Spinal commissural neurons mediating vestibular input to neck motoneurons in the cat upper cervical spinal cord.

Spinal commissural neurons (CNs) activated di- or trisynaptically by stimulation of ipsilateral vestibular afferents were stained with intraaxonal injection of horseradish peroxidase in the cat upper cervical spinal cord. Stem axons of CNs in lamina VIII or VII, after crossing the midline, had ascending and/or descending main branches that gave off multiple axon collaterals to laminae IX and VIII over a few cervical segments. Terminal boutons appeared to make contact with proximal dendrites and somata of retrogradely-labelled neck motoneurons. Therefore, these CNs were regarded as mediating vestibular afferent input to contralateral neck motoneurons trisynaptically at the shortest.     

Anatomical evidence for the re-crossing of lateral corticospinal fibers via the posterior gray commissure in the cat spinal cord

This study demonstrated the projections of the corticospinal tract (CST) by using the anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following injection of WGA-HRP into the pre- and post-cruciate cortices (somatosensory area) on one side, labeled fibers were found in the spinal cord both in the lateral and in the anterior CSTs; both crossed, and uncrossed. The most conspicuous labeling was found in the crossed lateral CST. In addition, labeled lateral CST fibers were seen to recross via the posterior gray commissure. These re-crossed lateral CST fibers were most frequently observed in the cervical spinal segments.     

Differential input to dorsal horn dorsal spinocerebellar tract neurons in mid- and low-lumbar segments from upper cervical spinal cord in the cat

Previous studies showed that ipsilaterally projecting dorsal horn dorsal spinocerebellar tract (dh-DSCT) neurons located outside Clarke's column in mid- and caudal-lumbar segments of the spinal cord receive different afferent inputs. Here, we examined, using extracellular recordings in anaesthetized cats, whether there are also input differences to these populations of dh-DSCT neurons from: (a) the spinocervical tract (SCT), estimated by stimulation of the ipsilateral dorsolateral funiculus at cervical cord C3 and rostral C1, below and above the termination of SCT axons in the lateral cervical nucleus (LCN), and (b) descending/ascending fibres activated by electrical stimulation at rostral C1. Seventy percent (21/30) of the lower-lumbar (L6-L7) dh-DSCT neurons received significantly greater excitation from C3 than rostral C1, whereas only 17% (5/30) of the mid-lumbar (L5) dh-DSCT neurons had greater responses from C3 than rostral C1. Inhibition of background activity was seen in 30% of the lower-lumbar neurons, but only in 3% of mid-lumbar neurons. These findings suggest that lower-lumbar dh-DSCT neurons are much more likely, than mid-lumbar ones, to be influenced by the SCT and by systems descending from the brain, LCN and/or ascending systems. The experiments provide further evidence for differences in input to the subpopulations of dh-DSCT neurons.     

Ultrastructure of the central gray region (lamina X) in cat spinal cord

The central gray region (lamina X) of the lumbar spinal cord in cat was examined by electron microscopy. This region consisted of three morphological zones. Medially, the first zone was comprised of ependyma which surrounded the central canal. The ependyma in the cat spinal cord was similar to most vertebrate spinal ependyma. Secondly, a subependymal zone consisted of glial processes arranged parallel to the long axis of the spinal cord. This glial zone was widest lateral to the central canal and extended approximately 75 microns. The lateral edge of the glial zone intermingled with a neuropil zone, the third zone. The components of the neuropil zone consisted of dendrites, myelinated and unmyelinated axons, synaptic terminals, astrocytes and neurons. The dendrites and neurons generally were oriented parallel with the long axis of the spinal cord. Three synaptic terminal types were categorized according to vesicular morphology, i.e. small round vesicles, flattened vesicles and dense core vesicles. The central gray region has been implicated in nociception and has been shown to receive both primary afferent and supraspinal input. The results from this study are consistent with the central gray region being an area of multiple synaptic inputs which may form the morphological basis of nociceptive processing that ascends to brainstem nuclei.     

Inhibition of spinal neuronal responses to noxious skin heating by stimulation of mesencephalic periaqueductal gray in the cat.

1. Discharges of lumbar dorsal horn neurons were evoked by noxious radiant skin heating, and inhibition of the heat-evoked responses by stimulation of the mesencephalic periaqueductal gray was investigated in N2O-anesthetized cats. 2. Thirty-seven units selected on the basis of receiving afferent C-fiber input from the posterior tibial and/or superficial peroneal nerves responded vigorously to 50 degrees C heating of the plantar surface of the ipsilateral hindpaw. All discharges were inhibited by periaqueductal gray stimulation (PAGS) at current strengths of 300--900 microA; the mean threshold for inhibition was 167 microamperemeter. The mean frequency of the inhibited discharge was 39% of the control response. 3. Effective PAGS sites were distributed throughout the ventral PAG bilaterally. Stimulus current-distance estimates indicate that small (0.5--1.2 mm diameter) volumes of tissue within the PAG were stimulated. 4. A monotonic relationship between temperature and unitary discharge was found for skin heating from threshold to about 50 degrees C. PAGS resulted in a decrease in the slope of the curve plotting discharge against temperature, without altering the threshold. 5. Inhibition of the heat-evoked discharges rarely outlasted the PAGS. 6. Possible neural substrates for descending inhibition and correlates with neural mechanisms of analgesia are discussed.     

The periaqueductal gray in the cat projects to lamina VIII and the medial part of lamina VII throughout the length of the spinal cord

The periaqueductal gray (PAG) plays an important role in analgesia as well as in motor activities, such as vocalization, cardiovascular changes, and movements of the neck, back, and hind limbs. Although the anatomical pathways for vocalization and cardiovascular control are rather well understood, this is not the case for the pathways controlling the neck, back, and hind limb movements. This led us to study the direct projections from the PAG to the spinal cord in the cat. In a retrograde tracing study horseradish peroxidase (HRP) was injected into different spinal levels, which resulted in large HRP-labeled neurons in the lateral and ventrolateral PAG and the adjacent mesencephalic tegmentum. Even after an injection in the S2 spinal segment a few of these large neurons were found in the PAG. Wheat germ agglutinin-conjugated HRP injections in the ventrolateral and lateral PAG resulted in anterogradely labeled fibers descending through the ventromedial, ventral, and lateral funiculi. These fibers terminated in lamina VIII and the medial part of lamina VII of the caudal cervical, thoracic, lumbar, and sacral spinal cord. Interneurons in these laminae have been demonstrated to project to axial and proximal muscle motoneurons. The strongest PAG-spinal projections were to the upper cervical cord, where the fibers terminated in the lateral parts of the intermediate zone (laminae V, VII, and the dorsal part of lamina VIII). These laminae contain the premotor interneurons of the neck muscles. This distribution pattern suggests that the PAG-spinal pathway is involved in the control of neck and back movements. Comparing the location of the PAG-spinal neurons with the results of stimulation experiments leads to the supposition that the PAG-spinal neurons play a role in the control of the axial musculature during threat display.     

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat.

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.     

Ultrastructural evidence for GABA-mediated disinhibitory circuits in the spinal cord of the cat.

The synaptic relationships between gamma-aminobutyric acid (GABA)-immunoreactive and enkephalin-immunoreactive profiles in the cat spinal cord were examined using combined pre-embedding immunoperoxidase and post-embedding immunogold electron microscopic immunocytochemistry. Although colchicine was not used, enkephalin-immunoreactive somata and dendrites were detected in regions associated with nociceptive transmission, including laminae I, II, V and X. In each of these laminae, many GABA-immunoreactive terminals were found presynaptic to enkephalin-immunoreactive cell bodies and dendrites. We propose that disinhibition of opioid-containing neurons may be a common feature of pain-related circuits in the cat spinal cord.     

Quantitative comparison of inhibition in spinal cord of nociceptive information by stimulation in periaqueductal gray or nucleus raphe magnus of the cat

The descending inhibition of spinal neuronal responses by focal electrical stimulation in the periaqueductal gray (PAG) or nucleus raphe magnus (NRM) was quantitatively studied and compared in the anesthetized, paralyzed cat. All 60 dorsal horn neurons studied were driven by electrical stimulation of hindlimb cutaneous nerves at strengths supramaximal for activation of A-alpha,delta- and C-fibers, and 52 also responded to noxious radiant heating (50 degrees C, 10 s) of the skin of the foot- or toepads; 8 units had receptive fields in the hairy skin of the hindlimb. All neurons studied also responded to mechanical stimuli; recording sites were located in laminae I-VI of the dorsal horn. The inhibition of spinal neuronal heat-evoked responses by stimulation in the PAG or NRM differed quantitatively when examined on the same spinal neurons. Inhibition of heat-evoked spinal neuronal responses occurred at a lower threshold of stimulation in the NRM than in the PAG. The mean intensity of stimulation in the NRM producing an attenuation to 50% of the control 50 degrees C heat-evoked response was significantly lower than the mean intensity of stimulation in the PAG producing a 50% attenuation of the same spinal units. The mean magnitude of inhibition produced by stimulation in the NRM was significantly greater than that produced on the same spinal units by the same intensity of stimulation in the PAG. However, stimulation in the NRM and PAG produced the same mean percent change in inhibition per 100-microA increase in the intensity of stimulation. Thus, the slopes of the recruitment of descending inhibition from the PAG and the NRM as a function of increasing intensities of stimulation are the same; the lines of recruitment of inhibition are parallel. When examined on the same dorsal horn units, stimulation in the PAG influenced their intensity coding to graded noxious heating of the skin differently than did stimulation in the NRM. The responses of the class 2 and class 3 spinal units examined to increasing temperatures of heat applied to the skin was a monotonic linear function throughout the temperature range studied (42-50 degrees C). Stimulation in the PAG decreased the slope of the stimulus-response function (SRF) without affecting unit thresholds of response, thus influencing the gain control of nociceptive transmission in the dorsal horn. Stimulation in the NRM produced a parallel shift to the right of the SRF, influencing the set point and threshold of response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Time course and effective sites for inhibition from midbrain periaqueductal gray of spinal dorsal horn neuronal responses to cutaneous stimuli in the cat

Summary Inhibition of spinal dorsal horn neuronal responses to noxious (50 °C) skin heating by stimulation of the midbrain periaqueductal gray (PAG) was quantitatively investigated in cats anesthetized with sodium pentobarbital and nitrous oxide. Systematic variation of the interval between onset of PAG stimulation (PAGS) and onset of noxious skin heating revealed that a marked reduction of spinal unit heat-evoked discharges occured immediately upon onset of PAGS, and ceased immediately at offset of PAGS with a post-stimulation excitatory rebound. Stimulation at sites in both ventral and dorsal PAG produced inhibition, the strength of which increased sometimes in a linear manner with increasing strength of PAGS. Thresholds for the generation of descending inhibition were higher in dorsal than ventral PAG. PAGS also inhibited spinal unit responses to non-noxious skin stimulation (brushing of hairs). Descending inhibition from PAG is considered as a possible mechanism for analgesia produced by stimulation of PAG and other brainstem structures.The work was supported by a grant from the Deutsche Forschungsgemeinschaft (Zi 110)     

Dorsal border periaqueductal gray neurons project to the area directly adjacent to the central canal ependyma of the C4-T8 spinal cord in the cat

In a previous study horseradish peroxidase (HRP) injections in the upper thoracic and cervical spinal cord revealed some faintly labeled small neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study describes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WGA-HRP) injections into cervical and upper thoracic spinal segments resulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not found after injections in more caudal segments. WGA-HRP injections in the dorsal border PAG region surprisingly resulted in anterogradely labeled fibers terminating in the area dorsally and laterally adjoining the central canal ependyma of the C4-T8 spinal cord. No anterogradely labeled fibers were found more caudal in the spinal cord. The labeled fibers found in the upper cervical cord were not located in the area immediately adjoining the ependymal layer of the central canal, but in the lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border PAG-spinal neurons terminate in the neuropil of the subependymal area and in the vicinity of the basal membranes of capillaries located laterally to the central canal. The terminal profiles contain electron-lucent and densecored vesicles, suggesting a heterogeneity of possible transmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the termination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypothalamohypophysial system is discussed.     

Descending projections of Forel's field H neurones to the brain stem and the upper cervical spinal cord in the cat

Summary 1. Descending projections from Forel's field H (FFH) to the brain stem and upper cervical spinal cord were studied in cats. 2. Following implantation of HRP pellets into the spinal gray matter (C1-C3) or in the ponto-medullary reticular formation, the nucleus reticularis pontis caudalis (NRPC) or in the nucleus reticularis gigantocellularis (NRG), numerous neurones were retrogradely labelled in FFH on the ipsilateral side. In the former cases, the sizes of labelled neurones were medium-large (2040 m in diametre) while both small and medium-large neurones were labelled in the latter cases. 3. The lowest levels of spinal projection of single FFH neurones (n=70) were assessed by antidromic spikes elicited by stimulating electrodes placed in C1, C3 and C7. The majority (59%) projected to C1 (but not to C3), about 27% to C3 (but not to C7), and only 14% to C7. 4. Axonal trajectories of single FFH neurones in C1-C3 segments were investigated by antidromic threshold mapping methods. The stem axons of spinal-projecting FFH neurones descended in the ventral or in the ventrolateral funicli and the collaterals were projected to neck motor nuclei (lamina IX, Rexed 1954) and laminae V–VIII. The conduction velocities were estimated as 8–37 m/s from the antidromic latencies. 5. Axonal trajectories of 7 FFH neurones were investigated in the ponto-medullary reticular formation. All were antidromically activated from C1. In six neurones, the stem axons were located in the ventral part of the central tegmental tract and collaterals were projected to the NRPC and/or the NRG. Some of them projected to the inferior olive and the nucleus prepositus hypoglossi as well. The stem axon, in the remaining cell, was in the most dorso-medial part of the medial longitudinal fasciculus and collaterals were projected mainly to the dorsal part of the NRPC and the NRG, and also to the medial vestibular nucleus. 6. Anterograde transport of WGA-HRP injected into FFH revealed that in the upper cervical spinal cord, stem axons were found in the ventral funiculus and ventral part of the lateral funiculus. Collateral projections and presumed bouton-like deposits were observed in the laminae VI–IX, especially in their medial part. In the brain stem, dense bundles of the descending fibres were found in the central and the medial tegmental tracts and in the medial longitudinal fasciculus. FFH neurones projected densely to the caudal half of the NRPC and to the rostral half of the NRG. Extremely dense projections to the inferior olive were noted.Abbreviations AM anteromedian nucleus of the Edinger-Westphal - BCC m. biventer cervicis and complexus - CCN central cervical nucleus - CP cerebral peduncle - CTT central tegmental tract - DAB diaminobenzidine - DAO dorsal accessory nucleus of the inferior olive - DW nucleus Darkschewitsch - FFH Forel's field H - FMN fasciculus mammillothalamics - FR fasciculus retroflexus - G genu of the facial nerve - HB habenula - HRP horseradish peroxidase - INC interstitial nucleus of Cajal - IO inferior olive - LGN lateral geniculate nucleus - LHT lateral hypothalamic nucleus - MAO medial accessory nucleus of the inferior olive - MB mammillary body - MLF medial longitudinal fasciculus - MTT medial tegmental tract - MVN medial vestibular nucleus - NRG nucleus reticularis gigantocellularis - NRPC nucleus reticularis pontis caudalis - NRTP nucleus reticularis tegmenti pontis - OT optic tract - PAG periaqueductal gray matter - PC posterior commissure - PCN posterior commissure nucleus - PF parafascicular nucleus - PH nucleus prepositus hypoglossi - PHT posterior hypothalamic nucleus - PN pontine nucleus - PO principal nucleus of the inferior olive - Py pyramid - RN red nucleus - RSNs reticulospinal neurones - SN substantia nigra - SNr substantia nigra pars reticulata - sPF subparafascicular nucleus - STH subthalamic nucleus - TB trapezoid body - TMB tetramethylbenzidine - V3 third ventricle - ZI zona incerta - VI abducens nucleus/nerve - XII nucleus hypoglossi     

Projections of reticular neurones to dorsal regions of the spinal cord in the cat

Reticulospinal neurones in the cat were identified by extracellular recording and antidromic stimulation of their axons in the cord. Approximately 34% of reticulospinal neurones in the medulla, and 28% in the pons, were found to project to dorsal regions of the cord, between T9 and L2. Most of these neurones had one branch situated dorsally and another in the ventral or ventrolateral funiculus. Some branches travelled for short distances in the dorsal columns. Microstimulation techniques demonstrated the presence of branches of reticulospinal fibres in the dorsal horn. The results may provide an anatomical basis for the widespread effects of stimulation of the reticular formation on afferent transmission in the spinal cord.     

Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey

In 4 rhesus monkeys wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the mesencephalic tegmentum. In 3 cases with injections involving the red nucleus (RN), rubrospinal fibers descended mainly contralaterally to terminate in laminae V, VI and dorsal VII of the spinal cord and in the lateral motoneuronal cell groups at the level of the cervical and lumbosacral enlargements. In all 4 cases the area of the interstitial nucleus of Cajal (INC) was injected, which resulted in labeled interstitiospinal fibers in the medial part of the ipsilateral ventral funiculus of the spinal cord. The results indicate that there is no major qualitative difference between the mesencephalic (RN and INC) and motor cortical projections to the spinal cord.     

Anatomical evidence that the pontine lateral tegmental field projects to lamina I of the caudal spinal trigeminal nucleus and spinal cord and to the Edinger-Westphal nucleus in the cat

Autoradiographical tracing results in the cat indicate that the lateral pontine tegmental field projects mainly contralaterally to the marginal layer of the spinal trigeminal nucleus, to laminae I and II and the lateral part of laminae V and VI of the spinal cord and the Edinger-Westphal nucleus. It is pointed out that the projections from the lateral pontine tegmentum are very similar to the ones derived from the Edinger-Westphal nucleus and that these two areas are reciprocally connected. It is postulated that both areas may play a role in supraspinal pain control.     

Fluorescent double‐label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA‐HRP into either the spinal cord or periaqueductal gray, labeled neurons were 7observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double‐labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double‐labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15–18 μ in their greatest transverse diameter. No double‐labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN‐spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions. Anat Rec 256:91–98, 1999. © 1999 Wiley‐Liss, Inc.