A study of axonal diameters and areas in lumbosacral roots and nerves in the rat

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

Dorsal root ganglion: a key to understanding the therapeutic effects of the erector spinae plane (ESP) and other intertransverse process blocks?

Since its description in 2016, the erector spinae plane block (ESPB) has become a widely employed regional anesthetic technique and kindled interest in a range of related techniques, collectively termed intertransverse process blocks. There has been ongoing controversy over mechanism of action of the ESPB, mainly due to incongruities between results of cutaneous sensory testing, clinical efficacy studies, and investigations into the neural structures that are reached by injected local anesthetic (LA). This paper reviews the spread of LA to the paravertebral and epidural space and the cutaneous anesthesia in ESPB, with specific emphasis on the dorsal root ganglion (DRG). We hypothesize that the DRG, due to its unique and complex microarchitecture, represents a key therapeutic target for modulation of nociceptive signaling in regional anesthesia. This paper discusses how the anatomical and physiological characteristics of the DRG may be one of the factors underpinning the clinical analgesia observed in ESPB and other intertransverse process blocks.

Dorsal root ganglion stimulation produces differential effects on action potential propagation across a population of biophysically distinct C-neurons

Dorsal root ganglion stimulation (DRGS) is a neurostimulation therapy used to manage chronic pain that does not respond to conventional therapies. Unfortunately, not all patients receive sufficient pain relief from DRGS, leaving them with few other treatment options. Presently, our understanding of the mechanisms of action of DRGS is incomplete, preventing us from determining why some patients do not receive analgesia from the therapy. One hypothesis suggests that DRGS augments the filtering of action potentials (APs) at the T-junction of nociceptive C-neurons. To test this hypothesis, we utilized a computational modeling approach in which we developed a population of one thousand biophysically distinct C-neuron models which each produced electrophysiological characteristics (e.g., AP height, AP duration) reported in previous experimental studies. We used this population of model C-neurons to study how morphological and electrophysiological characteristics affected the propagation of APs through the T-junction. We found that trains of APs can propagate through the T-junction in the orthodromic direction at a higher frequency than in the antidromic direction due to the decrease in axonal diameter from the peripheral to spinal axon. Including slow outward conductances in the axonal compartments near the T-junction reduced following frequencies to ranges measured experimentally. We next used the population of C-neuron models to investigate how DRGS affected the orthodromic propagation of APs through the T-junction. Our data suggest that suprathreshold DRGS augmented the filtering of APs at the T-junction of some model C-neurons while increasing the activity of other model C-neurons. However, the stimulus pulse amplitudes required to induce activity in C-neurons (i.e., several mA) fell outside the range of stimulation pulse amplitudes used clinically (i.e., typically ≤1 mA). Furthermore, our data suggest that somatic GABA currents activated directly or indirectly by the DRGS pulse may produce diverse effects on orthodromic AP propagation in C-neurons. These data suggest DRGS may produce differential effects across a population of C-neurons and indicate that understanding how inherent biological variability affects a neuron's response to therapeutic electrical stimulation may be helpful in understanding its mechanisms of action.

Analgesic dorsal root ganglionic field stimulation blocks conduction of afferent impulse trains selectively in nociceptive sensory afferents

Increased excitability of primary sensory neurons after peripheral nerve injury may cause hyperalgesia and allodynia. Dorsal root ganglion field stimulation (GFS) is effective in relieving clinical pain associated with nerve injury and neuropathic pain in animal models. However, its mechanism has not been determined. We examined effects of GFS on transmission of action potentials (APs) from the peripheral to central processes by in vivo single-unit recording from lumbar dorsal roots in sham injured rats and rats with tibial nerve injury (TNI) in fiber types defined by conduction velocity. Transmission of APs directly generated by GFS (20 Hz) in C-type units progressively abated over 20 seconds, whereas GFS-induced Aβ activity persisted unabated, while Aδ showed an intermediate pattern. Activity generated peripherally by electrical stimulation of the sciatic nerve and punctate mechanical stimulation of the receptive field (glabrous skin) was likewise fully blocked by GFS within 20 seconds in C-type units, whereas Aβ units were minimally affected and a subpopulation of Aδ units was blocked. After TNI, the threshold to induce AP firing by punctate mechanical stimulation (von Frey) was reduced, which was reversed to normal during GFS. These results also suggest that C-type fibers, not Aβ, mainly contribute to mechanical and thermal hypersensitivity (von Frey, brush, acetone) after injury. Ganglion field stimulation produces use-dependent blocking of afferent AP trains, consistent with enhanced filtering of APs at the sensory neuron T-junction, particularly in nociceptive units.

M-type K+ channels in peripheral nociceptive pathways

Pathological pain is a hyperexcitability disorder. Since the excitability of a neuron is set and controlled by a complement of ion channels it expresses, in order to understand and treat pain, we need to develop a mechanistic insight into the key ion channels controlling excitability within the mammalian pain pathways and how these ion channels are regulated and modulated in various physiological and pathophysiological settings. In this review, we will discuss the emerging data on the expression in pain pathways, functional role and modulation of a family of voltage-gated K+ channels called 'M channels' (KCNQ, Kv 7). M channels are increasingly recognized as important players in controlling pain signalling, especially within the peripheral somatosensory system. We will also discuss the therapeutic potential of M channels as analgesic drug targets.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc/.

The intriguing nature of dorsal root ganglion neurons: Linking structure with polarity and function

Dorsal root ganglion (DRG) neurons are the first neurons of the sensory pathway. They are activated by a variety of sensory stimuli that are then transmitted to the central nervous system. An important feature of DRG neurons is their unique morphology where a single process -the stem axon- bifurcates into a peripheral and a central axonal branch, with different functions and cellular properties. Distinctive structural aspects of the two DRG neuron branches may have important implications for their function in health and disease. However, the link between DRG axonal branch structure, polarity and function has been largely neglected in the field, and relevant information is rather scattered across the literature. In particular, ultrastructural differences between the two axonal branches are likely to account for the higher transport and regenerative ability of the peripheral DRG neuron axon when compared to the central one. Nevertheless, the cell intrinsic factors contributing to this central-peripheral asymmetry are still unknown. Here we critically review the factors that may underlie the functional asymmetry between the peripheral and central DRG axonal branches. Also, we discuss the hypothesis that DRG neurons may assemble a structure resembling the axon initial segment that may be responsible, at least in part, for their polarity and electrophysiological features. Ultimately, we suggest that the clarification of the axonal ultrastructure of DRG neurons using state-of-the-art techniques will be crucial to understand the physiology of this peculiar cell type.

Spike propagation in axons under stretch growth conditions in cultured neurons from dorsal root ganglion

Computational software NEURON was used to simulate the stretch growth neurons in order to investigate the ability of dorsal root ganglion neurons to generate and propagate action potentials after a period of rapid axon stretch growth in vitro, and under what stimulating parameters can evoke action potentials. In the simulation, we found the stretch growth neuron had higher spike amplitude than from the static culture neuron in the soma and all axonal branch. In addition, the conduction velocity was also faster in the stretch growth axon. When the stimulating frequency was less than 15 Hz or the stimulating voltage was lower than 15 mV, no spike was evoked. Increasing stimulating frequency from 15 Hz to 5000 Hz or stimulating voltage from 15 mV to 100 mV had almost no effect on the spike amplitude. Interestingly, the first spike time and absolute refractory period (ARP) in different axonal branches and somas decreased stepwise with incremental increase in the stimulating frequency. It is concluded that the stretch growth neuron had higher amplitude and faster conduction velocity than the static culture neuron. In addition, some stimulating parameters had been analyzed in this study, which provided guidelines for electrophysiological experiments in future.

Cauda equina repair in the rat: Part 3. Axonal regeneration across Schwann cell-Seeded collagen foam

INTRODUCTION

Treatments for patients with cauda equina injury are limited.

METHODS

In this study, we first used retrograde labeling to determine the relative contributions of cauda equina motor neurons to intrinsic and extrinsic rat tail muscles. Next, we transected cauda equina ventral roots and proceeded to bridge the proximal and distal stumps with either a type I collagen scaffold coated in laminin (CL) or a collagen-laminin scaffold that was also seeded with Schwann cells (CLSC). Regeneration was assessed by way of serial retrograde labeling.

RESULTS

After accounting for the axonal contributions to intrinsic vs. extrinsic tail muscles, we noted a higher degree of double labeling in the CLSC group (58.0 ± 39.6%) as compared with the CL group (27.8 ± 16.0%; P = 0.02), but not the control group (33.5 ± 18.2%; P = 0.10).

DISCUSSION

Our findings demonstrate the feasibility of using CLSCs in cauda equina injury repair. Muscle Nerve 57: E78-E84, 2018.

Analysis of compound action potentials elicited with specific current stimulating pulses in an isolated rat sciatic nerve

The ability to selectively stimulate Aα, Aβ-fibers and Aδ-fibers in an isolated rat sciatic nerve (SNR) was assessed. The stimulus used was a current, biphasic pulse with a quasitrapezoidal cathodic phase and rectangular anodic phase where parameters were systematically varied: intensity of the cathodic phase (i

Epidural Stimulation of Rat Spinal Cord at Lumbosacral Segment Using a Surface Electrode: A Computer Simulation Study

Clinical research indicates that the epidural spinal cord stimulation (ESCS) at lumbosacral segment has shown potential for promoting locomotor recovery in patients with incomplete spinal cord injury. However, the underlying neural mechanism needs to be determined by animal experiments. In order to refine experimental protocols, we used a finite element simulation to investigate the activation of nerve fibers in a rat spinal cord model. Our model is composed of a volume conductor model from L1 to S2 spinal segments and the McIntyre-Richard-Grill axon model, which is used to investigate the threshold of selected spinal fibers with different diameters at varied locations and predict the neural responses of any target fibers with bipolar electrode configuration. Mathematical modeling suggests that the electrode-fiber distance may play an important role in the recruitment of nerve fibers, whereas longer pulse width predicted greater activity of spinal root fibers and dorsal column fibers, as well as may exert an effective influence on the motor system by the ability to increase and even "steer" spatial selectivity with deeper penetration into the dorsal columns. The spikes were initiated at sites along the nerve fibers depending on which component was closest to the cathode among the longitudinal part of the fiber, its entrance into spinal cord, or strong bending at the entry. Our simulation results show good agreement with the previous findings from animal studies. It is concluded that the computational ESCS model is a valuable tool to obtain a better insight into the immediately evoked electrophysiological phenomena in animal models, and provides further guidelines for conducting animal experiments to enhance the exploration of basic neural mechanisms.

The Impact of Prestretch Induced Surface Anisotropy on Axon Regeneration

Nerve regeneration after spinal cord injury requires proper axon alignment to bridge the lesion site and myelination to achieve functional recovery. Significant effort has been invested in developing engineering approaches to induce axon alignment with less focus on myelination. Topological features, such as aligned fibers and channels, have been shown to induce axon alignment, but do not enhance axon thickness. We previously demonstrated that surface anisotropy generated through mechanical prestretch induced mesenchymal stem cells to align in the direction of prestretch. In this study, we demonstrate that static prestretch-induced anisotropy promotes dorsal root ganglion (DRG) neurons to extend thicker axon aggregates along the stretched direction and form aligned fascicular-like axon tracts. Moreover, Schwann cells, when cocultured with DRG neurons on the prestretched surface colocalized with the aligned axons and expressed P0 protein, are indicative of myelination of the aligned axons, thereby demonstrating that prestretch-induced surface anisotropy is beneficial in enhancing axon alignment, growth, and myelination.

Spike propagation through the dorsal root ganglia in an unmyelinated sensory neuron: a modeling study

Unmyelinated C-fibers are a major type of sensory neurons conveying pain information. Action potential conduction is regulated by the bifurcation (T-junction) of sensory neuron axons within the dorsal root ganglia (DRG). Understanding how C-fiber signaling is influenced by the morphology of the T-junction and the local expression of ion channels is important for understanding pain signaling. In this study we used biophysical computer modeling to investigate the influence of axon morphology within the DRG and various membrane conductances on the reliability of spike propagation. As expected, calculated input impedance and the amplitude of propagating action potentials were both lowest at the T-junction. Propagation reliability for single spikes was highly sensitive to the diameter of the stem axon and the density of voltage-gated Na⁺ channels. A model containing only fast voltage-gated Na⁺ and delayed-rectifier K⁺ channels conducted trains of spikes up to frequencies of 110 Hz. The addition of slowly activating KCNQ channels (i.e., K_V7 or M-channels) to the model reduced the following frequency to 30 Hz. Hyperpolarization produced by addition of a much slower conductance, such as a Ca²⁺-dependent K⁺ current, was needed to reduce the following frequency to 6 Hz. Attenuation of driving force due to ion accumulation or hyperpolarization produced by a Na⁺-K⁺ pump had no effect on following frequency but could influence the reliability of spike propagation mutually with the voltage shift generated by a Ca²⁺-dependent K⁺ current. These simulations suggest how specific ion channels within the DRG may contribute toward therapeutic treatments for chronic pain.

Control of somatic membrane potential in nociceptive neurons and its implications for peripheral nociceptive transmission

Peripheral sensory ganglia contain somata of afferent fibres conveying somatosensory inputs to the central nervous system. Growing evidence suggests that the somatic/perisomatic region of sensory neurons can influence peripheral sensory transmission. Control of resting membrane potential (E_rest) is an important mechanism regulating excitability, but surprisingly little is known about how E_rest is regulated in sensory neuron somata or how changes in somatic/perisomatic E_rest affect peripheral sensory transmission. We first evaluated the influence of several major ion channels on E_rest in cultured small-diameter, mostly capsaicin-sensitive (presumed nociceptive) dorsal root ganglion (DRG) neurons. The strongest and most prevalent effect on E_rest was achieved by modulating M channels, K2P and 4-aminopiridine-sensitive K_V channels, while hyperpolarization-activated cyclic nucleotide-gated, voltage-gated Na⁺, and T-type Ca²⁺ channels to a lesser extent also contributed to E_rest. Second, we investigated how varying somatic/perisomatic membrane potential, by manipulating ion channels of sensory neurons within the DRG, affected peripheral nociceptive transmission in vivo. Acute focal application of M or K_ATP channel enhancers or a hyperpolarization-activated cyclic nucleotide-gated channel blocker to L5 DRG in vivo significantly alleviated pain induced by hind paw injection of bradykinin. Finally, we show with computational modelling how somatic/perisomatic hyperpolarization, in concert with the low-pass filtering properties of the t-junction within the DRG, can interfere with action potential propagation. Our study deciphers a complement of ion channels that sets the somatic E_rest of nociceptive neurons and provides strong evidence for a robust filtering role of the somatic and perisomatic compartments of peripheral nociceptive neuron.

A computational model for estimating recruitment of primary afferent fibers by intraneural stimulation in the dorsal root ganglia

Primary afferent microstimulation has been proposed as a method for activating cutaneous and muscle afferent fibers to restore tactile and proprioceptive feedback after limb loss or peripheral neuropathy. Large populations of primary afferent fibers can be accessed directly by implanting microelectrode arrays in the dorsal root ganglia (DRG), which provide a compact and stable target for stimulating a diverse group of sensory fibers. To gain insight into factors affecting the number and types of primary afferents activated, we developed a computational model that simulates the recruitment of fibers in the feline L7 DRG. The model comprises two parts. The first part is a single-fiber model used to describe the current-distance relation and was based on the McIntyre-Richardson-Grill model for excitability. The second part uses the results of the singe-fiber model and published data on fiber size distributions to predict the probability of recruiting a given number of fibers as a function of stimulus intensity. The range of intensities over which exactly one fiber was recruited was approximately 0.5-5 µA (0.1-1 nC per phase); the stimulus intensity at which the probability of recruiting exactly one fiber was maximized was 2.3 µA. However, at 2.3 µA, it was also possible to recruit up to three fibers, albeit with a lower probability. Stimulation amplitudes up to 6 µA were tested with the population model, which showed that as the amplitude increased, the number of fibers recruited increased exponentially. The distribution of threshold amplitudes predicted by the model was similar to that previously reported by in vivo experimentation. Finally, the model suggested that medium diameter fibers (7.3-11.5 µm) may be recruited with much greater probability than large diameter fibers (12.8-16 µm). This model may be used to efficiently test a range of stimulation parameters and nerve morphologies to complement results from electrophysiology experiments and to aid in the design of microelectrode arrays for neural interfaces.

Neuronal and glial expression of the adhesion molecule TAG-1 is regulated after peripheral nerve lesion or central neurodegeneration of adult nervous system

Expression of the cell adhesion molecule TAG-1 is down-regulated in adult brain, with the exception of certain areas exhibiting structural plasticity. Here, we present evidence that TAG-1 expression persists also in adult rat spinal cord and dorsal root ganglia (DRG), and can be up-regulated after injury. On Western blots of adult tissue, TAG-1 is detected as a 135-kDa band, with an additional specific 90-kDa band, not present in developing tissue. TAG-1 expression is found both in DRG neurons and in Schwann cells, particularly those associated with the peripherally projecting DRG processes. Quantitative in situ hybridization revealed that TAG-1 expression is significantly higher in small neurons that give rise to unmyelinated fibers, than in large DRG neurons. The regulation of TAG-1 was then examined in two different lesion paradigms. After a sciatic nerve lesion, TAG-1 expression is not up-regulated in DRG neurons, but decreases with time. At the lesion site, reactive Schwann cells up-regulate TAG-1, as demonstrated by both immunohistochemistry and in situ hybridization. In a second paradigm, we injected kainic acid into the spinal cord that kills neurons but spares glia and axons. TAG-1 is up-regulated in the spinal neuron-depleted area as well as in the corresponding dorsal and ventral roots, associated with both target-deprived afferent fibers and with the non-neuronal cells that invade the lesion site. These results demonstrate a local up-regulation of TAG-1 in the adult that is induced in response to injury, suggesting its involvement in axonal re-modelling, neuron-glia interactions, and glial cell migration.

C-Fiber Structure Varies with Location in Peripheral Nerve

Recent advances in regeneration and pain research have revealed gaps in the understanding of normal C-fiber anatomy. In the rat PNS, C-fiber axons assemble into Remak bundles, but beyond this, features of C-fiber organization are not defined. Systematic sampling and quantitation reveals that Remak bundles exiting from the L5 dorsal root ganglion (DRG) contain large numbers of axons, for example, 56% of unmyelinated axons were in bundles of >20 axons. This is different from distal nerve segments such as the hindpaw plantar nerve where the median number of axons per bundle is 3. The cross-sectional area of unmyelinated axons in dorsal root was homogeneous near the DRG but variability in axonal area increased near the spinal cord (p = 0.00001) and the mean axonal area was unchanged. Unmyelinated axons in peripheral nerve were almost always isolated from one another by Schwann cell processes; however, in dorsal root 7% to 9% of unmyelinated axons were immediately adjacent within pockets containing 2 or more axons. Remak bundles in the distal peripheral nerve clustered with other Remak bundles. We observe that multiple unmyelinated axons are juxtaposed within the C-fiber/Remak bundle and that the close association of afferent axons may have important functional implications.

The Microtubular Pattern Changes at the Spinal Cord-Root Junction and Reverts at the Root-Peripheral Nerve Junction in Sensory and Motor Fibres of the Rat

In the rat, we studied the microtubular content of central nervous system (CNS) axons (pyramidal tract, dorsal funiculus, and intracord domain of motor axons), of radicular axons (ventral and dorsal roots), and of peripheral axons (sural and lateral gastrocnemius nerves). The microtubular density had an inverse relationship with the size of the axon. Within the CNS, values ranged from over 120 microtubules/microm2 for axons smaller than 0.1 microm2 of the pyramidal tract and dorsal funiculus to 24 for 3-microm motor axons (area, 7 microm2) in their spinal cord domain. Peripheral nerve and CNS axons of the same size had comparable microtubular densities. In contrast, the microtubular density of dorsal and ventral root axons was one half that of CNS or peripheral nerve axons of equal calibre. Considered along the axon, the microtubular density of motor and sensory fibres is high in the CNS domain, low in the root, and high again in the peripheral nerve domain. These observations are inconsistent with the notion that the cytoskeleton moves coherently away from the perikaryon. We conclude that the axonal microtubular content accords with the calibre of the fibre and with the anatomical region where it courses. We propose that axonal microtubules are regulated by local cues.

Activity of foot skin mechanoreceptors and afferent nerve fibres in the adult rat sciatic nerve are altered after central axotomy of sensory neurons

The functional properties of skin mechanoreceptors were examined in the hind foot of normal rats in comparison with animals subjected to dorsal rhizotomy. Evoked nerve impulses were recorded from afferent nerve fibres of the tibial nerve. The decentralized mechanoreceptors displayed evidence of autonomous functioning, but with several abnormalities as compared to normal animals. There was a decreased sensitivity to mechanical stimulation and a lower adaptive capacity as a consequence of rhizotomy. The underlying mechanism is suggested to be a loss of central trophic support because of the interrupted link between the central nervous system and the sensory ganglion cell periphery. The findings indicate that mechanical receptors continue functioning under conditions when sensory impulses flow cannot reach postsynaptic target neurons in the central nervous system, but stop at the level of the primary sensory neuron.

Distinct morphological characteristics of touch, temperature, and mechanical nociceptive neurons in the crotaline trigeminal ganglia

Intrasomal recording and horseradish peroxidase injection techniques were employed in vivo to determine the morphological characteristics of touch, temperature, and mechanical nociceptive neurons in the trigeminal ganglia of crotaline snakes. The touch neurons, with a peripheral axon conducting at the A-beta range, could be subdivided into tactile and vibrotactile neurons according to their response properties, but there were no morphological differences between them. These neurons exhibited a large and oval soma and possessed a set of large stem, peripheral, and central axons which were all myelinated and equal in diameter with a constriction at the bifurcation. The temperature neurons, which conducted peripherally at the A-delta range, were physiologically separated into thermosensitive and thermo-mechanosensitive neurons, which were also morphologically indistinguishable. The temperature neurons had a round soma of medium size and a set of medium axons with varied axonal bifurcation patterns. All axons of these neurons were myelinated, but the central axon was thinner than the stem and peripheral axons. The mechanical nociceptive neurons, which had a peripheral axon conducting at the A-delta range, were morphologically heterogeneous based on their conduction velocities. The neurons conducting at the fast A-delta range were morphologically similar to the temperature neurons in the ganglion excepting their thinner central axons, whereas those at the slow A-delta range had a thinner myelinated stem axon that gave rise to a thinner myelinated peripheral axon and an unmyelinated stem axon with a bifurcation of either a triangular expansion at the bifurcating point or a central axon arising straightforwardly from the constant stem and peripheral axons. This study revealed that distinct morphological characteristics do exist for the touch and temperature neurons and the subtypes of mechanical nociceptive neurons in the trigeminal ganglion, but not for the subfunctional types of touch neurons or temperature neurons.

Lack of feminization of the cremaster nucleus in cryptorchid androgen insensitive rats

Androgens may control rat testicular descent via effects on the genitofemoral nerve or cranial gonadal ligaments. Androgen-mediated release of calcitonin gene-related peptide from the genito-femoral nerve (whose motoneuron cell bodies reside in the sexually dimorphic cremaster nucleus) may stimulate cremaster sac formation and testicular descent. Alternatively, androgens may cause regression of cranial gonadal ligaments and thereby allow the testes to descend. To evaluate these theories testicular position, and the cremaster sac and nucleus were studied in Tfm (androgen insensitive) rats. Testes were abdominal, inguinal and scrotal in 20%, 67% and 13% of Tfm male rats, respectively, and cranial ligaments were present in all cases. Mean cremaster nucleus motoneuron number was lower in female rats (70 +/- 14) but not significantly different between normal male (256 +/- 44) and Tfm male (231 +/- 42) rats, and it correlated poorly with testicular position. Calcitonin gene-related peptide immunoreactivity was rarely observed in cremaster motoneurons. These data suggest that the cremaster nucleus is not androgen-dependent, calcitonin gene-related peptide release from cremaster motoneurons is not the likely mechanism of testicular descent and persistent cranial ligaments may cause cryptorchidism in the Tfm rat.

C mechanical nociceptive neurons in the crotaline trigeminal ganglia

Using 32 Crotaline snakes, Trimeresurus flavoviridis, intrasomal recordings were made from 44 neurons of the trigeminal ganglia in vivo. They were 10 C neurons from 9 snakes and 34 A-delta mechanical nociceptive neurons from 23 snakes. 5 of the 10 C neurons were identified as mechanical nociceptive neurons. The neurons were labeled with iontophoretically injected HRP. Each of the 5 C nociceptive neurons had one receptive field, on which 1 spike was elicited by pricking the skin or mucosa with a pin. They were sensitized after repeated stimulation. The fields were insensitive to thermal stimulation. No background discharge was observed. Average conduction velocity was 0.95 m/s (+/- 0.4 S.D., n = 5). Mean resting potential was -62.5 mV (+/- 6.0 S.D., n = 4), and mean action potential amplitude was 88.0 mV (+/- 10.9 S.D., n = 4). Two somata were successfully visualized with HRP (22 microns x 20 microns, 20 microns x 18 microns). Total lengths of labeled axons were 1260 and 1480 microns peripherally to the edge of the section, and 1810 and 770 microns centrally. Neither of the neurons had branching of the peripheral or central axons in the ganglion.

Experimental microsurgical repair of spinal roots

Three different methods of nerve repair were evaluated in an experimental model of spinal root injury. In adult rats, dorsal L4 roots were cleanly severed and repaired by microsurgical techniques. Anastomosis was performed by direct end-to-end suture, the arterial sleeve technique, or the interposition of a nerve graft. Results were evaluated 7, 10, and 14 weeks after surgery. Regeneration was studied by light and electron microscopy, showing a fair regenerative pattern in each group. The endoneurial connective response, including neovascularization, was more prominent after grafting. The artery sleeve technique is a very tedious procedure, and fibrosis around the artery and arachnoiditis were intense. A lack of continuity was found in 3 of 12 direct sutures. In conclusion, the best method for the reparation of nerve roots seems to be the interposition of a nerve graft.

Spike conduction properties of T-shaped C neurons in the rabbit nodose ganglion

The electrical activity of C-type neurons was recorded intracellularly in the rabbit nodose ganglion maintained in vitro. The initial segment of their axon is spirally wound close to the cell body and a primary branching point divides it into a central process (CP) projecting to the nucleus of solitary tract in the medulla oblongata and a peripheral process (PP) which conveys sensory inputs from the viscera. Stimulation of the CP induced either somatic ("S") spikes or low-amplitude axonal ("A") spikes ("A1" or "A2"). In some cases abrupt changes in the latency of "S" or "A" spikes (jumps) were observed by gradually increasing the stimulus intensity. They are discussed in relation to a secondary branching on the central axon located inside or near the ganglion. Collision experiments showed that antidromic "A" spikes are blocked at the primary bifurcation of the axon (T-shaped neuron). Stimulation of the PP induced either "S" spikes or high amplitude "A" spikes ("A3" or "A4"). Orthodromic spikes could be blocked either before or after the primary bifurcation. When blocking occurs after the bifurcation on the stem axon, the spike can invade the central axon without invading the soma. The study of the refractory periods of the two processes and the application of high frequency stimulation showed that the PP allows higher frequencies than the soma and the CP, and thus that branching and the CP act as low-pass filters. These data support the view that the primary branching point and the CP of these T-shaped cells represent a strategic area to modulate visceral afferent messages.

Morphological and membrane properties of young rat lumbar and thoracic dorsal root ganglion cells with unmyelinated axons

Membrane and morphological properties of thoracic (Th9-13) and lumbar (L2-5) dorsal root ganglion cells have been investigated in an in vitro dorsal root ganglion (DRG) preparation from 14-day-old rats using intracellular recordings and the intracellular injection of Neurobiotin. The passive and active membrane properties of 47 DRG cells with conduction velocities (CV) less than 0.81 m/s were studied, which were considered to possess unmyelinated axons. The action potentials elicited by the stimulation of peripheral nerves or the dorsal roots were characteristic of C-cells, with long duration, inflexion on the falling phase and long lasting after hyperpolarization. Input resistance of the C-cells varied between 16 and 158 M omega and were significantly higher in thoracic than in the lumbar ganglia. Cells in the more cranial levels also tended to be smaller than those in the caudal levels with a mean cross sectional area of 301 +/- 32.5 microns2. Twenty-five percent of the cells from both regions showed an inward rectification. The distribution of CVs, input resistances and cross sectional areas were non-normal. While a weak correlation was found between the conduction velocity and input resistance of the cells, no correlation was present between the size of the perikarya and conduction velocity or the input resistance. These results show that by the 14th day of postnatal development membrane and morphological parameters approach those of adult rats. They also suggest that in cells with unmyelinated fibres, the size of the perikaryon does not predict the thickness of the axon, and that this cell population is heterogeneous.

Physiological properties and morphological characteristics of cutaneous and mucosal mechanical nociceptive neurons with A-delta peripheral axons in the trigeminal ganglia of crotaline snakes

Primary A-delta nociceptive neurons in the trigeminal ganglia of immobilized crotaline snakes were examined by intrasomal recording and injection of horseradish peroxidase in vivo. Thirty-four neurons supplying the oral mucosa or facial skin were identified as A-delta nociceptive neurons which responded exclusively to noxious mechanical stimuli and had a peripheral conduction velocity ranging from 2.6 to 15.4 m/s. These neurons were subdivided into a fast-conducting type (FC-type) and a slowly conducting type (SC-type). Neurons of both types had a receptive field limited to a single spot which responded to pin prick stimulus with a threshold of more than 5 g. The FC-type neurons had a narrow spike followed by a shorter after-hyperpolarization. In contrast, SC-type neurons exhibited a broad spike with a hump on the falling phase and a longer after-hyperpolarization. The diameters of the stem, central and peripheral axons of the FC-type neurons were significantly thicker than those of the SC-type neurons, but there was no statistical difference in the soma size of the two types. Central axons of both types of neurons were thinner than their stem and peripheral axons. Dichotomizing fibers of peripheral axons were observed within the ganglion on 3 neurons. Central axons of the FC-type neurons terminated ipsilaterally in the nucleus principalis, the subnucleus oralis, interpolaris and caudalis and the interstitial nucleus, whereas those of the SC-type neurons generally projected only to the caudal half of the subnucleus interpolaris, subnucleus caudalis and interstitial nucleus ipsilaterally. The present data showed for the first time the physiological and morphological heterogeneity of the primary trigeminal A-delta nociceptive neurons and revealed that the trigeminal nucleus principalis and all the subdivisions of the trigeminal descending nucleus are involved in nociception as relay nuclei, but the subnucleus caudalis and the caudal half subnucleus interpolaris are the essential relay sites of the primary nociceptive afferents supplying the oral mucosa and facial skin. The interstitial nucleus also appears to play an important role in orofacial nociception.

Three types of sodium channels in adult rat dorsal root ganglion neurons

Several types of Na+ currents have previously been demonstrated in dorsal root ganglion (DRG) neurons isolated from neonatal rats, but their expression in adult neurons has not been studied. Na+ current properties in adult dorsal root ganglion (DRG) neurons of defined size class were investigated in isolated neurons maintained in primary culture using a combination of microelectrode current clamp, patch voltage clamp and immunocytochemical techniques. Intracellular current clamp recordings identified differing relative contributions of TTX-sensitive and -resistant inward currents to action potential waveforms in DRG neuronal populations of defined size. Patch voltage clamp recordings identified three distinct kinetic types of Na+ current differentially distributed among these size classes of DRG neurons. 'Small' DRG neurons co-express two types of Na+ current: (i) a rapidly-inactivating, TTX-sensitive 'fast' current and (ii) a slowly-activating and -inactivating, TTX-resistant 'slow' current. The TTX-sensitive Na+ current in these cells was almost completely inactivated at typical resting potentials. 'Large' cells expressed a single TTX-sensitive Na+ current identified as 'intermediate' by its inactivation rate constants. 'Medium'-sized neurons either co-expressed 'fast' and 'slow' current or expressed only 'intermediate' current. Na+ channel expression in these size classes was also measured by immunocytochemical techniques. An antibody against brain-type Na+ channels (Ab7493)10 labeled small and large neurons with similar intensity. These results demonstrate that three types of Na+ currents can be detected which correlate with electrogenic properties of physiologically and anatomically distinct populations of adult rat DRG neurons.

Soma neurofilament immunoreactivity is related to cell size and fibre conduction velocity in rat primary sensory neurons

1. Intracellular recordings were made in dorsal root ganglia in vitro at 37 degrees C. The L4, L5 and L6 ganglia from 46- to 51-day-old female Wistar rats were used. In each neuron conduction velocity (CV) was measured and fluorescent dye was injected. Later the intensity of the immunoreactivity to RT97 (a monoclonal antibody to the phosphorylated 200 kDa neurofilament subunit) as well as the cell size (cross-sectional area at the nuclear level) were measured in the dye-injected neurons. RT97 was used to distinguish between the L (light, neurofilament-rich) and the SD (small dark, neurofilament-poor) neuronal somata. 2. Neurons were classified as C neurons (CV less than 1.3 m/s), C/A delta neurons (1.3-2 m/s), A delta neurons (2-12 m/s) or A alpha/beta neurons (greater than 12 m/s). 3. All A-fibre somata were RT97 positive (L) and all C-fibre somata were RT97 negative (SD), although in the C/A delta group both positive and negative neurons were seen. Thus, RT97-negative somata had C (unmyelinated) or C/A delta fibres, while RT97-positive somata had A (myelinated) or C/A delta fibres. 4. The size distributions of A neurons and C neurons were consistent with their classification as L- and SD-cell neurons respectively. The size distribution of A delta cells was skewed with a peak of small cells and a tail of medium-sized cells. 5. There was a loose positive correlation between cell size and fibre CV. 6. RT97 intensity was positively correlated with CV if all neurons were considered together, but no correlation was seen within the C, A delta or A alpha/beta CV groups. 7. RT97 intensity was positively correlated with cell size when all neurons were considered together. Although no correlation was seen within the C or the A delta CV groups, a clear positive correlation was seen for A alpha/beta neurons. 8. The relationship of RT97 intensity to cell size was not demonstrably altered by axotomy, time in vitro or the presence of intracellular dye in control experiments. 9. RT97-negative and -positive neurons could be seen in neonatal rat ganglia. Their size distributions resembled, respectively, the SD- and L-neuron populations at this age. RT97 immunoreactivity may therefore be a useful predictor of the cell type and myelinated state which a sensory cell is destined to reach in the adult rat.

Cell types and axonal sizes of calcitonin gene-related peptide-containing primary sensory neurons of the rat

Neurons with immunoreactivity (IR) for calcitonin gene-related peptide (CGRP) in the rat dorsal root ganglia were examined by immunoelectron microscopy. About one-half of the neurons of the L5 dorsal root ganglia of animals treated with colchicine had CGRP-IR, and 40% of these were large neurons of type A. The proximal parts of their peripheral axons were, however, unmyelinated (91%) or thinly myelinated (9%). Thickly myelinated axons observed in the same sections were always devoid of CGRP-IR. The CGRP-IR neurons were various subtypes of type A and B neurons. No specific morphological characteristics were associated with CGRP-IR.

On the asymmetry of the primary branching of vagal sensory axons: possible role of the supporting tissue

Central and peripheral nonmedullated processes of vagal nodosal neurons of the cat were studied in normal nerves and after regeneration along their anatomical course and along the hypoglossal nerve. Nonmedullated fibers above the ganglion and in the root had comparable sizes (approximately 0.37 micron2) and caliber distribution. Below the ganglion, the cross-sectional area increased to 1.0 micron2. In axons of equal caliber, supranodosal and radicular fibers had similar microtubular densities while infranodosal fibers had two- to threefold that of the former. Regenerated fibers were studied after a recovery period of 6-9 months. Regrown axons were smaller than their parent axons; in turn, these were smaller than normal axons. This holds for central and peripheral nodosal branches, for homologous and heterologous regeneration. Regrown peripheral branches, either along their anatomical pathway or along the hypoglossal nerve, showed no change in microtubular density. Central branches exhibited their characteristic microtubular content when they regenerated along their anatomical course, but when regrowth took place along the hypoglossal nerve, the original low microtubular content of these branches increased to match the high content of peripheral fibers; parent central axons also shifted their microtubular content toward the pattern of peripheral fibers. We propose that the supporting tissue participates in specifying the organization of axonal microtubules.

Conduction velocity changes along the processes of rat primary sensory neurons

Conduction velocities of rat L4, L5 and L6 dorsal root ganglion neurons were measured in vitro, from several points on the peripheral nerve and dorsal root. Conduction velocities calculated from a single stimulation point (12-26 mm from the ganglion) proved accurate for fibres conducting up to 17 m/s in the peripheral nerve and up to 14 m/s in the dorsal root, but tended to underestimate the value for faster fibres. C-fibre neurons of the L4 and L5 ganglia had a unimodal distribution of conduction velocities below 1.3 m/s. These were discontinuous with A-fibre conduction velocities, which also had a unimodal distribution and had no clear A delta peak. In contrast, conduction velocities of L6 ganglion neurons showed no discontinuity between C- and A-fibres, but had a clear A delta peak. In A-fibre neurons, dorsal root conduction velocities were on average about 14% slower than, and linearly related to, those in the peripheral nerve. However, in individual neurons the dorsal root conduction velocity could be slower or faster than that in the peripheral nerve. In C-fibre neurons dorsal root conduction velocities were almost always slower (average 28%) but not correlated with those of the peripheral nerve. Slowing of conduction velocity along the sciatic nerve was seen in most fibres conducting at less than 2 m/s, but not in faster fibres. The slowing was substantial (up to 60%), sometimes from the A delta to the C-fibre range, and sudden, occurring at a distance of between 15 and 29 mm from the ganglion.

Rearrangement of microtubule associated protein parallels the morphological transformation of neurons from dorsal root ganglion

In primary cultures of dorsal root ganglion cells from rat embryos, neurons undergo a morphological transformation from a bipolar to a differentiated pseudo-unipolar shape, resembling their developmental stages in vivo. Cells present in these cultures are characterized here by immunological criteria using monoclonal and polyclonal antibodies against microtubule associated proteins MAP1 and MAP2 and against tubulin. After development for seven days in culture, antibodies against microtubule associated proteins MAP1 brightly labeled cells with neuronal morphology and lightly stained cells with the shape of Schwann cells. In addition, an extended network of neuronal processes was labeled with this antibody. Anti-microtubule associated protein MAP2 stained only neurons and a more restricted network of neuronal processes. The compartmentalization of microtubule associated protein MAP2 during the maturation process was followed by double-labeling with antibodies to microtubule associated proteins MAP1 and MAP2. Initially, microtubule associated protein MAP2 was present in the cell body and the two processes of bipolar neurons. Subsequently, the labeling of both processes changed, depending on neuronal morphology. In neurons in which both processes were approaching one another, one of these neurites was stained predominantly with anti-microtubule associated protein MAP2. Finally, in pseudo, unipolar neurons, anti-microtubule associated protein MAP2 labeling was found in the cell body and excluded from the more distal processes.

Do axons grow during adulthood? A study of caliber and microtubules of sural nerve axons in young, mature, and aging rats

Calibers and microtubules of sural nerve axons were studied in young (6-week-old), mature (14-week-old), and aging (2-year-old) rats. The mean cross-sectional area of nonmedullated fibers was about 0.50 micron 2 (range: 0.47-0.52) in the three age groups. Their caliber spectra were also similar. In contrast, myelinated axons grew from 6.6 to 16.7 micron 2 between the sixth and 14th week of age. The increase of cross-sectional area was greater, the greater the initial caliber of axon (range 44-154%). No further change of caliber was observed in the aging rat. The cross-sectional area of nerve allotted per myelinated fiber was 42, 66, and 97 micron 2 in young, mature, and aging rats, respectively. The fraction of nerve tissue occupied by the axoplasm, though, did not change substantially; it was 20, 28, and 21%, respectively. The microtubular density of 3-micron myelinated axons had a general average of 21 microtubules/micron 2. Differences between groups were not significant. In nonmedullated fibers, the microtubular density decreased as the size of the axon increased. No differences were observed between age groups. We conclude that nonmedullated fibers of the sural nerve stop growing before the sixth week whereas myelinated fibers keep growing until the 14th week of age. The correlation between microtubular content and axonal caliber is a lifelong feature of axons.

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

The arrangement of the ventral root afferent fibers was investigated in anesthetized and paralyzed cats. Single unit activity was recorded from a fascicle of the distal stump of the cut S1 dorsal root. Activity was elicited by stimulating the distal stump of the cut S1 ventral root. Attempts were then made to collide this activity with that elicited by stimulation of the S1 spinal nerve. Single unit activity elicited by ventral root stimulation was recorded from a total of 33 axons. In 17 of these, the activity collided with that elicited by peripheral stimulation. These results indicate that more than half the sampled population of ventral root afferent fibers are branches of dorsal root ganglion cells that have at least 3 processes: one in the dorsal root, one in the ventral root and one in a peripheral nerve. In 10 of these units, the conduction velocity of each of 3 processes was determined using the collision technique. The conduction velocities differed in the processes of a given ganglion cell, with conduction in the ventral root process generally being the slowest. The change in conduction velocity along the length of the ventral root was examined by comparing latency differences for the unit activity elicited by ventral root stimulation at different sites in the same root separated by known distances. The conduction velocity was found not to be uniform along the course of the ventral root. In many cases, the conduction velocity slowed down as the fiber approached the spinal cord. We conclude from the present study that many ventral root afferent fibers are the third branches of dorsal root ganglion cells that also have processes in the dorsal root and in a peripheral nerve. The sizes of each of these 3 processes of the dorsal root ganglion cell may differ; the ventral root process tends to be the smallest and is usually unmyelinated. Furthermore, many of the ventral root afferent fibers may taper as they approach the spinal cord.

Microtubules and calibers in normal and regenerating axons of the sural nerve of the rat

The calibers and microtubular content of axons were studied in normal and regenerating fibers of the sural nerve from 17 to 122 days after a lesion of the sciatic nerve of young adult rats. During this period (70-175 days of age), the cross-sectional area of control myelinated axons almost doubled but that of nonmedullated axons did not change. In regenerating nerves, after 122 days of recovery, the cross-sectional area of myelinated fibers was still 38% below that of the normal side. In contrast, the regenerating nonmedullated population was richer in fine (less than 0.2 micron2) and in coarse (greater than 0.9 micron2) fibers than on the control side; the cross-sectional area averages were 0.50 and 0.54-0.70 micron 2 for the normal and regenerating populations, respectively. The microtubular density of normal 3-micron myelinated fibers averaged 24.0 microtubules/micron2. In regenerating fibers of the same size the density varied between 19.2 and 23.2 microtubules/micron2. Microtubular density values of normal and regenerating fibers were not statistically different. In nonmedullated fibers, the microtubular content (expressed as microtubular density or number of microtubules per axon) correlated with the caliber of the fiber. In these correlations, only minor differences were observed between regenerating and uninjured fibers. Our results indicate that nonmedullated fibers terminate their radial growth well before myelinated fibers do, and that axonal microtubular content correlates with the local size of the fiber and is largely insensitive to regeneration.

Fiber composition of the rat sciatic nerve

The rat sciatic nerve originates from the spinal segments L4-L6. It is unifascicular at the trochanter; 5-7 mm distally, the nerve splits into two and then into four fascicles. The tibial portion gives rise to the tibial and the sural nerves, and the peroneal portion gives rise to the peroneal nerve and a cutaneous branch that perforates the lateral hamstring muscles to innervate the proximolateral face of the calf. The number and type of the axons in these branches were determined in light and electron micrographs of normal nerves, and after de-efferentation or sympathectomy. Deafferentation was technically not feasible because spinal ganglia and ventral roots were supplied by the same vascular plexus. The tibial nerve contained 1,000 motor and 3,500 myelinated afferent axons, 3,700 sympathetic axons, and 5,400 unmyelinated afferent axons. The peroneal nerve contained 600 motor and 1,300 myelinated afferent axons, 1,100 sympathetic axons and 3,000 unmyelinated afferent axons. The sural nerve contained 1,100 myelinated and 2,800 unmyelinated afferent axons; in addition, there were 1,500 unmyelinated sympathetic axons. The cutaneous branch consisted of 400 myelinated and 1,800 unmyelinated afferent axons. Thus, the entire sciatic nerve at midthigh is composed of about 27,000 axons; 6% are myelinated motor axons, 23% and 48% are myelinated and unmyelinated sensory axons, respectively, and 23% are unmyelinated sympathetic axons. The techniques used did not demonstrate sympathetic axons in the cutaneous branch and did not reveal the few motor axons contained in the sural nerve.

The amount of slow axonal transport is proportional to the radial dimensions of the axon

Axons are fundamentally cylindrical and their geometry is defined by two basic parameters, i.e. diameter and length. The average cross-sectional diameter of an axon is determined primarily by the number and density of cytoskeletal structures (i.e. microtubules and neurofilaments) in the axon. The proteins that constitute these structures are synthesized in the nerve cell body and are conveyed through the axon by slow axonal transport. In particular, slow component a (SCa) supplies all of the axonal neurofilament proteins and most of the microtubule proteins to the axon. To study the relationship between slow axonal transport and axonal diameter, the slowly transported proteins were radiolabelled in rat dorsal root ganglion (DRG) cells. The amount of radiolabelled SCa proteins transported in individual unmyelinated and myelinated DRG axons was measured by the electron microscopic autoradiographic method. We found that the amount of SCa transported in the axons is proportional to axonal cross-sectional area. These results indicate that slow axonal transport of microtubules and neurofilaments is a primary determinant of axonal diameter.

Correlation of cell body size, axon size, and signal conduction velocity for individually labelled dorsal root ganglion cells in the cat

Measurements of cell body and peripheral and central axon sizes were made for primary sensory neurons outlined by the intracellular injection of HRP. Conduction velocities were also measured on the outlined processes. The sensory neurons were then subdivided into A and C cells on the basis of the conduction velocity of the impulses carried by the processes of these cells. Central processes of both A and C cells are smaller than the peripheral processes, but the size differential is greater for the C cells. For A cells there is a linear relation between the size of the peripheral axon and the conduction velocity of the impulses carried by these axons, but the confidence limits are wide. For C cells there is a linear relation between the size of the central process and conduction velocity of the impulses carried by the processes, but for the peripheral processes two aberrant processes resulted in no correlation between process size and conduction velocity. For A cells, the size of the central and peripheral processes and the conduction velocity of the impulses carried by the peripheral processes are linearly correlated with cell body size. By contrast no such correlations can be demonstrated for C cells. This presumably implies an important difference in that the size of the cell body is correlated with axon size and impulse conduction velocity for A cells but not for C cells. A widely accepted generalization is that large sensory cells give rise to myelinated axons and small sensory cells to unmyelinated axons. In this study, myelinated and unmyelinated are defined on the basis of impulse conduction velocity. For those cells that are clearly large (greater than 50 microns in diameter), the conduction velocity of the impulses carried by their processes is always greater than 2.5 m/s, and for those cells that are clearly small (less than 35 microns in diameter), the conduction velocity is always less than 2.5 m/s. Thus for these cells the above generalization holds. For the intermediate-sized cells (35-50 microns), however, the size of the cell body bears no predictable relation to the conduction velocity of the impulses carried by those processes, and thus to whether the axons are myelinated or unmyelinated. Thus the above generalization does not hold for this intermediate group of cells, and since there are many cells in this size range, we feel that the generalization that large cells give rise to myelinated axons and small cells to unmyelinated axons is an oversimplification.

Microtubules and caliber of central and peripheral processes of sensory axons

The microtubular content and caliber of sensory axons were studied in the L7 dorsal root, at the distal pole of the L7 spinal ganglion, and in the sural nerve of cats. Calibers of myelinated axons were symmetrical about the ganglion. In contrast, nonmedullated axons were strikingly different; 80% of the population at the root were smaller than 0.4 micron2, whilst just across the ganglion the same group was less than 20%. The microtubule densities of myelinated axons of the root were 11.8 and 6.1 microtubules/micron2 for 3- and 10 microns diameter axons, respectively. Across the ganglion the densities of myelinated axons of equal sizes were 24.2 and 14.4 microtubules/micron2, respectively. These values represent an approximate ratio of 1:2 between central and peripheral microtubule densities. Microtubule densities for nonmedullated axons also decreased with the increase in the cross-sectional area. The densities of root nonmedullated axons ranged between 90 and 10 microtubules/micron2; these were smaller, usually by a factor of three, than the densities of peripheral axons of a similar size (range: 367-44). Contrasting with the differences observed across the ganglion, the microtubular content and caliber of sensory axons seems to be quite uniform along their peripheral course. This is supported by the similar values found in the juxtaganglionic and sural nerves. It is estimated that an axon that contains 90 microtubules/micron2 has 26.7 mg of tubulin per ml of axoplasm in its assembled form, and 3.0 mg/ml if it contains 10 microtubules/micron2; these values are the practical limits of assembled tubulin in axoplasms.(ABSTRACT TRUNCATED AT 250 WORDS)

Conduction velocity changes along lumbar primary afferent fibers in cats

Though peripheral conduction velocity is widely used to characterize afferent fibers according to somatosensory modality, disagreement exists as to whether or not conduction velocity varies along such an axon's length. Therefore, in this experiment, conduction velocities were measured over very short axonal segments (7.5 to 15 mm) within the posterior tibial nerve, sciatic nerve, and L7 dorsal root, using the method of spike-triggered averaging of neurograms recorded from tripolar electrodes. The conduction velocities for several units were also determined using electrical stimulation, so that the accuracy of the two techniques could be compared. For most units, dorsal root and sciatic nerve conduction velocities were not significantly different; however, they were not tightly correlated. Tibial nerve conduction velocity averaged 86% of that within the sciatic nerve. Variations in sciatic nerve conduction velocity within adjacent axonal segments (9 mm in length) rarely exceeded experimental error. It appears that spike-triggered averaging of signals from tripolar electrodes separated 15 mm or more apart is a more accurate method for measuring conduction velocity than electrical stimulation, which was subject to several large errors.

Developing dorsal root ganglion neurons require trophic support from their central processes: evidence for a role of retrogradely transported nerve growth factor from the central nervous system to the periphery

Injury to the peripheral processes produces a profound cell loss (40-50%) in the dorsal root ganglion of newborn rats. Although division of central processes produces little or no cellular change in sensory ganglion of adult animals, no information has been available on the effect of dorsal root section in developing dorsal root ganglion. We show that 6 days after dorsal rhizotomy on newborn rats, there is a 50% decrease in neuronal number in L5 dorsal root ganglion. A combined central and peripheral lesion of the sensory process results in a greater decrease in neuronal number (70%). Both of these effects can be prevented by the concomitant treatment with nerve growth factor. We also demonstrate that 125I-Ia-labeled nerve growth factor is retrogradely transported with high selectivity from the spinal cord to the dorsal root ganglion via the dorsal roots. The results indicate that trophic support for developing sensory neurons is provided through the central processes. This is presumably due to the uptake and retrograde transport of a trophic factor by the terminals of the central processes. The data suggest that nerve growth factor may be the trophic factor.