Search

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.

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)

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.

Collateral projections are an important feature of the organization of ascending projections from the spinal cord to the brain. Primate spinothalamic tract (STT) neurons with collaterals to the periaqueductal gray (PAG) were studied by means of a fluorescent double-labeling method. Granular Blue and rhodamine-labeled latex microspheres were placed in the ventral posterior lateral (VPL) nucleus of the thalamus and the periaqueductal gray, respectively. Single and double labeled neurons were studied in the upper cervical cord, cervical enlargement, thoracic cord, lumbar enlargement, and sacral segments. The laminar distribution of double labeled neurons was similar to that of spinomesencephalic tract (SMT) neurons. Most double labeled (STT-SMT) neurons were located in contralateral laminae I, V, VII, and X. Relatively more lamina I STT-SMT neurons were found in the cervical enlargement and more lamina V STT-SMT neurons in the lumbar enlargement. The density of STT-SMT neurons in the upper cervical segments and cervical enlargement was almost equal. The density of STT-SMT neurons in the lumbar enlargement was 40% of that in the cervical enlargement. The thoracic and sacral segments had the lowest density of STT-SMT neurons, about 10% of that in the cervical enlargement. STT-SMT neurons constituted 14.7% of SMT neurons and 6% of STT neurons in the cervical enlargement and 15.3% of SMT neurons and 2.9% of STT neurons in the lumbar enlargement. The branch points of eight STT-SMT axons were studied electrophysiologically. The average percentage of conduction time spent in the parent axon was more than 85% for an antidromic action potential from the VPL nucleus and 91% from the PAG. Branch points of STT-SMT axons were calculated to be 9–13 mm caudal to the PAG, in the pons or rostral medulla.

An anatomical technique based on the retrograde transport of horseradish peroxidase (HRP) was used to investigate the projections of spinal cord neurons to the mesencephalic tegmentum in the rat. Restricted unilateral injections were confined to central grey, cuneiformis areas, and superior colliculus. Injections into all these loci produced labeling in similar spinal areas. Only quantitative differences were noted. In the spinal grey matter, numerous labeled cells were regularly encountered in the marginal zone, the lateral part of the neck of the dorsal horn, and the dorsal grey commissure. Projections from the marginal zone and neck of the dorsal horn were predominantly contralateral. In the white matter, a pronounced bilateral labeling was observed in the nucleus of the dorsolateral funiculus, thus confirming our previous electrophysiological findings (Menetrey et al., '80). This distribution of labeled cells was commonly observed throughout the whole length of the cord. Additional sites of projecting cells have also been identified at the most rostral levels (obex, C1, C2). They mostly derived from spinal extensions of the dorsal column nuclei and lateral cervical nucleus contralaterally; from the lateral ventral horns bilaterally and from the nucleus commissuralis ipsilaterally. This study is thus a clear confirmation that the mesencephalic tegmentum constitutes a target for various somatosensory inputs originating from spinal cord, dorsal column nuclei, and lateral cervical nucleus. Moreover, from these results together with those obtained for the spinothalamic tract in the rat, it appears that marginal and dorsolateral funiculus neurons preferentially project to the mesencephalic tegmentum. The importance of marginal zone projections underlines the involvement of the spinomesencephalic tract in pain mechanisms.

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.

Spinothalamic tract cells in the lumbar, sacral and caudal segments of the primate spinal cord were labelled by the retrograde transport of horseradish peroxidase (HRP) injected into the thalamus. The laminar distribution of stained spinothalamic cells in the lumbosacral enlargement differed according to whether the HRP was injected into the lateral or the medial thalamus. Lateral injections labelled cells in most laminae, but the largest numbers of cells were in laminae I and V. The highest concentrations of cells labelled from the medial thalamus were in laminae VI-VIII. Ninety percent or more of the stained spinothalamic cells in the lumbosacral enlargement were contralateral to the injection site. In the conus medullaris stained spinothalamic cells were most numerous in laminae I, V and VI following lateral thalamic injections of HRP. Many of the cells of the conus were in Stilling's nucleus. Twenty-three percent of the cells in the conus were ipsilateral to the injection site in the lateral thalamus. Only a few cells in the conus were labelled by medial thalamic injections. The total number of spinothalamic cells from L5 caudally was estimated to be at least 1,200-2,500. An injection of HRP into the midbrain resulted in a laminar distribution of labelled cells much like that produced by a lateral thalamic injection. The types of spinothalamic tract cells and the sizes of their somata were determined for different laminae. The cells types resemble those already described from Golgi and other studies of the spinal cord gray matter. The spinothalamic tract cells in lamina I included Waldeyer cells and numerous small fusiform, pyriform or triangular cells. Those in lamina II included limitrophe and central cells. Spinothalamic cells in lamina III were central cells. Most of the labelled cells in laminae IV-X were polygonal, although there were also flattened cells in these layers. The smallest spinothalamic cells were in laminae I-III, while the largest were in laminae V and VII-IX. Spinothalamic cells in the conus medullaris included cells like those in the lumbosacral enlargement, but also a special cell type in Stilling's nucleus. Some cells in the conus had dendrites that crossed the midline. Spinothalamic axons could sometimes be traced to the ventral white commissure within one or a few sections. In longitudinal sections, most labelled axons were in the ventral part of the lateral funiculus on the side of the injection, although a few were in the ventral funiculus or on the contralateral side. The axons were widely dispersed, and a few were located adjacent to the pia-glial membrane.

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 method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify the locations of cells of origin of the spinothalamic tract in the cat. Injections of from 0.2–3.0 μl of 30% HRP were made unilaterally in various regions of the somatosensory thalamus. Massive injections of the caudal thalamus in several cats showed the spinothalamic cells of origin to be located mainly in laminae I, VII and VIII in the lumbar enlargement, and in laminae I, V and VII–VIII in the cervical enlargement. Small injections of HRP were made into the three major spinothalamic terminal zones in the thalamus, to determine the laminar origin(s) of the spinal projections to each zone. Neurons in lamina I in both cervical and lumbar enlargements were found to project almost exclusively to the rostral VB-caudal VL border zone. A small number of neurons in laminae VII and VIII also project there but a larger number project to the intralaminar region. Neurons projecting to the PO regions were located mainly in laminae IV and V. This anatomical segregation of thalamic afferents probably reflects a functional segregation of input, since the functional properties of spinal neurons vary according to their laminar location. Comparison of these data with the differential projection spinothalamic neurons in the rat and monkey indicate that it is unlikely that the proposed “paleo-” and “neospinothalamic” systems would arise from anatomically separate groups of spinal neurons.