Conduction along myelinated and demyelinated nerve fibres with a reorganized axonal membrane during the recovery cycle: model investigations

The changes in the excitability of the reorganized axonal membrane in myelinated and demyelinated nerve fibres as well as the causes conditioning such changes have been investigated by paired stimulation during the first 30 ms of the recovery cycle. The variations of the action potential parameters (amplitude and velocity) are traced also. The simulation of the conduction along the normal fiber is based on the Frankenhaeuser and Huxley (1964) and Goldman and Albus (1968) equations, while the demyelination is considered to be an elongation of the nodes of Ranvier. The axonal membrane reorganization is achieved by means of potassium channel blocking and increase of the sodium-channel permeability. It is shown that potassium channels block decreases membrane excitability for the myelinated and demyelinated fibres in the cases of initial and paired stimulation. With increasing sodium-channel permeability on the background of the blocked potassium channels, the membrane excitability is increased. For the fibres with a reorganized membrane, a supernormality of the membrane excitability is obtained, the latter remaining unrecovered during the 30 ms cycle under investigation. The supernormality of the excitability grows from the demyelinated fibre without reorganized membrane to the demyelinated fibre with reorganized one. For short interstimulus intervals, the second action potential propagates along the fibres with a reduced velocity and a decreased amplitude. No supernormality of the potential parameters (amplitude, velocity) is observed during the cycle up to 30 ms. The membrane properties of the myelinated and demyelinated fibres with blocked potassium channels recover in the interval from 15 to 20 ms depending on whether the sodium channels' increase of the permeability is added on the background of the blocked potassium channel or not. In the recovery cycle, the axonal membrane reorganization leads to an improvement of the conduction along most severely demyelinated fibres.     

Modelled temperature-dependent excitability behaviour of a generalised human peripheral sensory nerve fibre

The objective of this study was to determine if a recently developed human Ranvier node model, which is based on a modified version of the Hodgkin–Huxley model, could predict the excitability behaviour in human peripheral sensory nerve fibres with diameters ranging from 5.0 to 15.0 μm. The Ranvier node model was extended to include a persistent sodium current and was incorporated into a generalised single cable nerve fibre model. Parameter temperature dependence was included. All calculations were performed in Matlab. Sensory nerve fibre excitability behaviour characteristics predicted by the new nerve fibre model at different temperatures and fibre diameters compared well with measured data. Absolute refractory periods deviated from measured data, while relative refractory periods were similar to measured data. Conduction velocities showed both fibre diameter and temperature dependence and were underestimated in fibres thinner than 12.5 μm. Calculated strength–duration time constants ranged from 128.5 to 183.0 μs at 37°C over the studied nerve fibre diameter range, with chronaxie times about 30% shorter than strength–duration time constants. Chronaxie times exhibited temperature dependence, with values overestimated by a factor 5 at temperatures lower than body temperature. Possible explanations include the deviated absolute refractory period trend and inclusion of a nodal strangulation relationship. At the time of this research J. E. Smit was with the University of Pretoria.     

Selective deafferentation of hand cutaneous territory is followed by changes in fibre type distribution of a forearm muscle in the horse

Based on previous observations that capsaicin can selectively damage group III and IV afferents and induce muscle fibre transformation, we hypothesized that eliminating, by means of capsaicin, the group III and IV afferents of a peripheral territory it could lead to a fibre transformation in a muscle involved in the flexor reflexes of the same peripheral territory. Therefore, capsaicin was injected into the palmar nerves of the forelimb of the horse to investigate if eliminating group III and IV afferents from the hand of the horse a muscle fibre transition would occur in the flexor carpi radialis (FCR) muscle, which is involved in the flexor reflexes of the finger itself. 120 days after capsaicin injection, type I slow fibres increased and type IIA fast fibres decreased. We presume that the long lasting deafferentation of the ergo-nociceptive fibres causes a plastic remodelling in the central nervous system and indirectly influences the motoneuron excitability via short or long loop-pathways enhancing their tonic discharge.     

Effects of body suspension on soleus muscle fibres and spinal motoneurones in the rat.

1. After 14 days of body suspension, fibre type composition and fibre cross-sectional area in the soleus muscle of 17-week-old male Sprague-Dawley rats were investigated. Oxidative enzyme activity of soleus motoneurones in the spinal cord was also examined. 2. After suspension, soleus muscle weight decreased by 44.2%, the cross-sectional area of SO and FOG fibres decreased by 60.4% and 58.6%, respectively. 3. The percentage of fibre types was not changed by suspension. However, ATPase activity after alkaline preincubation was markedly inhibited in FOG fibres. 4. Oxidative enzyme activity of soleus motoneurones was not changed by suspension. 5. This study demonstrates that using mature animals body suspension induces atrophy of fast- and slow-twitch fibres accompanied with the selective inhibition in ATPase activity of fast-twitch fibres, and without changes in histochemical profiles of the corresponding motoneurones.     

The staining of elastic fibres with Direct Blue 152. A general hypothesis for the staining of elastic fibres

Summary Elastic fibres may be stained by a number of dyes, e.g. Direct Blue 1 (C.I. 24410), Direct Blue 10 (C.I. 24340), Direct Blue 15 (C.I. 24400), Direct Blue 152 (C.I. 24366) and Direct Violet 37 (C.I. 24370). A convenient method using Direct Blue 152 has been developed which is specific for elastic fibres. The method is simple and allows the demonstration of other connective tissue fibres. Staining of elastic fibres is unimpaired by numerous blocking procedures or by changes in dyebath pH. These properties are shared by several standard elastic fibre stains.As the Direct dyes and several of the standard elastic fibre stains possess numerous aromatic rings a wide range of dyes containing varying numbers of aromatic rings were examined for ability to stain elastic fibres. No association was observed between the ability to stain elastic fibres and dye class, formal charge or the presence of hydrogen bonding groups. Staining was, however, definitely associated with the presence in the dye molecule of 5 or more aromatic rings. This suggested that van der Waals forces of attraction may be responsible for elastic fibre staining both by Direct dyes and the standard elastic fibre stains. Staining of elastic fibres as a side-effect in many procedures is similarly explicable.     

The central consequences of the application of capsaicin to one peripheral nerve in adult rat

This paper reviews the central consequences of local application of capsaicin to one nerve in adult animals. 1) Marked chemical changes occur in the central terminals of C fibres. These include depletion of the enzyme FRAP and the peptides SP, CCK, somatostatin, CGRP and an increase of VIP. Maximal depletions occur if the nerve is soaked with capsaicin solutions with a concentration higher than 3 mM. The depletion begins by 7 days and is complete by 11. Recovery begins at about 110 days and is largely complete by 200. Our studies have concentrated on the effects of 40 mM capsaicin examined 14 days after the application. 2) Capsaicin treatment of a peripheral nerve decreased the ability of C fibres in that nerve to excite or to inhibit spinal cord cells. It produces a marked expansion of receptive fields of some cells in the dorsal horn which respond to A fibre stimulation. It is proposed that this change is not due to anatomical changes but to disinhibition. A further example of receptive field expansion is seen after treatment of the mouse infraorbital nerve which defocuses the normally precise projection of individual whiskers onto single cells in the barrel field of the somatosensory cortex. 3) Behavioural consequences follow the treatment of one adult nerve with capsaicin. In the area subserved by the treated nerve, there is a raised threshold to response to chemical and thermal stimuli, no change in the response to mechanical stimuli and an increase of autotomy following nerve section. 4) The aim of the experiments was to determine the role of C fibres in producing the changes seen in spinal cord following peripheral nerve section. Capsaicin treatment of nerve imitates the central effect of complete nerve section in certain important ways. Both result in a marked expansion of the receptive field of some cells. The effect is produced by a change of chemical transport. The results show that C fibres influence the connection of A fibres onto spinal cord cells.     

Innervation of radicular and extraparenchymal arteries of spinal cord

Summary The perivascular innervation of extraparenchymal arteries of spinal cord and the radicular arteries was examined using histochemical and immunohistochemical technics in monkey. The radicular and the extraparenchymal arteries of spinal cord were found to be invested with adrenergic, neuropeptide Y, vasoactive intestinal peptide, substance P and calcitonin gene-related peptide containing nerve fibres. The pattern of arrangement of fibres differed among the various fibre types. SP-and CGRP-containing fibres were less in density as compared to other nerve plexus. There was no difference in density of an individual type of nerve fibre in arteries of different cord segments or between the radicular arteries from different levels. The study reveals the existence of a comprehensive perivascular adrenergic and peptidergic innervation of spinal cord arterial system, with a possible role in neurogenic regulation of spinal cord circulation.     

Innervation of radicular and extraparenchymal arteries of spinal cord. Histochemical and immunohistochemical study in primate

The perivascular innervation of extraparenchymal arteries of spinal cord and the radicular arteries was examined using histochemical and immunohistochemical technics in monkey. The radicular and the extraparenchymal arteries of spinal cord were found to be invested with adrenergic, neuropeptide Y, vasoactive intestinal peptide, substance P and calcitonin gene-related peptide containing nerve fibres. The pattern of arrangement of fibres differed among the various fibre types. SP- and CGRP-containing fibres were less in density as compared to other nerve plexus. There was no difference in density of an individual type of nerve fibre in arteries of different cord segments or between the radicular arteries from different levels. The study reveals the existence of a comprehensive perivascular adrenergic and peptidergic innervation of spinal cord arterial system, with a possible role in neurogenic regulation of spinal cord circulation.     

Changes in the reflex excitability of spinal motor neurons in newborn infants during sleep and increases in environmental temperature

Reflex excitability of the spinal motor centres was studied in newborns by the monosynaptic testing (H-reflex) method during the rise of air temperature in the cunette up to 32 degrees and 34 degrees as compared to the control data obtained at 30 degrees C. It was shown that at temperatures of 32 degrees and 34 degrees C the reflex excitability of spinal motoneurones is lower than in the control. A narrowing of the range and a weakening of the stimuli were recorded at which the H-reflex could be elicited. The possible ways are discussed in which the surrounding temperature affects reflex excitability of the spinal motor centres.     

Vesicular glutamate transporter 2 is expressed in different nerve fibre populations that selectively contact pulmonary neuroepithelial bodies

Pulmonary neuroepithelial body (NEB) receptors in rats receive at least four different nerve fibre populations. In addition to a spinal sensory innervation that contacts NEBs at their basal side, extensive vagal nodose sensory terminals and separate nitrergic and cholinergic nerve endings protrude between NEB cells. In the present study, antibodies against the vesicular glutamate transporter 2 (VGLUT2), a transmembrane protein responsible for loading glutamate into synaptic vesicles, were used to investigate whether some of the nerve terminals contacting NEBs in rat lungs might use glutamate as a neurotransmitter. VGLUT2 immunoreactivity (IR) was detected in extensive intraepithelial arborising nerve terminals that appeared to contact most of the NEBs. Multiple immunostaining showed VGLUT2 IR in the vagal nodose and spinal sensory nerve terminals contacting NEBs, and in another, most likely sensory, intraepithelial nerve fibre population, the origin and further characteristics of which remain to be elucidated. At least part of the VGLUT2-immunoreactive nerve fibres that contact NEBs were shown to be myelinated. The expression of VGLUT2 indicates that glutamate is stored and released as a neurotransmitter in terminals of several pulmonary (sensory) nerve fibre populations that selectively relate to the complex NEB receptors. The present study strongly suggests an involvement of glutamatergic mechanisms in the peripheral transduction of sensory stimuli from the lungs, via the release of glutamate from nerve terminals, thereby modulating the activity of NEB receptor cells or the excitability of afferent nerves.     

The spinal origin of the sympathetic nerve fibres to the vascular and secretory components of the rat submaxillary salivary gland.

The spinal origin of the sympathetic vasoconstrictor and secretory fibres to the submaxillary gland of the rat was identified in the pithed rat preparation by means of selective stimulation of small segments of the spinal outflow. Secretory and vascular responses were similar following stimulation in pithed rats to those following stimulation of the isolated superior cervical nerve trunk in anaesthetized rats. The spinal origin of the secretory and vascular fibres was coincident and it is concluded that if a separate control of blood flow and secretion by sympathetic fibres does exist that it must occur at the level of C.N.S. but that the nerves share a common pathway to the gland.     

Conduction along myelinated and demyelinated nerve fibres during the recovery cycle: Model investigations

The membrane excitability changes as well as the underlying mechanisms of these changes in a normal and in a systematically paranodally demyelinated nerve fibre have been investigated by paired stimulation during the first 30 ms of the recovery cycle. The ionic current kinetics determining the observed changes in the action potential parameters are presented also. The simulation of the conduction in the normal fibre is based on the Frankenhaeuser and Huxley (1964) and Goldman and Albus (1968) equations, while in the case of a demyelinated fibre according to the same equations modified by Stephanova (1988a). It has been shown for the demyelinated membrane that increased demyelination increases both the threshold current for the second potential as well as the absolute refractory period. With increasing interpulse interval, the subnormality of the membrane excitability is followed by supernormality in the case of the demyelinated membrane. For the recovery cycle of 30 ms under consideration no supernormality of the normal membrane excitability is obtained. With interpulse interval from 8.8 to 10.9 ms, the highest degree of demyelination (l=30 m) is accompanied by a refractory period of transmission. The membrane properties of the normal and demyelinated fibres recover 20 ms after the first pulse. For short interpulse intervals, the amplitude of the second action potential is decreased, and a slower propagation velocity is obtained. The most sensitive phenomenon is the excitability of the demyelinated membrane, which remains unrecovered 30 ms after the first pulses has been applied.     

The role of the alpha2 chain in the stabilization of the collagen type I heterotrimer: a study of the type I homotrimer in oim mouse tissues

We have previously reported that the fragility of skin, tendon and bone from the oim mouse is related to a significant reduction in the intermolecular cross-linking. The oim mutation is unlikely to affect the efficacy of the lysyl oxidase, suggesting that the defect is in the molecule and fibre. We have therefore investigated the integrity of both the oim collagen molecules and the fibre by differential scanning calorimetry.The denaturation temperature of the oim molecule in solution and the fibre from tail tendon were found to be higher than the wild-type by 2.6deg.C and 1.9deg.C, respectively. With the loss of the alpha2 chain, the hydroxyproline content of the homotrimer is higher than the heterotrimer, which may account for the increase.There is a small decrease in the enthalpy of the oim fibres but it is not significant, suggesting that the amount of disorder of the triple-helical molecules and of the fibres is small and involves only a small part of the total bond energy holding the helical structure together. The difference in denaturation temperature of the skin collagen molecules (t(m)) and fibres (t(d)) is significantly lower for the oim tissues, 19.9deg.C against 23.1deg.C, indicating reduced molecular interactions and hence packing of the molecules in the fibre. Computation of the volume fraction of the water revealed that the interaxial separation of the oim fibres was indeed greater, increasing from 19.6A to 21.0A. This difference of 1.4A, equivalent to a C-C bond, would certainly decrease the ability of the telopeptide aldehyde to interact with the epsilon -amino group from an adjacent molecule and form a cross-link. We suggest, therefore, that the reduction of the cross-linking is due to increased water content of the fibre rather than a distortion of the molecular structure.The higher hydrophobicity of the alpha2 chain appears to play a role in the stabilisation of heterotrimeric type I collagen, possibly by increasing the hydrophobic interactions between the heterotrimeric molecules, thereby reducing the water content and increasing the binding of the molecules in the fibre.     

An experimental breakage ofReissner's fibre in the central canal of the pike (Esox lucius)

Summary The spinal cords of newly hatched pike (Esox lucius) fry were divided into two pieces by transverse cuts. After periods of different lengths, the appearances of the brokenReissner's fibres were investigated anatomically. The fibre normally terminates in the caudal end as a secretory accumulation, a caudal mass. After the operation this mass gradually disappears, apparently through the spinal cord wall cells. The new termination of the broken fibre, in front of the scar, forms a new caudal mass. This fact indicates thatReissner's fibre is a secretory transport mechanism.Supported by a grant from the Swedish Natural Science Research Council.     

Future trends in pain research

In the future we shall need to modify the classical view that nerve impulses which signal the presence of injury are reliably transmitted by specified and automatic relay cells. We must investigate at least four different modifying mechanisms that are likely to generate chronic intractable pains. With a latency of milliseconds, combinations of afferent signals and of descending controls operate a rapid and powerful gate control. With a latency of minutes, impulses in C fibres change the excitability of peripheral endings and of spinal cord circuits. With a latency of days, chemical transport in C fibres from areas of damage further modifies cord connectivity with a disappearance of inhibitions and an expansion of receptive fields. With a latency of weeks and months, anatomical degeneration produces secondary changes in deafferented cells with atrophy, sprouting and abnormal firing patterns.     

Morphological changes in continuously stretched skeletal muscles in sheep

The effects of continuous elongation of skeletal muscles were studied on six sheep who underwent a lengthening osteotomy of the right tibia. Open muscle biopsies were taken from the biceps femoris muscle preoperatively (Group A), after 5 weeks of bone distraction (Group B) and after another 5 weeks without further distraction (Group C). The size and distribution of type 1 (slow-twitch) and type 2 (fast-twitch) muscle fibres were determined from sections stained for myofibrillar ATPase activity. All sections were also evaluated by light microscopy, especially with regard to myopathic changes. The type 2 fibres showed a significant decrease in size from group A to B and from group B to C. The reduction in fibre size from group A to C was 44.2%. The type 1 fibres, on the other hand, showed no significant differences in mean fibre size between the groups. However, there were considerable individual variations in type 1 fibre size between the groups. The distribution of both fibre types was similar in groups A and B (appr. 17% type 1 fibres) whereas the relative number of type 1 fibres was reduced to 12.4% in group C (P less than 0.01). Myopathic changes, i.e. muscle fibre necroses, were not seen in any of the groups. It is concluded that the type 2 fibre atrophy is mainly caused by muscular inactivity during the postoperative period, but an additional effect of continuous stretching of the muscle cannot be excluded.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contralateral input modulates the excitability of dorsal horn neurons involved in noxious signal processes. Potential role in neuronal sensitization

Wide Dynamic Range (WDR) neurons in the spinal cord receive inputs from the contralateral side that, under normal conditions, are ineffective in generating an active response. These inputs are effective when the target WDRs change their excitability conditions. To further reveal the mechanisms supporting this effectiveness shift, we investigated the weight of the excitation of the contralateral neurons on the target WDR responses. In the circuit of presynaptic (sending) and postsynaptic (receiving) neurons in crossed spinal connections the fibres that form the presynaptic neurons impinge on postsynaptic neurons can be considered the final relay of this contralateral pathway. The enhancement of the presynaptic neuron excitability may thus modify the efficacy of the contralateral input. Pairs of neurons each on a side of the spinal cord, at the L5-L6 lumbar level were simultaneously recorded in intact, anaesthetized, paralysed rats. The excitatory aminoacid NMDA and strychnine, the antagonist of the inhibitory aminoacid glycine, were iontophoretically administrated to presynaptic neurons to increase their excitability. Before and during the drug administration, spontaneous and noxious-evoked activities of the neurons were analysed. During the iontophoresis of the two substances we found that noxious stimuli applied to the receptive field of presynaptic neurons activated up to 50% of the previously unresponsive postsynaptic neurons on the opposite side. Furthermore, the neurons on both sides of the spinal cord showed significantly increased spontaneous activity and amplified responses to ipsilateral noxious stimulation. These findings indicate that the contralateral input participates in the circuit dynamics of spinal nociceptive transmission, by modulating the excitability of the postsynaptic neurons. A possible functional role of such a nociceptive transmission circuit in neuronal sensitization following unilateral nerve injury is hypothesized.     

The effect of submillisecond temperature jump on the mechanical tension of skinned muscle fibers of the frog in a state of rigor

Modification of temperature jump method for skinned skeletal muscle fibres is described. The fibre heating was obtained applying the high-voltage high-frequency current impulse to the fibre after removing the surrounding solution. The method permits to obtain the heating for 15 degrees C in 0.25 msec. The temperature jump in rigorized fibre induced instantaneous drop of tension. This effect is probably induced by thermal expansion of contractile proteins in the fibre.     

Contralateral input modulates the excitability of dorsal horn neurons involved in noxious signal processes. Potential role in neuronal sensitization

Wide Dynamic Range (WDR) neurons in the spinal cord receive inputs from the contralateral side that, under normal conditions, are ineffective in generating an active response. These inputs are effective when the target WDRs change their excitability conditions. To further reveal the mechanisms supporting this effectiveness shift, we investigated the weight of the excitation of the contralateral neurons on the target WDR responses. In the circuit of presynaptic (sending) and postsynaptic (receiving) neurons in crossed spinal connections the fibres that form the presynaptic neurons impinge on postsynaptic neurons can be considered the final relay of this contralateral pathway. The enhancement of the presynaptic neuron excitability may thus modify the efficacy of the contralateral input. Pairs of neurons each on a side of the spinal cord, at the L5–L6 lumbar level were simultaneously recorded in intact, anaesthetized, paralysed rats. The excitatory aminoacid NMDA and strychnine, the antagonist of the inhibitory aminoacid glycine, were iontophoretically administrated to presynaptic neurons to increase their excitability. Before and during the drug administration, spontaneous and noxious-evoked activities of the neurons were analysed. During the iontophoresis of the two substances we found that noxious stimuli applied to the receptive field of presynaptic neurons activated up to 50% of the previously unresponsive postsynaptic neurons on the opposite side. Furthermore, the neurons on both sides of the spinal cord showed significantly increased spontaneous activity and amplified responses to ipsilateral noxious stimulation. These findings indicate that the contralateral input participates in the circuit dynamics of spinal nociceptive transmission, by modulating the excitability of the postsynaptic neurons. A possible functional role of such a nociceptive transmission circuit in neuronal sensitization following unilateral nerve injury is hypothesized.     

Cortical Presynaptic Control of Dorsal Horn C–Afferents in the Rat

Lamina 5 sensorimotor cortex pyramidal neurons project to the spinal cord, participating in the modulation of several modalities of information transmission. A well-studied mechanism by which the corticospinal projection modulates sensory information is primary afferent depolarization, which has been characterized in fast muscular and cutaneous, but not in slow-conducting nociceptive skin afferents. Here we investigated whether the inhibition of nociceptive sensory information, produced by activation of the sensorimotor cortex, involves a direct presynaptic modulation of C primary afferents. In anaesthetized male Wistar rats, we analyzed the effects of sensorimotor cortex activation on post tetanic potentiation (PTP) and the paired pulse ratio (PPR) of dorsal horn field potentials evoked by C–fiber stimulation in the sural (SU) and sciatic (SC) nerves. We also explored the time course of the excitability changes in nociceptive afferents produced by cortical stimulation. We observed that the development of PTP was completely blocked when C-fiber tetanic stimulation was paired with cortex stimulation. In addition, sensorimotor cortex activation by topical administration of bicuculline (BIC) produced a reduction in the amplitude of C–fiber responses, as well as an increase in the PPR. Furthermore, increases in the intraspinal excitability of slow-conducting fiber terminals, produced by sensorimotor cortex stimulation, were indicative of primary afferent depolarization. Topical administration of BIC in the spinal cord blocked the inhibition of C–fiber neuronal responses produced by cortical stimulation. Dorsal horn neurons responding to sensorimotor cortex stimulation also exhibited a peripheral receptive field and responded to stimulation of fast cutaneous myelinated fibers. Our results suggest that corticospinal inhibition of nociceptive responses is due in part to a modulation of the excitability of primary C–fibers by means of GABAergic inhibitory interneurons.