The normal sural nerve in man. I. Ultrastructure and numbers of fibres and cells

NRV: An open framework for in silico evaluation of peripheral nerve electrical stimulation strategies

Electrical stimulation of peripheral nerves has been used in various pathological contexts for rehabilitation purposes or to alleviate the symptoms of neuropathologies, thus improving the overall quality of life of patients. However, the development of novel therapeutic strategies is still a challenging issue requiring extensive in vivo experimental campaigns and technical development. To facilitate the design of new stimulation strategies, we provide a fully open source and self-contained software framework for the in silico evaluation of peripheral nerve electrical stimulation. Our modeling approach, developed in the popular and well-established Python language, uses an object-oriented paradigm to map the physiological and electrical context. The framework is designed to facilitate multi-scale analysis, from single fiber stimulation to whole multifascicular nerves. It also allows the simulation of complex strategies such as multiple electrode combinations and waveforms ranging from conventional biphasic pulses to more complex modulated kHz stimuli. In addition, we provide automated support for stimulation strategy optimization and handle the computational backend transparently to the user. Our framework has been extensively tested and validated with several existing results in the literature.

Discoidin domain receptor regulates ensheathment, survival and caliber of peripheral axons

Most invertebrate axons and small-caliber axons in mammalian peripheral nerves are unmyelinated but still ensheathed by glia. Here, we use Drosophila wrapping glia to study the development and function of non-myelinating axon ensheathment, which is poorly understood. Selective ablation of these glia from peripheral nerves severely impaired larval locomotor behavior. In an in vivo RNA interference screen to identify glial genes required for axon ensheathment, we identified the conserved receptor tyrosine kinase Discoidin domain receptor (Ddr). In larval peripheral nerves, loss of Ddr resulted in severely reduced ensheathment of axons and reduced axon caliber, and we found a strong dominant genetic interaction between Ddr and the type XV/XVIII collagen Multiplexin (Mp), suggesting that Ddr functions as a collagen receptor to drive axon wrapping. In adult nerves, loss of Ddr decreased long-term survival of sensory neurons and significantly reduced axon caliber without overtly affecting ensheathment. Our data establish essential roles for non-myelinating glia in nerve development, maintenance and function, and identify Ddr as a key regulator of axon-glia interactions during ensheathment and establishment of axon caliber.

Peripheral glia diversity

Recent years have seen an evolving appreciation for the role of glial cells in the nervous system. As we move away from the typical neurocentric view of neuroscience, the complexity and variability of central nervous system glia is emerging, far beyond the three main subtypes: astrocytes, oligodendrocytes, and microglia. Yet the diversity of the glia found in the peripheral nervous system remains rarely discussed. In this review, we discuss the developmental origin, morphology, and function of the different populations of glia found in the peripheral nervous system, including: myelinating Schwann cells, Remak Schwann cells, repair Schwann cells, satellite glia, boundary cap-derived glia, perineurial glia, terminal Schwann cells, glia found in the skin, olfactory ensheathing cells, and enteric glia. The morphological and functional heterogeneity of glia found in the periphery reflects the diverse roles the nervous system performs throughout the body. Further, it highlights a complexity that should be appreciated and considered when it comes to a complete understanding of the peripheral nervous system in health and disease.

Offset analgesia is increased intra-orally

BACKGROUND

Offset analgesia (OA) is commonly used to quantify endogenous pain inhibition. However, the potential role of afferent inputs and the subsequent peripheral factors from different body areas on the underlying mechanisms are still unclear.

OBJECTIVES

The aim of this cross-sectional study was to compare the magnitude of OA in four different body areas representing a) glabrous and non-glabrous skin, b) trigeminal and extra-trigeminal areas, and c) intra- and extra-oral tissue.

METHODS

OA was assessed at the oral mucosa of the lower lip, at the skin of the cheek, the forearm and the palm of the hand in 32 healthy and pain-free participants. OA testing included two trials: (1) a constant trial (30 seconds of constant heat stimulation at an individualized temperature of Pain₅₀ (pain intensity of 50 out of 100)), and (2) an offset trial (10 seconds of individualized Pain₅₀ , followed by 5 seconds at Pain₅₀ +1°C and 15 seconds at Pain₅₀ ). Participants continuously rated their pain during each trial with a computerized visual analog scale.

RESULTS

A significant OA response was recorded at the oral mucosa (p<0.001, d=1.24), the cheek (p<0.001, d=0.84) and the forearm (p<0.001, d=1.04), but not at the palm (p=0.19, d=0.24). Significant differences were shown for OA recorded at the cheek versus the mucosa (p=0.02), and between palm and mucosa (p=0.007), but not between the remaining areas (p>0.05).

CONCLUSION

This study suggests that intra-oral endogenous pain inhibition assessed with OA is enhanced and supports the role of peripheral mechanisms contributing to the OA response.

Optic nerve-associated connective tissue structures revisited: a histological study using human fetuses and adult cadavers

Unlike the usual peripheral nerve, the optic nerve accompanies a thick "dural sheath," a thin "sheath of pia mater" (SPM), and multiple "septa," which divides the nerve fibers into fascicles. We collected specimens from 25 adult cadavers and 15 fetuses and revisited the histological architecture of the optic and oculomotor nerves. In the optic chiasma, the meningeal layer of the dura joins the pia to form a thick SPM, and the periosteum of the sphenoid is continuous with the dural sheath at the orbital exit of the bony optic canal. The septa appeared as a cluster of irregularly arrayed fibrous plates in the intracranial course near the chiasma. Thus, the septa were not derived from either the SPM or the dural sheath. In the orbit, the central artery of the retina accompanies collagenous fibers from the dural sheath and the SPM to provide the vascular sheath in the optic nerve. These connective tissue configurations were the same between adult and fetal specimens. At the optic disk, the dural sheath and SPM merged with the sclera, whereas the septa appeared to end at the lamina cribrosa. However, in fetuses without lamina cribrosa, the septa extend into the nerve fiber layer of the retina. The SPM and septa showed strong elastin immunoreactivity, in contrast to the absence of reactivity in the sheaths of the oculomotor nerve. Each S100 protein-positive Schwann sheath of the oculomotor nerve was surrounded by collagenous endoneurium. Glial fibrillary acidic protein-positive astrocytes showed a linear arrangement along the septa. This article is protected by copyright. All rights reserved.

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.

Excitation properties of computational models of unmyelinated peripheral axons

Biophysically based computational models of nerve fibers are important tools for designing electrical stimulation therapies, investigating drugs that affect ion channels, and studying diseases that affect neurons. Although peripheral nerves are primarily composed of unmyelinated axons (i.e., C-fibers), most modeling efforts focused on myelinated axons. We implemented the single-compartment model of vagal afferents from Schild et al. (1994) (Schild JH, Clark JW, Hay M, Mendelowitz D, Andresen MC, Kunze DL. J Neurophysiol 71: 2338-2358, 1994) and extended the model into a multicompartment axon, presenting the first cable model of a C-fiber vagal afferent. We also implemented the updated parameters from the Schild and Kunze (1997) model (Schild JH, Kunze DL. J Neurophysiol 78: 3198-3209, 1997). We compared the responses of these novel models with those of three published models of unmyelinated axons (Rattay F, Aberham M. IEEE Trans Biomed Eng 40: 1201-1209, 1993; Sundt D, Gamper N, Jaffe DB. J Neurophysiol 114: 3140-3153, 2015; Tigerholm J, Petersson ME, Obreja O, Lampert A, Carr R, Schmelz M, Fransén E. J Neurophysiol 111: 1721-1735, 2014) and with experimental data from single-fiber recordings. Comparing the two models by Schild et al. (1994, 1997) revealed that differences in rest potential and action potential shape were driven by changes in maximum conductances rather than changes in sodium channel dynamics. Comparing the five model axons, the conduction speeds and strength-duration responses were largely within expected ranges, but none of the models captured the experimental threshold recovery cycle-including a complete absence of late subnormality in the models-and their action potential shapes varied dramatically. The Tigerholm et al. (2014) model best reproduced the experimental data, but these modeling efforts make clear that additional data are needed to parameterize and validate future models of autonomic C-fibers.NEW & NOTEWORTHY Peripheral nerves are primarily composed of unmyelinated axons, and there is growing interest in electrical stimulation of the autonomic nervous system to treat various diseases. We present the first cable model of an unmyelinated vagal nerve fiber and compare its ion channel isoforms and conduction responses with other published models of unmyelinated axons, establishing important tools for advancing modeling of autonomic nerves.

The Somatosensory World of the African Naked Mole-Rat

The naked mole-rat (Heterocephalus glaber) is famous for its longevity and unusual physiology. This eusocial species that lives in highly ordered and hierarchical colonies with a single breeding queen, also discovered secrets enabling somewhat pain-free living around 20 million years ago. Unlike most mammals, naked mole-rats do not feel the burn of chili pepper's active ingredient, capsaicin, nor the sting of acid. Indeed, by accumulating mutations in genes encoding proteins that are only now being exploited as targets for new pain therapies (the nerve growth factor receptor TrkA and voltage-gated sodium channel, Na_V1.7), this species mastered the art of analgesia before humans evolved. Recently, we have identified pain-insensitivity as a trait shared by several closely related African mole-rat species. In this chapter we will show how African mole-rats have evolved pain insensitivity as well as discussing what the proximate factors may have been that led to the evolution of pain-free traits.

Aging alters signaling properties in the mouse spinal dorsal horn

A well-recognized relationship exists between aging and increased susceptibility to chronic pain conditions, underpinning the view that pain signaling pathways differ in aged individuals. Yet despite the higher prevalence of altered pain states among the elderly, the majority of preclinical work studying mechanisms of aberrant sensory processing are conducted in juvenile or young adult animals. This mismatch is especially true for electrophysiological studies where patch clamp recordings from aged tissue are generally viewed as particularly challenging. In this study, we have undertaken an electrophysiological characterization of spinal dorsal horn neurons in young adult (3–4 months) and aged (28–32 months) mice. We show that patch clamp data can be routinely acquired in spinal cord slices prepared from aged animals and that the excitability properties of aged dorsal horn neurons differ from recordings in tissue prepared from young animals. Specifically, aged dorsal horn neurons more readily exhibit repetitive action potential discharge, indicative of a more excitable phenotype. This observation was accompanied by a decrease in the amplitude and charge of spontaneous excitatory synaptic input to dorsal horn neurons and an increase in the contribution of GABAergic signaling to spontaneous inhibitory synaptic input in aged recordings. While the functional significance of these altered circuit properties remains to be determined, future work should seek to assess whether such features may render the aged dorsal horn more susceptible to aberrant injury or disease-induced signaling and contribute to increased pain in the elderly.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

Independent evolution of pain insensitivity in African mole-rats: origins and mechanisms

The naked mole-rat (Heterocephalus glaber) is famous for its longevity and unusual physiology. This eusocial species that lives in highly ordered and hierarchical colonies with a single breeding queen, also discovered secrets enabling somewhat pain-free living around 20 million years ago. Unlike most mammals, naked mole-rats do not feel the burn of chili pepper's active ingredient, capsaicin, nor the sting of acid. Indeed, by accumulating mutations in genes encoding proteins that are only now being exploited as targets for new pain therapies (the nerve growth factor receptor TrkA and voltage-gated sodium channel, NaV1.7), this species mastered the art of analgesia before humans evolved. Recently, we have identified pain insensitivity as a trait shared by several closely related African mole-rat species. One of these African mole-rats, the Highveld mole-rat (Cryptomys hottentotus pretoriae), is uniquely completely impervious and pain free when confronted with electrophilic compounds that activate the TRPA1 ion channel. The Highveld mole-rat has evolved a biophysical mechanism to shut down the activation of sensory neurons that drive pain. In this review, we will show how mole-rats have evolved pain insensitivity as well as discussing what the proximate factors may have been that led to the evolution of pain-free traits.

Biology of the human blood-nerve barrier in health and disease

A highly regulated endoneurial microenvironment is required for normal axonal function in peripheral nerves and nerve roots, which structurally consist of an outer collagenous epineurium, inner perineurium consisting of multiple concentric layers of specialized epithelioid myofibroblasts that surround the innermost endoneurium, which consists of myelinated and unmyelinated axons embedded in a looser mesh of collagen fibers. Endoneurial homeostasis is achieved by tight junction-forming endoneurial microvessels that control ion, solute, water, nutrient, macromolecule and leukocyte influx and efflux between the bloodstream and endoneurium, and the innermost layers of the perineurium that control interstitial fluid component flux between the freely permeable epineurium and endoneurium. Strictly speaking, endoneurial microvascular endothelium should be considered the blood-nerve barrier (BNB) due to direct communication with circulating blood. The mammalian BNB is considered the second most restrictive vascular system after the blood-brain barrier (BBB) based on classic in situ permeability studies. Structural alterations in endoneurial microvessels or interactions with hematogenous leukocytes have been described in several human peripheral neuropathies; however major advances in BNB biology in health and disease have been limited over the past 50 years. Guided by transcriptome and proteome studies of normal and pathologic human peripheral nerves, purified primary and immortalized human endoneurial endothelial cells that form the BNB and leukocytes from patients with well-characterized peripheral neuropathies, validated by in situ or ex vivo protein expression studies, data are emerging on the molecular and functional characteristics of the human BNB in health and in specific peripheral neuropathies, as well as chronic neuropathic pain. These early advancements have the potential to not only increase our understanding of how the BNB works and adapts or fails to adapt to varying insult, but provide insights relevant to pathogenic leukocyte trafficking, with translational potential and specific therapeutic application for chronic peripheral neuropathies and neuropathic pain.

Cerebellar ataxia, neuropathy, vestibular areflexia syndrome due to RFC1 repeat expansion

Ataxia, causing imbalance, dizziness and falls, is a leading cause of neurological disability. We have recently identified a biallelic intronic AAGGG repeat expansion in replication factor complex subunit 1 (RFC1) as the cause of cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) and a major cause of late onset ataxia. Here we describe the full spectrum of the disease phenotype in our first 100 genetically confirmed carriers of biallelic repeat expansions in RFC1 and identify the sensory neuropathy as a common feature in all cases to date. All patients were Caucasian and half were sporadic. Patients typically reported progressive unsteadiness starting in the sixth decade. A dry spasmodic cough was also frequently associated and often preceded by decades the onset of walking difficulty. Sensory symptoms, oscillopsia, dysautonomia and dysarthria were also variably associated. The disease seems to follow a pattern of spatial progression from the early involvement of sensory neurons, to the later appearance of vestibular and cerebellar dysfunction. Half of the patients needed walking aids after 10 years of disease duration and a quarter were wheelchair dependent after 15 years. Overall, two-thirds of cases had full CANVAS. Sensory neuropathy was the only manifestation in 15 patients. Sixteen patients additionally showed cerebellar involvement, and six showed vestibular involvement. The disease is very likely to be underdiagnosed. Repeat expansion in RFC1 should be considered in all cases of sensory ataxic neuropathy, particularly, but not only, if cerebellar dysfunction, vestibular involvement and cough coexist.

Definition and diagnosis of small fiber neuropathy: consensus from the Peripheral Neuropathy Scientific Department of the Brazilian Academy of Neurology

The aim of this study was to describe the results of a Brazilian Consensus on Small Fiber Neuropathy (SFN). Fifteen neurologists (members of the Brazilian Academy of Neurology) reviewed a preliminary draft. Eleven panelists got together in the city of Fortaleza to discuss and finish the text for the manuscript submission. Small fiber neuropathy can be defined as a subtype of neuropathy characterized by selective involvement of unmyelinated or thinly myelinated sensory fibers. Its clinical picture includes both negative and positive manifestations: sensory (pain/dysesthesias/pruritus) or combined sensory and autonomic complaints, associated with an almost entirely normal neurological examination. Standard electromyography is normal. A growing list of medical conditions is associated with SFN. The classification of SFN may also serve as a useful terminology to uncover minor discrepancies in the normal values from different neurophysiology laboratories. Several techniques may disclose sensory and/or autonomic impairment. Further studies are necessary to refine these techniques and develop specific therapies.

Effect of Age, Sex and Gender on Pain Sensitivity: A Narrative Review

Introduction:

An increasing body of literature on sex and gender differences in pain sensitivity has been accumulated in recent years. There is also evidence from epidemiological research that painful conditions are more prevalent in older people. The aim of this narrative review is to critically appraise the relevant literature investigating the presence of age and sex differences in clinical and experimental pain conditions.

Methods:

A scoping search of the literature identifying relevant peer reviewed articles was conducted on May 2016. Information and evidence from the key articles were narratively described and data was quantitatively synthesised to identify gaps of knowledge in the research literature concerning age and sex differences in pain responses.

Results:

This critical appraisal of the literature suggests that the results of the experimental and clinical studies regarding age and sex differences in pain contain some contradictions as far as age differences in pain are concerned. While data from the clinical studies are more consistent and seem to point towards the fact that chronic pain prevalence increases in the elderly findings from the experimental studies on the other hand were inconsistent, with pain threshold increasing with age in some studies and decreasing with age in others.

Conclusion:

There is a need for further research using the latest advanced quantitative sensory testing protocols to measure the function of small nerve fibres that are involved in nociception and pain sensitivity across the human life span.

Implications:

Findings from these studies should feed into and inform evidence emerging from other types of studies (e.g. brain imaging technique and psychometrics) suggesting that pain in the older humans may have unique characteristics that affect how old patients respond to intervention.

Autonomic components of Complex Regional Pain Syndrome (CRPS) are favourably affected by Electrical Twitch-Obtaining Intramuscular Stimulation (ETOIMS): effects on blood pressure and heart rate

INTRODUCTION

Favourable pain relief results on evoking autonomous twitches at myofascial trigger points with Electrical Twitch Obtaining Intramuscular Stimulation (ETOIMS).

AIM

To document autonomic nervous system (ANS) dysfunction in Complex Regional Pain Syndrome (CRPS) from blood pressure (BP) and pulse/heart rate changes with ETOIMS.

METHODS AND MATERIALS

A patient with persistent pain regularly received serial ETOIMS sessions of 60, 90, 120 or ≥150 min over 24 months. Outcome measures include BP: systolic, diastolic, pulse pressure and pulse/heart rate, pre-session/immediate-post-session summed differences (SDPPP index), and pain reduction. His results were compared with that of two other patients and one normal control. Each individual represented the following maximal elicitable twitch forces (TWF) graded 1-5: maximum TWF2: control subject; maximum TWF3: CRPS patient with suspected ANS dysfunction; and maximum TWF4 and TWF5: two patients with respective slow-fatigue and fast-fatigue twitches who during ETOIMS had autonomous twitching at local and remote myotomes simultaneously from denervation supersensitivity. ETOIMS results between TWFs were compared using one-way analysis of variance test.

RESULTS

The patients showed immediate significant pain reduction, BP and pulse/heart rate changes/reduction(s) except for diastolic BP in the TWF5 patient. TWF2 control subject had diastolic BP reduction with ETOIMS but not with rest. Linear regression showed TWF grade to be the most significant variable in pain reduction, more so than the number of treatments, session duration and treatment interval. TWF grade was the most important variable in significantly reducing outcome measures, especially pulse/heart rate. Unlike others, the TWF3 patient had distinctive reductions in SDPPP index.

CONCLUSIONS

Measuring BP and pulse/heart rate is clinically practical for alerting ANS dysfunction maintained CRPS. SDPPP index (≥26) and pulse/heart rate (≥8) reductions with almost every ETOIMS treatment, plus inability to evoke autonomous twitches due to pain-induced muscle hypertonicity, are pathognomonic of this problem.

The biology of skin wetness perception and its implications in manual function and for reproducing complex somatosensory signals in neuroprosthetics

Our perception of skin wetness is generated readily, yet humans have no known receptor (hygroreceptor) to signal this directly. It is easy to imagine the sensation of water running over our hands or the feel of rain on our skin. The synthetic sensation of wetness is thought to be produced from a combination of specific skin thermal and tactile inputs, registered through thermoreceptors and mechanoreceptors, respectively. The present review explores how thermal and tactile afference from the periphery can generate the percept of wetness centrally. We propose that the main signals include information about skin cooling, signaled primarily by thinly myelinated thermoreceptors, and rapid changes in touch, through fast-conducting, myelinated mechanoreceptors. Potential central sites for integration of these signals, and thus the perception of skin wetness, include the primary and secondary somatosensory cortices and the insula cortex. The interactions underlying these processes can also be modeled to aid in understanding and engineering the mechanisms. Furthermore, we discuss the role that sensing wetness could play in precision grip and the dexterous manipulation of objects. We expand on these lines of inquiry to the application of the knowledge in designing and creating skin sensory feedback in prosthetics. The addition of real-time, complex sensory signals would mark a significant advance in the use and incorporation of prosthetic body parts for amputees in everyday life.NEW & NOTEWORTHY Little is known about the underlying mechanisms that generate the perception of skin wetness. Humans have no specific hygroreceptor, and thus temperature and touch information combine to produce wetness sensations. The present review covers the potential mechanisms leading to the perception of wetness, both peripherally and centrally, along with their implications for manual function. These insights are relevant to inform the design of neuroengineering interfaces, such as sensory prostheses for amputees.

Characterization of cutaneous and articular sensory neurons

BACKGROUND

A wide range of stimuli can activate sensory neurons and neurons innervating specific tissues often have distinct properties. Here, we used retrograde tracing to identify sensory neurons innervating the hind paw skin (cutaneous) and ankle/knee joints (articular), and combined immunohistochemistry and electrophysiology analysis to determine the neurochemical phenotype of cutaneous and articular neurons, as well as their electrical and chemical excitability.

RESULTS

Immunohistochemistry analysis using RetroBeads as a retrograde tracer confirmed previous data that cutaneous and articular neurons are a mixture of myelinated and unmyelinated neurons, and the majority of both populations are peptidergic. In whole-cell patch-clamp recordings from cultured dorsal root ganglion neurons, voltage-gated inward currents and action potential parameters were largely similar between articular and cutaneous neurons, although cutaneous neuron action potentials had a longer half-peak duration (HPD). An assessment of chemical sensitivity showed that all neurons responded to a pH 5.0 solution, but that acid-sensing ion channel (ASIC) currents, determined by inhibition with the nonselective acid-sensing ion channel antagonist benzamil, were of a greater magnitude in cutaneous compared to articular neurons. Forty to fifty percent of cutaneous and articular neurons responded to capsaicin, cinnamaldehyde, and menthol, indicating similar expression levels of transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and transient receptor potential melastatin 8 (TRPM8), respectively. By contrast, significantly more articular neurons responded to ATP than cutaneous neurons.

CONCLUSION

This work makes a detailed characterization of cutaneous and articular sensory neurons and highlights the importance of making recordings from identified neuronal populations: sensory neurons innervating different tissues have subtly different properties, possibly reflecting different functions.

Myelinated and unmyelinated endoneurial axon quantitation and clinical correlation

INTRODUCTION

Different disease patterns result from loss of myelinated and unmyelinated axons, but quantitation to define their loss has been difficult.

METHODS

We measured large and small endoneurial axons in axonal neuropathies by staining them with peripherin and comparing their area to that of nonmyelinating Schwann cells stained with neural cell adhesion molecule (NCAM).

RESULTS

Loss of myelinated and unmyelinated axons was typically proportional, with predominant myelinated or unmyelinated axon loss in a few patients. Myelinated axon loss was associated with loss of distal vibration sense and sensory potentials (P < 0.0001) and was selective in patients with bariatric and bowel resection surgery (P < 0.001). Unmyelinated axon measurements correlated with skin (ankle P = 0.01; thigh P = 0.02) and vascular (nerve P < 0.0001; muscle P = 0.01) innervation.

CONCLUSIONS

Myelinated and unmyelinated axons can be quantitated by comparing areas of axons and nonmyelinating Schwann cells. Clinical features correlate with myelinated axon loss, and unmyelinated axon loss correlates with skin and vascular denervation.

Transcriptional regulator PRDM12 is essential for human pain perception

Pain perception has evolved as a warning mechanism to alert organisms to tissue damage and dangerous environments. In humans, however, undesirable, excessive or chronic pain is a common and major societal burden for which available medical treatments are currently suboptimal. New therapeutic options have recently been derived from studies of individuals with congenital insensitivity to pain (CIP). Here we identified 10 different homozygous mutations in PRDM12 (encoding PRDI-BF1 and RIZ homology domain-containing protein 12) in subjects with CIP from 11 families. Prdm proteins are a family of epigenetic regulators that control neural specification and neurogenesis. We determined that Prdm12 is expressed in nociceptors and their progenitors and participates in the development of sensory neurons in Xenopus embryos. Moreover, CIP-associated mutants abrogate the histone-modifying potential associated with wild-type Prdm12. Prdm12 emerges as a key factor in the orchestration of sensory neurogenesis and may hold promise as a target for new pain therapeutics.

Assessment of small fibers using evoked potentials

Background and purpose

Conventional neurophysiological techniques do not assess the function of nociceptive pathways and are inadequate to detect abnormalities in patients with small-fiber damage. This overview aims to give an update on the methods and techniques used to assess small fiber (Aδ- and C-fibers) function using evoked potentials in research and clinical settings.

Methods

Noxious radiant or contact heat allows the recording of heat-evoked brain potentials commonly referred to as laser evoked potentials (LEPs) and contact heat-evoked potentials (CHEPs). Both methods reliably assess the loss of Aδ-fiber function by means of reduced amplitude and increased latency of late responses, whereas other methods have been developed to record ultra-late C-fiber-related potentials. Methodological considerations with the use of LEPs and CHEPs include fixed versus variable stimulation site, application pressure, and attentional factors. While the amplitude of LEPs and CHEPs often correlates with the reported intensity of the stimulation, these factors may also be dissociated. It is suggested that the magnitude of the response may be related to the saliency of the noxious stimulus (the ability of the stimulus to stand out from the background) rather than the pain perception.

Results

LEPs and CHEPs are increasingly used as objective laboratory tests to assess the pathways mediating thermal pain, but new methods have recently been developed to evaluate other small-fiber pathways. Pain-related electrically evoked potentials with a low-intensity electrical simulation have been proposed as an alternative method to selectively activate Aδ-nociceptors. A new technique using a flat tip mechanical stimulator has been shown to elicit brain potentials following activation of Type I A mechano-heat (AMH) fibers. These pinprick-evoked potentials (PEP) have a morphology resembling those of heat-evoked potentials following activation of Type II AMH fibers, but with a shorter latency. Cool-evoked potentials can be used for recording the non-nociceptive pathways for cooling. At present, the use of cool-evoked potentials is still in the experimental state. Contact thermodes designed to generate steep heat ramps may be programmed differently to generate cool ramps from a baseline of 35◦C down to 32◦C or 30◦C. Small-fiber evoked potentials are valuable tools for assessment of small-fiber function in sensory neuropathy, central nervous system lesion, and for the diagnosis of neuropathic pain. Recent studies suggest that both CHEPs and pinprick-evoked potentials may also be convenient tools to assess sensitization of the nociceptive system.

Conclusions

In future studies, small-fiber evoked potentials may also be used in studies that aim to understand pain mechanisms including different neuropathic pain phenotypes, such as cold- or touch-evoked allodynia, and to identify predictors of response to pharmacological pain treatment.

Implications

Future studies are needed for some of the newly developed methods.

Age-related decline in thermal adaptation capacities: an evoked potentials study

Aging is associated with changes in thermosensitivity and decreases in the functionality of the autonomic thermoregulation. The underlying mechanisms are, however, not fully understood. Elderly subjects may undergo functional changes in the integration process of the thermal sensory system, especially in their thermal adaptation capacities. To verify this hypothesis, we compared thermal evoked responses in younger and older subjects exposed to thermoneutral (27 °C) and warm (30 °C) environments. In the warm environment, the amplitudes of thermal evoked potentials (EPs) were significantly lower in older than in younger subjects, whereas in the thermoneutral environment, the EP amplitudes were similar in both groups. These findings suggest that thermal adaptation capacities are reduced in elderly individuals, due to a dysfunction of C-fibers with aging, particularly expressed by lowered adaptation capacities to temperature variations.

Reliable EEG responses to the selective activation of C-fibre afferents using a temperature-controlled infrared laser stimulator in conjunction with an adaptive staircase algorithm

Brain responses to the activation of C-fibres are obtained only if the co-activation of Aδ-fibres is avoided. Methods to activate C-fibres selectively have been proposed, but are unreliable or difficult to implement. Here, we propose an approach combining a new laser stimulator to generate constant-temperature heat pulses with an adaptive paradigm to maintain stimulus temperature above the threshold of C-fibres but below that of Aδ-fibres, and examine whether this approach can be used to record reliable C-fibre laser-evoked brain potentials. Brief CO2 laser stimuli were delivered to the hand and foot dorsum of 10 healthy subjects. The stimuli were generated using a closed-loop control of laser power by an online monitoring of target skin temperature. The adaptive algorithm, using reaction times to distinguish between late detections indicating selective activation of unmyelinated C-fibres and early detections indicating co-activation of myelinated Aδ-fibres, allowed increasing the likelihood of selectively activating C-fibres. Reliable individual-level electroencephalogram (EEG) responses were identified, both in the time domain (hand: N2: 704 ± 179 ms, P2: 984 ± 149 ms; foot: N2: 1314 ± 171 ms, P2: 1716 ± 171 ms) and the time-frequency (TF) domain. Using a control dataset in which no stimuli were delivered, a Receiver Operating Characteristics analysis showed that the magnitude of the phase-locked EEG response corresponding to the N2-P2, objectively quantified in the TF domain, discriminated between absence vs presence of C-fibre responses with a high sensitivity (hand: 85%, foot: 80%) and specificity (hand: 90%, foot: 75%). This approach could thus be particularly useful for the diagnostic workup of small-fibre neuropathies and neuropathic pain.

Specific paucity of unmyelinated C-fibers in cutaneous peripheral nerves of the African naked-mole rat: comparative analysis using six species of Bathyergidae

In mammalian peripheral nerves, unmyelinated C-fibers usually outnumber myelinated A-fibers. By using transmission electron microscopy, we recently showed that the saphenous nerve of the naked mole-rat (Heterocephalus glaber) has a C-fiber deficit manifested as a substantially lower C:A-fiber ratio compared with other mammals. Here we determined the uniqueness of this C-fiber deficit by performing a quantitative anatomical analysis of several peripheral nerves in five further members of the Bathyergidae mole-rat family: silvery (Heliophobius argenteocinereus), giant (Fukomys mechowii), Damaraland (Fukomys damarensis), Mashona (Fukomys darlingi), and Natal (Cryptomys hottentotus natalensis) mole-rats. In the largely cutaneous saphenous and sural nerves, the naked mole-rat had the lowest C:A-fiber ratio (∼1.5:1 compared with ∼3:1), whereas, in nerves innervating both skin and muscle (common peroneal and tibial) or just muscle (lateral/medial gastrocnemius), this pattern was mostly absent. We asked whether lack of hair follicles alone accounts for the C-fiber paucity by using as a model a mouse that loses virtually all its hair as a consequence of conditional deletion of the β-catenin gene in the skin. These β-catenin loss-of function mice (β-cat LOF mice) displayed only a mild decrease in C:A-fiber ratio compared with wild-type mice (4.42 compared with 3.81). We suggest that the selective cutaneous C-fiber deficit in the cutaneous nerves of naked mole-rats is unlikely to be due primarily to lack of skin hair follicles. Possible mechanisms contributing to this unique peripheral nerve anatomy are discussed.

Endoneurial fibroblast-like cells

Endoneurial fibroblast-like cells (EFLCs) have been described for more than 60 years, but the embryology, functions, and pathology of these cells are not well defined. Several hypotheses of their origin have been proposed. A previous study suggesting that they were of neural crest origin is supported by our data in humans. This lineage might account for EFLCs having multiple biologic functions and involvement in pathological processes. Here, we review what is known about the origin; functions in collagen synthesis, phagocytosis, inflammatory responses, and immune surveillance; and the pathological alterations of EFLCs based on the literature and on our personal observations.

Biology of Schwann cells

The fundamental roles of Schwann cells during peripheral nerve formation and regeneration have been recognized for more than 100 years, but the cellular and molecular mechanisms that integrate Schwann cell and axonal functions continue to be elucidated. Derived from the embryonic neural crest, Schwann cells differentiate into myelinating cells or bundle multiple unmyelinated axons into Remak fibers. Axons dictate which differentiation path Schwann cells follow, and recent studies have established that axonal neuregulin1 signaling via ErbB2/B3 receptors on Schwann cells is essential for Schwann cell myelination. Extracellular matrix production and interactions mediated by specific integrin and dystroglycan complexes are also critical requisites for Schwann cell-axon interactions. Myelination entails expansion and specialization of the Schwann cell plasma membrane over millimeter distances. Many of the myelin-specific proteins have been identified, and transgenic manipulation of myelin genes have provided novel insights into myelin protein function, including maintenance of axonal integrity and survival. Cellular events that facilitate myelination, including microtubule-based protein and mRNA targeting, and actin based locomotion, have also begun to be understood. Arguably, the most remarkable facet of Schwann cell biology, however, is their vigorous response to axonal damage. Degradation of myelin, dedifferentiation, division, production of axonotrophic factors, and remyelination all underpin the substantial regenerative capacity of the Schwann cells and peripheral nerves. Many of these properties are not shared by CNS fibers, which are myelinated by oligodendrocytes. Dissecting the molecular mechanisms responsible for the complex biology of Schwann cells continues to have practical benefits in identifying novel therapeutic targets not only for Schwann cell-specific diseases but other disorders in which axons degenerate.

Peripheral nerve: from the microscopic functional unit of the axon to the biomechanically loaded macroscopic structure

Peripheral nerves are composed of motor and sensory axons, associated ensheathing Schwann cells, and organized layers of connective tissues that are in continuity with the tissues of the central nervous system. Nerve fiber anatomy facilitates conduction of electrical impulses to convey information over a distance, and the length of these polarized cells necessitates regulated axonal transport of organelles and structural proteins for normal cell function. Nerve connective tissues serve a protective function as the limb is subjected to the stresses of myriad limb positions and postures. Thus, the tissues are uniquely arranged to control the local nerve fiber environment and modulate physical stresses. In this brief review, we describe the microscopic anatomy and physiology of peripheral nerve and the biomechanical properties that enable nerve to withstand the physical stresses of everyday life.

Laser heat stimulation of tiny skin areas adds valuable information to quantitative sensory testing in postherpetic neuralgia

Patients suffering from postherpetic neuralgia often complain about hypo- or hypersensation in the affected dermatome. The loss of thermal sensitivity has been demonstrated by quantitative sensory testing as being associated with small-fiber (Aδ- and C-fiber) deafferentation. We aimed to compare laser stimulation (radiant heat) to thermode stimulation (contact heat) with regard to their sensitivity and specificity to detect thermal sensory deficits related to small-fiber dysfunction in postherpetic neuralgia. We contrasted detection rate of laser stimuli with 5 thermal parameters (thresholds of cold/warm detection, cold/heat pain, and sensory limen) of quantitative sensory testing. Sixteen patients diagnosed with unilateral postherpetic neuralgia and 16 age- and gender-matched healthy control subjects were tested. Quantitative sensory testing and laser stimulation of tiny skin areas were performed in the neuralgia-affected skin and in the contralateral homologue of the neuralgia-free body side. Across the 5 thermal parameters of thermode stimulation, only one parameter (warm detection threshold) revealed sensory abnormalities (thermal hypoesthesia to warm stimuli) in the neuralgia-affected skin area of patients but not in the contralateral area, as compared to the control group. In contrast, patients perceived significantly less laser stimuli both in the affected skin and in the contralateral skin compared to controls. Overall, laser stimulation proved more sensitive and specific in detecting thermal sensory abnormalities in the neuralgia-affected skin, as well as in the control skin, than any single thermal parameter of thermode stimulation. Thus, laser stimulation of tiny skin areas might be a useful diagnostic tool for small-fiber dysfunction.

Thermal detection thresholds of Aδ- and C-fibre afferents activated by brief CO2 laser pulses applied onto the human hairy skin

Brief high-power laser pulses applied onto the hairy skin of the distal end of a limb generate a double sensation related to the activation of Aδ- and C-fibres, referred to as first and second pain. However, neurophysiological and behavioural responses related to the activation of C-fibres can be studied reliably only if the concomitant activation of Aδ-fibres is avoided. Here, using a novel CO(2) laser stimulator able to deliver constant-temperature heat pulses through a feedback regulation of laser power by an online measurement of skin temperature at target site, combined with an adaptive staircase algorithm using reaction-time to distinguish between responses triggered by Aδ- and C-fibre input, we show that it is possible to estimate robustly and independently the thermal detection thresholds of Aδ-fibres (46.9±1.7°C) and C-fibres (39.8±1.7°C). Furthermore, we show that both thresholds are dependent on the skin temperature preceding and/or surrounding the test stimulus, indicating that the Aδ- and C-fibre afferents triggering the behavioural responses to brief laser pulses behave, at least partially, as detectors of a change in skin temperature rather than as pure level detectors. Most importantly, our results show that the difference in threshold between Aδ- and C-fibre afferents activated by brief laser pulses can be exploited to activate C-fibres selectively and reliably, provided that the rise in skin temperature generated by the laser stimulator is well-controlled. Our approach could constitute a tool to explore, in humans, the physiological and pathophysiological mechanisms involved in processing C- and Aδ-fibre input, respectively.

A multi-scale view of skin thermal pain: from nociception to pain sensation

All biological bodies live in a thermal environment, including the human body, where skin is the interface with a protecting function. When the temperature is out of the normal physiological range, skin fails to protect, and the pain sensation is evoked. Furthermore, in medicine, with advances in laser, microwave and similar technologies, various thermal therapeutic methods have been widely used to cure disease/injury involving skin tissue. However, the corresponding problem of pain relief has limited further application and development of these thermal treatments. Skin thermal pain is induced through both direct (i.e. an increase/decrease in temperature) and indirect (e.g. thermomechanical and thermochemical) ways, and is governed by complicated thermomechanical-chemical-neurophysiological responses. However, a complete understanding of the underlying mechanisms is still far from clear. In this article, starting from an engineering perspective, we aim to recast the biological behaviour of skin in engineering system parlance. Then, by coupling the concepts of engineering with established methods in neuroscience, we attempt to establish multi-scale modelling of skin thermal pain through ion channel to pain sensation. The model takes into account skin morphological plausibility, the thermomechanical response of skin tissue and the biophysical and neurological mechanisms of pain sensation.

GABA increases electrical excitability in a subset of human unmyelinated peripheral axons

Background

A proportion of small diameter primary sensory neurones innervating human skin are chemosensitive. They respond in a receptor dependent manner to chemical mediators of inflammation as well as naturally occurring algogens, thermogens and pruritogens. The neurotransmitter GABA is interesting in this respect because in animal models of neuropathic pain GABA pre-synaptically regulates nociceptive input to the spinal cord. However, the effect of GABA on human peripheral unmyelinated axons has not been established.

Methodology/Principal Findings

Electrical stimulation was used to assess the effect of GABA on the electrical excitability of unmyelinated axons in isolated fascicles of human sural nerve. GABA (0.1–100 µM) increased electrical excitability in a subset (ca. 40%) of C-fibres in human sural nerve fascicles suggesting that axonal GABA sensitivity is selectively restricted to a sub-population of human unmyelinated axons. The effects of GABA were mediated by GABA_A receptors, being mimicked by bath application of the GABA_A agonist muscimol (0.1–30 µM) while the GABA_B agonist baclofen (10–30 µM) was without effect. Increases in excitability produced by GABA (10–30 µM) were blocked by the GABA_A antagonists gabazine (10–20 µM), bicuculline (10–20 µM) and picrotoxin (10–20 µM).

Conclusions/Significance

Functional GABA_A receptors are present on a subset of unmyelinated primary afferents in humans and their activation depolarizes these axons, an effect likely due to an elevated intra-axonal chloride concentration. GABA_A receptor modulation may therefore regulate segmental and peripheral components of nociception.

Biology and pathology of nonmyelinating Schwann cells

The CNS contains relatively few unmyelinated nerve fibers, and thus benefits from the advantages that are conferred by myelination, including faster conduction velocities, lower energy consumption for impulse transmission, and greater stability of point-to-point connectivity. In the PNS many fibers or regions of fibers the Schwann do not form myelin. Examples include C fibers nociceptors, postganglionic sympathetic fibers, and the Schwann cells associated with motor nerve terminals at neuromuscular junctions. These examples retain a degree of plasticity and a capacity to sprout collaterally that is unusual in myelinated fibers. Nonmyelin-forming Schwann cells, including those associated with uninjured fibers, have the capacity to act as the "first responders" to injury or disease in their neighborhoods.

Nucleotide signaling and cutaneous mechanisms of pain transduction

Sensory neurons that innervate the skin provide critical information about physical contact between the organism and the environment, including information about potentially-damaging stimuli that give rise to the sensation of pain. These afferents also contribute to the maintenance of tissue homeostasis, inflammation and wound healing, while sensitization of sensory afferents after injury results in painful hypersensitivity and protective behavior. In contrast to the traditional view of primary afferent terminals as the sole site of sensory transduction, recent reports have lead to the intriguing idea that cells of the skin play an active role in the transduction of sensory stimuli. The search for molecules that transduce different types of sensory stimuli (mechanical, heat, chemical) at the axon terminal has yielded a wide range of potential effectors, many of which are expressed by keratinocytes as well as neurons. Emerging evidence underscores the importance of nucleotide signaling through P2X ionotropic and P2Y metabotropic receptors in pain processing, and implicates nucleotide signaling as a critical form of communication between cells of the skin, immune cells and sensory neurons. It is of great interest to determine whether pathological changes in these mechanisms contribute to chronic pain in human disease states such as complex regional pain syndrome (CRPS). This review discusses recent advances in our understanding of communication mechanisms between cells of the skin and sensory axons in the transduction of sensory input leading to pain.

Differential brain activation associated with laser-evoked burning and pricking pain: An event-related fMRI study

An important question remains as to how the brain differentially processes first (pricking) pain mediated by Adelta-nociceptors versus second (burning) pain mediated by C-nociceptors. In the present cross-over randomized, within-subjects controlled study, brain activity patterns were examined with event-related fMRI while pricking and burning pain were selectively evoked using a diode laser. Stimuli evoking equivalent pain intensities were delivered to the dorsum of the left foot. Different laser parameters were used to elicit pricking (60ms pulse duration) and burning (2.0s pulse duration) pain. Whole brain group analysis showed that several brain areas were commonly activated by pricking and burning pain, including bilateral thalamus, bilateral anterior insula, bilateral posterior parietal lobule, contralateral dorsolateral prefrontal cortex, ipsilateral cerebellum, and mid anterior cingulate cortex. These findings show that pricking and burning pain were associated with activity in many of the same nociceptive processing brain regions. This may be expected given that Adelta-and C-nociceptive signals converge to a great extent at the level of the dorsal horn. Other brain regions showed differential processing. Stronger activation in the pricking pain condition was found in the ipsilateral hippocampus, bilateral parahippocampal gyrus, bilateral fusiform gyrus, contralateral cerebellum and contralateral cuneus/parieto-occipital sulcus. Stronger activation in the burning pain condition was found in the ipsilateral dorsolateral prefrontal cortex. These differential activation patterns suggest preferential importance of Adelta-fiber signals versus C-fiber signals for these specific brain regions.

Brain activation upon selective stimulation of cutaneous C- and Adelta-fibers

Thermal and nociceptive cutaneous stimuli activate the brain via two types of nerve fibers, slightly myelinated Adelta-fibers with moderate conduction velocity and unmyelinated C-fibers with slow conduction velocity. Differences in central processing upon selective stimulation of these two fiber types in healthy human subjects still remain poorly understood. By means of event-related functional magnetic resonance imaging the present study investigated brain activation in response to stimulation of Adelta- and C-fibers in healthy subjects. We used the stimulation of tiny skin areas to perform a selective stimulation upon cutaneous C-fibers. Besides similar activation in several brain areas in response to both kinds of stimulation, we observed pronounced brain activation to selective C-fiber stimulation as compared to Adelta-fiber stimulation in the right frontal operculum and anterior insula. Based on a putative function of these structures we suggest that the C-fiber system might be engaged in homeostatic and interoceptive functions in a manner other than the Adelta-fiber system, producing a signal of greater emotional salience.