Ectopic impulse generation in focally demyelinated trigeminal nerve

A TRPM7 mutation linked to familial trigeminal neuralgia: Omega current and hyperexcitability of trigeminal ganglion neurons

Trigeminal neuralgia (TN) is a unique pain disorder characterized by intense paroxysmal facial pain within areas innervated by the trigeminal nerve. Although most cases of TN are sporadic, familial clusters of TN suggest that genetic factors may contribute to this disorder. Whole-exome sequencing in patients with TN reporting positive family history demonstrated a spectrum of variants of ion channels including TRP channels. Here, we used patch-clamp analysis and Ca²⁺ and Na⁺ imaging to assess a rare variant in the TRPM7 channel, p.Ala931Thr, within transmembrane domain 3, identified in a man suffering from unilateral TN. We showed that A931T produced an abnormal inward current carried by Na⁺ and insensitive to the pore blocker Gd³⁺. Hypothesizing that replacement of the hydrophobic alanine at position 931 with the more polar threonine destabilizes a hydrophobic ring, near the voltage sensor domain, we performed alanine substitutions of F971 and W972 and obtained results suggesting a role of A931-W972 hydrophobic interaction in S3-S4 hydrophobic cleft stability. Finally, we transfected trigeminal ganglion neurons with A931T channels and observed that expression of this TRPM7 variant lowers current threshold and resting membrane potential, and increases evoked firing activity in TG neurons. Our results support the notion that the TRPM7-A931T mutation located in the S3 segment at the interface with the transmembrane region S4, generates an omega current that carries Na⁺ influx in physiological conditions. A931T produces hyperexcitability and a sustained Na⁺ influx in trigeminal ganglion neurons that may underlie pain in this kindred with trigeminal neuralgia.

Exome Sequencing Implicates Impaired GABA Signaling and Neuronal Ion Transport in Trigeminal Neuralgia

Trigeminal neuralgia (TN) is a common, debilitating neuropathic face pain syndrome often resistant to therapy. The familial clustering of TN cases suggests that genetic factors play a role in disease pathogenesis. However, no unbiased, large-scale genomic study of TN has been performed to date. Analysis of 290 whole exome-sequenced TN probands, including 20 multiplex kindreds and 70 parent-offspring trios, revealed enrichment of rare, damaging variants in GABA receptor-binding genes in cases. Mice engineered with a TN-associated de novo mutation (p.Cys188Trp) in the GABA_A receptor Cl⁻ channel γ-1 subunit (GABRG1) exhibited trigeminal mechanical allodynia and face pain behavior. Other TN probands harbored rare damaging variants in Na⁺ and Ca⁺ channels, including a significant variant burden in the α-1H subunit of the voltage-gated Ca²⁺ channel Ca_v3.2 (CACNA1H). These results provide exome-level insight into TN and implicate genetically encoded impairment of GABA signaling and neuronal ion transport in TN pathogenesis.

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Genomic analysis of trigeminal neuralgia (TN) using exome sequencing

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Rare mutations in GABA signaling and ion transport genes are enriched in TN cases

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Generation of a genetic TN mouse model engineered with a patient-specific mutation

Mechanism underlying cranial nerve rhizopathy

The cranial nerve rhizophathy, commonly presented with trigeminal neuralgia (TN) or hemifacial spasm (HFS), is a sort of hyperexcitability disorders with higher incidence in senior Asian. In this paper, a novel hypothesis on the pathogenesis is proposed and with which some clinical phenomena are explained. In those with crowded cerebellopontine angle in anatomy, the cranial nerve root and surrounding vessel are getting closer and closer to each other with aging and finally the neurovascular conflict happens. As the interfacial friction associated with pulse, the nerve incurs demyelination. Since this pathological change develops to a certain degree, some transmembrane proteins emerge from the nerve due to a series of signaling pathway mediated by inflammatory cytokines. Among them, voltage-gated (Na_v1.3) and mechanosensitive (Piezo2) ion channels may play the important role. With pulsatile compressions, the Piezo2 drives the resting potential toward depolarization forming a state of subthreshold membrane potential oscillation. Under this condition, just an appropriate pressure can make the membrane potential easy to reach threshold and activate the sodium channel, eventually generating conductible action potentials from the axon. When these ectopic action potentials propagate to the central nerve system, an illusion of sharp pain is perceived; while to the nerve-muscle junctions, an attack of irregular muscle constriction occurs. This hypothesis can well explain the symptomatic manifestation of paroxysmal attacks aroused by emotions. When we get nervous or excited, our heart rate and blood pressure alter correspondingly, which may give rise to "a just right pressure" - with specific frequency, amplitude and angle - impacting the suffered nerve to reach the threshold of impulse ignition. After a successful microvascular decompression surgery, the trigger is gone (there is no compression anymore) and the symptom is alleviated. While the postoperative recurrence could be attributable to Teflon granuloma development if had been placed improperly - for this nerve root has been susceptible no matter to arteries or to neoplasms. Besides, it may illustrate the clinical phenomenon that secondary TN or HFS cases are seldom caused by schwannoma: with a proliferative sheath, the nerve root is actually insulated. By contrast, not all neurovascular contacts can lead to the onset: it demands an exclusive extent of demyelination firstly.

The Spectrum of Trigeminal Neuralgia Without Neurovascular Compression

BACKGROUND

In trigeminal neuralgia type 1 (TN1), neurovascular compression (NVC) is often assumed to be the pain initiating mechanism. NVC can be surgically addressed by microvascular decompression (MVD). However, some patients with TN1 present without NVC (WONVC).

OBJECTIVE

To characterize and analyze the clinical spectrum of a TN1 patient population WONVC.

METHODS

A retrospective chart review of patients presenting with TN1 between 2007 and 2017 was performed. Patients who were potential candidates for MVD surgery underwent high-resolution imaging with 3-dimensional (3D) reconstruction to address the presence, or absence, of NVC. Demographic data about the populations with NVC (WNVC) and WONVC were collected.

RESULTS

Of 242 patients with TN1, 32% did not have NVC. Patients WONVC were on average 10.6 yr younger than those WNVC. TN1 onset in patients WONVC was more frequent below 48.7 yr, and the opposite was found in patients WNVC. Compared to patients WNVC, those WONVC were predominantly female (odds ratio 4.8), on average were 4 yr younger at symptom onset (34.7 yr) and 7.8 yr younger at first clinic visit, and had a 3.7 yr shorter symptom duration.

CONCLUSION

Patients presenting with TN1 WONVC were predominantly females in their mid-30s with short symptom duration. In the absence of NVC, this subgroup of TN1 patients has limited surgical options, and potentially a longer condition duration that must be managed medically or surgically. This population WONVC might provide insights into the true pathophysiology of TN1.

A Review of Neuropathic Pain: From Diagnostic Tests to Mechanisms

Neuropathic pain develops when the somatosensory nervous system is affected by a lesion or disease. Diagnostic tests aimed at assessing somatosensory afferent pathway damage are therefore useful for diagnosing neuropathic pain. Neuropathic pain manifests with a range of different symptoms such as ongoing burning pain, squeezing or pressure pain, paroxysmal electric shock-like sensations, stabbing pain, or mechanical dynamic allodynia. The various types of neuropathic pain are associated with different underlying nerve fiber abnormalities. This article summarizes the available methods of somatosensory afferent pathway assessment and discusses the potential pathophysiology underlying the most representative neuropathic pain types, i.e., ongoing burning pain, paroxysmal pain, and mechanical dynamic allodynia.

FUNDING

Pfizer, Italy.

How diagnostic tests help to disentangle the mechanisms underlying neuropathic pain symptoms in painful neuropathies

Neuropathic pain, ie, pain arising directly from a lesion or disease affecting the somatosensory afferent pathway, manifests with various symptoms, the commonest being ongoing burning pain, electrical shock-like sensations, and dynamic mechanical allodynia. Reliable insights into the mechanisms underlying neuropathic pain symptoms come from diagnostic tests documenting and quantifying somatosensory afferent pathway damage in patients with painful neuropathies. Neurophysiological investigation and skin biopsy studies suggest that ongoing burning pain primarily reflects spontaneous activity in nociceptive-fiber pathways. Electrical shock-like sensations presumably arise from high-frequency ectopic bursts generated in demyelinated, nonnociceptive, Aβ fibers. Although the mechanisms underlying dynamic mechanical allodynia remain debatable, normally innocuous stimuli might cause pain by activating spared and sensitized nociceptive afferents. Extending the mechanistic approach to neuropathic pain symptoms might advance targeted therapy for the individual patient and improve testing for new drugs.

Reappraising neuropathic pain in humans--how symptoms help disclose mechanisms

Neuropathic pain--that is, pain arising directly from a lesion or disease that affects the somatosensory system--is a common clinical problem, and typically causes patients intense distress. Patients with neuropathic pain have sensory abnormalities on clinical examination and experience pain of diverse types, some spontaneous and others provoked. Spontaneous pain typically manifests as ongoing burning pain or paroxysmal electric shock-like sensations. Provoked pain includes pain induced by various stimuli or even gentle brushing (dynamic mechanical allodynia). Recent clinical and neurophysiological studies suggest that the various pain types arise through distinct pathophysiological mechanisms. Ongoing burning pain primarily reflects spontaneous hyperactivity in nociceptive-fibre pathways, originating from 'irritable' nociceptors, regenerating nerve sprouts or denervated central neurons. Paroxysmal sensations can be caused by several mechanisms; for example, electric shock-like sensations probably arise from high-frequency bursts generated in demyelinated non-nociceptive Aβ fibres. Most human and animal findings suggest that brush-evoked allodynia originates from Aβ fibres projecting onto previously sensitized nociceptive neurons in the dorsal horn, with additional contributions from plastic changes in the brainstem and thalamus. Here, we propose that the emerging mechanism-based approach to the study of neuropathic pain might aid the tailoring of therapy to the individual patient, and could be useful for drug development.

Cranial neuralgias

After a description of the anatomical-functional organization of the human trigeminal system, this chapter discusses the diagnostic and therapeutic options for trigeminal neuralgia (TN). In about 15% of patients who present with the clinical picture of typical TN, this is secondary to a major neurological disease, i.e., benign tumors of the cerebellopontine angle or multiple sclerosis. Some clinical criteria that were used to distinguish between classic and symptomatic TN, such as age at onset, involvement of the ophthalmic division, and responsiveness to medical treatment, are no longer considered reliable. It is recommended that all patients undergo magnetic resonance imaging (MRI) or trigeminal reflex recording. Carbamazepine (CBZ) and oxcarbazepine (OXC) are the first-choice medical treatments. Although other drugs may be effective, these are indicated when the patient cannot reach the therapeutic dosage of CBZ/OXC because of adverse events. Patients unresponsive to CBZ/OXC should be made aware of the available surgical interventions. Surgical procedures (including percutaneous lesions to the ganglion/root, microvascular decompression (MVD) in the posterior fossa, and gamma knife radiosurgery) are extremely efficacious with relatively few complications: each procedure has some advantage and disadvantage with respect to the other. Only MVD is a non-destructive procedure. This chapter also describes management of glossopharyngeal neuralgia, which is often misdiagnosed, and some other chronic pain conditions mediated by the trigeminal system, such as ophthalmic postherpetic neuralgia (PHN).

Increased sodium channel immunofluorescence at myelinated and demyelinated sites following an inflammatory and partial axotomy lesion of the rat infraorbital nerve

The localization of sodium channels (NaChs) change following nerve lesions and this change may contribute to the development of increased pain states. Here we examine the change in distribution of NaChs within the rat infraorbital nerve (ION) two weeks after a combined inflammatory/partial axotomy lesion that results in behavior showing increased sensitivity to mechanical stimuli. Sections from experimental and normal control IONs were double-stained for indirect immunofluorescence using an antibody that identifies all NaCh isoforms and caspr-antibody to identify nodes of Ranvier, and a confocal microscope z-series of optically sectioned images were then obtained. ImageJ (NIH) software was used to quantify the area of pixels showing maximum NaCh intensity within both caspr and non-caspr associated accumulations. Analysis showed that the lesioned IONs had many more split nodes, heminodes and caspr-negative "naked" accumulations, a significantly increased area of NaCh staining within typical nodes and "naked" accumulations, as well as an increased density and size of significant accumulations when compared to normal IONs. This study demonstrates a dramatic redistribution and increased immunofluorescence of NaChs especially at myelinated and demyelinated sites in fibers located just proximal to the lesion. The remodeling of NaChs seen in this study may represent an important event associated with the development of increased nerve excitability after lesions.

Ectopic impulse generation and autoexcitation in single myelinated afferent fibers in patients with peripheral neuropathy and positive sensory symptoms

Microneurographic studies were performed using cutaneous nerves of 8 patients with documented peripheral neuropathy who expressed positive sensory symptoms. Intraneural recordings in single myelinated fibers revealed: (i) ectopic generation of bursts of spontaneous action potentials; (ii) ectopic generation of ongoing repetitive discharges transiently interrupted by natural stimulation of the receptive field; and (iii) repetitive discharges triggered by a preceding action potential. These results provide direct evidence of a peripheral pathophysiological basis for spontaneous and stimulus-induced paresthesias and dysesthesias in patients with peripheral neuropathy.

Paradoxical clinical consequences of peripheral nerve injury: a review of anatomical, neurophysiological and psychological mechanisms

This paper reviews some of the possible explanations and mechanisms that may be responsible for variation from expected clinical findings soon after nerve injury in certain patients, and for subjective sensations and objective sensibility which can appear to arise from within the autonomous zone of the cutaneous distribution of a divided nerve. A number of features of peripheral innervation, central nervous system physiology and sensory psychology are discussed. These include: (1) the normal extent of overlap- or cross-innervation between the territories of adjacent peripheral nerves; (2) anomalous innervation due to normal anatomical variation; (3) ectopic impulse generation and cross-excitation between neurons in the peripheral nervous system after nerve injury; (4) neurophysiological responses and mechanisms of re-innervation other than axon regeneration across the site of nerve repair; (5) cortical somatotopic reorganisation in response to nerve injury; and (6) phantom sensory phenomena including the psychology of sensory perception.

Fos expression in the medullary dorsal horn of the rat after chronic constriction injury to the infraorbital nerve

Chronic constriction injury to the rat's infraorbital nerve (IoN-CCI) induces asymmetric face grooming directed to the injured nerve territory and, beginning at 7-12 days postoperative, hyperresponsiveness to mechanical stimulation in this territory (B.P. Vos, A.M. Strassman, and R.J. Maciewicz, 1994, J. Neurosci. 14:2708-2723). To examine central mechanisms involved in these behavioral alterations, changes in nonevoked and mechanical stimulation-evoked fos-like immunoreactivity (fos-LI) following IoN-CCI were quantified in the medullary dorsal horn. Following the appearance of hyperresponsiveness in IoN-CCI rats, experimental and matched sham-operated rats were anesthetized with urethane and received either no stimulation or repeated stimulation with either a 2- or 15-g von Frey hair applied to the hairy skin between vibrissae B3-4/C3-4 on the operated side. Unstimulated IoN-CCI rats had increased fos-LI in laminae I-IV of the ipsilateral medullary dorsal horn. In both groups, mechanical stimulation produced a distinct pattern of fos-LI in the ipsilateral medullary dorsal horn, the quantity of which was related to stimulus intensity. For both stimulus intensities, the total amount and the rostrocaudal spread of evoked fos-LI were significantly larger in IoN-CCI rats. In IoN-CCI rats, stimulation-evoked increases in fos-LI were proportionally larger in laminae I-II than in III-IV. This laminar effect was also present in sham-operated rats but only for 15-g stimulation. Neither condition nor stimulus intensity affected fos-LI in the contralateral medullary dorsal horn. Positive correlations were found between the behavioral parameters of increased trigeminal nociceptive activity and the total amount of fos-LI in the ipsilateral medullary dorsal horn. The results demonstrate that IoN-CCI induces significant alterations in the central processing of afferent signals, which may underlie behavioral manifestations of increased nociceptive activity.

Na+ conductance and the threshold for repetitive neuronal firing

The Hodgkin-Huxley equation for electrogenesis in the voltage clamped squid giant axon was used to predict the effect of altering maximal Na+ conductance (gNa+max) on the repetitive firing process. The main finding was that increasing gNa+max, without changing any other membrane parameter, reduced the threshold current required to evoke repetitive firing. That is, it rendered the membrane hyperexcitable. Threshold for evoking single action potentials was also affected, but much less so. Other consequences of increasing gNa+max were a decrease in the minimum sustainable rhythmic firing frequency (mRFF), a monotonic increase in firing frequency at any given suprathreshold stimulus intensity, an increase in the current value at which intense depolarizing stimuli block rhythmogenesis, an increase in the maximal sustainable firing frequency using intense currents (MRFF), and the consequent expansion of the dynamic range for stimulus encoding. Thus, the control of gNa+max through the regulation of Na+ channel synthesis and membrane incorporation at sites of rhythmogenesis (e.g. axon hillock-initial segment region, or peripheral sensory endings) is a potential regulatory mechanism for neuronal excitability and stimulus encoding.

Ectopic discharge in injured nerves: comparison of trigeminal and somatic afferents

Using the teased fiber recording method, we have compared pathophysiological properties of afferent axons injured in the infraorbital nerve (ION) vs the sciatic nerve in rats. Both myelinated and unmyelinated axons ending in ION neuromas produced much less ongoing discharge than those ending in sciatic nerve neuromas. Similarly, mechanosensitivity and acute injury discharge in ION neuromas were minimal. These differences may be related to the different spectrum of neuropathic symptomatology associated with nerve injury in the trigeminal vs the segmental innervation fields.

Acute and remitting painful diabetic polyneuropathy: a comparison of peripheral nerve fibre pathology

The cause of the neuropathic pain that is experienced by some patients with diabetic neuropathy remains to be established. Early neuropathological reports, based on comparisons between diabetic patients and non-diabetic control subjects, emphasised associations between pathological changes in specific classes of peripheral nerve fibre and the presence of pain. By making comparisons with more appropriate control subjects, namely diabetic patients without neuropathic pain, more recent studies have found that there are few clear morphological correlates for this type of pain. To investigate this further, we have conducted a detailed morphometric study of sural nerve biopsies from six diabetic patients, four with active acute painful neuropathy and two with recent remission from the same condition. Normal values for the neuropathological parameters were obtained from six non-diabetic control subjects. Teased fibre analysis showed that similar axonal and Schwann cell abnormalities were present in both groups of diabetic patients. Electron microscopical studies revealed that evidence of both myelinated and unmyelinated fibre degeneration and regeneration was also present in the nerves of all diabetic patients, irrespective of whether they had pain. Within the constraints of interpreting results from small numbers of patients, our observations suggested that remission from pain might be associated with a less abnormal axon/Schwann cell calibre ratio, more successful myelinated fibre regeneration and less active unmyelinated fibre regeneration. However, the inescapable finding of this study was, in fact, the similarity in the nerve fibre pathology in diabetic patients with active and remitting painful neuropathy. We conclude that the occurrence of nerve fibre degeneration and regeneration is in itself unlikely to be sufficient to account fully for diabetic neuropathic pain. However, it is conceivable that events occurring during certain stages in the pathological cycle of degeneration and regeneration create the necessary circumstances which lead to pain.

Abnormal impulse discharge in primary afferent axons injured in the peripheral versus the central nervous system

Chronic injury to sensory axons in the rat peripheral nerve induces pathophysiologic changes in the axolemma at the cut nerve end, which are reflected in spontaneous ectopic impulse discharge and hyperexcitability to a range of depolarizing stimuli. We asked whether sensory axons injured in the central nervous system (CNS) also respond in this way. Primary afferent axons were severed in the sciatic nerve and, alternatively, in the midcervical or upper lumbar dorsal column (DC). Measurements of abnormal discharge from myelinated afferents showed high levels of spontaneous activity generated at the nerve injury site, especially during the period 3-16 days postoperatively, but comparatively little activity generated at the DC lesion site at any postoperative time. There was a corresponding difference in ectopic hyperexcitability to mechanical and adrenergic stimulation, and to depolarization with topical K+. DC lesion sites were not made more excitable by concurrent transection of the sciatic nerve, or by placing an autologous graft of excised sciatic nerve tissue into the DC defect at the time of initial surgery. Transection sites on dorsal roots L4 and L5 yielded abnormal discharge similar to that of sciatic nerve neuromas, indicating that the relative silence of DC transection sites was related to the CNS environment and not to position with respect to the sensory cell body.

Ligature-induced injury in peripheral nerve: electrophysiological observations on changes in action potential characteristics following blockade of potassium conductance

The effects of the potassium channel blocking agent 4-aminopyridine (4-AP) on action potential properties were studied in chronically injured rat sciatic nerves. In normal, mature myelinated fibers, application of 4-AP does not lead to any significant change in action potential waveform or firing pattern in response to single stimuli. In contrast, application of 4-AP to nerves injured by the placement of loose ligatures results in the appearance of late rippled components in the compound action potential. This alteration in waveform is present at the injury site, but not at nerve segments proximal or distal to this region. Paired stimulation experiments demonstrate that this oscillation of the whole nerve response reflects repetitive firing in response to single stimuli following application of 4-AP. Intra-axonal recording following 4-AP application demonstrates bursts of action potentials, with several spikes of reduced amplitude arising from a depolarizing potential following the initial spike. Refractory period for the late spike is greater than that of the primary action potential. These results demonstrate that potassium channels are present and functional in chronically injured nerves, where blockage of these channels results in repetitive firing in response to single stimuli.

Changes with age in canine tooth pulp-nerve fibres of the cat

Light microscopical observations have been made on teased pulpal nerve fibres from mandibular canine teeth of cats aged 3 years or more. Both internodal lengths and external fibre diameters appeared to be reduced compared to the young adult. Qualitative myelin sheath changes were commonly observed. These consisted of extremely short, smooth or distorted intercalated internodes, myelin wrinkling, nodal lengthening and formation of myelin ovoids. Features suggesting de-myelination were also present. These findings should be considered when functional aspects of pulpal axons in the ageing tooth are discussed.

Trigeminal neuralgia as a consequence of circulatory disorders in the infratemporal and pterygopalatine fossae

An extensive review of the well documented experimental and clinical findings related to trigeminal neuralgia shows, that it is possible to set up an unifying theory for trigeminal neuralgia, which would explain most clinical features of the disease and also would be congruent with several other theories that have previously been presented. Thus, trigeminal neuralgia is proposed to be the result of a combination of predisposing anatomical variations, impaired arterial and venous circulation, and morphological changes in the infratemporal fossa and cavum Meckeli. The process is initiated by disturbances of dental occlusion and mandibular function, resulting in muscular hypertrophy, which finally leads to increased perineural pressure and hypoxia in the nervous tissue. Through a series of related events this results in massive afferent discharges in the trigeminal nerve, which are strong enough to break through an inhibitory gating mechanism in the nucleus caudalis, and which are experienced by the patient as a pain attack. But the tissue hypoxia, which partly has elicited the pain, will finally also be the critically delimiting factor for further signal transmission, thereby stopping the pain attack.

Myelin sheath thickness and internodal length of nerve fibres in the developing feline inferior alveolar nerve

The relation between the number of myelin lamellae (nl) and axon size (d) was examined in the developing and adult feline inferior alveolar nerve (IAN). The internodal lengths (L) and total diameters (D) were measured on teased IAN specimens from kittens and cats. The results show that relations nl/d and L/D principally similar to those in young adult cats had been established 2 months after birth. This coincides with the maturation of the primary dentition. During the first 3 postnatal weeks signs of a developmental demyelination were common. Comparisons between the internodal elongation of early myelinating axons and the longitudinal growth of the IAN in the mandibular canal indicated that some 50% of all prospective large internodes must be removed. Between 2 months and the young adult stage, when the permanent dentition is established, the relations nl/d and L/D were essentially unaltered, but the ranges extended towards larger sizes. In the young adult the average g-value was 0.67. In the old adult cat the relation nl/d was less uniform than in young adults, and the average g-value had decreased to 0.59. In addition, successive short (100-150 micrometer) internodes were found, indicating a senescent de- and remyelination in the IAN. These alterations may be related to the age-dependent deterioration of the mandibular dentition.

Lysosomal abnormalities in cultured schwann cells from a patient with peripheral neuropathy and continuous muscle fiber activity

Schwann cell cultures were established from a sural nerve containing large membrane-bound vacuoles in its Schwann cells, obtained from a patient with neuropathy and continuous muscle fiber activity. Cultured Schwann cells contained many large membrane-bound vacuoles, presumably lysosomes, resembling those present in the biopsied nerve. The acid phosphatase reaction was excessive in the patient's cultured Schwann cells but practically negative in normal cultured Schwann cells. This study indicates that the patient's neuropathy is primary dysschwannian with abnormal lysosomes as a major abnormality.

Ectopic impulse generation in demyelinated axons: effects of PaCO2, pH, and disodium edetate

Demyelinating lesions in the trigeminal root were produced by chronic implantation of chromic suture in 12 cats. Two types of abnormal repetitive action potential generation were recorded from demyelinated fibers: reflected spikes and afterdischarge. Both types of abnormal impulse generation were increased by hypocalcemia induced by disodium edetate infusion, hypocapnia produced by hyperventilation, and, to a lesser extent, alkalosis secondary to sodium bicarbonate administration. This hyperexcitability of fibers could be inhibited by hypoventilation with elevation of PaCO2 or by calcium chloride administration. These results may help explain the pathophysiological basis of paroxysmal symptoms in multiple sclerosis and other disorders in which demyelination is a prominent feature.