without any effect on inhibitory neuron numbers. To assess the behavioral consequences of Lmx1b deletion in the spinal cord, we used the brain-sparing driver Hoxb8::Cre. We show that such a deletion of Lmxb1 leads to a robust reduction in sensitivity to mechanical and thermal noxious stimulation. Furthermore, such conditional mutant mice show a loss of a subpopulation of glutamatergic dorsal horn neurons, abnormal sensory afferent innervations, and reduced spinofugal innervation of the parabrachial nucleus and the periaqueductal gray, important nociceptive structures. Together, our results demonstrate an important role for the intersection of Lmx1b and Hoxb8::cre expression in the development of nociceptive dorsal horn circuits critical for mechanical and thermal pain processing.

Role of primary somatosensory cortex in the coding of pain

The intensity and submodality of pain are widely attributed to stimulus encoding by peripheral and subcortical spinal/trigeminal portions of the somatosensory nervous system. Consistent with this interpretation are studies of surgically anesthetized animals, demonstrating that relationships between nociceptive stimulation and activation of neurons are similar at subcortical levels of somatosensory projection and within the primary somatosensory cortex (in cytoarchitectural areas 3b and 1 of somatosensory cortex, SI). Such findings have led to characterizations of SI as a network that preserves, rather than transforms, the excitatory drive it receives from subcortical levels. Inconsistent with this perspective are images and neurophysiological recordings of SI neurons in lightly anesthetized primates. These studies demonstrate that an extreme anterior position within SI (area 3a) receives input originating predominantly from unmyelinated nociceptors, distinguishing it from posterior SI (areas 3b and 1), long recognized as receiving input predominantly from myelinated afferents, including nociceptors. Of particular importance, interactions between these subregions during maintained nociceptive stimulation are accompanied by an altered SI response to myelinated and unmyelinated nociceptors. A revised view of pain coding within SI cortex is discussed, and potentially significant clinical implications are emphasized.

The location of the major ascending and descending spinal cord tracts in all spinal cord segments in the mouse: actual and extrapolated

Information on the location of the major spinal cord tracts in the mouse is sparse. We have collected published data on the position of these tracts in the mouse and have used data from other mammals to identify the most likely position of tracts for which there is no mouse data. We have plotted the position of six descending tracts (corticospinal, rubrospinal, medial and lateral vestibulospinal, rostral and caudal reticulospinal) and eight ascending tracts (gracile; cuneate; postsynaptic dorsal columns; dorsolateral, lateral, and anterior spinothalamic; dorsal and ventral spinocerebellar) on diagrams of transverse sections of all mouse spinal cord segments from the first cervical to the third coccygeal segment.

The thalamostriatal systems: anatomical and functional organization in normal and parkinsonian states

Although we have gained significant knowledge in the anatomy and microcircuitry of the thalamostriatal system over the last decades, the exact function(s) of these complex networks remain(s) poorly understood. It is now clear that the thalamostriatal system is not a unique entity, but consists of multiple neural systems that originate from a wide variety of thalamic nuclei and terminate in functionally segregated striatal territories. The primary source of thalamostriatal projections is the caudal intralaminar nuclear group which, in primates, comprises the centromedian and parafascicular nuclei (CM/Pf). These two nuclei provide massive, functionally organized glutamatergic inputs to the whole striatal complex. There are several anatomical and physiological features that distinguish this system from other thalamostriatal projections. Although all glutamatergic thalamostriatal neurons express vGluT2 and release glutamate as neurotransmitter, CM/Pf neurons target preferentially the dendritic shafts of striatal projection neurons, whereas all other thalamic inputs are almost exclusively confined to the head of dendritic spines. This anatomic arrangement suggests that transmission of input from sources other than CM/Pf to the striatal neurons is likely regulated by dopaminergic afferents in the same manner as cortical inputs, while the CM/Pf axo-dendritic synapses do not display any particular relationships with dopaminergic terminals. A better understanding of the role of these systems in the functional circuitry of the basal ganglia relies on future research of the physiology and pathophysiology of these networks in normal and pathological basal ganglia conditions. Although much remains to be known about the role of these systems, recent electrophysiological studies from awake monkeys have provided convincing evidence that the CM/Pf-striatal system is the entrance for attention-related stimuli to the basal ganglia circuits. However, the processing and transmission of this information likely involves intrinsic GABAergic and cholinergic striatal networks, thereby setting the stage for complex physiological responses of striatal output neurons to CM/Pf activation. Finally, another exciting development that will surely generate significant interest towards the thalamostriatal systems in years to come is the possibility that CM/Pf may be a potential surgical target for movement disorders, most particularly Tourette syndrome and Parkinson's disease. Although the available clinical evidence is encouraging, these procedures remain empirical at this stage because of the limited understanding of the thalamostriatal systems.

Processing noxious information at the subnucleus reticularis dorsalis (SRD) of anesthetized cats: wind-up mechanisms

With the exception of one monkey's study, where wind-up was not reported, electrophysiological data from SRD neurons were obtained in rodents where they show wind-up. This work was designed to examine the response properties of SRD neurons in anesthetized cats to study how general the data from rats may be. Since cat's SRD cells showed wind-up, its underlying mechanisms were approached, an issue not previously addressed at supraspinal level. Electrical stimulation, extracellular (combined with microiontophoresis) and intracellular techniques revealed that A delta information reaches the SRD via the ventrolateral cord, whereas C information preferentially follows a dorsal route. Wind-up was usually generated by spinal and peripheral stimulation, but it was also evoked either by stimulating the nucleus reticularis gigantocellularis (NRGc), even after spinal cord section and bilateral full thickness removal of the cerebral cortex, or by applying microiontophoretic pulses of l-glutamate at 0.3-1 Hz. Wind-up relied on afferent repetitive activity gradually depolarizing the SRD neurons leading 3-4.5 Hz subthreshold membrane rhythmic activity to threshold. Riluzole retarded wind-up generation and decreased the number of spikes per stimulus during wind-up. GABA or glycine abolished spontaneous and sensory-evoked activity and bicuculline, but not strychnine, increased spontaneous and stimulus-evoked activity. These results demonstrate that wind-up at the SRD is not merely the reflection of spinal wind-up, but (i) can be locally generated, (ii) is partially dependent upon persistent sodium currents, and (iii) is under the modulation of a tonic GABAa-dependent inhibition decreasing SRD excitability. Injury and/or inflammation producing tonic C-fiber activation will surpass tonic inhibition generating wind-up.

Multiple pathways for noxious information in the human spinal cord

To investigate the pathways of noxious information in the spinal cord in humans, we recorded cortical potentials following the stimulation of A-delta fibers using a YAG laser applied to two cutaneous points on the back at the C7 and Th10 level, 4cm to the right of the vertebral spinous process. A multiple source analysis showed that four sources were activated; the primary somatosensory cortex (SI), bilateral parasylvian region (Parasylvian), and cingulate cortex. The activity of the cingulate cortex had two components (N2/P2). The mean peak latencies of the activities obtained by C7 and Th10 stimulation were 166.9 and 186.0 ms (SI), 144.3 and 176.8 ms (contralateral Parasylvian), 152.7 and 185.5 ms (ipsilateral Parasylvian), 186.2 and 215.8 ms (N2), and 303.0 and 332.3 ms (P2). Estimated spinal conduction velocities (CVs) of the respective activities were 16.8, 9.3, 8.7, 10.1 and 10.7 m/s. CV of SI was significantly faster than the others (P<0.05). Therefore, our results suggested that noxious signals were conveyed through at least two distinct pathways of the spinal cord probably reaching distinct groups of thalamic nuclei. Further studies are required to clarify the functional significance of these two pathways.

Psychophysics of CNS Pain-Related Activity: Binary and Analog Channels and Memory Encoding

The forebrain neuronal system signaling pain has been poorly characterized. The pain pathway afferent to the thalamus may be a labeled line consisting of neurons in the pain-signaling pathway to the brain (spinothalamic tract, STT) that respond only to painful stimuli. It has recently been proposed that the STT contains a series of analog-labeled lines, each signaling a different aspect of the internal state of the body (interoception), for example, visceral/cold/itch sensations. In this view, pain is the unpleasant emotion produced by disequilibrium of the internal state. The authors now show that stimulation of an STT receiving zone (thalamic principal somatic sensory nucleus, ventral caudal) in awake humans produces two different exteroceptive responses. The first is a binary response signaling the presence of painful stimuli. The second is an analog response in which nonpainful and painful sensations are graded with intensity of the stimulus. Such stimulation can evoke both the sensory and emotional components of previously experienced pain. These results illustrate the diverse functions of human pain signaling pathways.

Pain and temperature encoding in the human thalamic somatic sensory nucleus (Ventral caudal): inhibition-related bursting evoked by somatic stimuli

Stimulus-evoked inhibitory events have not been demonstrated in thalamic spike trains encoding of pain and temperature stimuli. We have now tested the hypothesis that the human thalamic response to mechanical and thermal stimuli is characterized by low-threshold calcium spike (LTS)-associated bursts of high-frequency action potentials preceded by prolonged inhibition. The results included 57 neurons recorded in the human thalamic principal somatic sensory nucleus (ventral caudal, Vc) of 24 patients during awake surgery. Neurons were classified by the grading of their response with stimulus intensity into the painful range (graded or non-graded) and the stimulus response (to mechanical, cold, or heat stimuli). Firing rates were analyzed by the response to all stimuli combined (stimuli overall) and to the stimulus characteristic of the stimulus response type (optimal stimulus), e.g., cold stimuli for neurons of the cold stimulus response type. All neuronal categories had clear stimulus-evoked LTS bursting as identified by the criteria for selecting bursts in the spike train, by significant preburst inhibition, and by preburst inter-spike interval not significantly <100 ms. Stimulus-evoked LTS burst rates were significantly higher for neurons in the cold stimulus response type independent of the firing rate between bursts. The parameters of preburst inhibition were largely independent of the neuronal category and the stimuli included in the analysis, which suggests inhibitory mechanisms are similar across neuronal types. Therefore LTS bursting is a substantial, nonlinear component of the spontaneous and stimulus-evoked activity of thalamic neurons in awake humans.

The functional anatomy of neuropathic pain

The generation of neuropathic pain is a complex phenomenon involving a process of peripheral and central sensitization producing enhanced transmission of nociceptive inputs to the brain associated with the loss of discriminatory processing of noxious and innocuous stimuli. This increased flow of abnormally processed nociceptive inputs to the brain may overcome the ability of descending modulatory pathways to produce analgesia, causing further worsening of the pain. Several crucial locations involved in the physiologic generation of pain inputs (eg, peripheral nociceptors, dorsal horns, thalamus, cortex) show evidence of functional reorganization and altered nociceptive processing in association with chronic pain. These locations present the best targets for therapeutic intervention, including systemic administration of drugs able to counteract the chemical storm induced by neural injuries in the nociceptive afferents and dorsal horns, or for more focused intervention, such as neuroablative procedures; intrathecal drug delivery; and spinal cord, deep brain, or motor cortex stimulation.

Brain processing of esophageal sensation in health and disease

This case study demonstrates that patients with NCCP can be subclassified on the basis of sensory responsiveness and neurophysiologic profiles. This approach identifies specific abnormalities within the CNS processing of esophageal sensation in individual patients, allowing us to objectively differentiate those with sensitized esophageal afferents from those that are hypervigilant to esophageal sensations. The importance of this approach is to underline that NCCP comprises a heterogeneous group of patients. and only when we have defined the phenotype of this condition and identified groups of patients with specific CNS abnormalities will it be possible to perform clinical studies aimed at answering specific hypotheses. The development of a comprehensive pathophysiologic model that identifies the specific causes of symptoms in patients with esophageal hypersensitivity will allow the future management strategies of these patients to be targeted more specifically and efficiently. This will have great benefits to patients'well-being and health care use.

Widespread thalamic terminations of fibers arising in the superficial medullary dorsal horn of monkeys and their relation to calbindin immunoreactivity

The relay of pain fibers from the spinal and medullary dorsal horn in the thalamus has become a controversial issue. This study analyzed the relationship of fibers arising in lamina I to nuclei in and around the caudal pole of the ventral posterior nuclear complex and especially to a zone of calbindin-dense immunoreactivity (VMpo) identified by some authors as the sole thalamic relay for these fibers. We show that the densest zone of calbindin immunoreactivity is part of a more extensive, calbindin-immunoreactive region that lies well within the medial tip of the ventral posterior medial nucleus (VPM), as delineated by other staining methods, and prove that the use of different anti-calbindin antibodies cannot account for differences in interpretations of the organization of the posterior thalamic region. By combining immunocytochemical staining with anterograde tracing from injections involving lamina I, we demonstrate widespread fiber terminations that are not restricted to the calbindin-rich medial tip of VPM and show that the lamina I arising fibers are not themselves calbindin immunoreactive. This study disproves the existence of VMpo as an independent thalamic pain nucleus or as a specific relay in the ascending pain system.

[Pain information pathways from the periphery to the cerebral cortex]

A recent PET study revealed that the first and second somatosensory cortices (SI, SII), and the anterior cingulate cortex are activated by painful peripheral stimulation in humans. It has become clear that painful signals (nociceptive information) evoked at the periphery are transmitted via various circuits to the multiple cerebral cortices where pain signals are processed and perceived. Human or clinical pain is not merely a modality of somatic sensation, but associated with the affect that accompanies sensation. Consequently, pain has a somatosensory-discriminative aspect and an affective-cognitive aspect that are processed in different but correlated brain structures in the ascending circuits. Considering the physiologic characteristics and fiber connections, the SI and SII cortices appear to be involved in somatosensory-discriminative pain, and the anterior cingulate cortex (area 24) in the affective-cognitive aspect of pain. This paper deals with the ascending pain pathways from the periphery to these cortices and their interconnections. Our recent findings on the protease-activated receptors 1 and 2 (PAR-1, and -2), which are confirmed to exist in the dorsal root ganglion cells, are also described. Activation of PAR-2 during inflammation or tissue injury at the periphery is pronociceptive, while PAR-1 appears to be antinociceptive. Based on the these findings, PAR-1 and PAR-2 are attracting interest as target molecules for new drug development.

Cardiac nociception in rats: neuronal pathways and the influence of dermal neurostimulation on conveyance to the central nervous system

Neurostimulation for refractory angina pectoris is often advocated for its clinical efficacy. However, the recruited pathways to induce electroanalgesia are partially unknown. Therefore, we sought to study the effect of neurostimulation on experimentally induced cardiac nociception, using capsaicin as nociception-induced substance. Four different groups of male Wistar rats were pericardially infused with either saline or capsaicin with or without neurostimulation. Group StimCap was infused with capsaicin, and group StimVeh was infused with saline. Both groups were treated with neurostimulation. Group ShamCap was only infused with capsaicin without stimulation, whereas group ShamVeh was only infused with saline. Neuronal activation differences were assessed with cytochemical staining, revealing the cellular expression of c-fos. Pain behavior was registered on video and was quantitatively analyzed. In the StimCap and ShamCap groups, all animals exerted typical pain behavior, whereas in the StimVeh group only moderate changes in behavior were observed. Group ShamVeh animals were unaffected by the procedure. The upper thoracic spinal cord showed high numbers of c-fos-positive cells, predominantly in laminae III and IV in both StimCap and StimVeh groups. Almost no c-fos expression was noticed in groups ShamCap and ShamVeh in these sections of the spinal cord. In groups StimCap and ShamCap a significantly higher number of c-fos-positive cells in comparison with groups StimVeh and ShamVeh were noticed in the periambigus region, the nucleus tractus solitarius, and the paraventricular hypothalamus. In the paraventricular thalamus, periaqueductal gray, and central amygdala, no significant differences were noticed among the first three groups, and the c-fos concentration in these three groups was significantly higher than in group ShamVeh. It is concluded that neurostimulation does not influence capsaicin-induced cardiac nociceptive pain pulses to the central nervous system. Furthermore, capsaicin-induced cardiac pain and neurostimulation may utilize two different pathways.

Pain mechanisms and their disorders

The purpose of this article is to summarise how functional imaging techniques have changed our understanding of normal and abnormal pain mechanisms, how they inform a change in clinical practice and to speculate on possible future clinical uses.

Association of spinothalamic lamina I neurons and their ascending axons with calbindin-immunoreactivity in monkey and human

The calbindin-immunoreactivity of spinothalamic (STT) lamina I neurons and their ascending axons was examined in two experiments. In the first experiment, lamina I STT neurons in macaque monkeys were double-labeled for calbindin and for retrogradely transported WGA*HRP following large (n=2) or small (n=1) injections that included the posterior thalamus. Most, but not all (78%) of the contralateral retrogradely labeled lamina I STT cells were positive for calbindin. Calbindin-immunoreactivity was not selectively associated with any particular anatomical type of lamina I STT cell; 82% of the fusiform cells, 78% of the pyramidal cells and 67% of the multipolar cells were double-labeled. In the second experiment, oblique transverse sections from upper cervical spinal segments of three macaque monkeys, one squirrel monkey and five humans were stained for calbindin-immunoreactivity. In each case, a distinct bundle of fibers was densely stained in the middle of the lateral funiculus. This matches the location of anterogradely labeled ascending lamina I axons observed in prior work in cats and monkeys, and it matches the location of the classically described 'lateral spinothalamic tract' in humans. This bundle had variable shape across cases, an observation that might have clinical significance. These findings support the view that lamina I STT neurons are involved in spinal cordotomies that reduce pain, temperature and itch sensations.

Functional plasticity in primate somatosensory thalamus following chronic lesion of the ventral lateral spinal cord

The long-term consequences of thoracic spinothalamic tract lesion on the physiological properties of neurons in the ventral posterior lateral nucleus of the thalamus in monkeys were assessed. Neurons responding to both compressive and phasic brush stimuli (multireceptive neurons), but not brush-specific (low-threshold) neurons, in the partially deafferented thalamus showed increased spontaneous activity, increased responses evoked by cutaneous stimuli and larger mean receptive field size than the same types of cells in the thalamus with intact innervation. The spike train properties of both the spontaneous and evoked discharges of cells were also altered so that there was an increased incidence of spike-bursts in cells of deafferented thalamus. These changes were widespread in the thalamus, and included cells in both the fully innervated forelimb representation and the partially denervated hindlimb representation ipsilateral to the lesion. The spontaneous and evoked spike trains in the ipsilateral thalamus also show increased frequency of both spike-burst and non-burst events compared to the intact thalamus. These results indicate that chronic spinothalamic tract lesion produces widespread changes in the physiological properties of a discrete cell population of the thalamus.The findings in this study indicate that the thalamic processing of somatosensory information conveyed by the lemniscal system is altered by transection of the spinothalamic tract. This change in sensory processing in the thalamus would result in altered cortical processing of innocuous somatosensory inputs following deafferentation and so possibly contribute to the generation of the central pain syndrome.

Neurophysiology and functional neuroanatomy of pain perception

The traditional view that the cerebral cortex is not involved in pain processing has been abandoned during the past decades based on anatomic and physiologic investigations in animals, and lesion, functional neuroimaging, and neurophysiologic studies in humans. These studies have revealed an extensive central network associated with nociception that consistently includes the thalamus, the primary (SI) and secondary (SII) somatosensory cortices, the insula, and the anterior cingulate cortex (ACC). Anatomic and electrophysiologic data show that these cortical regions receive direct nociceptive thalamic input. From the results of human studies there is growing evidence that these different cortical structures contribute to different dimensions of pain experience. The SI cortex appears to be mainly involved in sensory-discriminative aspects of pain. The SII cortex seems to have an important role in recognition, learning, and memory of painful events. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in affective aspects of pain-related learning and memory. The ACC is closely related to pain unpleasantness and may subserve the integration of general affect, cognition, and response selection. The authors review the evidence on which the proposed relationship between cortical areas, pain-related neural activations, and components of pain perception is based.

Position of spinothalamic tract axons in upper cervical spinal cord of monkeys

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 microA) 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.

Effect of midthoracic spinal cord constriction on catalytic nitric oxide synthase activity in the white matter columns of rabbit

The distribution and changes of catalytic nitric oxid synthase (cNOS) activity in the dorsal, lateral and ventral white matter columns at midthoracic level of the rabbit's spinal cord were studied in a model of surgically-induced spinal cord constriction performed at Th7 segment level and compared with the occurrence of nicotinamide adenine dinucleotide phosphate diaphorase expressing and neuronal nitric oxide synthase immunoreactive axons in the white matter of the control thoracic segments. Segmental and white-column dependent differences of cNOS activity were found in the dorsal (141.5 +/- 4.2 dpm/microm protein), lateral (87.3 +/- 11.5 dpm/microm protein) and ventral (117.1 +/- 7.6 dpm/microm protein) white matter columns in the Th5-Th6 segments and in the dorsal (103.3 +/- 15.5 dpm/microm protein), lateral (54.9 +/- 4.9 dpm/microm protein), and ventral (86.1 +/- 6.8 dpm/microm protein) white matter columns in the Th8-Th9 segments. A surgically-induced constriction of Th7 segment caused a disproportionate response of cNOS activity in the rostrally (Th5-Th6) and caudally (Th8-Th9) located segments in both lateral and ventral white matter columns. While a statistically significant decrease of cNOS activity was detected above the constriction site in the ventral columns, a considerable, statistically significant increase of cNOS activity was noted in the white lateral columns below the site of constriction. It is reasoned that the changes of cNOS activity may have adverse effects on nitric oxide (NO) production in the white matter close to the site of constriction injury, thus broadening the scope of the secondary mechanisms that play a role in neuronal trauma.

Locations of spinothalamic tract axons in cervical and thoracic spinal cord white matter in monkeys

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 microA) within the lateral funiculus from T(9) to C(6). 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.

Parallel activation of primary and secondary somatosensory cortices in human pain processing

Cerebral processing of pain has been shown to involve primary (SI) and secondary (SII) somatosensory cortices. However, the temporal activation pattern of these cortices in nociceptive processing has not been demonstrated so far. We therefore used whole-head magnetoencephalography to record cortical responses to cutaneous laser stimuli in six healthy human subjects. By using selective nociceptive stimuli our results confirm involvement of contralateral SI and bilateral SII in human pain processing. Beyond they show for the first time simultaneous activation onset of contralateral SI and SII after approximately 130 ms, indicating parallel thalamocortical distribution of nociceptive information. This contrasts to the serial cortical organization of tactile processing in higher primates and instead corresponds to the parallel cortical organization in lower primates and nonprimates. Thus our finding suggests preservation of the basic mammalian parallel organizational scheme in human pain processing, whereas in the tactile modality parallel organization appears to be abandoned in favor of a serial processing scheme. Functionally, preservation of direct access to SII underscores the relevance of this area in human pain processing, probably reflecting an important role of SII in nociceptive learning and memory.

Mechanisms of cardiac pain

Angina pectoris often results from ischemic episodes that excite chemosensitive and mechanoreceptive receptors in the heart. Ischemic episodes release a collage of chemicals, including adenosine and bradykinin, that excites the receptors of the sympathetic and vagal afferent pathways. Sympathetic afferent fibers from the heart enter the upper thoracic spinal cord and synapse on cells of origin of ascending pathways. This review focuses on the spinothalamic tract, but other pathways are excited as well. Excitation of spinothalamic tract cells in the upper thoracic and lower cervical segments, except C7 and C8 segments, contributes to the anginal pain experienced in the chest and arm. Cardiac vagal afferent fibers synapse in the nucleus tractus solitarius of the medulla and then descend to excite upper cervical spinothalamic tract cells. This innervation contributes to the anginal pain experienced in the neck and jaw. The spinothalamic tract projects to the medial and lateral thalamus and, based on positron emission tomography studies, activates several cortical areas, including the anterior cingulate gyrus (BA 24 and 25), the lateral basal frontal cortex, and the mesiofrontal cortex.

The contribution of functional imaging techniques to our understanding of rheumatic pain

The main cerebral components of the human pain matrix have been defined using functional imaging techniques. The experience of pain is likely to be elaborated as a result of parallel processing within this matrix. There is not, therefore, a single pain center. The determinants of pain are as likely to be determined by top-down as by bottom-up processes. The precise function of the different components of the matrix are just beginning to be defined. There appear to be important adaptive responses in the forebrain components of the matrix during arthritic pain. Endogenous opioid peptides are strong candidates for the modulation of some of these responses. More extensive and sequential behavioral and functional imaging studies are required to establish the contribution these adaptive responses make to the perception of pain.

Primary and secondary somatosensory projections in direct-developing plethodontid salamanders

In three species of plethodontid salamanders (Plethodon jordani, Hydromantes italicus, and Bolitoglossa subpalmata), primary and secondary somatosensory pathways were investigated by means of tract-tracing in vivo and in vitro using biocytin, horseradish peroxidase, and neurobiotin. Afferent sensory fibers of cranial nerves V, VII, and X and the brachial nerve run in the dorsal funiculus of the medulla oblongata and spinal cord. Fibers ascend to the level of, but do not enter, the cerebellum. In the caudal medulla oblongata, sensory tracts of the cranial nerves descend in a dorsal and a dorsolateral bundle and reach the level of the fourth spinal nerve. Two bundles are likewise formed by spinal afferent fibers, which descend to the level of the seventh spinal nerve. Secondary somatosensory projections ascend in contralateral ventral, contralateral lateral, and ipsilateral lateral tracts, the latter two corresponding to the spinal lemniscal tracts of Herrick. These tracts reach the cerebellum, mesencephalic, and diencephalic targets (tegmentum, torus, tectum, tuberculum posterius, pretectum, and ventral thalamus) ipsi- and contra-laterally. The projection to the tectum is confined to fiber layer 4. Fibers of the ascending tracts cross in the cerebellar and tectal commissure. Our study demonstrates that the ascending secondary somatosensory pathways of plethodontid salamanders differ remarkably from those of other amphibians.

Three times as many lamina I neurons project to the periaqueductal gray than to the thalamus: a retrograde tracing study in the cat

The number and distribution of lamina I neurons projecting to the periaqueductal gray (PAG) were examined by a retrograde tracing study in the cat. WGA-HRP injections in the intermediate and caudal PAG resulted in as much as 1600 labeled lamina I neurons throughout the length of the spinal cord, counted in a 1:4 series of sections. The lamina I-PAG projection was predominantly contralateral and most labeled lamina I neurons were found in the enlargements. Comparing these results with the number of lamina I-thalamic neurons leads to the conclusion that in the cat about three times as many lamina I neurons project to the PAG than to the thalamus. Considering this, one can conclude that the spino-PAG system is a virtually neglected area in pain research.

Neurons in the human thalamic somatosensory nucleus (Ventralis caudalis) respond to innocuous cool and mechanical stimuli

A population of neurons in the principal somatosensory nucleus of human thalamus (ventralis caudalis, Vc) had a significant tonic, graded response to cool stimuli and also responded to innocuous mechanical stimuli (mechanical-cool neurons). These neurons were clustered at the dorsal aspect of medial Vc. Stimulation at the sites where these neurons were recorded evoked tingling sensations in the part of the body including or adjacent to the receptive field of these neurons. These neurons may contribute to the mechanism that mediates the perception of cold in man.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Cerebral mechanisms operating in the presence and absence of inflammatory pain

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Evidence for segregated pain and temperature conduction within the spinothalamic tract

The lateral spinothalamic tract, located in the anterolateral quadrant of the white matter of the spinal cord, is one of the most important structures in transmitting pain within the central nervous system. It has been known for almost a century that destruction of fibers in this tract results in analgesia contralateral to the lesion. The effectiveness and clinical importance of interruption of the lateral spinothalamic tract has been proven in many studies. Today cordotomies are still a useful neurosurgical treatment modality, especially when pain can no longer be sufficiently controlled by analgesic drugs. Although analgesia on the contralateral side is the desired effect, one must also expect to cause disturbance in temperature sensation when performing a cordotomy. The authors' observations showed that after a cordotomy the dermatome level of analgesia can be variable within certain limits, which is in accordance with the literature. Surprisingly, however, the loss of temperature sensation may differ significantly from the loss of pain sensation. It was also found to be possible to perform a successful cordotomy without altering the sensation of temperature at all. This indicates that pain and temperature sensations may be conducted via separate pathways. Possible mechanisms underlying this phenomenon are discussed.

A population of cells in the human thalamic principal sensory nucleus respond to painful mechanical stimuli

A population of neurons located in the cutaneous core of the principal sensory nucleus of human thalamus (ventralis caudalis, Vc) has been identified that had their maximal response to mechanical stimuli which were perceived as painful by the patients involved. None of these cells responded to painful thermal stimuli. The graded responses of these cells to mechanical stimuli extending into the painful range suggest they both mediate acute pain in response to mechanical stimuli and participate in mechanical hyperalgesia.

Percutaneous cervical cordotomy: a review of 181 operations on 146 patients with a study on the location of "pain fibers" in the C-2 spinal cord segment of 29 cases

The authors present a review of 146 patients who underwent 181 percutaneous cervical cordotomies for intractable pain. In addition, an anatomical-clinical correlation was carried out for 29 of these patients. It was found that the fibers subserving pain sensation in the C-2 segment lie in the anterolateral funiculus between the level of the denticulate ligament and a line drawn perpendicularly from the medial angle of the ventral gray-matter horn to the surface of the cord. The best analgesic results have been obtained by creating lesions that extend 5.0 mm deep to the surface of the cord and destroy about 20% of the hemicord. There is a somatotopic organization with sacral fibers running ventromedially and cervical fibers running dorsolaterally. The authors believe that the ascending fibers subserving the distinct sensations of pain induced by tissue damage and pinprick, although mixed (overlapping) in the anterolateral funiculus of the spinal cord, are physiologically distinct from one another. Whereas some cordotomies, both in the current series and as reported in the literature, may affect these functions differentially, optimum pain relief seems to be obtained only when pinprick sensation is also abolished in the affected segments. Evoked pain sensation is not abolished by cordotomy, but its threshold is greatly raised. When pathological pain is completely abolished, so is pinprick sensation. However, in a number of cases where pathological pain was only partially alleviated, pinprick sensation remained intact. The significance of these and other cases reported in the literature is discussed. The importance of clinically distinguishing between pain caused by tissue damage and pinprick sensation is emphasized, as well as that between return of pre-existing or new tissue-damage pain and painful dysesthesia.

Spinothalamocortical projections to the secondary somatosensory cortex (SII) in squirrel monkey

Anterograde labeling of the cervical spinothalamic tract was combined with retrograde labeling of thalamocortical cells projecting to the hand region of the second somatosensory cortex (hSII) to identify likely sites in the thalamus for processing and transmitting nociceptive information to hSII. Anterograde labeling of terminals was done with 2% WGA-HRP injections in the cervical enlargement; thalamocortical cells were retrogradely labeled with fluorescent tracers. In one experiment, the contralateral primary somatosensory cortex hand region (hSI) was injected to provide a direct comparison with hSII thalamic label. Both labeled cells and terminal-like structures were visualized in single thalamic sections and their numbers and positions quantitatively analyzed. The number of labeled cells within 100 microns from the STT terminals were counted as overlapping cells. Four thalamic nuclei, ventroposterior inferior (VPI), ventroposterior lateral (VPL), posterior nucleus (PO) and centrolateral nucleus (CL) combined to contain 86.5% of all hSII-projecting overlapping cells. Of all hSII-projecting thalamic overlapping cells, VPI contained the largest number (36.4% of the total) followed by the anterior portion of the posterior nuclear complex (POa; 20.4%), VPL (18.3%) and CL (11.4%). Results of the hSI injection show a different pattern of overlap in agreement with our earlier study. The relative distribution of overlapping cells was dependent on the antero-posterior position of the SII injections. The most anterior injections resulted in small numbers of labeled cells, with the majority of overlapping cells located in PO and CL. The more posterior injections resulted in overlapping cells mainly in VPI and VPL.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurons in the area of human thalamic nucleus ventralis caudalis respond to painful heat stimuli

A population of neurons in the area of human thalamic nucleus ventralis caudalis (Vc) respond to noxious heat stimuli. In the cutaneous core of Vc 6% (6/108) of recorded neurons had a significantly greater response to noxious heat stimuli than to innocuous control stimuli. Half of these neurons (n = 3) also responded to innocuous cold stimuli. Within the region posterior and inferior to the cutaneous core of Vc 5% (4/77) of neurons responded exclusively to noxious heat stimuli. Cells responding to noxious heat were recorded at a greater proportion (66%) of sites where painful sensations were evoked by microstimulation than at sites where nonpainful sensations were evoked (1.5%). The results suggest that neurons in the region of human Vc mediate the sensory aspect of pain.

Alternative spinal, somatosensory pathways investigated with the tactile orienting reaction in the cat

Previous results indicated the possibility of abolishing the orienting reaction to light tactile stimulation of specific areas below lesions encompassing three sectors of the transverse spinal plane, if all sectors were transected simultaneously. Hence presumably interrupting three different ascending pathways. Two sectors corresponded to the sites of the well known, somatosensory, dorsal column and spino-cervical pathways. Single stage lesion technique now has been used to pinpoint the site of the third pathway. Immediate orienting reactions to both sides were seen before surgery. The orienting reactions remained postoperatively to stimuli applied on the hind limb contralateral to the dorsal column and the spino-cervical lesions. When the hind limb ipsilateral to the dorsal column and the spino-cervical lesions was stimulated five cats showed an absence of orienting reactions. The cats' lesions included the dorsal column and the spino-cervical on one side and the border area between the lateral and ventral funiculi on the other side of the cord. The remaining cats showed either partial or no deficiency of the orienting reactions. These cats' spinal lesions spared the area between the ventral and lateral funiculi. The findings show the possibility of abolishing the tactile orienting reactions from one hind limb with single stage lesions, which include the dorsal column and the spino-cervical pathway on one side, and a pathway located in the border area between the contralateral lateral, and ventral funiculi. This site corresponds to the morphological position of ascending spino-mesencephalic and/or spino-thalamic fibres. Consequently, all of these pathways might provide alternative routes for information about the place of tactile stimuli, which may evoke orienting reactions.

The primate dorsal spinothalamic tract: evidence for a specific termination in the posterior nuclei (Po/SG) of the thalamus

The spinothalamic tract in primates and other mammals arises primarily from cells in lamina I of the dorsal horn, from lamina V cells and to a lesser extent from other laminae. Most of the neurons of lamina I respond only to noxious mechanical or thermal stimuli. Spinothalamic tract (STT) cells of lamina V tend to respond to both innocuous and noxious stimuli. Recent studies have suggested that the classical STT in the anterolateral quadrant (ALQ) contains primarily the axons of lamina V cells and that the axons of lamina I cells travel more dorsally in the dorsolateral quadrant (DLQ) to constitute the dorsal spinothalamic tract (DSTT). Using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) injected into the spinal cord in conjunction with a contralateral anterolateral cordotomy, we have found there is a substantial projection of the DSTT to the posterior nuclei of the caudal-ventral thalamus, designated Po/SG. This projection is almost entirely abolished when the lesion includes the area of spinal cord white matter at the level of the denticulate ligament. Larger lesions that destroy the ALQ and much of the lateral column white matter, but that spare the dorsolateral column white matter in the region of the corticospinal tract, abolish all transport of WGA-HRP to the thalamus. We conclude that the spinothalamic pathway in the non-human primate encompasses a continuous fiber bundle that extends dorsally to include the region of lateral column white matter opposite the denticulate ligament and that the more dorsal aspect of this pathway projects primarily to Po/SG of thalamus.

Spinal distribution of ascending lamina I axons anterogradely labeled with Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat

The location of the ascending axons of spinal lamina I cells was studied in cats that received injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the superficial dorsal horn of the cervical or lumbosacral enlargement. Lamina I axons that could be ascribed to the spinothalamic tract (STT) were of particular interest. The cases were divided into three sets: in seven optimal cases the injections were restricted to lamina I; in ten nominal cases the injections involved laminae I-II or laminae I-III and occasionally lamina IV; and in eight mixed cases laminae I-V were injected. Since ipsilateral propriospinal and bilateral supraspinal axons originate from laminae I and V, but only ipsilateral propriospinal axons from laminae II-IV, this categorization facilitated a comparative analysis. Ascending axons labeled immunohistochemically with avidin/Texas Red were observed in oblique transverse sections from the C1, C3/4, T6, T12, and L3/4 levels. Incidental axonal labeling occurred in the ipsilateral dorsal columns because of passing primary afferent fiber uptake and, in nominal and mixed cases with involvement of laminae III-IV, in the superficial dorsolateral funiculus at the location of the spinocervical tract. Ipsilateral ascending lamina I axons in optimal cases were located in Lissauer's tract and in the white matter adjacent to the dorsal horn. Since these appeared to terminate in lamina I, and few remained at C1, they were ascribed to propriospinal projections. Contralateral ascending lamina I axons in optimal and nominal cases were distributed throughout the dorsal and ventral portions of the lateral funiculus (LF), but, despite considerable variability between animals in their location and dispersion, they were consistently concentrated in the middle of the LF (i.e., at the level of the central canal). This concentration was observed in a slightly more ventral location at C1, and a similar but weaker concentration of lamina I axons was located slightly more dorsally in C1 on the ipsilateral side. These supraspinal lamina I projections were ascribed to the spinomesencephalic tract (SMT) and to the STT. In mixed cases, additional ascending axons ascribed to lamina V cells were labeled in the ventrolateral and ventral funiculi. Many labeled axons were found in this region following a large injection of biocytin into lumbosacral laminae V-VIII in a supplementary case. These results thus together support previous descriptions of a dorsoventral distribution of STT axons according to laminar origin, but they contradict recent reports that lamina I axons ascend in the dorsolateral funiculus.(ABSTRACT TRUNCATED AT 400 WORDS)