Morphology of action potentials recorded from human nerves using microneurography

Coupling of single cutaneous afferents in the hand with ankle muscles, and their response to rapid light touch displacements

Light touch reduces sway during standing. Unexpected displacement of a light touch reference at the finger can produce rapid responses in ankle muscles when standing, suggesting cutaneous receptors in the hand are functionally coupled with ankle muscles. Using microneurography in the median nerve, we tested the hypotheses: 1) that cutaneous afferent activity of mechanoreceptors of the hand would modulate electromyographic (EMG) activity of ankle muscles, and 2) that displacement of a light touch contact across a receptor's sensory territory would be encoded in the afferent activity. Spike-triggered averaging of EMG activity of tibialis anterior (TA) and soleus (SOL) demonstrated thirty-four of forty-two (81%) cutaneous afferents recorded modulated activity of ankle muscles with latencies between 40 to 119 ms. Cutaneous afferents of all types (slow and fast adapting, types I and II) demonstrated responses in TA and SOL, in both the ipsilateral and contralateral leg. Activity from eleven cutaneous afferents were recorded while a light touch contact was displaced across their receptive fields. Afferent activity increased with stimulus onset and remained elevated for the stimulus duration for all afferents recorded. These results suggest cutaneous afferents from the hand consistently form connections with motor pools of the leg at latencies implicating spinal pathways. In addition, the same population of afferents is readily excited by the displacement of a light touch contact. Therefore, cutaneous receptors of the hand can be recruited and utilized to alter motoneuron pool excitability in muscles important to balance control, at latencies relevant for rapid balance responses.

Median amplitude and frequency analysis of sensory nerve responses to intraepidermal stimulation

BACKGROUND

In clinical practice, small myelinated sensory fibers conveying pain and other sensations, Aδ-fibers, cannot be examined with available nerve conduction study techniques.

NEW METHOD

Equipment available in clinical neurophysiology laboratories is used to record from human sensory nerves multiple averaged responses to non-painful stimulation of intraepidermal nerves. Ten averaged responses are analyzed in all possible pair combinations with an algorithm applied to a 0.45 ms period of amplitude and frequency (power spectrum). The median of the algorithms is compared to control data to identify potentials generated as response to intraepidermal stimulation.

RESULTS

Median analysis of the algorithm applied to amplitude and frequency of multiple record pairs identifies potentials with conduction velocities of Aδ-fibers. The analysis of frequency (power spectrum) adds data to the analysis of amplitude. Median analysis of multiple record pairs yields more data than analysis of one pair of alternate averages with the same algorithms.

COMPARISON WITH EXISTING METHOD(S)

At present, analysis of one pair of alternate average records with an algorithm is the only method to identify Aδ-fiber generated potentials. Median analysis of the same algorithm applied to the amplitude of multiple record pairs increases the number of Aδ-fiber generated potentials identified. Neither median analysis of amplitude nor frequency of multiple records pairs has ever been used for conduction studies of nerve fibers, including Aδ-fibers.

CONCLUSIONS

Stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify the conduction velocities of Aδ-fibers in human sensory nerves.

Slowly conducting potentials in human sensory nerves

BACKGROUND

In clinical practice, small myelinated sensory fibers, Aδ-fibers, conveying mainly pain and temperature sensations, cannot be examined with available nerve conduction study techniques. Currently, these fibers can only be examined with experimental or very specialized and not commonly available nerve conduction techniques, or only indirectly with cerebral evoked potentials.

NEW METHOD

This study uses equipment and methods available in clinical neurophysiology laboratories to record from human sensory nerves ≥1000 averaged responses to focal, non-painful stimuli applied by a special electrode to epidermal nerves. The averaged responses to odd numbered stimuli are compared to the averaged responses to even numbered stimuli. An algorithm identifies potentials common in both averages. The 99^th and 99.9^th percentiles for this algorithm are obtained from control records without stimulation and applied to records with stimulation to identify potentials resulting from stimulation of intraepidermal nerves.

RESULTS

The algorithm identifies numerous negative and positive potentials as being different from controls at the 99th and 99.9^th percentile levels. The conduction velocities of the potentials range from of 1.3-29.9 m/s and are compatible with conduction velocities of Aδ-fibers.

COMPARISON WITH EXISTING METHOD(S)

No existing methods.

CONCLUSIONS

The stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify Aδ-fibers in human sensory nerves.

Cutaneous afferent innervation of the human foot sole: what can we learn from single-unit recordings?

Cutaneous afferents convey exteroceptive information about the interaction of the body with the environment and proprioceptive information about body position and orientation. Four classes of low-threshold mechanoreceptor afferents innervate the foot sole and transmit feedback that facilitates the conscious and reflexive control of standing balance. Experimental manipulation of cutaneous feedback has been shown to alter the control of gait and standing balance. This has led to a growing interest in the design of intervention strategies that enhance cutaneous feedback and improve postural control. The advent of single-unit microneurography has allowed the firing and receptive field characteristics of foot sole cutaneous afferents to be investigated. In this review, we consolidate the available cutaneous afferent microneurographic recordings from the foot sole and provide an analysis of the firing threshold, and receptive field distribution and density of these cutaneous afferents. This work enhances the understanding of the foot sole as a sensory structure and provides a foundation for the continued development of sensory augmentation insoles and other tactile enhancement interventions.

Precise coding of ankle angle and velocity by human calf muscle spindles

Human standing balance control requires the integration of sensory feedback to produce anticipatory, stabilizing ankle torques. However, the ability of human triceps surae muscle spindles to provide reliable sensory feedback regarding the small, slow ankle movements that occur during upright standing has recently come under question. We performed microneurography to directly record axon potentials from single muscle spindle afferents in the human triceps surae during servo-controlled movement of the ankle joint. To simulate movements of the ankle while standing, we delivered random 90-s dorsiflexion/plantar flexion oscillations of the ankle joint, with a peak-to-peak amplitude of 0.7° and frequency content below 0.5Hz. In roughly half of the trials (46%), participants held a low-level, near-isometric contraction of the triceps surae muscles. We demonstrate that afferent activity in a population of muscle spindles closely reflects ankle movements at frequencies and amplitudes characteristic of human standing. Four out of five soleus spindles, and three out of seven gastrocnemius spindles coded for at least a single frequency component of anteroposterior ankle rotation. Concatenating within muscles, coherence was significantly greater for soleus spindles at all stimulus frequencies. Voluntary contraction of the parent muscle reduced spindle sensitivity, but only significantly near the mean power frequency of the stimulus (∼0.3Hz). In conclusion, these results provide direct evidence that triceps surae muscle spindles are potentially capable of providing important sensory feedback for the control of human standing balance.

Estimation of the Electrode-Fiber Bioelectrical Coupling From Extracellularly Recorded Single Fiber Action Potentials

Selective peripheral neural interfaces are currently capable of detecting minute electrical signals from nearby nerve fibers as single fiber action potential (SFAP) waveforms. Each detected single unit has a distinct shape originating from the unique bioelectrical coupling that exists between the neuroprosthetic electrode, the nerve fiber and the extracellular milieu. The bioelectrical coupling manifests itself as a series of low-pass Bessel filters acting on the action currents along the nerve fiber. Here, we present a method to estimate the electrode-fiber bioelectrical coupling through a quantitative analysis of the spectral distribution of the single units extracellularly recorded with the thin-film longitudinal intrafascicular electrode (tfLIFE) in an in vivo mammalian peripheral nerve animal model. The bioelectrical coupling estimate is an estimate of the electrode sensitivity function traversed by the nerve fiber, suggesting that it is as a means to directly measure the spatial relationship between the nerve fiber and electrode. It not only reflects a shape change to the SFAP, but has implications for in situ nerve fiber location tracking, in situ diagnostics of nerves and neuroproshetic electrodes, and assessment of the biocompatibility of neural interfaces and the health of the reporting nerve fibers.

Cooling reduces the cutaneous afferent firing response to vibratory stimuli in glabrous skin of the human foot sole

Skin on the foot sole plays an important role in postural control. Cooling the skin of the foot is often used to induce anesthesia to determine the role of skin in motor and balance control. The effect of cooling on the four classes of mechanoreceptor in the skin is largely unknown, and thus the aim of the present study was to characterize the effects of cooling on individual skin receptors in the foot sole. Such insight will better isolate individual receptor contributions to balance control. Using microneurography, we recorded 39 single nerve afferents innervating mechanoreceptors in the skin of the foot sole in humans. Afferents were identified as fast-adapting (FA) or slowly adapting (SA) type I or II (FA I n = 16, FA II n = 7, SA I n = 6, SA II n = 11). Receptor response to vibration was compared before and after cooling of the receptive field (2-20 min). Overall, firing response was abolished in 30% of all receptors, and this was equally distributed across receptor type (P = 0.69). Longer cooling times were more likely to reduce firing response below 50% of baseline; however, some afferent responses were abolished with shorter cooling times (2-5 min). Skin temperature was not a reliable indicator of the level of receptor activation and often became uncoupled from receptor response levels, suggesting caution in the use of this parameter as an indicator of anesthesia. When cooled, receptors preferentially coded lower frequencies in response to vibration. In response to a sustained indentation, SA receptors responded more like FA receptors, primarily coding "on-off" events.

Baroreflex mechanisms regulating the occurrence of neural spikes in human muscle sympathetic nerve activity

This study tested the hypothesis that the discharge patterns of action potentials (APs) within bursts of postganglionic muscle sympathetic nerve activity (MSNA) are subject to arterial baroreflex control but in a manner that varies inversely with AP size. MSNA data were collected over 5 min of supine rest in 15 young and healthy individuals (8 males; 24 ± 4 yr of age; means ± SD). The baroreflex threshold and sensitivity diagrams were constructed for both the integrated sympathetic bursts and for the AP clusters. For the integrated bursts, a strong linear relationship between burst probability and diastolic blood pressure (DBP) was observed (P < 0.05). There was little relationship between integrated burst strength (amplitude) and DBP. On average, 12 AP clusters were observed across individuals. Larger APs tended to appear in the larger bursts. Linear regression analysis was used to study the baroreflex threshold (probability of AP cluster occurrence vs. DBP) as well as the baroreflex sensitivity (AP cluster size vs. DBP). A significant reflex threshold relationship was observed in 75-100% of AP clusters across all individuals. In contrast, significant reflex sensitivity relationships were observed in only 9 of 15 individuals and for limited APs. Overall, the slope of the AP baroreflex threshold relationship was greater for the small-medium sized AP clusters than that of the larger APs. Therefore, within each burst, the small-medium sized APs are governed by the baroreflex mechanism. However, the large APs, which tend to appear in the large integrated bursts, are weakly associated with a baroreflex control feature. The variable impact of baroreflex control over AP occurrence provides a plausible explanation for the overall weak baroreflex control over integrated burst strength, a feature that is determined by both the number and size of the AP complement.

Relationship between size and latency of action potentials in human muscle sympathetic nerve activity

We employed a novel action potential detection and classification technique to study the relationship between the recruitment of sympathetic action potentials (i.e., neurons) and the size of integrated sympathetic bursts in human muscle sympathetic nerve activity (MSNA). Multifiber postganglionic sympathetic nerve activity from the common fibular nerve was collected using microneurography in 10 healthy subjects at rest and during activation of sympathetic outflow using lower body negative pressure (LBNP). Burst occurrence increased with LBNP. Integrated burst strength (size) varied from 0.22 ± 0.07 V at rest to 0.28 ± 0.09 V during LBNP. Sympathetic burst size (i.e., peak height) was directly related to the number of action potentials within a sympathetic burst both at baseline ( r = 0.75 ± 0.13; P < 0.001) and LBNP ( r = 0.75 ± 0.12; P < 0.001). Also, the amplitude of detected action potentials within sympathetic bursts was directly related to the increased burst size at both baseline ( r = 0.59 ± 0.16; P < 0.001) and LBNP ( r = 0.61 ± 0.12; P < 0.001). In addition, the number of detected action potentials and the number of distinct action potential clusters within a given sympathetic burst were correlated at baseline ( r = 0.7 ± 0.1; P < 0.001) and during LBNP ( r = 0.74 ± 0.03; P < 0.001). Furthermore, action potential latency (i.e., an inverse index of neural conduction velocity) was decreased as a function of action potential size at baseline and LBNP. LBNP did not change the number of action potentials and unique clusters per sympathetic burst. It was concluded that there exists a hierarchical pattern of recruitment of additional faster conducting neurons of larger amplitude as the sympathetic bursts become stronger (i.e., larger amplitude bursts). This fundamental pattern was evident at rest and was not altered by the level of baroreceptor unloading applied in this study.

Spike detection in human muscle sympathetic nerve activity using a matched wavelet approach

Sympathetic nerve recordings associated with blood pressure regulation can be recorded directly using microneurography. A general characteristic of this signal is spontaneous burst activity of spikes (action potentials) separated by silent periods against a background of considerable Gaussian noise. During measurement with electrodes, the raw muscle sympathetic nerve activity (MSNA) signal is amplified, band-pass filtered, rectified and integrated. This integration process removes information regarding action potential content and their discharge properties. This paper proposes a new method for detecting action potentials from the raw MSNA signal to enable investigation of post-ganglionic neural discharge properties. The new method is based on the design of a mother wavelet that is matched to an actual mean action potential template extracted from a real raw MSNA signal. To detect action potentials, the new matched wavelet is applied to the MSNA signal using a continuous wavelet transform following a thresholding procedure and finding of a local maxima that indicates the location of action potentials. The performance of the proposed method versus two previous wavelet-based approaches was evaluated using (1) real MSNA recorded from seven healthy participants and, (2) simulated MSNA. The results show that the new matched wavelet performs better than the previous wavelet-based methods that use a non-matched wavelet in detecting action potentials in the MSNA signal.

A study on synaptic coupling between single orofacial mechanoreceptors and human masseter muscle

The connection between individual orofacial mechanoreceptive afferents and the motoneurones that innervate jaw muscles is not well established. For example, although electrical and mechanical stimulation of orofacial afferents in bulk evokes responses in the jaw closers, whether similar responses can be evoked in the jaw muscles from the discharge of type identified single orofacial mechanoreceptive afferents is not known. Using tungsten microelectrodes, we have recorded from 28 afferents in the inferior alveolar nerve and 21 afferents in the lingual nerve of human volunteers. We have used discharges of single orofacial afferents as the triggers and the electromyogram (EMG) of the masseter as the source to generate spike-triggered averaged records to illustrate time-based EMG modulation by the nerve discharge. We have then used cross correlation analysis to quantify the coupling. Furthermore, we have also used coherence analysis to study frequency-based relationship between the nerve spike trains and the EMG. The discharge patterns of the skin and mucosa receptors around the lip and the gingiva generated significant modulation in EMGs with a success rate of 40% for both cross correlation and coherence analyses. The discharge patterns of the periodontal mechanoreceptors (PMRs) generated more coupling with a success rate of 70% for cross correlation and about 35% for coherence analyses. Finally, the discharges of the tongue receptors displayed significant coupling with the jaw muscle motoneurones with a success rate of about 40% for both analyses. Significant modulation of the jaw muscles by single orofacial receptors suggests that they play important roles in controlling the jaw muscle activity so that mastication and speech functions are executed successfully.

Intraneural microstimulation of motor axons in the study of human single motor units

Single motor unit activity has been studied in depth since the first intramuscular electrodes were developed more than 70 years ago. Many techniques have been combined or used in isolation since then. Intraneural motor axon microstimulation allows the detailed study of single motor units in awake human subjects in a manner most analogous to that used in reduced animal preparations. A microelectrode, inserted percutaneously into a peripheral nerve, stimulates the axon of a single alpha-motoneuron at a site remote from the contracting muscle, allowing detailed analyses of the contractile properties of a single motor unit in an otherwise quiescent muscle, that is, without interference of simultaneously active motor units or the presence of an electrode within the muscle. The methods and results obtained using this technique are described and compared to those of other studies of single motor units in human subjects. Differences have been found between human and animal motor units and between motor units of various muscles. Studying human and animal motor units using an analogous technique provides insight into the interpretation of human data when results differ from animal data, and when human motor units cannot be examined in the same way, or at a similar level of detail, as animal motor units.

Spontaneous bursting neuronal discharges recorded from peripheral nerve in human: injury discharges or not?

This paper deals with a spontaneous, bursting neuronal activity which can not be altered by any stimulation in the periphery or voluntary actions or by cognitive tasks. An initial description of such units led to the conclusion that this activity was generated ectopically at the site of a previous or present impalement of a nerve fibre. The aim of the current study was to record a larger number of these units by using microneurography, in order to characterise their firing properties and particularly, see if any subtypes of units could be identified. In conclusion, this paper suggests that some of these discharges could be related to an injury of the nerve fibre, however most of them could not. Some hypothesis regarding the nature of these bursting activities are suggested.

Receptive field properties of unmyelinated tactile afferents in the human skin

We recorded, with the microneurography technique, single-unit impulses from nine cutaneous mechanoreceptive afferents with conduction velocities in the C range and receptive fields in the hairy skin of the forearm. The units responded with high impulse rates to light touch and had low monofilament thresholds. The geography of receptive fields was explored with a scanning method: a lightweight probe with a small and rounded tip was made to scan the field area in a series of closely adjacent tracks while single-unit activity was recorded. The fields of the nine units varied considerably in size as well as complexity. The individual field consisted of one to nine small responsive spots distributed over an area of 1-35 mm(2) when explored with a moving indentation of 5 mN. The fields were roughly round or oval in shape with no preferred orientation. The size of the response differed between individual sensitive spots in a field, suggesting a highly nonuniform terminal organization. The properties of the fields seem consistent with a role of tactile C afferents to provide information about pleasant touch and skin-to-skin contacts to central structures controlling emotions and affiliative behavior.

Novel information on peripheral tactile mechanisms in man acquired with concentric needle electrode microneurography

Microneurography with tungsten electrodes has provided a wealth of new data on peripheral nerve fibre function in man. Yet, some lingering controversies pertaining to the technique and its results have not been resolved. In particular, the working principles of microneurography allowing single unit sampling in man are not fully understood. Additionally debated, especially during recent years, was the validity of some neurographic data which supported the long standing conventional concept that myelinated fibres are randomly distributed intraneurally. A novel approach to address these issues was provided by microneurography with concentric needle electrodes. Data obtained with the latter technique suggested that these electrodes record activity extraaxonally from single myelinated fibres in man, possibly at or close to a node of Ranvier. The mechanisms described, which allow single unit resolution in humans, might well also be valid when performing microneurography with tungsten electrodes. Other sets of data indicated that Ranvier nodes tend to occur in clusters within certain regions of a nerve fascicle. Interestingly, the nerve fibres belonging to these clustering nodes were of the same modality and tended to innervate the same skin area in the hand. The discovered nerve fibre segregation involved all the four main classes of myelinated low threshold skin afferents in the hand (RA, PC, SAI and SAII units). The fact that sensory nerve fibres with clustering nodes and of the same modality tend to run together suggests at least a partially ordered intrafascicular nerve fibre organisation. The demonstrated intraneural fibre systematisation could be of profound functional significance both under normal conditions and in disease

Distribution and behaviour of glabrous cutaneous receptors in the human foot sole

To document the activity of cutaneous mechanoreceptors in the glabrous skin of the foot sole, tungsten microelectrodes were inserted through the popliteal fossa and into the tibial nerve of thirteen healthy human subjects. A total of 104 cutaneous mechanoreceptors were identified in the glabrous skin of the foot. This sample consisted of 15 slow adapting type I (14 %), 16 slow adapting type II (15 %), 59 fast adapting type I (57 %), and 14 fast adapting type II units (14 %). The location of the receptors and the outline of the receptive fields were determined by using nylon monofilaments perpendicularly applied against the surface of the skin. This revealed that the receptors were widely distributed without an accumulation of receptors in the toes. There were also larger receptive fields predominantly isolated on the plantar surface of the metatarsal-tarsal region of the foot sole. Furthermore, with the foot in an unloaded position, there was no background discharge activity in any of the cutaneous receptors in the absence of intentionally applied stimulation. These findings suggest that skin receptors in the foot sole behave differently from those receptors found on the glabrous skin of the hand. This may reflect the role of foot sole skin receptors in standing balance and movement control.

Fitting pieces in the peripheral nerve puzzle

Findings from comparative microneurography are reviewed, i.e., data obtained by exploring human nerves with tungsten electrodes or concentric needle electrodes under similar conditions. It has emerged that activity in single myelinated fibers originates near nodes of Ranvier. Other data have shown that Ranvier nodes tend to cluster in certain regions of a fascicle and belong to fibers of the same modality which innervate the same skin area. This segregation involves all four main classes of myelinated low-threshold skin afferents. Fiber populations of the same modality may act as peripheral projection modules involved in somatosensory processing of tactile stimuli to cognitive levels. The fiber bundle arrangement of the nerves may be important for conserving functional gnosis in conditions where peripheral nerve fibers are lost. This organization may also be critical as a substrate to promote reinnervation after nerve cut followed by peripheral nerve suture. It is therefore less critical for an outgrowing fiber to find its exact distal counterpart. Even if misguided outgrowth occurs into the endoneurial tube of a neighboring distal fiber of the same modality with an adjacent receptive field, function can be reestablished. A precise nerve topography might also be of significance for obtaining a functionally satisfactory recovery after avulsion injuries treated by nerve root implantation into the spinal cord. Thus, there is in man an ordered nerve fiber organization, both in the periphery and in the CNS, which may have profound functional significance both under normal conditions and in disease.

Intrafascicular electrodes for stimulation and recording from mudpuppy spinal roots

This paper presents a technique for stimulating and recording from multiple intact spinal roots in the in vitro mudpuppy (Necturus maculatus) spinal cord-forearm preparation using fine wire electrodes, a modified intrafascicular electrode. We found that multiple spinal roots of the preparation could be implanted with these modified electrodes for independent stimulation or recording of the roots without inducing mechanical vibrations, disrupting conduction, or obscuring the view of or access to the spinal cord. Recording and stimulation performance using these electrodes was compared with results obtained using conventional hook electrodes. We found that intrafascicular electrodes were more efficient than hook electrodes for stimulating nerve fibers, being able to produce equivalent levels of activation using stimulation levels that were an order of magnitude smaller. Compound action potential signals recorded from electrodes implanted in the spinal roots were found to be larger than those from hook electrodes placed around the corresponding spinal nerve, showing that intrafascicular electrodes are more efficient at recording activity in the nerve. Moreover, it was possible to record evoked activity from cutaneous mechanoreceptors, even though the signal to noise ratio was low. Rough estimates of the conduction velocities for the fastest components in the compound action potentials were calculated and found to be around 17.5 m/s for both dorsal and ventral roots.

Evidence for strong synaptic coupling between single tactile afferents and motoneurones supplying the human hand

1. Electrical stimulation of digital nerves elicits short-latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short-latency EMG responses could be evoked from the discharge of a single cutaneous afferent. 2. Thirty-three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike-triggered averaging revealed EMG events time-locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. 3. Eleven cutaneous afferents (33 %) showed a short-latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0-41.1 ms). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval (r = 0.99). 4. The EMG associated with two rapidly adapting (FA) type I afferents (29 %) showed a short-latency excitation while five showed neither excitation nor inhibition. Seven SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. 5. We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short-latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.

The development of conduction block in single human axons following a focal nerve injury

1. Using microneurography with a conventional monopolar electrode, the action potentials of ten myelinated axons in the peripheral nerves of human subjects were followed while they developed conduction block. 2. The action potentials had initially (n = 6) or developed (n = 4) a positive double-peaked morphology. The time interval between the two positive peaks represents the conduction time across the impaled internode. 3. When the interpeak interval was < 500 micros, conduction across the site of impalement was secure, even if the conduction time was markedly prolonged. When the interval was > 600 microseconds, intermittent conduction failure occurred. For all units the longest interpeak interval recorded just prior to complete conduction failure was, on average, 1.12 ms (range, 0.8-1.4 ms). 4. For five axons, there was evidence that natural activity triggered the conduction failure. 5. Impalement of the nerve fibre by the microelectrode impairs the ability of the axon to conduct impulses across the site of injury, but impulse transmission can be secure even when the conduction time across individual internodes is prolonged to 500 microseconds. These findings are therefore relevant to the conduction deficits that occur in focal injuries of human axons.

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.

Protocol for microneurography with concentric needle electrodes

In 1968, the method of human percutaneous microneurography with solid tungsten electrodes was introduced. Since then many investigators used this technique to study peripheral mechanisms in the somatosensory, motor and autonomic systems of conscious humans. Although some modifications of the method were described, the basic construction of the recording electrode has remained the same over the years. In the present protocol we describe in detail the procedures of microneurography using a thin diameter concentric needle electrode. There are some advantages with the concentric electrodes in comparison with the tungsten needles: (1) the electrical and mechanical properties of the electrode are stable which allows repeated use, (2) its restricted and one-dimensionally directed recording area provides the possibility to study topographical aspects within even a part of a peripheral nerve fascicle, and (3) multi-channel recordings can be achieved by adding more recording surfaces to the electrode. Based on recent investigations evaluating the recording properties of concentric electrodes we propose a novel procedure for signal analysis where template matching is incorporated. The analyses described in this protocol might also be applicable for extracellular recordings from muscle or elsewhere within the nervous system.

The methodology and scope of human microneurography

Microneurography was introduced in 1967 and has developed into an invaluable tool for investigating human somatosensory, motor and cardiovascular physiology and pathophysiology. It involves percutaneous insertion of a metal microelectrode into fascicles of limb and facial nerves. This review covers the procedures and equipment necessary for microneurography and provides a current circuit for a preamplifier. Evidence is presented that (i) most recordings from myelinated axons involve an effective penetration of the myelin by the electrode; (ii) based on physiological criteria, microstimulation through the electrode can be used to activate single axons although the probability of this is relatively low and (iii) despite 'micro' lesions caused by the electrode insertion into the nerve and its fascicles, the morbidity with the procedure is acceptably low.