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The spinothalamic tract (STT), well known for its role in the relay of information about noxe, temperature, and crude touch, is usually associated with projections from lamina 1, but spinothalamic neurons in other laminae have also been reported. In cat, no complete overview exists of the precise location and number of spinal cells that project to the thalamus In the present study the laminar distribution of retrogradely labeled cells in all spinal segments (C1-Coc2) was investigated after large WGA-HRP injections in the thalamus. The results show that this distribution of STT cells differed greatly between the different spinal segments. Quantitative analysis showed that there exist at least five separate clusters of spinothalamic neurons. Lamina I neurons in cluster A and lamina V neurons in cluster B are mainly found contralaterally throughout the length of the spinal cord. Cluster C neurons are located bilaterally in the ventrolateral part of laminae VI-VII and lamina VIII of the C1-C3 spinal cord. Cluster D neurons were found contralaterally in lamina VI in the C1-C2 segments, and cluster E neurons were located mainly contralaterally in the medial part of laminae VI-VII and lamina VIII of the lumbosacral cord. Most spinothalamic neurons are not located in the enlargements and most spinothalamic neurons are not located in lamina 1, as suggested by several other authors. The location of the spinothalamic neurons shows remarkable similarities, but also differences, with the location of spino-periaqueductal gray neurons.

The present retrograde tracing study in the cat describes the spinal cord projections to the periaqueductal gray (PAC), taking into account different regions of the PAG and all spinal segments. Results show that injecting different parts of the PAC leads to different laminar and segmental distributions of labeled spinal neurons. The impression was gained that at least five separate clusters of spinal neurons exist. Cluster I neurons are found in laminae I and V throughout the length of the cord and are probably involved in relaying nociceptive information to the PAG. Cluster II neurons lie in the ventrolateral part of laminae VI-VII of the C1-C4 spinal cord and were labeled by injecting the ventrolateral or lateral part of the rostrocaudal PAG or the deep tectum. Cluster III neurons are located in lamina X of the thoracic and upper lumbar cord and seem to target the PAG and the deep tectum. Cluster IV neurons are located in the medial part of laminae VI-VII of the lumbosacral cord and seem to project predominantly to the lateral and ventrolateral caudal PAC. These neurons may play a role in conveying tactile stimuli to the PAG during mating behavior. Neurons of cluster V are located in the lateral part of lamina I of L6-S2 and in laminae V-VII and X of S1-S3. They are labeled only after injections into the central portion of the lateral and ventrolateral caudal PAG and probably relay information concerning micturition and mating behavior. (C) 2000 Wiley-Liss, Inc.

The possibility of collateral segmental actions of spinocervical tract (SCT) neurones upon interneurones with input from cutaneous and group II muscle afferents was investigated in deeply anaesthetized cats. To this end, intracellular and/or extracellular recordings were made from 35 dorsal horn and 15 intermediate zone interneurones in midlumbar segments of the spinal cord and effects of stimulation of the ipsilateral dorso-lateral funiculus (DLF) at C3 and C1 levels, i.e. below and above the lateral cervical nucleus where axons of SCT cells terminate, were compared. The stimuli applied at the C3 segment were within the range of stimuli (50-100 microA) required for antidromic activation of SCT neurones in the same experiment. Those applied at the C segment (200-500 microA) were at least 3 times stronger than C3 stimuli. Under the same experimental conditions, long ascending and descending tract neurones (dorsal spino-cerebellar and rubro-spinal tract neurones) with axons in the DLF were activated at similar thresholds from the C and C3 segments. Intracellular recordings were made from 29 interneurones of which 19 (65%) were dorsal horn and 10 (35%) were intermediate zone interneurones. Excitatory postsynaptic potentials (EPSPs) evoked by single stimuli applied at the C3 segment, but not the C segment, were found in 14 (48%) of those interneurones; their latencies (3.0-5.7 ms) and frequency following with only minimal temporal facilitation were as required for potentials being evoked monosynaptically by the fastest conducting SCT neurones. Extracellular recordings were made from 30 interneurones (24 dorsal horn and 6 intermediate zone interneurones), and in these neurones spike potentials induced from the C3, but not from the C segment, were evoked only by short trains of stimuli. However, their latencies from the first effective stimulus (4.3-5.4 ms) were compatible with mono- or oligosynaptically mediated collateral actions of SCT neurones. They were found in 10 (33%) of the 30 investigated interneurones. Similar effects of C3 stimuli were found in similar proportions of dorsal horn interneurones and intermediate zone interneurones. Indications were also found for synaptic actions evoked by C3 stimuli that could not be attributed to direct collateral actions of SCT neurones. In some intracellularly recorded dorsal horn interneurones, short-latency EPSPs were evoked from the C3 segment by the 2nd or 3rd stimulus in the train, but not by single stimuli. In other dorsal horn and intermediate zone interneurones, inhibitory postsynaptic potentials (IPSPs) were evoked from the C3 segment at minimal latencies (2.7-3.2 ms), which might be too short to allow their mediation via SCT neurones. We conclude that SCT neurones might be used to forward information from muscle group II and cutaneous afferents not only to neurones in the lateral cervical nucleus and via them to thalamus and cerebral cortex but also to interneurones in spinal reflex pathways. Thereby reflex actions evoked from group II and cutaneous afferents might be co-ordinated with responses mediated by supraspinal neurones. We conclude also that dorsal horn and intermediate zone mid-lumbar interneurones might contribute to the previously reported di-and poly-synaptic excitation or inhibition of postsynaptic dorsal column (PSDC), spinothalamic tract (STT) and spinomesencephalic tract (SMT) neurones by collateral actions of SCT cells. Thereby these interneurones might contribute to the co-ordination of responses mediated by various populations of supraspinal neurones.

The segmental and laminar organization of spinal projections to the functionally distinct ventrolateral (vlPAG) and lateral periaqueductal gray (lPAG) columns was examined by using retrograde and anterograde tracing techniques. It was found 1) that spinal input to both vlPAG and lPAG columns arose predominantly from neurons in the upper cervical (C1-4) and sacral spinal cord; 2) that there was a topographical separation of vl-PAG projecting and lPAG-projecting neurons within the upper cervical spinal cord; but 3) that below spinal segment C4, vlPAG-projecting and lPAG-projecting spinal neurons were similarly distributed, predominantly within contralateral lamina I, the nucleus of the dorsolateral fasciculus (the lateral spinal nucleus) and the lateral (reticular) part of lamina V. Consistent with the retrograde results, the greatest density of anterograde label, within both the vlPAG and lPAG, was found after tracer injections made either in the superficial or deep dorsal horn of the upper cervical spinal cord. Tracer injections made within the thoraco-lumbar spinal cord revealed that the vlPAG column received a convergent input from both the superficial and deep dorsal horn. However, thoraco-lumbar input to the lPAG was found to arise uniquely from the superficial dorsal horn; whereas the deep dorsal horn was found to innervate the "juxta-aqueductal" PAG region rather than projecting to the lPAG. These findings suggest that similar to spino-parabrachial projections, spinal projections to the lPAG (and juxta-aqueductal PAG) are topographically organised, with distinct subgroups of spinal neurons projecting to specific lPAG or juxta-aqueductal PAG subregions. In contrast, the vlPAG receives a convergent spinal input which arises from the superficial and deep dorsal horn of cervical, thoracic, lumbar, and sacral spinal segments.

We have reported that the lateral region of the caudal third of the midbrain periaqueductal grey (PAG), which mediates flight and hypertension, receives inputs from lamina I and IIo and the lateral cervical nucleus (LCN) of the upper cervical spinal cord (UCC); whereas the ventrolateral PAG region, which mediates hypotension, quiescence and immobility, is targeted by cells in laminae VII, VIII and X. In the UCC the cells of laminae VII and VIII receive a significant afferent input from the deep neck muscles, whereas cells of laminae I and IIo and the LCN receive a large input from cutaneous nociceptors. Thus we investigated the hypothesis that nociceptive activation of the deep neck muscles would activate the spinal-ventrolateral PAG projection, whereas cutaneous nociceptive stimulation would activate the spinal-lateral PAG projection, by examining the expression of Fos protein. We found that deep noxious stimulation led to Fos-positive cells predominantly in the ventrolateral PAG and superficial noxious stimulation led to Fos-positive cells predominantly in the lateral PAG. The results: (i) indicate that the UCC afferent regulation of the PAG arises from topographically separable and functionally dissociable populations of neurons and (ii) raise the possibility that the ventrolateral and lateral PAG play important but different roles in mediating the distinctive affective, emotional and autonomic responses evoked by pain arising from deep or superficial structures.

The lateral ventral horn. of the upper cervical (C(1-3vl)) cord in rat, cat and monkey contains many cells that project to the periaqueductal gray matter (PAG). Until now it was assumed that these cells only project to the ventrolateral part of the caudal PAG. Because the ventrolateral caudal PAG is involved in quiescence and hypotension, it was hypothesized that the C(1-3vl)-PAG projecting cells play a role in immobility behavior, possibly activated by neck muscle afferents. However, in the present anterograde and retrograde study in cat we showed that C(1-3vl) cells do not only target the caudal PAG, but terminate even more abundantly in the intermediate and rostral parts of the PAG. There, projections target the ventrolateral column, like in the caudal PAG, but also terminate in the lateral and dorsomedial columns. This finding, combined with the current, albeit limited, physiological data on C(1-3vl)-PAG and C(1-3vl)-thalamic projecting cells, sheds a new light on the possible functions of C(1-3vl) cells. it might be that the C(1-3vl) cells with complex response properties and large receptive fields have a relay function similar to cells in the dorsal column nuclei, lateral cervical or central cervical nucleus. Other C(1-3vl) cells might receive somatic or visceral input that was never tested before, or perhaps other input, like vestibular information. It might also be that these cells project to the more caudal cord to modulate visceral input, with ascending collaterals to more rostrally, located structures, including mesencephalon and thalamus. (C) 2009 Elsevier B.V. All rights reserved.

Projection neurons in lamina I, together with those in laminae III–IV that express the neurokinin 1 receptor (NK1r), form a major route through which nociceptive information reaches the brain. Axons of these cells innervate various targets, including thalamus, periaqueductal gray matter (PAG), and lateral parabrachial area (LPb), and many cells project to more than one target. The aims of this study were to quantify projections from cervical enlargement to PAG and LPb, to determine the proportion of spinothalamic neurons at lumbar and cervical levels that were labelled from PAG and LPb, and to investigate morphological differences between projection populations. The C7 segment contained fewer lamina I spinoparabrachial cells than L4, but a similar number of spino-PAG cells. Virtually all spinothalamic lamina I neurons at both levels were labelled from LPb and between one-third and one-half from PAG. This suggests that significant numbers project to all three targets. Spinothalamic lamina I neurons differed from those labelled only from LPb in that they were generally larger, were more often multipolar, and (in cervical enlargement) had stronger NK1r immunoreactivity. Most lamina III/IV NK1r cells at both levels projected to LPb, but few were labelled from PAG. The great majority of these cells in C7 and over one-fourth of those in L4 were spinothalamic, and at each level some projected to both thalamus and LPb. These results confirm that neurons in these laminae have extensive collateral projections and suggest that different neuronal subpopulations in lamina I have characteristic patterns of supraspinal projection. J. Comp. Neurol. 515:629–646, 2009. © 2009 Wiley-Liss, Inc.

The location of spinal cells projecting rostrally to central areas that process visceroception and visceronociception were studied in rat using the retrograde transport of a protein - gold complex. Origins of afferents to the nucleus tractus solitarius (the spinosolitary tract), the parabrachial area (the spinoparabrachial tract), the hypothalamus (the spinohypothalamic tract) and the amygdala (the spinoamygdalar tract) were studied at thoracic, lumbar and sacral levels, where spinal visceroceptive areas are concentrated. All of the afore-mentioned pathways have common origins in the lateral spinal nucleus and in the reticular formation of the neck of the dorsal horn at all the levels studied, and also in the dorsal grey commissure and adjacent areas at sacral levels. The spinosolitary and the spinoparabrachial tracts are dense pathways, both of which are also characterized by afferents from the superficial layers of the dorsal horn at all the levels studied and from cells lying in close proximity to some autonomic spinal areas. These autonomic areas are the central autonomic nucleus (dorsal commissural nucleus) of lamina X at thoracolumbar levels and the parasympathetic column at sacral levels; some projections from the intermediolateral cell column at thoracic levels were also noted. Projections from all these autonomic structures to the parabrachial area have not yet been recognized. Thus, the origin of the spinoparabrachial tract closely resembles that of the spinomesencephalic tract that reaches the periaquaductal grey and adjacent areas. The spinohypothalamic and the spinoamygdalar tracts are smaller pathways. Direct spinal connections to the amygdala have not been reported previously. Both the hypothalamus and amygdala receive projections from lamina VII cells at low thoracic and upper lumbar levels in a pattern that resembles that of the preganglionic cells of the intercalated nucleus. Hypothalamic projections from the sacral parasympathetic area were also noted. The use of c-fos as a functional marker to identify spinal neurons that are activated by noxious visceral stimulation suggests that both the spinoparabrachial and the spinosolitary tracts contribute significantly to the central transmission of visceronoceptive messages. Most of the visceronociceptive ascending projections in these pathways issued from lamina I cells. The results presented here confirm previous observations regarding the spinosolitary and the spinohypothalamic tracts and also demonstrate, for the first time, the complex origin of the spinoparabrachial tract and the existence of direct spinal afferents to the amygdala. These findings suggest that rostral transmission and central integration of visceral inputs require several parallel routes. The spinosolitary and spinoparabrachial tracts clearly play a role in conveying information regarding visceronociception.

1. The effects of electrical stimulation at different rostrocaudal levels of the midbrain, and at sites in the rostral medulla ipsilateral and contralateral to spinal recording sites, were evaluated against the responses of 46 cells belonging to the cat spinomesencephalic tract (SMT). 2. Inhibitory and/or excitatory effects of brain stem stimulation were observed on SMT cells that responded best (26 cells) or exclusively (12 cells) to noxious mechanical or thermal stimuli, as well as on 7 cells responding only to tap and/or stimulation of deep tissues. Recording sites for 32 cells were located in laminae V-VIII (27 cells) and laminae I-III (5 cells). 3. Midbrain stimulation sites were located in the superior colliculus, central gray (CG), red nucleus, and the midbrain reticular formation. Both inhibitory-only and excitatory-only effects were observed, although the most common effect of midbrain stimulation was excitation followed by inhibition (mixed effects). The effects of stimulation at different midbrain levels were determined for each cell. Stimulation in the caudal, middle, or rostral midbrain was often found to exert different effects on the same SMT cell. 4. Stimulation in the rostral medulla at sites located in nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis, and nucleus reticularis magnocellularis produced the same complement of effects observed with midbrain stimulation. Excitation followed by inhibition was the most common effect observed. 5. Stimulus intensities required to produce excitatory or inhibitory effects from midbrain were 114 +/- 85 (SD) microA and 210 +/- 91 microA, respectively. Stimulus currents required to produce excitatory or inhibitory effects from medullary stimulation sites were 124 +/- 56 microA and 70 +/- 60 microA, respectively. The mean currents required to produce mixed effects were 221 +/- 120 microA (midbrain) and 127 +/- 71 microA (medulla). Increasing the stimulus intensity used to evaluate brain stem effects increased the magnitude and duration of effects for 33 cells. Mixed effects were observed on 11 cells at stimulus intensities greater than those required to produce inhibitory-only effects. 6. Significant differences were found between the latencies of excitation and inhibition produced from different brain stem levels. These differences suggest that midbrain and medullary stimulation activate descending pathways with a wide range of conduction velocities and/or supraspinal and spinal connectivities. 7. The spinal trajectory of pathways contributing to the varied effects of brain stem stimulation as well as the complex receptive fields (RFs) of SMT cells were evaluated by the placement of lesions in the cervical spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

The midbrain periaqueductal gray matter (PAG) integrates the basic responses necessary for survival of individuals and species. Examples are defense behaviors such as fight, flight, and freezing, but also sexual behavior, vocalization, and micturition. To control these behaviors the PAG depends on strong input from more rostrally located limbic structures, as well as from afferent input from the lower brainstem and spinal cord. Mouton and Holstege (2000, J Comp Neurol 428:389-410) showed that there exist at least five different groups of spino-PAG neurons, each of which is thought to subserve a specific function. The lateral cervical nucleus (LCN) in the upper cervical cord is not among these five groups. The LCN relays information from hair receptors and noxious information and projects strongly to the contralateral ventroposterior and posterior regions of thalamus and to intermediate and deep tectal layers. The question is whether the LCN also projects to the PAG. The present study in cat, using retrograde and anterograde tracing techniques, showed that neurons located in the lateral two-thirds of the LCN send fibers to the lateral part of the PAG, predominantly at rostrocaudal levels A0.6-P0.2. This part of the PAG is known to be involved in flight behavior. A concept is put forward according to which the LCN-PAG pathway alerts the animal about the presence of cutaneous stimuli that might represent danger, necessitating flight. J. Comp. Neurol. 471:434-445, 2004.

Estrogen affects female sexual behavior, analgesia, and micturition in mammals. One of the possible sites at which estrogen might exert its effect on these functions is the periaqueductal gray (PAG). The PAG is involved in each of these functions, it receives sensory input relevant to these functions from the lumbosacral cord, and contains estrogen receptor-alpha immunoreactive (ER-alpha IR) neurons. The present light (LM) and electron microscopic (EM) study seeks to determine whether there are monosynaptic projections from the lumbosacral cord to ER-alpha IR neurons in the PAG of the female rhesus monkey. Tracer was injected into the lumbosacral cord to visualize the lumbosacral-PAG projection, and the distribution of ER-alpha IR neurons in the PAG was studied immunohistochemically. The medial part of the ventrolateral caudal PAG received the densest projection from the lumbosacral cord. Another prominent projection was found in the lateral PAG at the intercollicular level. Although ER-alpha IR neurons were widely distributed throughout the PAG, approximately 40%, of ER-alpha IR PAG neurons were located as a distinct cluster in the medial portion of the ventrolateral, caudal PAG. Double labeling experiments showed that the location of this cluster precisely overlapped with the densest lumbosacral-PAG projection. EM revealed that axons from the lumbosacral cord made asymmetrical synaptic contacts with unlabeled dendrites and ER-alpha IR neuronal somata in the ventrolateral PAG. It is concluded that there exists a specific, monosynaptic pathway from lumbosacral neurons to ER-alpha expressing PAG neurons in the rhesus monkey. This pathway might be involved in the mechanism of analgesia, blood pressure, mating behavior, and micturition. J. Comp. Neurol. 443:27-42, 2002. (C) 2002 Wiley-Liss, Inc.

The periaqueductal gray is well known for its involvement in nociception control, but it also plays an important role in the emotional motor system. To accomplish these functions the periaqueductal gray receives input from the limbic system and from the caudal brainstem and spinal cord. Earlier studies gave the impression that the majority of the periaqueductal gray projecting cells in caudal brainstem and spinal cord are located in the contralateral lamina I, which is involved in nociception. The present study in the cat, however, demonstrates that of all periaqueductal gray projecting neurons in the contralateral caudal medulla less than 7% was located in lamina I. Of the spinal periaqueductal gray projecting neurons less than 29% was located in lamina I. However, within the spinal cord large segmental differences exist: in few segments of the enlargements the lamina I-periaqueductal gray projecting neurons represent a majority. In conclusion, although the lamina I-periaqueductal gray projection is a very important nociceptive pathway, it constitutes only a limited part of the total projection from the caudal medulla and spinal cord to the periaqueductal gray. These results suggest that a targe portion of the medullo- and spino-periaqueductal gray pathways conveys information other than nociception. (C) 2001 Elsevier Science Inc.

The responses of 32 spinomesencephalic tract cells to intradermal capsaicin were examined in anesthetized monkeys. Wide dynamic range (n = 20) and nociceptive specific (n = 6) cells showed two types of excitatory responses to intradermal injection of capsaicin. The first excitatory response shown by the majority of wide dynamic range (n = 13) and nociceptive specific (n = 4) cells was consistent with their sensitization by capsaicin. The cells showed an acute and prolonged increase in ongoing activity with capsaicin injection. Responses to mechanical stimuli were substantially increased after capsaicin and an expansion of receptive field was often observed. The responses of the same cells to excitatory amino acid agonists applied locally by iontophoresis also increased. All cells showing sensitization were antidromically activated from periaqueductal gray regions dorsal to the sulcus limitans. Electrical stimulation at these sites did not affect the ongoing or evoked discharges of these cells. The second excitatory response of wide dynamic range (n = 5) and nociceptive specific (n = 1) cells was a novel pattern not consistent with sensitization. These cells nevertheless showed an acute and prolonged increase in background activity after capsaicin injection. Yet, there was no change or a decrease in responses to cutaneous stimuli, no evidence for change in receptive field size and no increase in responses to locally released excitatory amino acids. These cells projected to regions in the periaqueductal gray ventral to the sulcus limitans. Electrical stimulation at these sites produced a decrease in spontaneous activity of the same cell. Low threshold mesencephalic-projecting neurons (n = 6) showed a single inhibitory pattern (n = 4) of responses to capsaicin. The injection produced an acute decrease in spontaneous activity that was sustained for at least 30 min after injection. The responses to cutaneous stimuli and to excitatory amino acids were also substantially reduced. Low threshold cells were found that projected to both dorsal-lateral and ventral-lateral regions of the periaqueductal gray. In summary, three patterns of responses shown by primate spinomesencephalic tract cells to intradermal capsaicin appear dependent on the functional regions of the periaqueductal gray to which they project. These results suggest that inputs of spinomesencephalic tract neurons to the periaqueductal gray may evoke important components of the systemic response to the neurogenic hyperalgesia produced by intradermal capsaicin.

The periaqueductal gray (FAG) is involved in motor activities, such as movements of the neck, back and hind limbs, cardiovascular regulation, micturition, vocalization, and mating behavior, as well as in nociception control. To accomplish these functions the FAG uses information from other parts of the limbic system, from the lower brainstem, and from the spinal cord. To study the ascending projections from the spinal cord to the PAG, tracer was injected in different parts of the FAG, and the number of retrogradely labeled neurons were counted for each spinal segment. Results show that large segmental differences exist in the number of FAG projecting neurons throughout the length of the spinal cord and that different parts of the spinal cord project to specific areas in the FAG. (C) 1997 Elsevier Science Ireland Ltd.

Extracellular microelectrode recordings were made from spinomesencephalic tract (SMT) neurons in the lumbosacral spinal cord of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The SMT cells were antidromically fired from the posterolateral parts of the superior colliculus and the intercollicular region, were located in laminae IV to VIII, and had response properties and axonal conduction velocities similar to those described previously. The effects of stimulating the dorsolateral funiculus of the cervical cord at C3 and rostral C1, below and above the termination of spinocervical tract (SCT) axons in the lateral cervical nucleus, were examined on 33 SMT cells. The strength of stimulation was adjusted so that at C3 it was above threshold for antidromic activation of SCT cells and at C1 was below threshold for activation of the same cells. Seven (21%) SMT neurons were excited from C3 but not from C1. The remaining 26 (79%) were excited from both C3 and rostral C1 and 23 (70% of these) were excited significantly more from C3. That is, 91% of the total sample were either excited only from C3 or more strongly from C3 than from rostral C1. We discuss the possible neuronal systems involved and conclude that the greater excitatory effects from C3 are most likely due to antidromic activation of the SCT. The shortest latency effects from C3 indicate a monosynaptic linkage between SCT cells with the fastest axons and the SMT. The longer latency actions may be due to monosynaptic connexions from SCT cells with slower conducting axons, to di- or polysynaptic actions from SCT cells with fast axons, or a combination of both. SMT cells are another population of spinal neurons, in addition to postsynaptic dorsal column, spinothalamic and dorsal horn spinocerebellar neurons, which receive excitation via SCT collaterals.

The nucleus retroambiguus (NRA) projects to distinct brainstem and cervical and thoracic cord motoneuronal cell groups. The present paper describes NRA projections to distinct motoneuronal cell groups in the lumbar enlargement. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify the retrogradely labeled neurons in the caudal medullary lateral tegmentum. These injections were combined with spinal hemisections to distinguish between neurons having ipsi- or contralaterally descending axons. The NRA-lumbosacral fibers descend almost exclusively contralaterally, but neurons in areas surrounding the NRA project mainly ipsilaterally.In an anterograde tracing study, injections of WGA-HRP or tritiated leucine were made in the region of the NRA to determine the NRA. targets in the lumbosacral cord. Hemisections in C2 made it possible to distinguish between NRA projections and projections from neurons in the adjoining lateral tegmentum. The results show delicate NRA projections to distinct lumbosacral motoneuronal cell groups innervating specific hindlimb muscles (iliopsoas, adductors, and hamstrings) as well as axial muscles (medial longissimus and proximal tail muscles). The projection is bilateral, with a contralateral predominance. Ipsilaterally terminating fibers are derived from NRA neurons whose axons cross the midline at the level of the obex, descend through the contralateral spinal white matter, and recross at the level of termination. A conceptual description is presented in which the periaqueductal gray-NRA-lumbosacral projections form the final common pathway for lordosis in the cat. (C) 1995 Wiley-Liss, Inc.

Information concerning the rate of bladder filling is determined by receptors in the bladder wall and conveyed via afferent fibers in the pelvic nerve to sensory neurons in the lumbosacral cord. It was assumed that this information is relayed from the lumbosacral cord to a medial cell group in the dorsolateral pontine tegmentum, called the M-region, the pontine micturition center, or Barrington's nucleus. The M-region, in turn, projects via long descending pathways to the sacral parasympathetic motoneurons. In the present electron microscopic study, it was investigated in cats whether monosynaptic projections from lumbosacral neurons to the M-region indeed exist. Wheat-germ agglutinin-horseradish peroxidase injections were made into the lumbosacral, cord. Many retrogradely labeled dendrites and somata were found in the M-region, but no labeled terminals were found on retrogradely labeled dendrites or somata. Only a small number of anterogradely labeled terminals, which were filled with mainly round vesicles, contacted unlabeled dendrites in the M-region. In contrast, many more anterogradely labeled terminals, which were filled with mainly round and, to a limited extent, dense core vesicles and with asymmetrical synapses, were found on dendrites in the lateral part of the periaqueductal gray (PAG). Previously (Blok and Holstege [1994] Neurosci. Lett. 166:93-96), it was demonstrated that the lateral part of the FAG contains neurons projecting to the M-region. A concept for the central organization of the micturition reflex is presented in which ascending projections from the lumbosacral cord convey information on bladder filling to the FAG. When the bladder contains so much urine that voiding is necessary, the FAG, in turn, triggers the M-region. The M-region, however, also receives afferents from the preoptic area, which might be involved in the final decision to start micturition. (C) 1995 Wiley-Liss, Inc.

Retrograde transport of the fluorescent tracer Fluorogold was used in combination with immunohistochemical staining for the enzyme glutaminase to identify putative glutamatergic neurons belonging to the rat spinomesencephalic tract. Glutaminase-like staining in spinal projection neurons suggests that the relay of nociceptive information from the spinal cord to midbrain may involve the excitatory amino acid glutamate.

Attempts were made to determine the cells of origin of the spinal projections to the PAG using retrograde axoplasmic transport techniques of HRP and fluorescent markers. Following injections limited to the lateroventral subnucleus of the PAG, labels were transported to the contralateral laminae I, III, V, VII and X primarily. A striking number of labeled neurons were found in the lumbosacral region. Discussion followed on the possible dichotomic axonal projections of the labeled spinal neurons.

This study used the retrograde transport of a protein-gold complex to examine the distribution of spinal cord and trigeminal nucleus caudalis neurons that project to the nucleus of the solitary tract (NST) in the rat. In the spinal grey matter, retrogradely labeled cells were common in the marginal zone (lamina I), in the lateral spinal nucleus of the dorsolateral funiculus, in the reticular part of the neck of the dorsal horn (lamina V), around the central canal (lamina X), and in the region of the thoracic and sacral autonomic cell columns. The pattern of labeling closely resembled that seen for the cells at the origin of the spinomesencephalic tract and shared some features with that of the spinoreticular and spinothalamic tracts. Labeled cells in lamina IV of the dorsal horn were only observed when injections spread dorsally, into the dorsal column nuclei, and are thus not considered to be at the origin of the spinosolitary tract. They are probably neurons of the postsynaptic fibers of the dorsal column. Retrogradely labeled cells were also numerous in the superficial laminae of the trigeminal nucleus caudalis, through its rostrocaudal extent. The pattern of marginal cell labeling appeared to be continuous with that of labeled neurons in the paratrigeminal nucleus, located in the descending tract of trigeminal nerve. Since the NST is an important relay for visceral afferents from both the glossopharyngeal and vagus nerves, we suggest that the spinal and trigeminal neurons that project to the NST may be part of a larger system that integrates somatic and visceral afferent inputs from wide areas of the body. The projections may underlie somatovisceral and/or viscerovisceral reflexes, perhaps with a significant afferent nociceptive component.

We have examined the physiological and morphological characteristics of spinal dorsal horn lamina I neurons with projections to the midbrain in the cat by combining physiological recording of neurons with the intracellular injection of HRP. Lamina I spinomesencephalic neurons were antidromically activated from the region that included the cuneiform nucleus and lateral periaqueductal gray at the intercollicular level. The majority of mesencephalic projection neurons (50 of 55) responded exclusively to noxious stimulation (nociceptive-specific) of their peripheral receptive fields. Lamina I spinomesencephalic neurons were activated from both the ipsilateral and contralateral midbrain and had slow antidromic conduction velocities (1 to 18 m/second). We identified eight cells with projections to both the midbrain and the thalamus and eight cells that were antidromically activated only from the thalamus. Intracellular injection of HRP revealed that lamina I spinomesencephalic neurons were of diverse morphological types, but generally had extensive, rostrocaudally oriented, dendritic arbors confined to lamina I and the overlying white matter. Axons were observed on nine of the HRP-filled spinomesencephalic neurons; five of the axons issued collateral branches. The morphological characteristics of these neurons did not appear to correlate with functional categories (i.e., wide-dynamic-range- or nociceptive-specific-type neurons). The large number of nociceptive-specific neurons with projections to the midbrain and the interconnections of these midbrain sites with hypothalamic and limbic structures suggest that the lamina I spinomesencephalic pathway plays an important role in the autonomic and affective responses to pain.

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase was used to determine the terminal domain of the projection from the lumbosacral spinal cord to the midbrain in the rat, cat, and monkey. Results have shown that several midbrain regions receiving afferent input from this level of the spinal cord are common to the three species examined. Structures innervated by this projection were located throughout the full rostrocaudal extent of the midbrain. The strongest projections were to the intercollicular region and caudal midbrain contralateral to injection sites in the spinal cord. Terminal labeling in the rostral midbrain, except that observed in the nucleus of Darkschewitsch, was substantially less than that observed at more caudal midbrain levels. Structures receiving the strongest input from the spinal cord included the central gray, nucleus cuneiformis, the deep and intermediate layers of the superior colliculus, and the intercollicular nucleus. Other structures receiving afferent input from the lumbosacral spinal cord included the anterior and posterior pretectal nuclei, red nucleus, Edinger-Westphal nucleus, interstitial nucleus of Cajal, and the mesencephalic reticular formation. It is concluded that the spinal projection to the midbrain is a multicomponent projection consisting of several pathways terminating in discrete midbrain regions. Considering the diverse functions associated with midbrain regions receiving spinal input and the response and receptive field properties of cells belonging to this pathway, the results of the present study are discussed in relation to the potential role of the spinomesencephalic tract in somatic, visceral, and motor function.

The ascending degeneration resulting from experimental lesions of the ventral funiculus of the spinal cord of Macaca mulatta has been studied using the Nauta technique and its variants. The ventral spinothalamic tract is shown to be an independent entity with respect to the lateral spinothalamic tract; its fibers are widely distributed in the ventral funiculus and it establishes connections with the brain stem and thalamus which are analogous but not identical to those of the latter. Its role in the relay of nociceptive input is discussed in view of the similarity in hodology of the two systems and it is proposed that it may be responsible for the failure of anterolateral cordotomy to control pain on a long term basis. Other ascending systems in the ventral funiculus include the spino-olivary and spino-reticular tracts, as well as minor connections to the N. of Edinger-Westphal, the red nucleus and the superior colliculus. The projections from the ventral quadrant of the spinal cord to the brain stem are almost entirely ipsilateral until the rostral mesencephalon is reached, at which level the N. of Darkschewitz receives both ipsilateral and crossed input; the magnocellular nucleus of the medial geniculate body receives a small contribution which is mainly ipsilateral. In the thalamus the VPL receives predominantly ipsilateral projections while the input to the paralaminar nuclei is only slightly less pronounced contralaterally than ipsilaterally.

Recordings were made from 90 identified spinomesencephalic tract (SMT) cells in the lumbosacral spinal cord of cats anesthetized with alpha-chloralose and pentobarbital sodium. Recording sites were located in laminae I-VIII. Antidromic stimulation sites were located in different regions of the rostral and caudal midbrain including the periaqueductal gray, midbrain reticular formation, and the deep layers of the superior colliculus. Twelve SMT cells were antidromically activated from more than one midbrain level or from sites in the medial thalamus. The mean conduction velocity for the population of cells sampled was 45.2 +/- 21.4 m/s. Cells were categorized based on their responses to graded intensities of mechanical stimuli and the location of excitatory and/or inhibitory receptive fields. Four major categories of cells were encountered: wide dynamic range (WDR); high threshold (HT); deep/tap; and nonresponsive. WDR and HT cells had excitatory and/or inhibitory receptive fields restricted to the ipsilateral hindlimb or extending to other parts of the body including the tail, forelimbs, and face. Some cells had long afterdischarges following noxious stimulation, whereas others had high rates of background activity that was depressed by nonnoxious and noxious stimuli. Deep/tap cells received convergent input from muscle, joint, or visceral primary afferent fibers. The placement of mechanical lesions at different rostrocaudal levels of the cervical spinal cord provided information related to the spinal trajectory of SMT axons. Six axons were located contralateral to the recording electrode in the ventrolateral/medial or lateral funiculi while two were located in the ventrolateral funiculus of the ipsilateral spinal cord. Stimulation at sites used to antidromically activate SMT cells resulted in the inhibition of background and evoked responses for 22 of 25 cells tested. Inhibitory effects were observed on responses evoked by low/high intensity cutaneous stimuli and by the activation of joint or muscle primary afferent fibers. Based on the response and receptive-field properties of SMT cells it is suggested that the SMT may have an important role in somatosensory mechanisms, particularly those related to nociception.

To obtain a comprehensive map of the brainstem and spinal cord areas that project to the mesencephalic central gray small injections of hors-radish peroxidase were made into various regions of the periaqueductal gray in a series of monkeys. Despite the fact that different regions of the central gray were injected in separate animals, the majority of the brainstem areas containing retrogradely filled neurons remained the same. Labeled neurons were observed in the superior colliculus, periaqueductal gray, lateral parabrachial, locus coeruleus, nucleus raphe magnus and pallidus, and a variety of brainstem reticular nuclei. In contrast to labeled brainstem areas, where labeled neurons were present predominantly ipsilateral to the injection site, the spinal trigeminal nucleus pars caudalis and the spinal cord displayed labeled cells chiefly on the side contralateral to the injection. Also in contrast to the labeled brainstem sites, where medial and lateral injection sites produced a similar pattern of labeling, medial injections in the PAG labeled almost exclusively neurons in the deep laminae (V-X) in the spinal trigeminal nucleus pars caudalis and spinal cord while more lateral injections labeled neurons in both the deep (V-X) and superficial (I) laminae. No consistent differences were noted in the location of labeled neurons in either brainstem or spinal sites after dorsal vs. ventral injections or caudal vs. rostral injection sites. The present study has demonstrated that the central gray receives afferent projections from a number of brainstem and spinal areas which are known to be involved in the modulation andor conduction of nociception, while other inputs are probably involved in the regulation of visceral functions. These data support the hypothesis that the mesencephalic periaqueductal gray functions as a visceral, nociceptive, and cognitive integrator.

Anatomical and electrophysiological methods were used to investigate the projections and response properties of neurons in the second cervical (C2) spinal segment of the cat giving origin to a previously undescribed projection to the ipsilateral thalamus. The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify neurons in C2 giving rise to thalamic projections. Following large (3.0 microliter) thalamic HRP injections, a large number of labeled neurons was observed in lateral laminae VII-VIII of C2 ipsilateral to the injections. They occurred as small clusters of cells along the longitudinal axis of C2. Labeled neurons were also observed contralaterally in the lateral cervical nucleus, dorsal horn (especially medial lamina VI), and loosely distributed in the ventral horn. The ipsilaterally projecting neurons were also labeled following small (0.2--0.5 microliter) HRP injections restricted to individual spinothalamic terminal zones (intralaminar nuclei, ventrobasal complex-nucleus ventralis lateralis border zone, medial division of the posterior nuclei), indicating that as a group they project widely throughout the thalamus. Single unit recording methods were used to obtain complementary information on the functional properties of these neurons. The antidromic stimulation method was applied to identify units in C2 projecting to the ipsilateral thalamus in anesthetized, paralyzed cats. Three categories of ipsilaterally projecting C2 units were identified: (1) units not driven by any type of natural stimulation; (2) units having large cutaneous receptive fields (RFs) and wide dynamic response ranges ("widefield"), and (3) units with smaller RFs and varied properties ("other"). Widefield units with bilaterally symmetrical and asymmetrical RFs were observed. Co-stimulation of different portions of an excitatory RF produced summation of the unit response. Inhibitory RF components were identified in one-third of the widefield units. Unit recordings after spinal tract lesions revealed that the afferent input passed via the ipsilateral lateral and/or ventral funiculi. Widefield unit responses to somatosensory stimuli could be inhibited by dorsal column conditioning stimulation. Several "other" units resembled widefield units, while a second group had small RFs restricted to the C2 dermatome. Possible functional roles of the projecting C2 neurons in somatosensory and non-specific systems are discussed.

1. Spinoreticular tract neurons at the rat lumbar cord level were identified by antidromic activation following stimulation at mainly pontine and mesencephalic levels. These units, which were found in the dorsal half of the cord, could be separated into two groups according to their spinal location, electrophysiological properties, and their central projections. 2. Units in the dorsolateral funiculus nucleus projected mainly to the cuneiformis area and adjacent structures with frequent bilateral projections. They had the slowest conduction velocities, sometimes in the unmyelinated range. Generally, they were driven only by stimulation of subcutaneous and/or deep structures. 3. Neurons located in the dorsal horn mainly projected contralaterally to pontine and mesencephalic levels. their conduction velocities and the electrophysiological properties were identical to those observed for the rat spinothalamic tract (22). Almost all (86%) had clear cutaneous sensitivity and generally large receptive fields: 40% responded to nonnoxious and noxious mechanical cutaneous stimuli and frequently to noxious radiant heat, 26% were exclusively excited by light tactile stimuli, and 20% required noxious cutaneous mechanical stimulation for activation. There was a good correlation between responses to natural and transcutaneous electrical stimulation: units driven by noxious mechanical stimuli received A-delta- and/or C-fiber inputs. The remaining units (14%) had more complex receptive fields associated with both excitatory and inhibitory inputs originating from a single peripheral area. 4. The functional heterogeneity of the rat spinoreticular tract is reminiscent of that demonstrated for the rat and monkey spinothalamic tracts. Similarly, the rat spinoreticular neurons are under the influence of descending inhibitory controls originating from the nucleus raphe magnus and bulbar reticular formation. 5. Responses of the rat spinoreticular tract neurons are consistent with the involvement of this pathway in the transmission of messages of both innocuous and noxious origins.

HIGH-THRESHOLD MECHANORECEPTORS and their centrally projecting myelinated fibers make up a functionally distinct group of cutaneous sensory units that have been suggested as part of the afferent apparatus for pain resulting from mechanical damage to the skin (3, 17). The argument for their relation to pain was based on two points: 1) the ability of such afferent elements, unique among those with medullated fibers, to provide signals differentiating noxious from innocuous mechanical events affecting the skin; and 2) the well-established correlation between pain and activity in thin myelinated afferent fibers