Dorsal Root Fibers Research Articles

Direct excitant action of convulsant barbiturates.

5-(2-cyclohexylideneethyl)-5-ethylbarbituric acid (CHEB) and other convulsant barbiturates produced depolarization of isolated dorsal root fibres and other unmyelinated nerves from the rat. CHEB-induced depolarizations, unlike kainate-induced depolarizations were abolished by omission of Ca2+ from, or addition of ruthenium red (5 μM) to, the bathing medium.

Internodal length of nerve fibres in dorsal roots of cat spinal cord

Internodal length of myelinated fibres adjacent to the borderline between the central and the peripheral nervous system in S1 dorsal roots of cats during development was determined. Numerical data was obtained from reconstructions of individual nerve fibres as recorded on scale using several long series of consecutive sections. At the initiation of myelination (foetus 60 days), the internodal length was ∼ 100 μm. During the period 7–30 days, internodes shorter than 50 μm occurred. In older cats the shortest internodes were about 100 μm in length. In the adult cat a linear relationship between axon diameter and internodal length was not found.

Dual mode of junctional transmission at synapses between single primary afferent fibres and motoneurones in the amphibian.

1. The isolated hemisected frog spinal cord has been used to examine the effects of changes in ionic composition of perfusing medium on the intracellularly recorded e.p.s.p.s produced in single motoneurones by direct stimulation of individual dorsal root fibres through a separate intracellular micro-electrode.2. The monosynaptic single-fibre e.p.s.p.s usually reveal two distinct components, early and late. The early component is resistant to replacement of external Ca(2+) by Mn(2+) whereas the later is reversibly abolished. It is concluded that the junction between primary afferent fibre and motoneurone provides joint electrical and chemical transmission.3. The average amplitude of the unitary electrical e.p.s.p. varies at different junctions from 25 to 430 muV, mean 124 +/- 17 muV (n = 50). It is relatively stable and changes its amplitude only with changes in the height of presynaptic spike. The individual amplitudes observed in a given cell usually have a normal distribution suggesting that any variability in electrical response is entirely due to noise.4. The average amplitude of the single-fibre chemically mediated e.p.s.p. varies from less than 20 muV to 1.7 mV, mean 222 +/- 33 muV (n = 71). A positive correlation was found between the amplitudes of chemical and electrical e.p.s.p.s recorded from different motoneurones.5. Chemical e.p.s.p.s evoked by consecutive impulses in a single presynaptic fibre show statistical fluctuations in amplitude. The fluctuations occur in quantal steps in a manner described by binomial or Poisson statistics. Only in a few cases the deviation from stochastic distribution can be attributed to variable invasion of nerve impulses into the terminal region.6. The amplitude of the unit e.p.s.p. varies between 33 and 104 muV, mean 66.4 +/- 4.3 muV (n = 19). The mean quantum content (m) varies from less than 1 to more than 10. The m is reversibly reduced by Ca(2+) lack and by Mn(2+) until the chemically mediated response fluctuates according to a Poisson distribution with the unit e.p.s.p. equivalent to the single quantum of transmitter.7. When paired or repetitive stimuli are applied to the presynaptic fibre the facilitation of the chemically mediated unitary e.p.s.p. can be observed as well as the post-tetanic potentiation. The amplitude of the electrical e.p.s.p. remains unchanged during the period of post-tetanic potentiation, suggesting that the latter is attributable to some change restricted to the specific presynaptic mechanism responsible for the transmitter release but not to changes in presynaptic spike height.

Direct dorsal root projection onto the cerebellum in the frog

With the aid of an improved cobalt labeling technique, a relatively large number of dorsal root fibers were shown to terminate ipsilaterally as mossy fibers in the granular layer of the frog cerebellum. These direct dorsal root-cerebellar fibers originated exclusively from the limb-innervating segments of the spinal cord.

Segregation of electro- and mechanoreceptive inputs to the elasmobranch medulla

The anterior lateral line nerve of the thornback ray consists of fibers that innervate head electroreceptive ampullary organs and mechanoreceptive neuromasts. As the anterior lateral line nerve enters the medulla it divides into dorsal and ventral roots. Single unit responses of dorsal root fibers to electric field and mechanical stimuli indicate that the dorsal root consists only of ampullary fibers, whereas the ventral root consists only of mechanoreceptove fibers. The dorsal and ventral roots of the anterior lateral line nerve terminate in the dorsal and medial octavolateralis nuclei respectively, indicating that the dorsal nucleus is the primary electroreceptive nucleus of the elasmobranch medulla and the medial nucleus is the mechanoreceptive nucleus. Averaged evoked potential responses to electric field stimuli could be recorded from the dorsal but not the medial nucleus, further evidence that the dorsal nucleus is the electroreceptive nucleus. A second evoked response to electric field stimuli was elicited from the lateral reticular nucleus, suggesting that the reticular formation may be a secondary target of efferents of the dorsal octavolateralis nucleus. A dorsal octavolateralis nucleus exists not only in elasmobranchs, but also in agnathan, chondrostean, dipnoan, and crossopterygian fishes, suggesting that all of these taxa are also electroreceptive.

Longitudinal extent of dorsal root fibres in the spinal cord and brain stem of the frog

The longitudinal arrangement of dorsal root fibres was investigated with a modified cobalt labelling technique in the spinal cord and brain stem of frogs. The topographical order of dorsal root fibres in the dorsal white column closely resembles the well-known scheme of the mammalian spinal cord. A significant difference between frogs and mammals is the extension of fibres up to the cerebellar plate. The ascending fibres of different origin are organized in concentric rings in the medulla. An oval-shaped area and a triangular area in the dorsal horn, and the motor horn, receive fibre collaterals in the spinal cord. Thoracic dorsal root fibres terminate exclusively in the oval-shaped area. Fibre terminations clearly outline the dorsal column nuclei which begin in the obex region and end at the level of the glossopharyngeal nucleus. The spinal nucleus of the trigeminus is richly supplied by both thin and thick calibre dorsal root fibres in its entire rostrocaudal extension. Two parts of the reticular formation receive dorsal root fibres; the first is in the dorsal gray matter ventral and lateral to the solitary fascicle in the medulla, the second is the lateral reticular zone. In the vestibular region, the medial, lateral and superior vestibular nuclei are innervated by dorsal root fibres. The granular layer of the cerebellum receives a significant contingent of dorsal root fibres. Fibres terminating in the vestibular region and in the cerebellum arise from limb-innervating spinal ganglia. The results indicate a close similarity in the longitudinal arrangement of dorsal root fibres in frogs and in higher vertebrates. The several collaterals that terminate in the hindbrain may modulate the function of the receiving structures. On the basis of present and previous findings the aggregation of primary sensory fibres and the convergence of their terminations are surveyed in the hindbrain.

Selective synaptic changes following spinal motoneuron axotomy

Fibers descending in the lateral columns (LC) of the frog spinal cord form synapses more proximal to the soma than those derived from dorsal root (DR) fibers. Following ventral root transection (motoneuron axotomy), the risetime of the LC-EPSP increased while that of the DR-EPSP decreased such that the risetime of the LC-EPSP was now longer than that of the DR-EPSP. Focal-potential analysis showed that the location of LC synapses remained in the region of the motor nucleus while DR synapses were located closer to the motor nucleus after axotomy. Proximal synapses thus persist after axotomy, but they undergo physiological changes that alter the LC-EPSP time course. In addition membrane properties of axotomized motoneurons were investigated. Input resistance, rheobasic current, and threshold depolarization all increased. Resting membrane potentials and after hyperpolarizations were unchanged. Because axotomy had opposite effects on LC- and DR-EPSPs, changes in membrane properties cannot account for changes in EPSPs. Other possible mechanisms are discussed.

Hyperpolarization of frog primary afferent fibres caused by activation of a sodium pump.

1. In the isolated frog spinal cord repetitive stimulation of a lumbar dorsal root produced a sustained negative potential recorded from an adjacent inactive dorsal root by sucrose gap techniques. This negative potential was followed by a positive potential, an indication that the dorsal root terminals were hyperpolarized. Increasing the duration of the tetanus applied to the active root increased the amplitude and duration of the after-hyperpolarization which could be up to 6 mV and 3 min respectively. 2. The hyperpolarization presumably reflected an increased rate of active sodium pumping. Since it was reversibly reduced by metabolic inhibitors (dinitrophenol, NaCN) and cooling (Q10, 2 . 6) it was clearly dependent upon intact metabolic activity. In addition, a variety of procedures used to inhibit sodium pumps (including application of ouabain, elimination of potassium from the superfusate, and partial substitution of lithium for sodium ions) significantly and reversibly decreased the potential. 3. The hyperpolarization was not dependent upon intact chemical synaptic transmission since it could survive prolonged immersion of the cord in Ringer solution containing manganese or magnesium ions. 4. It is suggested that the hyperpolarization of inactive fibres resulted from a decreased extracellular potassium concentration in the dorsal horn produced as a result of a pumping mechanism which extruded sodium and transported potassium inwards by dorsal root fibres directly activated by the tetanus.

Recurrent interactions between individual motoneurones and dorsal root fibres in the frog

Direct intracellular stimulation of amphibian motoneurones can elicit unitary recurrent responses in some individual primary afferent fibres. These effects are probably mediated through GABA-ergic terminals of spinal interneurones.

A horseradish peroxidase study of primary afferent projections to the medullary cuneate nucleus in the rat

HRP was used to label first order afferent cells of the dorsal root ganglia which synapse at the cuneate nucleus. The results show that it is a population of predominantly large cells that project monosynaptically to the medullary cuneate nucleus and thus support anatomical and physiological studies showing that directly projecting axons to the dorsal column nuclei are larger, faster and conduct information from a select class of sensitive cutaneous mechanoreceptors. The finding, that approximately 20% of DRG neurons can be labeled from the dorsal column nuclei, is consistent with previous estimates of dorsal root fiber projections in the dorsal column based on Marchi degeneration.

Histological changes after transection of the spinal cord of fetal and neonatal mice

The spinal cords of fetal (days 12 to 17 gestation) and neonatal (0 to 3 days postnatal) mice were transected and examined histologically to as long as 4 months postoperatively. Serial reconstructions were made of spinal cord sections which were prepared within the vertebral column. Based upon the application of several histological stains, we found: (i) Several months after transection the cut ends of the spinal cord were separated by a space of as many as one to two vertebral segments in transection animals and two to four vertebral segments in resection animals. (ii) After complete transection, the lesion site contained longitudinally oriented glial processes which occasionally connected the rostral and caudal segments of the spinal cord. (iii) Very few nerve fibers traversed the lesion site continuously from one stump to the other. (iv) Connective tissue scarring at the site of transection was absent and neuroglial scar formation minimal to both fetal and neonatal mice. (v) In the undamaged regions of the spinal cord, microcysts occurred more frequently in neonatal than in fetal animals. (vi) Dorsal root fibers regenerated across the gap between the ends of the cord and reentered the rostral stump of the cord. We conclude that despite the absence of connective tissue and glial scars, there was insignificant outgrowth of spinal nerve fibers. Thus, there is no greater regenerative capacity in the fetal and neonatal spinal cords than in the adult spinal cord.

Acute Hyperesthesia after Spinal Trauma

Four cases of acute hyperesthesia after spinal trauma are presented. An attempt is made to characterize the natural history of this syndrome. In the three cases with truncal sensory aberration, hyperesthetic zones developed only anteriorly. All four patients demonstrated spontaneous relief of pain within 2 weeks after injury. The pain was modified by transcutaneous stimulation in one case and by l-dopa in another case. It is suggested that the "lesion" responsible for the hyperesthetic response is partial damage to dorsal root fibers at a point medial to the dorsal root ganglion.

Selective regeneration of sensory fibers following nerve crush injury

We investigated whether or not all classes of myelinated sensory fibers regenerate simultaneously after nerve crush injury and whether each parent fiber produces and maintains a regenerated branch. Electrophysiologic recordings were made from isolated lumbar dorsal root fibers in rats at various times after sciatic nerve crush injury. For all fibers that responded to electrical stimulation distal to the crush, the conduction velocity of the parent axon was established using the collision method. During the first few months following sciatic nerve injury, Aδ parent fibers were under-represented in the sample of successfully regenerated fibers. Only later did sprouts of the bulk of Aδ fibers grow out. In time, however, all fibers regenerated. Combined with histologic data indicating that fiber number was restored in the distal nerve stump, this finding indicated that after crush injury the majority of parent fibers maintained one, but only one, regenerated branch.

Reexamination of the dorsal root projection to the spinal dorsal horn including observations on the differential termination of coarse and fine fibers

Primary afferent fibers in the lumbar, sacral, and caudal spinal segments of several mammals (rat, cat, monkey) were stained by applying horseradish peroxidase to the proximal part of cut dorsal rootlets and reacting the tissue histochemically after several hours of survival. The stained fibers' pattern of termination in the dorsal horn was similar in all three species, with many bouton-like enlargements in the ipsilateral marginal zone, substantia gelatinosa, and nucleus proprius, as well as a few projections at each level to the dorsal commissure and contralaterally to the ventral border of the nucleus proprius. Partial lesions of dorsal rootlets in monkey revealed that the thin fibers comprising the lateral division end principally in the marginal zone and substantial gelatinosa, while the thick fibers of the medial division terminate in the nucleus proprius and deeper regions, contributing little to the substantia gelatinosa and marginal zone. On the basis of the termination patterns observed for whole and partly sectioned rootlets, the superficial dorsal horn can be divided into at least four regions. (1) The marginal zone (lamina I of cat) appears to receive terminations from intermediate (smaller myelinated) fibers; (2) the outer substantia gelatinosa (outer lamina II) receives many terminations from the very finest afferent fibers; (3) the inner substantia gelatinosa (inner lamina II) receives endings from some of the finest fibers and also from intermediate (smaller myelinated) fibers; and (4) the superficial part of the nucleus proprius (lamina III) receives endings from intermediate and large diameter dorsal root fibers.

Potentiation of the effects of GABA by pentobarbitone

Pentobarbitone (50 microM) has been shown to potentiate GABA-induced responses in rat isolated superior cervical ganglia and in dorsal or ventral root fibres of immature rat isolated spinal cords. Measurements of dose ratios for the antagonism of GABA-induced responses of dorsal root fibres by bicuculline showed that pentobarbitone (50 microM) did not significantly alter this antagonism. This result indicates that the antagonism of GABA by bucuculline at dorsal root fibres is not reversed by pentobarbitone. Ventral root responses to GABA (but not glycine) were also potentiated by pentobarbitone (+/-)-Nipecotic acid (300 microM) potentiated responses to GABA much more than those to 3-aminopropane-sulphonic acid.

Afferent fibers in the ventral root.

The dorsal rhizotomy was one of the earliest operations used for the treatment of chronic, intractable, segmental pain. The theoretical basis underlying the use of this procedure was the law of separation of function of the spinal roots that implies, among other things, that sensory fibers are located in the dorsal root and motor fibers are located in the ventral roots. On this basis, it should be possible by cutting the appropriate dorsal roots to relieve pain for any particular part of the body at the cost of making that part of the body anesthetic. Unfortunately, however, the dorsal rhizotomy seems to have a relatively low success rate in that the pain is often not relieved or is relieved for a variable period of time and then returns. Many different reasons may account for these failures, but one reason that is not often considered is that the law of separation of function of the spinal roots may not be accurate. This is pertinent at the present time, because large numbers of ventral root afferent fibers have recently been discovered. This review summarizes the work, both old and new, on ventral root afferent with particular emphasis on possible clinical applications. In particular, it is pointed out that the ventral root afferent fibers are probably present in humans and, if so, they may be involved in explanations for the failure of dorsal rhizotomy to relieve chronic pain. If these fibers are important, then they can be removed by a dorsal root ganglionectomy, which would remove both the dorsal and ventral root afferent fibers, as opposed to a dorsal rhizotomy, which would remove only the dorsal root afferent fibers.

Maturation of axonal sprouts after nerve crush

We traced the gradual increase in conduction velocity of axons regenerating after nerve crush injury. Electrophysiologic recordings were made from populations of single myelinated lumbar dorsal root fibers in adult rats at various times after the sciatic nerve had been crushed. Conduction velocity was established for the parent fiber as well as for the maturing sprout at several points along its length. There was initially a sharp gradient in conduction velocity along the length of sprouts, but this returned to normal values by about the 80th day after injury. The conduction velocity of a regenerating sprout was always proportional to that of its parent fiber, even at very short survival times before the sprout had become connected to the periphery. The conduction velocity increased at a steadily decreasing rate until about the 150th day by which time it had achieved normal values. Sprouts of the fastest conducting parent fibers increased their conduction velocity at a greater rate than those of slowly conducting parent fibers. As a result, sprouts of all size-classes of myelinated parent fibers reached maturity at about the same time. Regenerating axons played a sensory role as soon as they arrived in the skin of the foot, long before they had reached full maturity.

The distribution of dorsal root axons in laminae I, II and III of the macaque spinal cord: a quantitative electron microscope study.

AbstractThis study examines the projection of dorsal root fibers to the upper dorsal horn of the monkey lumbar spinal cord utilizing degeneration and autoradiographic methods. The animals survived dorsal rhizotomy for periods varying from 18 hours to 28 days. Electron microscopy reveals the earliest degeneration to be neurofilamentous alteration of large synaptic profiles in lamina III and the inner zone of the substantia gelatinosa (IIi). This degeneration begins 18 hours after rhizotomy, reaches a peak at three days postoperatively and disappears by the end of the first week. Degenerating myelinated axons in the spinal gray matter, dorsal column white matter and Lissauer's tract first appear three days postoperatively.The second tye of degeneration of synapses occurs in lamina I and outer gelatinosa (IIo) and consists of electron lucent alteration of moderate size synapses, especially those having large granular vesicles (LGVs) and some neurofilamentous and dense degeneration. This synaptic degeneration in lamina I begins two days following rhizotomy and reaches a peak between five to seven days, declines markedly by ten days and is absent at four weeks survival.The third type of degeneration occurs in the substantia gelatinosa (laminae IIo and IIi) initially as an enlargement of synaptic vesicles at two days and then progresses to large numbers of electron dense small synapses, the peak of degeneration occurring at seven days and persisting as long as four weeks postoperatively. Some of the dense synapses can be seen to arise from small, nonmyelinated axons. These axons are first seen to be degenerating in the gelatinosal and marginal layers at four days survival and the first definite degeneration of nonmyelinated axons in Lissauer's tract is at seven days postoperatively.It is concluded that the largest axons projecting to this region of the dorsal horn degenerate most rapidly and that these axons are distributed to laminae III and IIi. Axons of intermediate diameter degenerate next and are distributed principally to laminae I and IIo. Fine diameter axons, probably nonmyelinated, degenerate more slowly and terminate principally in the substantia gelatinosa (IIi and IIo). There is some overlap in these projection domains, in that the principal projection to lamina III extends into the lower part of the gelatinosa and the projection to the marginal layer overlaps the outer gelatinosa.The axon terminals in gelatinosa of C fibers are sometimes postsynaptic in axoaxonal synapses as are several of the axon terminals of larger A fibers in lamina III. Most of the synapses of primary afferent origin in lamina I are not involved in axoaxonal synapses. It is likely that the terminations of many primary afferent fibers in laminae II and III are subject to presynaptic inhibition and those in lamina I are not. Some of the primary afferents in all three laminae synapse upon presynaptic dendrites and thus may influence transmitter release from these profiles.The LGV profiles are distributed in a manner similar to the distribution of substance P and it is suggested that the degenerating LGV profiles may contain substance P. Most of the LGV profiles and many of the round vesicle profiles do not appear to be derived from dorsal root, but most of the central synaptic profiles are of primary afferent origin. In no case was there evidence that flat vesicle synapses were derived from primary afferents.Following dorsal root ganglia injections with H3 leucine, light microscopic autoradiography at short postoperative survival times demonstrated heavy grain distribution over marginal and gelatinosal layers with somewhat less numbers of grains over lamina III. There were also many grains over the dorsal column white matter and Lissauer's tract. Electron microscopic autoradiography revealed that the majority of labeled structures seen with fast axonal transport in the upper dorsal horn are not synapses but are myelinated and nonmyelinated axons. Labeled synapses were the same types as those undergoing degeneration following rhizotomy: round vesicle profiles, central synaptic profiles and LGV profiles. Each of the labeled types was distributed throughout the upper laminae, with the exception of LGV profiles which are uncommon in layers deep to the outer zone of the gelatinosa. It is concluded that fast axon transport autoradiography is not a selective label for synapses in the cord and light microscopic autoradiography does not provide direct estimates of synaptic densities in the dorsal horn.

Identification of dorsal root synaptic terminals on monkey ventral horn cells by electron microscopic autoradiography.

The projection of dorsal root fibres to the motor nucleus of the macaque monkey spinal cord has been examined utilizing light and electron microscopic autoradiography. Light microscopy demonstrates a very sparse labelling of primary afferent fibres in the ventral horn. Silver grains overlying radioactive sources are frequently clustered into small groups, often adjacent to dendritic profiles. Under the electron microscope, myelinated axons and a few large synaptic profiles containing rounded synaptic vesicles were overlain by numerous silver grains. These labelled profiles made synaptic contact with dendrites 1-3 micron in diameter. The labelled profiles did not contact cell bodies or large proximal dendrites of ventral horn neurons. Frequently, small synaptic profiles containing flattened vesicles were presynaptic to the large labelled terminals and it is suggested that these axoaxonal synapses may mediate presynaptic inhibition of the primary afferent fibres. The relationship of the present findings to previously published physiological and anatomical studies is discussed.

The origin, distribution and synaptic relationships of substance P axons in rat spinal cord

AbstractSubstance P positive (SP+) immunoperoxidase reaction product has been localized in light and electron microscopic preparations of rat lumbar spinal cord using an immunocytochemical method. SP+ reaction product was found to be highly concentrated in dorsal horn laminae I, II, portions of III, Lissauer's tract, s small nucleus in the dorsal part of the lateral funiculus, and in a longitudinal bundle of fibers just ventral to the central canal. Moderate accumulations of SP+ reaction product were observed in portions of laminae III‐V, lamina X and in a narrow zone bordering fasciculus gracilis which expanded in its ventral aspect to include nucleus cornucommissuralis dorsalis and nucleus dorsalis. Remaining portions of the spinal gray matter exhibited extremely sparse staining. Light microscopic observations indicated that SP+ product was concentrated in fine axons and collaterals, which displayed densely‐stained varicosities and terminal puncta. These varicosities and puncta were closely associated with neurons and blood vessels.Dorsal rhizotomy produced a marked reduction in the number of SP+ fibers in Lissauer's tract, dorsal horn laminae I‐III and the nucleus of the dorsolateral funiculus. However, dorsal root fibers were not the only source of SP+ structures in these regions since some SP+ fibers remained following dorsal root resections. In addition, this residual population of SP+ fibers did not appear to be altered by a combination of dorsal rhizotomy and ipsilateral transverse hemisection of the spinal cord. In conjunction with the results of other investigators, this finding suggests that the SP+ fibers which remain after dorsal rhizotomy are derived from local circuit interneurons of the spinal cord.Electron microscopic observations revealed that certain axons and axon terminals contained concentrations of SP+ product which were associated with large, granular vesicles, while lesser amounts were in the terminal cytoplasm and associated with the exterior surfaces of small, agranular synaptic vesicles. SP+ terminals formed axodendritic, axosomatic and axoaxonal synapses, but the deposition of SP+ product often was not concentrated precisely at the “active sites” of these synaptic junctions. Some SP+ axons also made rudimentary contacts with astrocytic processes including those surrounding blood vessels. Since many of the SP+ terminals are similar in several respects to neuroendocrine terminals, it is possible that substance P may be released at several different types of non‐synaptic sites as well as at conventional synaptic junctions. Thus our findings suggest that substance P could participate in a variety of neural functions ranging from those which are limited to synaptic junctions to those which are more generally distributed via an involvement of axonal plexuses as well as the glial, vascular and ventricular systems.