Changes in excitability of human cutaneous afferents following prolonged high-frequency stimulation

Treatment of Painful Diabetic Neuropathy-A Narrative Review of Pharmacological and Interventional Approaches

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus that is associated with a significant decline in quality of life. Like other painful neuropathic conditions, PDN is difficult to manage clinically, and a variety of pharmacological and non-pharmacological options are available for this condition. Recommended pharmacotherapies include anticonvulsive agents, antidepressant drugs, and topical capsaicin; and tapentadol, which combines opioid agonism and norepinephrine reuptake inhibition, has also recently been approved for use. Additionally, several neuromodulation therapies have been successfully used for pain relief in PDN, including intrathecal therapy, transcutaneous electrical nerve stimulation (TENS), and spinal cord stimulation (SCS). Recently, 10 kHz SCS has been shown to provide clinically meaningful pain relief for patients refractory to conventional medical management, with a subset of patients demonstrating improvement in neurological function. This literature review is intended to discuss the dosage and prospective data associated with pain management therapies for PDN.

Motor Skill Acquisition and Retention after Somatosensory Electrical Stimulation in Healthy Humans

Somatosensory electrical stimulation (SES) can increase motor performance, presumably through a modulation of neuronal excitability. Because the effects of SES can outlast the period of stimulation, we examined the possibility that SES can also enhance the retention of motor performance, motor memory consolidation, after 24 h (Day 2) and 7 days (Day 7), that such effects would be scaled by SES duration, and that such effects were mediated by changes in aspects of corticospinal excitability, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF). Healthy young adults (n = 40) received either 20 (SES-20), 40 (SES-40), or 60 min (SES-60) of real SES, or sham SES (SES-0). The results showed SES-20 increased visuomotor performance on Day 2 (15%) and Day 7 (17%) and SES-60 increased visuomotor performance on Day 7 (11%; all p < 0.05) compared with SES-0. Specific responses to transcranial magnetic stimulation (TMS) increased immediately after SES (p < 0.05) but not on Days 2 and 7. In addition, changes in behavioral and neurophysiological parameters did not correlate, suggesting that paths and structures other than the ones TMS can assay must be (also) involved in the increases in visuomotor performance after SES. As examined in the present study, low-intensity peripheral electrical nerve stimulation did not have acute effects on healthy adults' visuomotor performance but SES had delayed effects in the form of enhanced motor memory consolidation that were not scaled by the duration of SES.

Axonal conduction block as a novel mechanism of prepulse inhibition

In prepulse inhibition (PPI), the startle response to a strong, unexpected stimulus is diminished if shortly preceded by the onset of a different stimulus. Because deficits in this inhibitory gating process are a hallmark feature of schizophrenia and certain other psychiatric disorders, the mechanisms underlying PPI are of significant interest. We previously used the invertebrate model system Tritonia diomedea to identify the first cellular mechanism for PPI--presynaptic inhibition of transmitter release from the afferent neurons (S-cells) mediating the startle response. Here, we report the involvement of a second, more powerful PPI mechanism in Tritonia: prepulse-elicited conduction block of action potentials traveling in the startle pathway caused by identified inhibitory interneurons activated by the prepulse. This example of axo-axonic conduction block--neurons in one pathway inhibiting the propagation of action potentials in another--represents a novel and potent mechanism of sensory gating in prepulse inhibition.

Long-term effects of repetitive transcranial magnetic stimulation on markers for neuroplasticity: differential outcomes in anesthetized and awake animals

Long-term effects of repetitive transcranial magnetic stimulation (rTMS) have been associated with neuroplasticity, but most physiological studies have evaluated only the immediate effects of the stimulation on neurochemical markers. Furthermore, although it is known that baseline excitability state plays a major role in rTMS outcomes, the role of spontaneous neural activity in metaplasticity has not been investigated. The first aim of this study was to evaluate and compare the long-term effects of high- and low-frequency rTMS on the markers of neuroplasticity such as BDNF and GluR1 subunit of AMPA receptor. The second aim was to assess whether these effects depend on spontaneous neural activity, by comparing the neurochemical alterations induced by rTMS in anesthetized and awake rats. Ten daily sessions of high- or low-frequency rTMS were applied over the rat brain, and 3 d later, levels of BDNF, GluR1, and phosphorylated GluR1 were assessed in the hippocampus, prelimbic cortex, and striatum. We found that high-frequency stimulation induced a profound effect on neuroplasticity markers; increasing them in awake animals while decreasing them in anesthetized animals. In contrast, low-frequency stimulation did not induce significant long-term effects on these markers in either state. This study highlights the importance of spontaneous neural activity during rTMS and demonstrates that high-frequency rTMS can induce long-lasting effects on BDNF and GluR1 which may underlie the clinical benefits of this treatment in neuroplasticity-related disorders.

Influence of activity-induced axonal hypoexcitability on transmission of descending and segmental signals

In this experiment, the changes in excitability of motor axons produced after natural activity were measured in nine healthy subjects using 1 min of maximal voluntary contractions (MVC) of the abductor digiti minimi (ADM) by studying the relationship between stimulus intensity applied to the ulnar nerve and the size of the ADM compound muscle action potential (CMAP). On cessation of the contraction, there was a prominent right-shift of the input-output curve: the intensity required to produce a control CMAP approximately 60% of maximum, generated a post-contraction response approximately 25% of maximum. Similar changes occurred in the input-output curves obtained by recording the ulnar nerve volley evoked by same test stimulus for CMAP. Motor-evoked potential (MEP) and F-waves (and H-reflex in one subject) were recorded from ADM before and after 1 min of MVC. On cessation of contraction, the MEP input-output curves exhibited a significant right-shift: the stimulus required to evoke a pre-contraction maximum MEP ( approximately 60% of maximum CMAP) generated a post-contraction response approximately 65% of initial values. One minute of MVC produced similar decreases of F ( approximately 35%)- and H ( approximately 30%)-ADM responses. All responses recovered their control value in 15-20 min after the end of contraction. The almost identical depressive effect produced by 1 min of MVC on peripherally and centrally generated muscle responses suggests a common conditioning factor. These findings are discussed within the context of activity-induced motor axonal hyperpolarizion.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Refractoriness of sural nerve in humans

High-frequency stimulation of peripheral nerve bundles is frequently used in clinical tests and physiologic experiments to study presynaptic and postsynaptic effects. To understand the postsynaptic effects, it is important to ensure that each pulse in the train is equally effective in stimulating the presynaptic nerve bundle; however, the optimal interpulse interval (IPI) and the stimulus intensity at which each pulse is equally effective in stimulating the same number of axons are not known. The magnitude of the compound action potential produced by each pulse in a train was tested on the sural nerve of 4 healthy human subjects. The stimulus train (2–4 pulses) was applied to the sural nerve at the lateral malleolus, and neural responses were recorded from just below the knee. With 2-pulse trains, families of curves between IPIs (1–6 ms) and normalized amplitudes of the second response were plotted for different stimulus intensities. Visual inspection of the data showed that the curves fell into 2 groups: with stimulus intensities <2.5× perception threshold (Th), the test response appeared partially at longer IPIs, whereas with stimulus intensities ≥3× Th, partial recovery of the test response was earlier. The interval for complete recovery was statistically the same for low- and high-intensity stimulation. With more than 2 pulses in a stimulus train (IPI = 5 ms), the amplitude of the compound action potential (CAP) was not affected significantly. These results are important in understanding both the presynaptic and postsynaptic responses when presynaptic axon bundles are stimulated at high frequencies.

Conditioning transcutaneous electrical nerve stimulation induces delayed gating effects on cortical response: A magnetoencephalographic study

The present study was undertaken to investigate after-effects of 7 Hz non-painful prolonged stimulation of the median nerve on somatosensory-evoked fields (SEFs). The working hypothesis that conditioning peripheral stimulations might produce delayed interfering ("gating") effects on the response of somatosensory cortex to test stimuli was evaluated. In the control condition, electrical thumb stimulation induced SEFs in ten subjects. In the experimental protocol, a conditioning median nerve stimulation at wrist preceded 6 electrical thumb stimulations. Equivalent current dipoles fitting SEFs modeled responses of contralateral primary area (SI) and bilateral secondary somatosensory areas (SII) following control and experimental conditions. Compared to the control condition, conditioning stimulation induced no amplitude modulation of SI response at the initial stimulus-related peak (20 ms). In contrast, later response from SI (35 ms) and response from SII were significantly weakened in amplitude. Gradual but fast recovery towards control amplitude levels was observed for the response from SI-P35, while a slightly slower cycle was featured from SII. These findings point to a delayed "gating" effect on the synchronization of somatosensory cortex after peripheral conditioning stimulations. This effect was found to be more lasting in SII area, as a possible reflection of its integrative role in sensory processing.

Frequency-dependent conduction block in carpal tunnel syndrome

Frequency-dependent conduction block (FDB) across segments of demyelination in response to high-frequency nerve stimulation has been well demonstrated in animals and has been explored in humans. However, attempts to demonstrate this phenomenon in sensory fibers involved in entrapment neuropathies have been unsuccessful. Therefore, we investigated the effects of high-frequency nerve stimulation in the median motor nerve in patients with carpal tunnel syndrome (CTS) with moderate to severely increased distal motor terminal latencies (MTL). As a group, the mean decrease in negative peak amplitude (npAmp) during 20 stimuli at 30-HZ frequency was significantly greater in CTS subjects (-11.3%) than in controls (+7.9%). The degree of FDB was greater when MTL was more prolonged (i.e., -4.9% at 5.0 ms and -25.3% at 9.4 ms) and FDB was more pronounced at higher stimulation frequencies (20 and 30 HZ). Our results suggest that the safety margin for impulse transmission is impaired in the motor axons of patients with a focal demyelinating lesion. These findings may correlate with the observation of weakness in the absence of conduction block in patients with entrapment neuropathies.

Differences in activity-dependent hyperpolarization in human sensory and motor axons

The present study was undertaken to determine whether activity-dependent changes in axonal excitability are greater in motor axons than cutaneous afferents for the same impulse load. In nine healthy subjects, supramaximal stimulation at 8 Hz was delivered to the median nerve at the wrist. Changes in the threshold current required to generate compound motor and sensory potentials approximately 50% of maximum and other indices of axonal excitability were tracked before and after repetitive stimulation for 10 min. The long-lasting stimulation produced a prolonged depression in the excitability of both cutaneous afferents and motor axons, with gradual recovery to control levels over 15-20 min. These changes in threshold were associated with a reduction in refractoriness, an increase in supernormality and a decrease in the strength-duration time constant, changes consistent with axonal hyperpolarization. Greater changes in threshold occurred in motor axons: threshold increased by 9.9% and 16.4% for test stimulus durations of 0.1 and 1 ms, respectively, for motor axons and by 5.4% and 8.3% for cutaneous afferents. With higher stimulus frequencies and thereby greater impulse loads, greater threshold changes could be induced in cutaneous afferents. It is argued that the hyperpolarization resulted from activity of the electrogenic Na(+)-K+ pump, that it requires > 125 ms to restore the resting state following an action potential, and that significant intracellular Na+ accumulation occurs during a steady 8-Hz train. These findings imply that physiological discharge rates will activate the pump and thereby produce axonal hyperpolarization, the extent of which will vary with impulse load. A plausible explanation is that greater activity-dependent hyperpolarization in motor axons is due to less inward rectification as a result of less activity of the hyperpolarization-activated cation conductance (IH) than in cutaneous afferents.

Evidence for axonal membrane hyperpolarization in multifocal motor neuropathy with conduction block

Multiple nerve excitability measurements were used to investigate axonal membrane properties of patients diagnosed with multifocal motor neuropathy (MMN). Six patients were selected, all with evidence of distal focal motor conduction block involving the median nerve in the forearm. In all patients, the median nerve was stimulated at the wrist, just distal to the site of block, and the resulting compound muscle action potentials were recorded from abductor pollicis brevis. Stimulus-response behaviour, the strength--duration time constant, threshold electrotonus to 100 ms polarizing currents, a current-threshold relationship and the recovery of excitability following supramaximal activation were recorded using a protocol described recently. When compared with control values, patients demonstrated significantly greater superexcitability, a 'fanning out' of threshold electrotonus recordings, and a significant change in the slope of the current--threshold relationship. These abnormalities in axonal membrane excitability parameters closely resembled those in normal axons hyperpolarized following release from ischaemia. To test for axonal hyperpolarization, DC depolarizing currents were applied to the nerves of three patients, and all the excitability parameters were normalized by depolarization. Attempts to trace excitability measures proximally towards the site of block were unsuccessful, as the nerve became inexcitable in all cases. It is suggested that the distal hyperpolarization is probably linked to focal depolarization and that the clinical features of MMN are consistent with a depolarizing/hyperpolarizing lesion.

Excitability of human axons

The excitability of human axons can be studied reliably using the technique of threshold tracking, which allows the strength of a test stimulus to be adjusted by computer to activate a defined fraction of the maximal nerve or muscle action potential. The stimulus current that just evokes the target response is considered the "threshold" for that response. More useful than the resting threshold are other indices of axonal excitability derived from pairs of threshold measurements, such as refractoriness, supernormality, strength-duration time constant and "threshold electrotonus" (i.e. the changes in threshold produced by long-lasting depolarizing or hyperpolarizing current pulses). Each of these measurements depends on membrane potential and on other biophysical properties of the axons. Together they can provide new information about the pathophysiology underlying abnormalities in excitability in neuropathy.

The behavioral complications of pallidal stimulation: a case report

We report a case of recurrent manic episodes associated with chronic deep brain stimulation (DBS) targeting globus pallidus (GP) in the treatment of Parkinson's disease (PD). Cardinal PD symptoms and dyskinesia improved with DBS, and neuropsychological testing found improvements in visuospatial measures associated with left DBS and in verbal memory with right DBS when compared to the patient's preoperative baseline. Under conditions of right, left, and bilateral DBS, the patient experienced bouts of mania and hypomania lasting several days at a time. Positron emission tomography (PET) with (15)O-labeled water was performed after his first manic episode under four conditions: no stimulation, right DBS, left DBS, and bilateral DBS. Although no manic switch occurred during the course of the PET study, all three DBS conditions were associated with decreases in regional flow in the left parahippocampus and hippocampus and right mid-cingulate gyrus. Increases in flow in left inferior frontal area, bilateral insula, dorsolateral prefrontal cortex, and cuneus were common to all DBS conditions. GP stimulation in PD may be associated with behavioral and cognitive effects. Distributed blood flow changes observed with pallidal DBS support a role for the pallidum in cognition and affective regulation.

Activity-dependent excitability changes in chronic inflammatory demyelinating polyneuropathy: A microneurographic study

The purpose of this study was to investigate activity-dependent excitability changes in polyneuropathy and their correlation with symptomatology. First, we recorded sensory nerve action potentials (SNAPs) with an intraneural microelectrode during impulse trains in 11 patients with chronic inflammatory demyelinating polyneuropathy. When the stimulus frequency was increased to >/=20 Hz, all patients showed marked decreases in the amplitudes of averaged SNAPs (128 responses) associated with latency increases. The amplitude decreases were much greater than those in patients with axonal neuropathies. In single-unit recordings, responses showed latency increases, which were small but sufficient to cause decreases in the averaged responses. Clinical sensory impairment was correlated with the degree of preexisting conduction block or axonal loss, but not with the degree of rate-dependent amplitude decreases. Activity-dependent changes occur preferentially in demyelinating neuropathy and are a sensitive measure of demyelination. The mechanism responsible for the amplitude decreases could be conduction slowing or block caused by activity-dependent hyperpolarization.

Spontaneous and evoked ectopic discharges recorded from single human axons

A quantitative assessment was made of the firing characteristics of repetitive axonal discharges encountered during microneurographic recordings from human peripheral nerves. Spontaneous activity was recorded from 16 single axons using tungsten microelectrodes inserted percutaneously into fascicles of the median or peroneal nerves in normal subjects. These discharges typically consisted of brief bursts of 2-5 spikes occurring at a frequency of 7-10 Hz. Peak instantaneous frequencies usually exceeded 300 Hz. Based on their similarity with spontaneous high-frequency discharges recorded from single axons following nerve damage, ischemia, prolonged electrical stimulation, or hyperventilation, it is concluded that they are generated ectopically at the site of a previous impalement of a nerve fiber. It is suggested that short-term damage to the nerve fiber caused by the microelectrode may allow accumulation of K+ underneath the myelin, triggering an inward flow of K+ and regenerative depolarizations. Alternatively, internodal channels may be exposed following damage to the myelin, resulting in the generation of spontaneous pacemaker potentials and repetitive discharges.

Activity-dependent hyperpolarization of human motor axons produced by natural activity

1. The changes in excitability of motor axons produced by natural activity were measured in six healthy subjects using voluntary contractions lasting 15 s, 30 s and 1 min, by recording the changes in stimulus current required to produce a compound muscle action potential of approximately 60 % of maximum. 2. On cessation of the contractions there was a prominent increase in the current required to produce the target potential, accompanied by an increase in rheobase, a decrease in strength-duration time constant, and an increase in axonal supernormality. These changes indicate that the hypoexcitability was due to axonal hyperpolarization. 3. The activity-dependent hypoexcitability increased in depth and duration the longer the contraction. Following a 1 min contraction, it produced a 24 % increase in threshold, waning over 15 min. The hypoexcitability was greater than in cutaneous afferents tetanized to produce an equivalent rate-dependent stress. 4. It is concluded that natural activity results in substantial hyperpolarization of active axons and that, for similar discharge rates, the degree of hyperpolarization is greater in motor axons than cutaneous afferents. The greater effect of activity on the excitability of motor axons could be due to less inward rectification and less persistent Na+ conductance than in sensory axons. It is suggested that motor axons may therefore be more susceptible than cutaneous afferents to conduction block at sites of impaired safety margin for impulse conduction.

Paraesthesiae induced by prolonged high frequency stimulation of human cutaneous afferents

1. The present study has explored the behaviour of human cutaneous afferents following conduction of prolonged trains of impulses at 200 Hz for 10-20 min, correlating the resultant changes in excitability with the perception of paraesthesiae. 2. Tetanization for 10 min resulted in activity-dependent changes in axonal excitability, with an initial period of hyperexcitability, followed by a long-lasting subexcitability. All subjects experienced paraesthesiae soon after cessation of the tetanic train, and these subsided gradually over 16 min. 3. Longer tetanic trains of 20 min duration resulted in greater changes in axonal excitability, but with paraesthesiae of a similar time course. The post-tetanic increase in excitability was abolished when short tetanic trains were delivered > 30 min before long trains, but all subjects still experienced paraesthesiae. 4. Threshold distributions following tetanic stimulation for both 10 and 20 min established that all axons contributing to the sensory volley underwent a uniform pattern of post-tetanic threshold changes. There was no evidence of a bimodal distribution with some axons hyperpolarized and others depolarized, as occurs with motor axons. However, the excitability changes were graded, with axons of lowest threshold undergoing a proportionately greater increase in excitability than axons of higher threshold. 5. The post-tetanic excitability changes were greater at the site of stimulation than elsewhere along the peripheral nerve. However, DC polarizing currents applied at this site failed to alter the sensation of paraesthesiae in the post-tetanic period. Furthermore, local anaesthetic block of the peripheral nerve proximal to the stimulation site failed to suppress the paraesthesiae. 6. The uniform pattern of post-tetanic threshold changes for cutaneous afferents differs from the bimodal distribution seen with post-ischaemic and post-tetanic motor axons. This difference in behaviour may reflect greater inward rectification and greater expression of a non-inactivating threshold conductance in cutaneous afferents. It is suggested that the ectopic activity responsible for paraesthesiae in the post-tetanic period arises from a more central site than the peripheral nerve.

Excitability changes in human cutaneous afferents induced by prolonged repetitive axonal activity

1. The present study was undertaken to document the excitability changes produced by prolonged high-frequency trains of impulses in cutaneous afferents of six human subjects. 2. Trains of supramaximal stimuli at 200 Hz for 2 min or less produced a prolonged depression in excitability, consistent with activation of the electrogenic Na+-K+ pump. Trains of longer duration resulted in an initial period of hyperexcitability which, with 10 min trains, was associated with the sensation of paraesthesiae in all subjects. This transient hyperexcitability gradually gave way to a long-lasting period of hypoexcitability. 3. The excitability changes were reproducible, and were accompanied by corresponding changes in supernormality, refractoriness, strength-duration time constant and rheobase current, suggesting that the changes in axonal excitability reflected a change in membrane potential. 4. The transient increase in excitability that follows tetanic trains of 10 min had qualitatively similar effects on cutaneous axons as ischaemia or application of a depolarizing current. The post-tetanic changes in the supernormal period of sensory axons were those expected from the changes in excitability, without evidence of a gross distortion in its time course, as has been previously demonstrated in a hyperstimulated human motor axon. 5. It is concluded that the post-tetanic hyperexcitability of human sensory axons is probably driven by increased K+ accumulation in the restricted diffusion space under the myelin sheath, much as in motor axons, the differences in behaviour of sensory and motor axons being explicable by greater inward rectification in sensory axons.

Autoantibodies detected to expressed K+ channels are implicated in neuromyotonia

Antibody-mediated autoimmunity underlies a diverse range of disorders, particularly in the nervous system where the extracellular domains of ion channels and receptors are especially vulnerable targets. We present here a novel means of detecting autoantibodies where the genes of the suspected target proteins are known, and use it to detect specific autoantibodies in acquired neuromyotonia (Isaacs' syndrome), a disorder characterized by hyperexcitable motor nerves and sometimes by central abnormalities. We expressed different human brain voltage-gated potassium channels in Xenopus oocytes by injecting the relevant alpha-subunit complementary RNA, and detected antibody binding by immunohistochemistry on frozen sections. Antibodies were detected to one or more human brain voltage-gated potassium channel in 12 of 12 neuromyotonia patients and none of 18 control subjects. The results establish neuromyotonia as a new antibody-mediated channelopathy and indicate the investigative potential of this molecular immunohistochemical assay.

Mechanoreceptive afferents exhibit functionally-specific activity dependent changes in conduction velocity

Impulse activity in axons generates aftereffects on membrane excitability that can alter the conduction velocity of subsequently conducted impulses. We used a computerized stimulus pattern (a 1 Hz stimulus period followed by a period of repeated short bursts at 200 Hz) to assess in vivo activity-dependent changes in conduction latency of functionally identified rat cutaneous afferents conducting in the A beta range. Several different parameters of activity dependence were measured: burst supernormality, the average increase in conduction latency following conditioning with a single preceding impulse during high frequency burst stimulation; burst subnormality, the average latency increase during each burst; depression, a long-term increase in latency caused by the high frequency stimulation. The data show that different mechanosensitive A beta afferents with overlapping resting conduction velocities exhibit activity-dependent changes in conduction latency that are characteristic of their particular functions.

Short latency median nerve somatosensory evoked potential (SEP): increase in stimulation frequency from 3 to 30 Hz

Somatosensory evoked potentials were obtained by electrical stimulation of the median nerve in 10 normal subjects at 3 and 30 Hz. At the higher rate of stimulation, a reduction was observed in amplitudes and prolongation of latencies of the N9, N/P13 and N20 components as well as increase of the interpeak latency N9-N/P13. A significant increase between the onsets of the N11 and N20 components was also seen; however, no significant increase of the N/P13-N20 interpeak latency was observed. Analysis suggested that an important reason for this last finding was related to the fact that in some cases different fast frequency components (FFC) determined the N20 peak in the different situations. It was further observed that, in those cases in which at least 3 peaks in the fast frequency components were detected (7/10), a significantly different increase in latency between the first and the third peaks was noted. A possible thalamo-cortical generation of the FFC is discussed.

Modality-dependent modulation of conduction by impulse activity in functionally characterized single cutaneous afferents in the rat

Cutaneous afferents exhibit changes in excitability after impulse activity that are correlated with functional modality but are independent of axonal diameter, as studied in 39 cold fibers and 51 nociceptors of the rat. Latency of conducted impulses was used to indicate changes in axonal excitability caused by electrical stimulation. Stimuli were applied both at fixed frequencies and at the time intervals of impulses previously recorded during response to natural stimulation. Latency increased following both these forms of electrical stimulation, as well as after natural stimulation of the receptive fields. The latency increase was correlated with the number of impulses and the frequency of the preceding discharge in all of 4 nociceptors and 13 cold fibers studied for this feature. Increase of latency by electrical or natural stimulation led to reduced responsiveness to natural stimulation. The magnitude and time course of latency changes were correlated with fiber modality. In 32 nociceptors the latency increased continuously with time during a stimulus train, whereas in 21 cold fibers there was only an initial increase in latency over the first few seconds, after which the latency remained at a plateau even as the firing response continued. Paralleling this slowing, impulse failure occurred more frequently during repetitive stimulation in both A delta and C nociceptors than in velocity-matched cold fibers of either class. Based on the magnitude of latency increases during stimulus trains at different frequencies, two distinct patterns were discerned in A nociceptors: "Type II" fibers slowed significantly more than "Type I" or cold fibers. The results support the hypotheses (1) that the pattern of latency changes during activity are signatures for the modality in a given fiber; and (2) that endogenous, activity-dependent processes of the axon contribute to adaptation and encoding in cutaneous sensory afferents.

Microneurography, impulse conduction, and paresthesias

It is possible to learn more about peripheral nerve function in human subjects than is obtainable with routine nerve conduction studies, and thereby to study the basis of "positive" symptoms, such as paresthesias. Using microneurography, ectopic impulse activity in cutaneous afferents has been recorded in patients suffering from neurologic disorders and in normal subjects in whom paresthesias were provoked by hyperventilation, prolonged tetanization of cutaneous nerves and ischemia. Using relatively simple modifications of standard nerve conduction techniques, the increases in axonal excitability responsible for this ectopic activity have been documented in human volunteers. Hyperventilation increases axonal excitability but does not change supernormality, probably because Na+ channels are activated by the decrease in [Ca2+] on the axonal membrane. Prolonged tetanic stimulation and ischemia probably share similar mechanisms. At least in motor axons, postischemic ectopic activity occurs when the hyperpolarization that results from activation of the Na+/K+ pump lowers the membrane potential below the equilibrium potential for K+. A high extracellular [K+] can then result in an inward current producing depolarization and possibly triggering regenerative processes.

Reduced capability of transmitting high frequency impulses in tail nerves of diabetic rats

The effect of long-term (40 min) high frequency stimulation (143 Hz) of sensory-motor tail nerves was studied in normal and in streptozotocin-diabetic rats. The study comprised a 6-week period, repeating the test at 2-week intervals. In the initial (prediabetic) study, single experiments showed a mild depression of the peak-to-peak amplitude during high frequency stimulation, which reversed completely during a subsequent rest period. In normal rats, the amplitude depression was unchanged in repeated tests over a 6-week period. Diabetic rats showed a greater amplitude depression during high frequency stimulation. The difference was statistically significant after 2 weeks, but showed no further change at subsequent tests. The greater decrease in the amplitude in diabetic rats may reflect a depression of the axon membrane function, which may be the functional correlate to the inhibition of the Na/K ATPase activity in diabetes, described by others. Monitoring of the axon membrane functional capacity may have clinical implications in the control of peripheral neuropathies.

Physiological evidence for a slow K+ conductance in human cutaneous afferents

1. The depression in axonal excitability that follows short trains of impulses (H1) may lead to spike frequency adaptation to a sustained stimulus, and has been attributed to a slow K+ conductance. The present experiments sought indirect evidence for slow K+ channels at the node of Ranvier of human cutaneous afferents based on the demonstration of post-tetanic changes in excitability typical of H1. 2. The excitability changes in low-threshold cutaneous afferents in the digital nerves of the index finger were explored using a submaximal test pulse conditioned by trains of supramaximal stimuli, containing up to 100 impulses. Changes in the amplitude of the compound sensory action potential set up by a constant test stimulus were used as a measure of the changes in excitability. These changes in amplitude were paralleled by inverse changes in latency. 3. When the conditioning stimulus was a single supramaximal pulse, excitability was enhanced at conditioning-test intervals of 4-40 ms, with a peak at 6-8 ms. When the conditioning stimulus consisted of a train of ten pulses delivered at 200 Hz, the recovery cycle was dominated by subnormality that was maximal at 20 ms and subsided gradually over 50 ms. 4. The post-train depression in excitability increased as the number of pulses in the conditioning train increased to ten but changed little with further increases in train duration. The degree of depression increased with the pulse frequency within the train. Cooling the hand from a skin temperature of 35 to 25 degrees C slowed the recovery processes but did not alter the magnitude of the post-train depression. 5. These characteristics are typical of the H1 phase of post-tetanic depression in axonal excitability. The extent of the depression in excitability suggests, first, that there may be a significant K+ conductance at the nodes of human cutaneous afferents and, secondly, that H1 may play a significant role in limiting repetitive discharge in normal and pathological afferents.

Selective temporal shift in the somatosensory evoked potential produced by chronic stimulation of the human index finger

The present study determined whether the cortical potential from the human index finger changed with chronic nerve stimulation. Cerebral potentials were repeatedly recorded to stimulation of the ulnar nerve and the digital nerves of thumb, index and middle fingers, before and during a 7-day period in which the index was electrically stimulated (80 Hz) for 8-10 h daily. Cerebral potentials were recorded at three scalp sites over the contralateral "hand" area. Chronic stimulation produced no significant changes in the amplitudes or distribution of the cerebral potentials from the individual digits or the ulnar nerve. However, for the stimulated index finger there was a significant, progressive increase in latency of N20 and P25 without a detectable change in conduction velocity of distal peripheral axons. Timing in human central somatosensory pathways may be altered by the previous pattern of peripheral nerve inputs.

Changes in excitability of human motor axons underlying post-ischaemic fasciculations: evidence for two stable states

1. We have investigated the origin of post-ischaemic ectopic discharges in human nerve by recording changes in electrical excitability following periods of ischaemia (15-20 min) sufficient to induce spontaneous motor fasciculations. The ulnar nerve was stimulated beneath a pressure cuff on the upper arm, and compound motor action potentials recorded from abductor digiti minimi. 2. On releasing the cuff after 15 min of ischaemia, thresholds to short current pulses increased in two distinct phases: a slow phase followed by a rapid rise to a peak threshold. The rapid rise was too fast to track (i.e. 100% threshold increase in less than 4 s), and was sometimes followed after 30-40 s by an equally rapid fall. Small polarizing currents affected the timing of the rapid threshold increase, as if it was occurring at a particular membrane potential. 3. By recording complete stimulus-response curves every few seconds, we found that the rapid threshold changes were associated with a bimodal distribution of thresholds. Most fibres were found in either a high-threshold or low-threshold state, and these two states converged over a period of about 10 min. 4. Spontaneous motor fasciculations were only recorded after the rapid rise in threshold and when the fibres existed in two threshold states. The spontaneous activity was not responsible for inducing the two states, since they could also be recorded in its absence. 5. A computer model of a human motor axon node and internode was constructed, incorporating channel types demonstrated in other axons, and channel densities adjusted to match the responses of human axons to depolarizing and hyperpolarizing current pulses. An increase in extracellular potassium concentration produced a region of negative slope conductance in the current-voltage relationship of the model, and the appearance of two stable states with enhanced activity of the electrogenic sodium pump. 6. Transitions between the two stable states of the model could account qualitatively for the rapid threshold changes recorded from post-ischaemic axons. In the model, spontaneous action potentials occurred following some transitions from the high potential state to the low potential state. We suggest that post-ischaemic motor fasciculations in man also involve transitions between two equilibrium states, occurring in axons with high extracellular potassium and high electrogenic pump activity.

Changes in excitability and accommodation of human motor axons following brief periods of ischaemia

1. The mechanism of post-ischaemic ectopic impulse generation in nerve is not known, and previous measurements of excitability changes in human motor axons have appeared to conflict. We have used automatic threshold tracking and different stimulus-response combinations to follow the effects on excitability of brief (5-10 min) periods of ischaemia, too short to induce motor fasciculations. Excitability changes have been compared at different sites in axons innervating hand, arm and foot muscles. 2. Threshold was determined as the percutaneous stimulus current required to excite a single motor unit, or to evoke a constant multiunit response, after rectifying and integrating the electromyogram (EMG). Three different waveforms of stimulus current were compared: short (less than or equal to 2 ms) pulses, long (100-200 ms) pulses to measure rheobase, and 100 ms current ramps. We also measured accommodation by recording the effects of subthreshold depolarizing currents on excitability. 3. Ischaemic and post-ischaemic excitability changes were greatest in the proximal parts of the longest motor axons, and greater if the sphygmomanometer cuff was inflated over, rather than proximal to, the stimulating site. 4. Using integrated EMG responses from abductor digiti minimi, the ulnar nerve stimulated above the elbow became rapidly much less excitable after ischaemia when tested with short pulses, but more excitable when tested with current ramps. The rheobase rose briefly, but then fell, often below resting level, always staying below the pulse and ramp thresholds. 5. The latency of the response to a rheobasic stimulus altered in parallel with the threshold to short current pulses, and increased dramatically after ischaemia. This latency increase was associated with a prolonged phase of 'negative accommodation', i.e. the continued increase in excitability to a maintained subthreshold depolarizing current. 6. Changes in excitability and accommodation similar to those occurring after ischaemia were recorded following high frequency trains of stimuli. They were attributed primarily to hyperpolarization by the electrogenic sodium pump, since comparable changes could be induced by passing a steady hyperpolarizing current through the stimulating electrode. 7. Threshold and latency recordings from single motor units during and after ischaemia resembled in most respects the multiunit responses, but single unit rheobase did not show a post-ischaemic fall below the resting level. Repetitive firing contributed to the low multiunit thresholds recorded with long current pulses during the post-ischaemic period. 8. We conclude that human motor nerves become simultaneously both more and less excitable than normal after 10 min of ischaemia, depending on the choice of stimulus and response.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-lasting depression of central synaptic transmission following prolonged high-frequency stimulation of cutaneous afferents: a mechanism for post-vibratory hypaesthesia

High-frequency vibration or electrical stimulation of cutaneous afferents may produce long-lasting hypaesthesia. Such stimulation alters the excitability of axons in the peripheral nerve but there is evidence that this does not completely explain the hypaesthesia. The present study was undertaken to determine whether a prolonged afferent barrage results in depression of synaptic transmission at a central site. Changes in central excitability to cutaneous inputs were examined in normal subjects by measuring the cerebral evoked potential at different stages after high-frequency conditioning stimulation of the digital nerves. Changes in peripheral excitability were eliminated by adjusting the stimulus intensity so that a constant afferent volley entered the central nervous system. Following the conditioning stimulation (4-5 T, 200 Hz, 10 min), the cortical potential evoked by constant submaximal test volleys was depressed by up to 50% for 25 min. The attenuation was less profound (10-20%) but more prolonged (greater than 45 min) when maximal test volleys were used, and occurred regardless of whether the high-frequency stimulation was applied to the test digit or to adjacent digits. It is concluded that prolonged activation of cutaneous afferents causes a depression in central excitability independent of and additional to peripheral changes, and it is suggested that this mechanism contributes to the associated perceptual disturbances. By analogy it is suggested that the hypaesthesia associated with prolonged vibration may be of central rather than peripheral origin.