Many ventral root afferent fibers in the cat are third branches of dorsal root ganglion cells

Ganglionic axons in motor roots and pia mater

In addition to motor axons and preganglionic axons, ventral roots contain unmyelinated or thin myelinated sensory axons and postganglionic sympathetic axons. It has been said that ventral roots channel sensory axons to the CNS. However, it now seems that these axons end blindly, shift to the pia or loop and return towards the periphery and that these units reach the CNS via dorsal roots. Sensory ventral root axons project from a variety of somatic or visceral receptors; some of them are third branches of dorsal root afferents and some seem to lack a CNS projection. Many ventral root afferents contain substance P (SP) and/or calcitonin gene-related peptide (CGRP). These fibres are not affected by neonatal capsaicin treatment and they cannot induce radicular or pial extravasation. Some thin ventral root axons are sympathetic and relate to blood vessels. Afferents containing SP and/or CGRP and sympathetic axons also occur in the spinal pia mater. The sensory axons mediate pain. They might also have vasomotor, tissue-regulatory and/or mechanoreceptive functions. The motor roots of cranial nerves IV, VI and XI contain unmyelinated axons arranged like in ventral roots outside the autonomic outflow. However, the motor root of cranial nerve V channels some unmyelinated axons into the CNS. The occurrence of thin axons in ventral roots and pia mater changes during development and ageing. After peripheral nerve injury, ipsilateral ventral roots and pia are invaded by new sensory and postganglionic sympathetic axons.

Node distribution and packing density in the rat CNS-PNS transitional zone

The density of nodes of Ranvier was examined at CNS, PNS, and transitional zone (TZ) levels of rat lumbar ventral motoneurone fibres. It was found to be significantly greater in the TZ than at the other levels: The difference was sevenfold for the ventral root and at least fourfold for central fibre levels. Node distribution and spacing was examined within the two main types of TZ found in rat ventral rootlets: the first, in which the TZ is short and is approximately on a level with the surface of the cord; and the second, in which it is much longer and extends into the proximal part of the rootlet. Node spacing was estimated as nearest neighbour distance, the true distance between adjacent node centres. This is a better estimate of node spacing than simple density since it measures the actual linear distance between nodes over which any interaction between them would be likely to take place. Despite marked differences in the dimensions of the two types of TZ, nearest neighbour distance distribution was very similar in each, suggesting that similar mechanisms may influence their spacing during development. The TZ contains especially large amounts of interstitial tissue, mainly composed of astrocyte processes, separating the fibres traversing it. The proportion of the TZ composed of interstitium was over three times that in the ventral root and nearly twice that at the CNS level studied. The large amounts of astrocytic tissue in the TZ may be related to the high packing density of nodes. It may function to regulate extracellular ionic concentrations in the TZ and to maintain a stable ionic environment for the transitional nodes.

Are the neurons in the dorsal root ganglion pseudounipolar? A comparison of the number of neurons and number of myelinated and unmyelinated fibres in the dorsal root

The neurons in the dorsal root ganglion have classically been described as pseudounipolar. Previous studies have questioned this simple organisation because an equality between the number of fibres in the dorsal root and neurons could not be established. In this study the number of neurons in the fifth lumbar dorsal root ganglion of the adult rat is compared to the number of fibres in the dorsal root. The methods used are founded on unbiased stereological principles and includes the optical disector, the Cavalieri principle, unbiased counting rules in two and three dimensions, and systematic random sampling. The number of A- and B-cells is estimated with light microscopy, and the number of myelinated and unmyelinated fibres is estimated with electron microscopy. The present study demonstrates that there is a 1:1 ratio (mean: 0.98, CV: 0.12, 95% confidence interval: 0.90-1.07) of fibres in the dorsal root to neurons in the dorsal root ganglion, as the classical theory predicts. Furthermore, the study of the two neuron subtypes supports the hypothesis that myelinated fibres originate from the A-cells and the unmyelinated fibres from the B-cells.

Nerve fibers immunoreactive for substance P and calcitonin gene-related peptide in the cervical spinal ventral roots of the mouse

We demonstrate the existence of nerve fibers possessing substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactivity in the mouse cervical ventral roots. The distribution of the SP and CGRP fibers was similar, but CGRP fibers were generally more numerous. Both types entered the ventral pia mater or formed hairpin loops, but they did not enter the spinal cord directly through these roots. SP and CGRP fibers in the ventral roots were thin and had many varicosities. We suggest that these SP and CGRP fibers are involved not only in a sensory mechanism, but also in other functions, via the release of SP and CGRP from varicosities in the ventral roots.

Sensory C-fibers in rat ventral roots are capsaicin-insensitive and they do not mediate extravasation from pial vessels

Mammalian ventral roots and pia mater contain sensory C-fibers, some of which exhibit a substance P- and/or calcitonin gene-related peptide (CGRP)-like immunoreactivity. At some locations, sensory axons containing these neuropeptides evoke peripheral plasma protein extravasation after antidromic electrical stimulation. Such axons usually disappear following treatment of neonatal rats with capsaicin. The purpose of the present study is to find out if afferent C-fibers in the rat ventral roots L4 and L5 are capsaicin-sensitive, and if antidromic stimulation of these fibers elicits extravasation in the root and/or the ventral pia mater. The results show (1) that the number of C-fibers in these ventral roots is unaffected by neonatal capsaicin treatment, as seen in the electron microscope; (2) that the occurrence and general configuration of axons with substance P- and CGRP-like immunoreactivity do not appear abnormal in neonatally capsaicin-treated rats, as revealed by fluorescence microscopy on longitudinal frozen sections; (3) that Evans blue albumin is not extravasated in the ventral root or pia mater after electrical ventral root stimulation or following systemic injection of capsaicin. We conclude, that ventral root afferents are functionally different from otherwise similar afferents at other locations.

Immunohistochemical evidence for sprouting of ventral root afferents after neonatal sciatic neurectomy in the rat

The present study demonstrates that afferent fibers in the rat ventral root sprout after transection of the neonatal sciatic nerve. The key data are that the number of unmyelinated fibers increased dramatically in the L5 ventral roots and about half of these unmyelinated fibers were labeled with antibody to growth associated phosphoprotein (GAP-43), suggesting that they are regenerating axons. Furthermore, these fibers survived in the distal stump of the acutely cut ventral root, suggesting that they are afferent fibers. The results provide direct evidence for sprouting of afferent fibers in the ventral root induced by a neonatal peripheral nerve lesion.

Neonatal sciatic nerve lesion triggers the sprouting of fibers in the contralateral ventral root of the rat

We explored the possibility that unilateral neurectomy of the sciatic nerve of the rat at the neonatal stage triggers sprouting of afferent fibers in the contralateral ventral root. 3 months after neonatal sciatic neurectomy, the numbers of both myelinated and unmyelinated fibers in the L5 and L3 ventral roots were counted on electron micrographic montages. Age-matched littermates were used as unoperated controls. To identify regenerating axons, electron microscopic immunohistochemistry was done on the ventral roots using antibody against growth-associated phosphoprotein (GAP-43). Neonatal sciatic neurectomy resulted in: (1) about a three-fold increase in the number of unmyelinated fibers in the contralateral L5 ventral root as compared with the unoperated control; (2) about a 25-fold increase in the number of unmyelinated fibers in the ipsilateral L5 ventral root as compared with the control; (3) approximately 25% of the unmyelinated fibers in the contralateral L5 ventral root expressing GAP-43; and (4) no significant change in the number of unmyelinated fibers in the L3 ventral root of either side as compared with the control. The data suggest that a neonatal sciatic neurectomy of the rat triggers sprouting of unmyelinated afferent fibers in the ventral root of the contralateral as well as the ipsilateral side. The sprouting is restricted, however, to spinal segments which receive inputs from the sciatic nerve.

Routes of putative afferent axons in rat lumbosacral ventral roots and pia mater

Unmyelinated sensory axons occur in mammalian ventral roots. On this basis the law of Magendie has been questioned. With few exceptions, the current ideas on the routes that these axons might follow have been based on electron microscopic examination of transverse sections, and evidence obtained through electrophysiological experiments or tracer studies. The purpose of the present study is to examine putative afferent rat ventral root axons, which have been visualized directly by immunohistochemical labelling. After immunolabelling of axons containing substance P or calcitonin gene-related peptide in longitudinal sections from the ventral roots L4-S1, and in wholemount pia mater-rootlet preparations, fluorescent fibres were searched for in the microscope. The results show that some immunolabelled axons end blindly in the root, while others shift to the pia mater, or make U-turns. Some axons make irregular 'zigzag' deviations or branch. No immunoreactive axons were seen entering the spinal cord via the ventral roots. These observations support the view that the law of Magendie is valid.

Evidence for invasion of regenerated ventral root afferents into the spinal cord of the rat subjected to sciatic neurectomy during the neonatal period

Sectioning the sciatic nerve of experimental animals at the neonatal stage triggers growth of afferent fibers in the ventral root. The present study examined the possibility that the regenerating fiber terminals grow into the spinal cord. The sciatic nerve on one side was cut in neonatal rats. After the rats were fully grown, either an electrophysiological or a histochemical study was performed. The results of electrophysiological experiments showed that stimulation of certain loci in the L5 spinal cord evoked antidromic potentials in the L5 ventral root with a long latency. Various evidence suggests that the long latency potentials are due to activation of C fibers. These C-fiber potentials were on average bigger and were elicited from more numerous loci on the side ipsilateral to the sciatic nerve lesion than on the contralateral side. Furthermore, stimulation of the spinal cord of unoperated normal rats rarely evoked such potentials. For the histochemical study, horseradish peroxidase (HRP) was injected into the L5 spinal cord after cutting the L4-L6 dorsal roots. A lot more cells in the L5 dorsal root ganglion (DRG) on the side ipsilateral to the sciatic nerve lesion were labeled with HRP transported retrogradely through the L5 ventral root than on the contralateral side. Control experiments showed that few DRG cells are labeled with HRP in normal unoperated rats. The combined results of the electrophysiological and histochemical studies suggest invasion of ventral root afferents into the spinal cord, given enough postoperative time. It is not known whether or not these terminals make functional synaptic contacts in the spinal cord.

Fibre composition of the ventral roots L7 and S1 in the owl monkey (Aotus trivirgatus)

The ventral roots L7 and S1 of the owl monkey Aotus trivirgatus, were examined by electron microscopy. On average, these roots contain 2950 and 1837 myelinated axons respectively. In both roots the myelinated axons have bimodal size distributions, but the S1 root contains more small myelinated axons. Both roots contain a substantial proportion of unmyelinated axon profiles (UAP). In the L7 root the proportion of UAP decreases as the spinal cord is approached, from 19% distally to 5% in the juxtamedullary rootlets. Unmyelinated and very small myelinated CNS-type axons have not been observed in the L7 transitional region. The average S1 root contains some 40% unmyelinated axons at all examined proximo-distal levels. Unmyelinated/very small myelinated axons are easily found on the CNS side of the S1 transitional region, in direct relation to motoraxon bundles. Bundles of unmyelinated and small myelinated axons occur in the ventral pia mater of both segments. The unmyelinated axons in the L7 root of the owl monkey appear to be arranged like those in the feline L7 ventral root, possibly representing afferents. It is likely that most unmyelinated and small myelinated axons in the ventral root S1 are autonomic efferents.

Increased number of unmyelinated fibers in the ventral root after peripheral neurectomy in adult rat

We examined the possibility that peripheral nerve injury in the adult rat triggers sprouting of unmyelinated ventral root afferent fibers. Three to 5 months after the sciatic nerve was sectioned on one side in the adult rat, myelinated and unmyelinated fibers were counted at 3 sites along the length of the ventral root. A sciatic nerve lesion resulted in about a 3-fold increase in the number of unmyelinated fibers in the L5 ventral root. Our data suggest that a peripheral nerve lesion in the adult rat triggers sprouting of unmyelinated afferent fibers in the ventral root. No evidence was found that dorsal rhizotomy triggers sprouting of afferent fibers.

A quantitative study of the central projection patterns of unmyelinated ventral root afferents in the cat

1. The ventral roots of the spinal cord contain a large number of unmyelinated primary afferent neurones. There is some controversy, however, about the function of these fibres and the route of their central projection. Here we have used electrophysiological techniques to quantify the central projection patterns of these neurones in the segment S2 of adult chloralose-anaesthesized cats. 2. A total of 1185 single unmyelinated units were recorded in small filaments isolated from intact and de-efferented ventral roots or intact dorsal roots of the segment S2 in nineteen cats. The projection patterns of these neurones were tested using supramaximal electrical stimulation of the pelvic and pudendal nerve (the main tributaries of the spinal nerve of this segment) and of the segmental companion root (dorsal or ventral as appropriate). 3. The principal finding of this study is that 85% of unmyelinated afferent axons in the ventral root are direct and exclusive projections. They constitute a separate class of afferents which is only capable of transmitting information from the periphery via the ventral roots. However, the proportion of these fibres that enter the central nervous system is unknown and it seems likely that some of them peter out as they approach the spinal cord and end blindly. The functional role of such afferents remains obscure. 4. For the remaining 15% of unmyelinated ventral root afferents, a projection into the segmental dorsal root was found. The majority of those fibres (about two-thirds) are primary afferent neurones innervating the pia mater. Some of these units had a small spot-like receptive field and responded to mechanical stimuli such as pressure and stretch of the root. They did not have axon projections in a peripheral nerve. 5. A few (5%) unmyelinated ventral root fibres are collateral branches of normal primary afferents projecting through the dorsal root. These trifurcating neurones are a small population which make up only some 0.5% of all dorsal root ganglion cells. The functional significance of this population too is unknown. 6. For none of the fibres that projected into both dorsal and ventral root was there positive evidence for the existence of looping axons that merely make a detour into one of the roots. Although the existence of loops cannot completely be excluded, our evidence suggests that they can constitute at most 5% of the unmyelinated ventral root afferents.

Fiber counts at multiple sites along the rat ventral root after neonatal peripheral neurectomy or dorsal rhizotomy

We hypothesized that the afferent fibers in the ventral root of the rat are the third branches of dorsal root ganglion cells; these afferent processes in the ventral root are of varying length and end bluntly along the length of the root. In the case of an injury at either the central or the peripheral processes of the dorsal root ganglion cells in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We cut either the sciatic nerve or the dorsal root on one side in neonatal rats. After the rats were fully grown, the number of both myelinated and unmyelinated fibers was counted in electron photomicrographs at multiple sites along the length of the ventral root. We observed a greatly increased number of unmyelinated fibers in the ventral root after the sciatic nerve had been cut at the neonatal stage. The magnitude of increase was more at the distal than at the proximal portion of the ventral root, suggesting that added fibers originated from the distal side. Neonatal dorsal rhizotomy, however, did not produce the same result. These results are consistent with our hypothesis that peripheral nerve injury at the neonatal stage triggers sprouting of the third branches of the dorsal root ganglion cells which end bluntly along the length of the ventral root in the normal animal.

Electrophysiological evidence for an increase in the number of ventral root afferent fibers after neonatal peripheral neurectomy in the rat

Our recent study has shown that many afferent fibers in the ventral root are third branches of dorsal root ganglion cells in addition to their processes in the peripheral nerve and the dorsal root. From results of this study, we hypothesized that most of the afferent fibers in the normal ventral root are extra processes of certain dorsal root ganglion cells. To accommodate experimental findings by others, we formulated several working hypotheses in the present study as an extension of our previous hypothesis: these afferent processes in the ventral root are of varying length; they end bluntly along the length of the root; and in an event such as peripheral neurectomy in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We tested the above hypotheses using electrophysiological methods. The sciatic nerve on one side in neonatal rats was cut. After the rat was fully grown, volleys of neural activity were recorded along the length of the ventral root while stimulating the dorsal root of the same segment. There was a great increase in the size of compound action potentials in the ventral root on the sciatic nerve-lesioned side. Various lines of evidence suggest that this enhancement of the evoked potentials is likely to be due to an increase in the number of afferent fibers in the ventral root in response to neonatal peripheral nerve injury. The results are consistent with our hypotheses.

Electrophysiological evidence for the presence of looping myelinated afferent fibers in the rat ventral root

We obtained neurophysiological evidence for the existence of looping fibers in the rat ventral root. In anesthetized rats, simultaneous recordings of spontaneous activity were made from the lumbar ventral root with two pairs of bipolar recording electrodes. In 6 ventral roots, single unit activity appeared as pairs of spikes with opposite polarity in both the distal and proximal recording electrodes. The timing and polarity of the spikes can only be logically explained by supposing that they are simultaneous recordings from two pairs of electrodes of two action potentials traveling opposite directions in a single myelinated fiber. This explanation was reinforced by recording the activity after sectioning various parts of the ventral root. These data suggest that there are spontaneously active ventral root myelinated fibers that enter the root and then loop back out toward the dorsal root ganglion.

Age changes in axon number along the cervical ventral spinal nerve roots in rats

Axon counts were made at two standardised levels of C7 ventral spinal nerve roots from 46 female rats representing nine ages between birth and 500 days. The objective was to provide a definitive account of proximodistal changes in axon numbers and of age changes in axon numbers both during postnatal development and at several stages during maturity. At each age there is a proximodistal increase in the numbers of axons in all categories examined (myelinated, promyelin, transitional, and fetal) between levels midway along the subarachnoid course of the root and where it is apposed to but separate from the dorsal root ganglion. During maturation and throughout maturity axon totals change similarly at both levels: After a slight increase immediately postnatum, they decline sharply between 4 and 20 days due to a marked loss of unmyelinated axons. A gradual decline in myelinated axon numbers continues to 500 days. While these changes are occurring, axon numbers in all categories show a proximodistal increase throughout. The magnitude of this increase lessens with age for all but the transitional category due to a preferential decrease in numbers distally. Though these observations do not differentiate between axon branching and looping of sensory axons into the ventral root as a cause of the proximodistal increase in numbers, they tend to support the former. At each age during maturation axon proportions at proximal and distal levels correspond well for each animal, indicating that axon segregation proceeds at related rates within each root. Age changes in axon proportions within the transitional and fetal categories indicate that the postnatal stage of axon segregation results from axon loss, rather than Schwann cell proliferation.

The proportions of sympathetic postganglionic and unmyelinated afferent axons in normal and regenerated cat sural nerves

Electrophysiological experiments have been carried out to see if the proportions of sympathetic postganglionic and unmyelinated afferent axons in a cutaneous nerve were changed after injury and regeneration. It seemed possible that an alteration in the relative numbers of the two groups of axons could contribute to the aetiology of reflex sympathetic dystrophy, but the experiments provided no evidence for such a change. There were, however, signs of a decrease in axon numbers in the regenerated nerves.

Proportion and location of spinal neurons receiving ventral root afferent inputs in the cat

Spinal neurons receiving ventral root afferent inputs were investigated in anesthetized and paralyzed cats. We were concerned with the afferent fibers in the ventral root that travel distally and then enter the spinal cord through the dorsal root. The questions to be answered included the proportion and distribution of spinal neurons receiving ventral root afferent inputs and their peripheral input characteristics. The 1.7 ventral root was cut near the spinal cord and the distal stump was stimulated while making a systematic search for neurons in the entire gray matter of the ipsilateral spinal cord that responded to the stimulation. The following conclusions were made: (i) the afferent fibers in the cat ventral root enter the spinal cord through the dorsal root and evoke a variety of responses (excitation, inhibition, or mixed) in a large proportion of spinal neurons (about 20%): (ii) these responses seem to be mediated largely by spinal mechanisms: (iii) spinal neurons receiving ventral root afferent inputs are situated in a wide region of the ventral spinal cord: (iv) ventral root fibers in a single root enter the spinal cord and exert their responses over a large region of the spinal cord (at least two spinal segments rostrally and caudally): (v) some of the spinal neurons that responded to ventral root stimulation were found to be ascending tract cells, suggesting that ventral root afferent inputs can reach supraspinal structures: (vi) ventral root afferent fibers converge onto spinal neurons that have a variety of peripheral receptive field characteristics: and (vii) with some exceptions, most neurons receiving ventral root inputs were excited best by mechanical and/or thermal noxious stimuli applied to the periphery.

Dorsal root ganglion neurons with central processes in both dorsal and ventral roots in rats

Axonal transport of fluorescent dyes, Diamidino yellow dihydrochloride (DY) and Fast blue (FB), applied to the cut distal ends of dorsal and ventral roots, respectively, was studied in order to determine whether any dorsal root ganglion (DRG) neurons have processes in both dorsal and ventral roots. A total of 359 DRG neurons are double labeled in 14 ganglia (L6 and S1) from 6 different animals, thus suggesting a possibility of many DRG neurons having multiple central processes. The somata of the double-labeled DRG neurons are small to medium in size with a mean diameter of 29 microns. These data are consistent with findings that the majority of ventral root afferent fibers are unmyelinated or small myelinated axons.