The hyperpolarization which follows activity in mammalian non-medullated fibres

Rapid cold hardening increases axonal Na+/K+-ATPase activity and enhances performance of a visual motion detection circuit in Locusta migratoria

Rapid cold hardening (RCH) is a type of phenotypic plasticity that delays the occurrence of chill coma in insects. Chill coma is mediated by a spreading depolarization of neurons and glia in the CNS, triggered by a failure of ion homeostasis. We used biochemical and electrophysiological approaches in the locust, Locusta migratoria, to test the hypothesis that the protection afforded by RCH is mediated by activation of the Na+/K+-ATPase (NKA) in neural tissue. RCH did not affect NKA activity measured in a biochemical assay of homogenized thoracic ganglia. However, RCH hyperpolarized the axon of a visual interneuron (DCMD) and increased the amplitude of an activity-dependent hyperpolarization (ADH) shown previously to be blocked by ouabain. RCH also improved performance of the visual circuitry presynaptic to DCMD to minimize habituation and increase excitability. We conclude that RCH enhances in situ NKA activity in the nervous system but also affects other neuronal properties that promote visual processing in locusts.

Measuring enzyme activities in crude homogenates: Na+/K+-ATPase as a case study in optimizing assays

In this review of assays of Na⁺/K⁺-ATPase (NKA), we explore the choices made by researchers assaying the enzyme to investigate its role in physiological regulation. We survey NKA structure and function in the context of how it is typically assayed, and how technical choices influence what can be said about the enzyme. In comparing different methods for extraction and assay of NKA, we identified a series of common pitfalls that compromise the veracity of results. We include experimental work to directly demonstrate how choices in detergents, salts and substrates influence NKA activities measured in crude homogenates. Our review of assay approaches integrates what is known from enzymology, biomedical physiology, cell biology and evolutionary biology, offering a more robust method for assaying the enzyme in meaningful ways, identifying caveats and future directions to explore its structure and function. The goal is to provide the sort of background on the enzyme that should be considered in exploring the function of the enzyme in comparative physiology.

The Slow Depolarization Following Individual Spikes in Thin, Unmyelinated Axons in Mammalian Cortex

An important goal in neuroscience is to understand how neuronal excitability is controlled. Therefore, Gardner-Medwin's 1972 discovery, that cerebellar parallel fibers were more excitable up to 100 ms after individual action potentials, could have had great impact. If this long-lasting effect were due to intrinsic membrane mechanisms causing a depolarizing after-potential (DAP) this was an important finding. However, that hypothesis met resistance because the use of K⁺ sensitive electrodes showed that synchronous activation, as commonly used in excitability tests, increased extracellular K⁺ concentration sufficiently to explain much of the hyperexcitability. It is still controversial because intra-axonal recordings, which could have settled the debate, have not been made from parallel fibers or other axons of similar calibers. If it had not been for the fact that such thin axons are, by far, the most common axon type in cortical areas and control almost all glutamate release, it would be tempting to ignore them until an appropriate intra-axonal recording technique is invented. I will go through the literature that, taken together, supports the hypothesis that a DAP is an intrinsic membrane mechanism in cerebellar parallel fibers and hippocampal Schaffer collaterals. It is most likely due to a well-controlled process that stops the fast repolarization at a membrane potential positive to resting membrane potential, leaving the membrane more excitable for ~100 ms during a slow, passive discharge of the membrane capacitance. The DAP helps reduce failures but can also cause uncontrolled bursting if it is not properly controlled. The voltage at which the fast repolarization stops, and the DAP starts, is close the activation range of both Na⁺ and Ca²⁺ voltage activated channels and is therefore essential for neuronal function.

Ischemic Brain Injury Leads to Brain Edema via Hyperthermia-Induced TRPV4 Activation

Brain edema is characterized by an increase in net brain water content, which results in an increase in brain volume. Although brain edema is associated with a high fatality rate, the cellular and molecular processes of edema remain largely unclear. Here, we developed an in vitro model of ischemic stroke-induced edema in which male mouse brain slices were treated with oxygen-glucose deprivation (OGD) to mimic ischemia. We continuously measured the cross-sectional area of the brain slice for 150 min under macroscopic microscopy, finding that OGD induces swelling of brain slices. OGD-induced swelling was prevented by pharmacologically blocking or genetically knocking out the transient receptor potential vanilloid 4 (TRPV4), a member of the thermosensitive TRP channel family. Because TRPV4 is activated at around body temperature and its activation is enhanced by heating, we next elevated the temperature of the perfusate in the recording chamber, finding that hyperthermia induces swelling via TRPV4 activation. Furthermore, using the temperature-dependent fluorescence lifetime of a fluorescent-thermosensitive probe, we confirmed that OGD treatment increases the temperature of brain slices through the activation of glutamate receptors. Finally, we found that brain edema following traumatic brain injury was suppressed in TRPV4-deficient male mice in vivo Thus, our study proposes a novel mechanism: hyperthermia activates TRPV4 and induces brain edema after ischemia.SIGNIFICANCE STATEMENT Brain edema is characterized by an increase in net brain water content, which results in an increase in brain volume. Although brain edema is associated with a high fatality rate, the cellular and molecular processes of edema remain unclear. Here, we developed an in vitro model of ischemic stroke-induced edema in which mouse brain slices were treated with oxygen-glucose deprivation. Using this system, we showed that the increase in brain temperature and the following activation of the thermosensitive cation channel TRPV4 (transient receptor potential vanilloid 4) are involved in the pathology of edema. Finally, we confirmed that TRPV4 is involved in brain edema in vivo using TRPV4-deficient mice, concluding that hyperthermia activates TRPV4 and induces brain edema after ischemia.

Sodium pump regulation of locomotor control circuits

Sodium pumps are ubiquitously expressed membrane proteins that extrude three Na⁺ ions in exchange for two K⁺ ions, using ATP as an energy source. Recent studies have illuminated additional, dynamic roles for sodium pumps in regulating the excitability of neuronal networks in an activity-dependent fashion. We review their role in a novel form of short-term memory within rhythmic locomotor networks. The data we review derives mainly from recent studies on Xenopus tadpoles and neonatal mice. The role and underlying mechanisms of pump action broadly match previously published data from an invertebrate, the Drosophila larva. We therefore propose a highly conserved mechanism by which sodium pump activity increases following a bout of locomotion. This results in an ultraslow afterhyperpolarization (usAHP) of the membrane potential that lasts around 1 min, but which only occurs in around half the network neurons. This usAHP in turn alters network excitability so that network output is reduced in a locomotor interval-dependent manner. The pumps therefore confer on spinal locomotor networks a temporary memory trace of recent network performance.

Ion dynamics during seizures

Changes in membrane voltage brought about by ion fluxes through voltage and transmitter-gated channels represent the basis of neural activity. As such, electrochemical gradients across the membrane determine the direction and driving force for the flow of ions and are therefore crucial in setting the properties of synaptic transmission and signal propagation. Ion concentration gradients are established by a variety of mechanisms, including specialized transporter proteins. However, transmembrane gradients can be affected by ionic fluxes through channels during periods of elevated neural activity, which in turn are predicted to influence the properties of on-going synaptic transmission. Such activity-induced changes to ion concentration gradients are a feature of both physiological and pathological neural processes. An epileptic seizure is an example of severely perturbed neural activity, which is accompanied by pronounced changes in intracellular and extracellular ion concentrations. Appreciating the factors that contribute to these ion dynamics is critical if we are to understand how a seizure event evolves and is sustained and terminated by neural tissue. Indeed, this issue is of significant clinical importance as status epilepticus—a type of seizure that does not stop of its own accord—is a life-threatening medical emergency. In this review we explore how the transmembrane concentration gradient of the six major ions (K⁺, Na⁺, Cl⁻, Ca²⁺, H⁺and HCO3−) is altered during an epileptic seizure. We will first examine each ion individually, before describing how multiple interacting mechanisms between ions might contribute to concentration changes and whether these act to prolong or terminate epileptic activity. In doing so, we will consider how the availability of experimental techniques has both advanced and restricted our ability to study these phenomena.

Sustained maximal voluntary contraction produces independent changes in human motor axons and the muscle they innervate

The repetitive discharges required to produce a sustained muscle contraction results in activity-dependent hyperpolarization of the motor axons and a reduction in the force-generating capacity of the muscle. We investigated the relationship between these changes in the adductor pollicis muscle and the motor axons of its ulnar nerve supply, and the reproducibility of these changes. Ten subjects performed a 1-min maximal voluntary contraction. Activity-dependent changes in axonal excitability were measured using threshold tracking with electrical stimulation at the wrist; changes in the muscle were assessed as evoked and voluntary electromyography (EMG) and isometric force. Separate components of axonal excitability and muscle properties were tested at 5 min intervals after the sustained contraction in 5 separate sessions. The current threshold required to produce the target muscle action potential increased immediately after the contraction by 14.8% (p<0.05), reflecting decreased axonal excitability secondary to hyperpolarization. This was not correlated with the decline in amplitude of muscle force or evoked EMG. A late reversal in threshold current after the initial recovery from hyperpolarization peaked at −5.9% at ∼35 min (p<0.05). This pattern was mirrored by other indices of axonal excitability revealing a previously unreported depolarization of motor axons in the late recovery period. Measures of axonal excitability were relatively stable at rest but less so after sustained activity. The coefficient of variation (CoV) for threshold current increase was higher after activity (CoV 0.54, p<0.05) whereas changes in voluntary (CoV 0.12) and evoked twitch (CoV 0.15) force were relatively stable. These results demonstrate that activity-dependent changes in motor axon excitability are unlikely to contribute to concomitant changes in the muscle after sustained activity in healthy people. The variability in axonal excitability after sustained activity suggests that care is needed when using these measures if the integrity of either the muscle or nerve may be compromised.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

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.

The H-current secures action potential transmission at high frequencies in rat cerebellar parallel fibers

Most axons in the mammalian brain are unmyelinated and thin with pre-synaptic specializations (boutons) along their entire paths. The parallel fibers in the cerebellum are examples of such axons. Unlike most thin axons they have only one branch point. The granule cell soma, where they originate, can fire bursts of action potentials with spike intervals of about 2 ms. An important question is whether the axons are able to propagate spikes with similarly short intervals. By using extracellular single-unit and population-recording methods we showed that parallel fibers faithfully conduct spikes at high frequencies over long distances. However, when adding 20 microm ZD7288 or 1 mm Cs(+), or reducing the temperature from 35 to 24 degrees C, the action potentials often failed even when successfully initiated. Ba(2+)(1 mm), which blocks Kir channels, did not reproduce these effects. The conduction velocity was reduced by ZD7288 but not by Ba(2+). This suggests that the parallel fibers have an H-current that is active at rest and that is important for their frequency-following properties. Interestingly, failures occurred only when the action potential had to traverse the axonal branch point, suggesting that the branch point is the weakest point in these axons.

Thermodynamic regulation of NKCC1-mediated Cl- cotransport underlies plasticity of GABA(A) signaling in neonatal neurons

In the adult brain, chloride (Cl-) influx through GABA(A) receptors is an important mechanism of synaptic inhibition. However, under a variety of circumstances, including acquired epilepsy, neuropathic pain, after trains of action potentials or trauma, and during normal early brain development, GABA(A) receptor activation excites neurons by gating Cl- efflux because the intracellular Cl- concentration (Cl(i)) is elevated. These findings require an inducible, active mechanism of chloride accumulation. We used gramicidin-perforated patch recordings to characterize Cl- transport via NKCC1, the principal neuronal Cl- accumulator, in neonatal CA1 pyramidal neurons. NKCC1 activity was required to maintain elevated Cl(i) such that GABA(A) receptor activation was depolarizing. Kinetic analysis of NKCC1 revealed reversible transmembrane Cl- transport characterized by a large maximum velocity (vmax) and high affinity (Km), so that NKCC1 transport was limited only by the net electrochemical driving force for Na+, K+, and Cl-. At the steady-state Cl(i), NKCC1 was at thermodynamic equilibrium, and there was no evidence of net Cl- transport. Trains of action potentials that have been previously shown to induce persistent changes in neuronal E(Cl) (reversal potential for Cl-) did not alter vmax or Km of NKCC1. Rather, action potentials shifted the thermodynamic set point, the steady-state Cl(i) at which there was no net NKCC1-mediated Cl- transport. The persistent increase in Cl(i) required intact alpha2/alpha3 Na+-K+-ATPase activity, indicating that trains of action potentials reset the thermodynamic equilibrium for NKCC1 transport by lowering Na(i). Activity-induced changes in Na+-K+-ATPase activity comprise a novel mechanism for persistent alterations in synaptic signaling mediated by GABA.

Uremic neuropathy: clinical features and new pathophysiological insights

Neuropathy is a common complication of end-stage kidney disease (ESKD), typically presenting as a distal symmetrical process with greater lower-limb than upper-limb involvement. The condition is of insidious onset, progressing over months. and has been estimated to be present in 60%-100% of patients on dialysis. Neuropathy generally only develops at glomerular filtration rates of less than 12 ml/min. The most frequent clinical features reflect large-fiber involvement, with paresthesias, reduction in deep tendon reflexes, impaired vibration sense, muscle wasting, and weakness. Nerve conduction studies demonstrate findings consistent with a generalized neuropathy of the axonal type. Patients may also develop autonomic features, with postural hypotension, impaired sweating, diarrhea, constipation, or impotence. The development of uremic neuropathy has been related previously to the retention of neurotoxic molecules in the middle molecular range, although this hypothesis lacked formal proof. Studies utilizing novel axonal excitability techniques have recently shed further light on the pathophysiology of this condition. Nerves of uremic patients have been shown to exist in a chronically depolarized state prior to dialysis, with subsequent improvement and normalization of resting membrane potential after dialysis. The degree of depolarization correlates with serum K(+), suggesting that chronic hyperkalemic depolarization plays an important role in the development of nerve dysfunction in ESKD. These recent findings suggest that maintenance of serum K(+) within normal limits between periods of dialysis, rather than simple avoidance of hyperkalemia, is likely to reduce the incidence and severity of uremic neuropathy.

Internodal function in normal and regenerated mammalian axons

AIM

Following Wallerian degeneration, peripheral myelinated axons have the ability to regenerate and, given a proper pathway, establish functional connections with targets. In spite of this capacity, the clinical outcome of nerve regeneration remains unsatisfactory. Early studies have found that regenerated internodes remain persistently short though this abnormality did not seem to influence recovery in conduction. It remains unclear to which extent abnormalities in axonal function itself may contribute to the poor outcome of nerve regeneration.

METHODS

We review experimental evidence indicating that internodes play an active role in axonal function.

RESULTS

By investigating internodal contribution to axonal excitability we have found evidence that axonal function may be permanently compromised in regenerated nerves. Furthermore, we illustrate that internodal function is also abnormal in regenerated human nerves.

CONCLUSION

The data suggest that persistently shorter regenerated internodes lead to increased Na+/K+-pump activity in response to increased Na+ entry during conduction. This may impair axonal function during prolonged repetitive activity and drain the energy reserves of the axons.

Axonal function and activity-dependent excitability changes in myotonic dystrophy

To investigate peripheral nerve function and its potential contribution to symptoms of weakness in myotonic dystrophy type 1 (MD), nerve excitability was assessed in 12 MD patients. Compound muscle action potentials (CMAPs) were recorded at rest from abductor pollicis brevis (APB) following stimulation of the median nerve. Stimulus-response behavior, threshold electrotonus, a current-threshold relationship, and recovery cycles were successfully recorded in each patient. Compared with controls, there was significant reduction in CMAP amplitude in MD patients. This was accompanied by reduction in depolarizing threshold electrotonus and an increase in refractoriness and in the duration of the relative refractory period. To determine whether alteration in axonal resting membrane potential was a factor underlying these changes, axonal excitability was assessed following maximal contraction of APB for 60 seconds. Following contraction, there was reduction in CMAP amplitude for a submaximal stimulus (by 51.5+/-11.8%) and an increase in super-excitability (of 22.2+/-12.0%), consistent with activity-dependent hyperpolarization, with a greater increase in threshold for MD patients compared to controls (MD group, 22.3+/-5.1%; controls, 11.7+/-2.1%; P<0.04) and prolonged recovery to baseline. The present study has established that greater activity-dependent changes in excitability may be induced in MD patients by maximal voluntary contraction when compared to controls. The excitability changes and prolonged recovery of threshold following contraction are likely to contribute to symptoms of fatigue and weakness in MD patients.

Presynaptic action potential amplification by voltage-gated Na+ channels in hippocampal mossy fiber boutons

Action potentials in central neurons are initiated near the axon initial segment, propagate into the axon, and finally invade the presynaptic terminals, where they trigger transmitter release. Voltage-gated Na(+) channels are key determinants of excitability, but Na(+) channel density and properties in axons and presynaptic terminals of cortical neurons have not been examined yet. In hippocampal mossy fiber boutons, which emerge from parent axons en passant, Na(+) channels are very abundant, with an estimated number of approximately 2000 channels per bouton. Presynaptic Na(+) channels show faster inactivation kinetics than somatic channels, suggesting differences between subcellular compartments of the same cell. Computational analysis of action potential propagation in axon-multibouton structures reveals that Na(+) channels in boutons preferentially amplify the presynaptic action potential and enhance Ca(2+) inflow, whereas Na(+) channels in axons control the reliability and speed of propagation. Thus, presynaptic and axonal Na(+) channels contribute differentially to mossy fiber synaptic transmission.

Excitability differences in lower-limb motor axons during and after ischemia

Neuropathic diseases typically begin distally and spread proximally. Irrespective of the etiology, pathological investigations often indicate changes consistent with ischemia. In the present study, threshold tracking was used to investigate length-dependent differences in ischemic susceptibility of lower-limb axons in 6 healthy volunteers, with ischemia induced by a sphygmomanometer cuff inflated to 200 mm Hg and maintained for 13 minutes. Following stimulation of the peroneal nerve at the fibula neck, compound muscle action potentials were recorded proximally from tibialis anterior (TA) and distally from extensor digitorum brevis (EDB). During ischemia, excitability changes were consistent with nerve depolarization, with a greater reduction in threshold in EDB than TA. This reduction in threshold was associated with an increase in refractoriness, decrease in superexcitability, and prolongation of strength-duration time constant, consistent with axonal depolarization. With release of ischemia, reversal of these changes was associated with an increase in threshold, greater in EDB than TA, indicating axonal hyperpolarization. The rate of recovery of threshold was similar proximally and distally, arguing against a gradient in Na(+)/K(+) pump function along the peroneal nerve. The greater changes in threshold in EDB during and after ischemia suggest an increased susceptibility of more distal axons to ischemia and are likely to contribute to the length-dependent development of neuropathy.

Mechanisms of hyperpolarization in regenerated mature motor axons in cat

We found persistent abnormalities in the recovery of membrane excitability in long-term regenerated motor nerve fibres in the cat as indicated in the companion paper. These abnormalities could partly be explained by membrane hyperpolarization. To further investigate this possibility, we compared the changes in excitability in control nerves and long-term regenerated cat nerves (3-5 years after tibial nerve crush) during manoeuvres known to alter axonal membrane Na(+)-K(+) pump function: polarization, cooling to 20 degrees C, reperfusion after 10 min ischaemia, and up to 60 s of repetitive stimulation at 200 Hz. The abnormalities in excitability of regenerated nerves were reduced by depolarization and cooling and increased by hyperpolarization and during postischaemia. Moreover, the time course of recovery of excitability from repetitive stimulation and ischaemia was prolonged in regenerated nerves. Our data are consistent with an increased demand for electrogenic Na(+)-K(+) pumping in regenerated nerves leading to membrane hyperpolarization. Such persistent hyperpolarization may influence the ability of the axon to compensate for changes in membrane potential following normal repetitive activity.

Unmyelinated axons in the rat hippocampus hyperpolarize and activate an H current when spike frequency exceeds 1 Hz

The mammalian cortex is densely populated by extensively branching, thin, unmyelinated axons that form en passant synapses. Some thin axons in the peripheral nervous system hyperpolarize if action potential frequency exceeds 1-5 Hz. To test the hypothesis that cortical axons also show activity-induced hyperpolarization, we recorded extracellularly from individual CA3 pyramidal neurons while activating their axon with trains consisting of 30 electrical stimuli. Synaptic excitation was blocked by kynurenic acid. We observed a positive correlation between stimulation strength and the number of consecutive axonal stimuli that resulted in soma spikes, suggesting that the threshold increased as a function of the number of spikes. During trains without response failures there was always a cumulative increase in the soma response latency. Intermittent failures, however, decreased the latency of the subsequent response. At frequencies of > 1 Hz, the threshold and latency increases were enhanced by blocking the hyperpolarization-activated H current (Ih)by applying the specific Ih blocker ZD7288 (25 microM) or 2 mM Cs+. Under these conditions, response failures occurred after 15-25 stimuli, independent of the stimulation strength. Adding GABA receptor blockers (saclofen and bicuculline) and a blocker of metabotropic glutamate receptors did not change the activity-induced latency increase in recordings of the compound action potential. We interpret these results as an activity-induced hyperpolarization that is partly counteracted by Ih. Such a hyperpolarization may influence transmitter release and the conduction reliability of these axons.

Axon-glia interactions modulate axonal excitability in mammalian unmyelinated nerves

A period of electrical activity in unmyelinated nerve fibers is followed by a post-tetanic hyperpolarization (PTH), generated by the hyperactivity of the electrogenic Na(+)-K(+) pump. In order to protect the membrane potential against these strong hyperpolarizations, different types of axonal inward currents are activated during the PTH. We investigated in the rabbit vagus nerve one of these currents, which was activated by carbamylcholine (CCh). We observed that the effect of CCh on the PTH amplitude could be blocked or reversed with scopolamine. Moreover, the PTH amplitude increased when scopolamine alone was added to the perfusate, indicating that an endogenous muscarinic agonist was liberated in the preparation during the period of electrical activity. This CCh-activated current was TEA but not Ba(2+) or Cs(+) sensitive. It has been shown previously that muscarinic acetylcholine receptors (mAChRs) in the rabbit vagus nerve are located on the axonal but not glial membrane and that Schwann cells express several types of purinergic receptors, which activation evoke Ca(2+) transients in Schwann cells. We hypothesise that during electrical activity axons release a transmitter, presumably ATP. This transmitter evoke in the neighbouring Schwann cells a Ca(2+)-dependent liberation of a endogenous muscarinic agonist, which in turn activates a TEA-sensitive inward current in axons. We suggest that the major purpose of this mechanism is the control of the membrane potential during and after a period of intense electrical activity when the Na(+)-K(+) pump generates a robust PTH.

Persistent TTX-resistant Na+ current affects resting potential and response to depolarization in simulated spinal sensory neurons

Small dorsal root ganglion (DRG) neurons, which include nociceptors, express multiple voltage-gated sodium currents. In addition to a classical fast inactivating tetrodotoxin-sensitive (TTX-S) sodium current, many of these cells express a TTX-resistant (TTX-R) sodium current that activates near -70 mV and is persistent at negative potentials. To investigate the possible contributions of this TTX-R persistent (TTX-RP) current to neuronal excitability, we carried out computer simulations using the Neuron program with TTX-S and -RP currents, fit by the Hodgkin-Huxley model, that closely matched the currents recorded from small DRG neurons. In contrast to fast TTX-S current, which was well fit using a m(3)h model, the persistent TTX-R current was not well fit by an m(3)h model and was better fit using an mh model. The persistent TTX-R current had a strong influence on resting potential, shifting it from -70 to -49.1 mV. Inclusion of an ultra-slow inactivation gate in the persistent current model reduced the potential shift only slightly, to -56.6 mV. The persistent TTX-R current also enhanced the response to depolarizing inputs that were subthreshold for spike electrogenesis. In addition, the presence of persistent TTX-R current predisposed the cell to anode break excitation. These results suggest that, while the persistent TTX-R current is not a major contributor to the rapid depolarizing phase of the action potential, it contributes to setting the electrogenic properties of small DRG neurons by modulating their resting potentials and response to subthreshold stimuli.

Axonal flip-flops and oscillators

The strange and unpleasant sensations (paraesthesiae) or asynchronous motor-unit activation (fasciculation) that result from a period of limb ischaemia are examples of ectopic discharge in peripheral nerves. Ectopic activity also results from demyelination and is associated with serious neurological conditions, such as multiple sclerosis. A build-up of extracellular K(+) in the internode and persistent Na(+) currents are now implicated in generating the different forms of activity arising in normal and demyelinated axons.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Activity-dependent slowing of conduction differentiates functional subtypes of C fibres innervating human skin

1. The effects of impulse activity on conduction in cutaneous C fibres have been examined in 46 microneurographic recordings from 11 normal subjects and 11 diabetic patients with normal nerve conduction. A tungsten microelectrode was inserted into a cutaneous nerve, usually the superficial peroneal close to the ankle, and intraneural microstimulation was used to identify an area of skin innervated. Three minute trains of 0.25 ms stimuli at 1, 2 and 4 Hz were then delivered to the surface of the skin, separated by intervals of 6 min with stimulation at 0.25 Hz. Slowing and block of conduction were measured from the nerve responses for up to seven C units per stimulation sequence. 2. Three types of C unit were distinguished by their responses to repetitive stimulation: type 1 units slowed progressively during the 3 min trains; slowing of type 2 units reached a plateau within 1 min; while type 3 units hardly slowed at all. Data from normal and diabetic subjects did not differ and were pooled. After 3 min at 2 Hz, the percentage increases in latency were for type 1, 28.3 +/- 9.7 (n = 63 units, mean +/- s.d.); for type 2, 5.2 +/- 1.6 (n = 14); and for type 3, 0.8 +/- 0.5 (n = 5), with no overlap. After 3 min at 4 Hz, 58 % of type 1 units (but no type 2 or 3 units) blocked intermittently. Recovery of latency after stimulation was faster for type 2 than for type 1 units, but conduction velocities of the three types were similar. 3. Type 1 units were identified as nociceptors and 7 type 2 units were identified as 'cold' fibres, activated by non-noxious cold, with no overlap in modality. None of the units tested was activated by weak mechanical stimuli or reflex sympathetic activation. 4. Spike waveforms were averaged for 18 type 1, 10 type 2 and 6 type 3 units. All units had predominantly triphasic action potentials with a major negative peak, but those of type 3 units were on average both smaller and briefer than those of type 1 and type 2 units. 5. It is concluded that repetitive electrical stimulation reliably differentiates nociceptive from cold-specific C fibres innervating human hairy skin, as has previously been shown for the rat. Cold fibres can propagate impulses continuously at much higher rates than nociceptive fibres. The nature of the type 3 units is unclear.

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.