Conduction In Myelinated Fibres Research Articles

Schwannomin-interacting Protein 1 Isoform IQCJ-SCHIP1 Is a Multipartner Ankyrin- and Spectrin-binding Protein Involved in the Organization of Nodes of Ranvier

The nodes of Ranvier are essential regions for action potential conduction in myelinated fibers. They are enriched in multimolecular complexes composed of voltage-gated Nav and Kv7 channels associated with cell adhesion molecules. Cytoskeletal proteins ankyrin-G (AnkG) and βIV-spectrin control the organization of these complexes and provide mechanical support to the plasma membrane. IQCJ-SCHIP1 is a cytoplasmic protein present in axon initial segments and nodes of Ranvier. It interacts with AnkG and is absent from nodes and axon initial segments of βIV-spectrin and AnkG mutant mice. Here, we show that IQCJ-SCHIP1 also interacts with βIV-spectrin and Kv7.2/3 channels and self-associates, suggesting a scaffolding role in organizing nodal proteins. IQCJ-SCHIP1 binding requires a βIV-spectrin-specific domain and Kv7 channel 1-5-10 calmodulin-binding motifs. We then investigate the role of IQCJ-SCHIP1 in vivo by studying peripheral myelinated fibers in Schip1 knock-out mutant mice. The major nodal proteins are normally enriched at nodes in these mice, indicating that IQCJ-SCHIP1 is not required for their nodal accumulation. However, morphometric and ultrastructural analyses show an altered shape of nodes similar to that observed in βIV-spectrin mutant mice, revealing that IQCJ-SCHIP1 contributes to nodal membrane-associated cytoskeleton organization, likely through its interactions with the AnkG/βIV-spectrin network. Our work reveals that IQCJ-SCHIP1 interacts with several major nodal proteins, and we suggest that it contributes to a higher organizational level of the AnkG/βIV-spectrin network critical for node integrity.

Nodal, paranodal and juxtaparanodal axonal proteins during demyelination and remyelination in multiple sclerosis

Saltatory conduction in myelinated fibres depends on the specific molecular organization of highly specialized axonal domains at the node of Ranvier, the paranodal and the juxtaparanodal regions. Voltage-gated sodium channels (Na(v)) have been shown to be deployed along the naked demyelinated axon in experimental models of CNS demyelination and in multiple sclerosis lesions. Little is known about aggregation of nodal, paranodal and juxtaparanodal constituents during the repair process. We analysed by immunohistochemistry on free-floating sections from multiple sclerosis brains the expression and distribution of nodal (Na(v) channels), paranodal (paranodin/Caspr) and juxtaparanodal (K(v) channels and Caspr2) molecules in demyelinated and remyelinated lesions. Whereas in demyelinated lesions, paranodal and juxtaparanodal proteins are diffusely distributed on denuded axons, the distribution of Na(v) channels is heterogeneous, with a diffuse immunoreactivity but also few broad Na(v) channel aggregates in all demyelinated lesions. In contrast to the demyelinated plaques, all remyelinated lesions are characterized by the detection of aggregates of Na(v) channels, paranodin/Caspr, K(v) channels and Caspr2. Our data suggest that these aggregates precede remyelination, and that Na(v) channel aggregation is the initial event, followed by aggregation of paranodal and then juxtaparanodal axonal proteins. Remyelination takes place in multiple sclerosis tissue but myelin repair is often incomplete, and the reasons for this remyelination deficit are many. We suggest that a defect of Na(v) channel aggregation might be involved in the remyelination failure in demyelinated lesions with spared axons and oligodendroglial cells.

Cutaneous field stimulation (CFS): a new powerful method to combat itch

Scratching the skin, while instantly relieving itch, often aggravates itch over time due to skin injury. To relieve itch, without damaging the skin, a new technique termed cutaneous field stimulation (CFS) was developed and tested on 21 subjects. CFS uses a flexible plate with needle-like electrodes ( n=16) to electrically stimulate nerve fibres in the superficial skin. The electrodes were stimulated consecutively (4 Hz per electrode, pulse duration 1 ms, intensity 0.4–0.8 mA, 25 min). CFS resulted in a pricking and burning sensation that usually faded rather quickly. The burning sensation was still present during a selective block of impulse conduction in myelinated fibres indicating that nociceptive C-fibres are activated by CFS. Furthermore, a flare reaction developed around the CFS electrodes indicating activation of axon reflexes in nociceptive C-fibres. Itch, elicited by transdermal iontophoresis of histamine, was abolished within the skin area pre-treated with CFS, and was reduced to 14% of control 10 cm distally. Contralateral effects were small or non-existent. After 4 h, itch was reduced ipsilaterally to 32% of control. In comparison, 2 h after transcutaneous electrical nerve stimulation (TENS; 10–20 mA, 100 Hz, 25 min) ipsilateral itch was reduced to 56% of control. In conclusion, CFS offers a powerful new method for combating itch. It is suggested that CFS acts through endogenous central inhibitory mechanisms that are normally activated by scratching the skin.

Heart rate responses to selective stimulation of cardiac vagal C fibres in anaesthetized cats, rats and rabbits.

1. The contribution of cardiac vagal C fibres to vagal chronotropic control in anaesthetized cats, rats and rabbits was analysed using electrical stimulation of the vagus nerve with a selective anodal block technique. 2. After bilateral vagotomy and pretreatment with atenolol, 10 Hz continuous selective stimulation of unmyelinated fibres in the cut peripheral end of the cervical vagus evoked a bradycardia in anaesthetized rats, cats and rabbits. With this stimulation protocol the three species exhibited a similar lengthening of the heart period (R-R interval) when expressed as a percentage of their basal cardiac interval. 3. The mechanism of action of the selective blocking technique was analysed by recording eighty-nine single A- (n = 12), B- (n = 22) and C-fibre (n = 55) vagal-projecting neurones in the medulla of the rat. This demonstrated that the technique can selectively block conduction in myelinated fibres and that 'break excitation' is seen mainly in unmyelinated fibres. Although thirty C fibres showed break excitation sixteen did not and this difference could not be correlated with their axonal conduction velocity, chronaxie or initial segment frequency following. 4. Using the anodal block technique the vagal effects on heart rate were reanalysed in the cat by incorporating a collision technique. B fibres were activated orthodromically to evoke cardioinhibition and simultaneously antidromically to collide with errant B-fibre spikes activated at the electrode producing anodal block. With this protocol it was noted that the B- and C-fibre bradycardias were not additive. Using a double anodal block and collision technique, it was demonstrated that this phenomenon was likely to be due to occlusion of the effects of B and C fibres. 5. In conclusion, in addition to the well-defined effects of vagal B fibres on heart rate, selective stimulation of vagal C fibres also had a cardioinhibitory effect in all three species studied. However, since the effects of cardiac C fibres on heart rate was small, these neurones alone cannot account for the cardioinhibition of the pulmonary chemoreflex. It is likely that activation of both B- and C-fibre cardiac vagal preganglionic neurones accounts for this reflex cardioinhibition.

RELEASE OF COLD-INDUCED BURNING PAIN BY BLOCK OF COLD-SPECIFIC AFFERENT INPUT

While the pure sensation of cold is evoked by activation of a specific set of afferent channels, an additional set is believed to be activated by noxious low-temperature stimuli evoking cold pain. At primary afferent level, the channels concerned with the cold fraction of cold pain are served by myelinated A delta cold-specific fibres, whereas those concerned with the pain fraction are served by unmyelinated C nociceptors. In the present study, interaction between the two types of afferent input underlying cold pain was investigated by selectively blocking conduction in myelinated fibres. When doing so to the point of abolishing cold sensation, ramps of low-temperature stimuli eventually evoked a first sensation of burning pain. In addition to, and contemporaneous with, this change in quality, a significant decrease in pain threshold (reduction in required stimulus energy) was recorded when applying a noxious low-temperature stimulus. Such exaggeration in magnitude of low temperature-induced pain and the unmasking of its burning quality by A fibre block imply release of central sensory transmission due to removal of inhibitory primary afferent input. Myelinated fibres transmitting either tactile, cold sensations or both could exert this inhibition. Previous evidence of suppression of pain by low-temperature stimuli indicates that it is the cold-specific input that normally exerts this central gating on nociceptor input. The present results may also offer an explanation for the occurrence of a syndrome of burning pain on cold exposure in neuropathic patients with impaired ability to perceive cold.

Central suppression of cold-induced C fibre pain by myelinated fibre input

Changes in thermal sensibility for warmth, cold, heat pain and cold pain during nerve compression block of impulse conduction in myelinated fibres were studied in 20 healthy subjects. When mainly unmyelinated fibres were conducting, after 30–36 min of nerve compression, the pain threshold, induced by cold stimulation, was shifted towards higher temperatures (from 19.1° C to 22.8°C, mean values). Furthermore, the sensation of cold pain became more unpleasant and had a hot burning rather than a cold quality. These results indicate that a change in central decoding of the afferent input has occurred, possibly due to lack of inhibition normally exerted by concomitant activation of myelinated fibres. Whereas dramatic changes in the sensation of cold pain were observed during the course of nerve compression, no alteration in heat pain threshold was seen. This implies that heat pain threshold in hairy skin is due to activation of C nociceptor fibres without any significant contribution from myelinated nociceptor fibres. Furthermore, no gating from heat-sensitive myelinated fibre input was evident on heat pain threshold.

Peripheral projections of nociceptive unmyelinated axons in the human peroneal nerve.

1. Previous knowledge of the anatomical course of unmyelinated (C) axons along a peripheral nerve has been scarce and has led to the concept of the axons in a constantly interchanging position. 2. Results obtained by microneurography in the peroneal nerve at knee or ankle levels in awake humans demonstrated that the receptive fields of neighbouring C units in the nerve cluster in close vicinity on the skin of the foot or the ankle. These findings indicate that C afferents run closely together throughout large portions of the peripheral nerve. 3. Intraneural microstimulation performed at neural sites where nociceptive C units were recorded induced painful sensations projected to the skin. When the stimulus intensity was increased, there was typically a concentric increase in the area of projected pain, rather than recruitment of several scattered pain projections. This finding further supports the hypothesis of a neighbouring relation of nociceptive C axons within nerve fascicles, implying spatial recruitment of adjacent axons in the nerve with adjacent peripheral projections. 4. A pain locognosia test performed during ischaemic block of impulse conduction in myelinated fibres demonstrated a fairly precise cerebral localization of noxious events on the foot from the input of C afferent fibres alone.

Stimulation of cat cutaneous nociceptive C fibres causing tonic and synchronous activity in climbing fibres.

1. The input from cutaneous nociceptors to climbing fibres projecting to the forelimb area of the C3 zone in the cerebellar anterior lobe was examined in barbiturate-anaesthetized cats. Climbing fibre responses were simultaneously recorded in single Purkinje cells and as field potentials from the cerebellar surface close to these cells. 2. The cutaneous receptive field of the climbing fibres studied were located on the ipsilateral forelimb. All climbing fibres were activated by both non-noxious tactile stimulation and noxious pinch of the skin. The location of the receptive field and the distribution of sensitivity in the receptive field appeared to be identical for noxious and tactile stimuli. 3. A phasic response in the climbing fibres was evoked by either short- or long-lasting non-noxious pressure applied to their cutaneous receptive field. By contrast, all climbing fibres studied were strongly and tonically activated (up to 4-11 Hz for the duration of the stimulation) by sustained noxious pinch in the most sensitive area of their receptive fields. 4. Experiments with anodal block of impulse conduction in myelinated fibres indicated that a major input to climbing fibres during sustained noxious pinch originates from nociceptive C fibres. 5. Sustained noxious pinch of the skin evoked large field potentials on the cerebellar surface. These field potentials were evoked simultaneously with climbing fibre responses in single Purkinje cells and were due to synchronous activation of many climbing fibres. These field potentials and discharges in single climbing fibres were elicited from the same area of the skin suggesting that many of the synchronously discharging climbing fibres have the same receptive field on the skin.

The role of vagal reflexes in experimental lung oedema, bronchoconstriction and inhalation of halothane

The role of vagal reflexes in experimental lung oedema, bronchoconstriction and inhalation of halothane has been assessed by comparing responses in rabbits with intact vagi, with cut vagi, and with vagal conduction in myelinated fibres blocked by direct current. The reflex increase in breathing frequency due to halothane is present during block of conduction in myelinated fibres, and is thought to be due to stimulation of alveolar J-receptors. The reflex increase in breathing frequency due to inhalation of histamine as a large particle (8 μ diameter) aerosol was prevented by myelinated-fibre block, and is thought to be due to stimulation of airway irritant receptors. The reflex increase in breathing frequency due to lung oedema was largely prevented by myelinated-fibre block, and is thought to be due to stimulation of irritant receptors, with pulmonary stretch and J-receptors playing a smaller part.

Role of pulmonary vagal afferent nerve fibres in the development of rapid shallow breathing in lung inflammation.

1. The administration of the polysaccharide carageenin through a catheter into the lungs of cats and rabbits has produced an inflammatory lesion confined to one lobe of a lung. The lesion consisted of an alveolar and interstitial infiltration with polymorphonuclear leucocytes and, subsequently, macrophages. There was no apparent damage to alveolar walls and no pleurisy. The rest of the lung remained normal. 2. In both conscious cats and anaesthetized rabbits there was an increased frequency of breathing dependent on an intact vagus nerve on the same side as the lesion. It was independent of changes in body temperature and was not due to hypoxaemia. 3. By using a direct current to the right cervical vagus nerve in the rabbits (with the left vagus nerve sectioned), it has been possible to block conduction in myelinated fibres; the non-myelinated fibres conduct normally. Studies with this differentially blocked nerve have shown that the increased frequency of breathing is dependent on activity in the non-myelinated vagal afferent fibres.

The role of non‐myelinated vagal afferent fibres from the lungs in the genesis of tachypnoea in the rabbit

1. The use of a direct current (d.c.) to produce a differential block of conduction in the cervical vagus nerves of rabbits is described; the myelinated fibres are blocked, while the non-myelinated ;C' fibres conduct normally. The method produces reproducible and reversible results.2. The block is equally effective for low and high frequencies of discharge (1-100 Hz). During recovery or development of the block, lower frequencies of discharge can pass but higher frequencies cannot.3. Block of conduction in myelinated fibres is associated with slower, deeper breathing, confirming previous work with cooling.4. A further slowing and deepening of breathing may occur when a differentially blocked (;non-myelinated') nerve is sectioned, and this is mainly apparent when there are pathological conditions in the lungs.5. The respiratory response to the right atrial injection of phenyl diguanide is mediated by non-myelinated thoracic vagal afferent fibres.6. The tachypnoeic response to lung deflation is not mediated by non-myelinated fibres.7. Head's Paradoxical reflex has been demonstrated during partial recovery of conduction in myelinated fibres when only lower frequencies of afferent discharge can pass the area of block.8. A standard technique for providing a reproducible vagally mediated, tachypnoeic response to pulmonary micro-embolism is described using inert carbon-coated microspheres of 50 mum diameter. This tachypnoeic response was unchanged during a differential block indicating that the response was mediated by non-myelinated ;C' fibres.9. Pathological changes such as haemorrhage, oedema, infarction and collapse were absent after micro-embolism, and there were no systematic changes in lung resistance and compliance. The walls of arterioles and adjacent alveoli are distorted by the emboli and these areas are the probable sites of afferent stimulation.

Computation of Impulse Conduction in Myelinated Fibers; Theoretical Basis of the Velocity-Diameter Relation

For myelinated fibers, it is experimentally well established that spike conduction velocity is proportional to fiber diameter. However no really satisfactory theoretical treatment has been proposed. To treat this problem a theoretical axon was described consisting of lengths of passive leaky cable (internode) regularly interrupted by short isopotential patches of excitable membrane (node). The nodal membrane was assumed to obey the Frankenhaeuser-Huxley equations. The explicit diameter dependencies of the various parameters were incorporated into the equations. The fiber diameter to axon diameter ratio was taken to be constant, and the internode length was taken to be proportional to the fiber diameter. Both these conditions reflect the situation that exists in real, experimental fibers. Dimensional analysis shows that these anatomical conditions are equivalent to Rushton's (1951) assumption of corresponding states. Hence, conduction velocity will be proportional to fiber diameter, in complete agreement with the experimental findings. Digital computer solutions of these equations were made in order to compute a set of actual velocities. Computations made with constant internode length or constant myelin thickness (i.e., nonconstant fiber diameter to axon diameter ratio) did not show linearity of the velocity-diameter relation.