Segregation by modality of myelinated and unmyelinated fibers in human sensory nerve fascicles

Polyethylene glycol fusion repair of severed rat sciatic nerves reestablishes axonal continuity and reorganizes sensory terminal fields in the spinal cord

JOURNAL/nrgr/04.03/01300535-202507000-00030/figure1/v/2024-09-09T124005Z/r/image-tiff Peripheral nerve injuries result in the rapid degeneration of distal nerve segments and immediate loss of motor and sensory functions; behavioral recovery is typically poor. We used a plasmalemmal fusogen, polyethylene glycol (PEG), to immediately fuse closely apposed open ends of severed proximal and distal axons in rat sciatic nerves. We have previously reported that sciatic nerve axons repaired by PEG-fusion do not undergo Wallerian degeneration, and PEG-fused animals exhibit rapid (within 2-6 weeks) and extensive locomotor recovery. Furthermore, our previous report showed that PEG-fusion of severed sciatic motor axons was non-specific, i.e., spinal motoneurons in PEG-fused animals were found to project to appropriate as well as inappropriate target muscles. In this study, we examined the consequences of PEG-fusion for sensory axons of the sciatic nerve. Young adult male and female rats (Sprague-Dawley) received either a unilateral single cut or ablation injury to the sciatic nerve and subsequent repair with or without (Negative Control) the application of PEG. Compound action potentials recorded immediately after PEG-fusion repair confirmed conduction across the injury site. The success of PEG-fusion was confirmed through Sciatic Functional Index testing with PEG-fused animals showing improvement in locomotor function beginning at 35 days postoperatively. At 2-42 days postoperatively, we anterogradely labeled sensory afferents from the dorsal aspect of the hindpaw following bilateral intradermal injection of wheat germ agglutinin conjugated horseradish peroxidase. PEG-fusion repair reestablished axonal continuity. Compared to unoperated animals, labeled sensory afferents ipsilateral to the injury in PEG-fused animals were found in the appropriate area of the dorsal horn, as well as inappropriate mediolateral and rostrocaudal areas. Unexpectedly, despite having intact peripheral nerves, similar reorganizations of labeled sensory afferents were also observed contralateral to the injury and repair. This central reorganization may contribute to the improved behavioral recovery seen after PEG-fusion repair, supporting the use of this novel repair methodology over currently available treatments.

Polyethylene Glycol-Mediated Axonal Fusion Promotes Early Sensory Recovery after Digital Nerve Injury: A Randomized Clinical Trial

BACKGROUND

Peripheral nerve repair is limited by Wallerian degeneration coupled with the slow and inconsistent rates of nerve regrowth. In more proximal injuries, delayed nerve regeneration can cause debilitating muscle atrophy. Topical application of polyethylene glycol (PEG) during neurorrhaphy facilitates the fusion of severed axonal membranes, immediately restoring action potentials across the coaptation site. In preclinical animal models, PEG fusion resulted in remarkable early functional recovery.

METHODS

This is the first randomized clinical trial comparing functional outcomes between PEG fusion and standard neurorrhaphy. Participants with digital nerve transections were followed up at 2 weeks, 1 month, and 3 months postoperatively. The primary outcome was assessed using the Medical Research Council Classification (MRCC) rating for sensory recovery at each time point. Semmes-Weinstein monofilaments and static 2-point discrimination determined MRCC ratings. Postoperative quality of life was measured using the Michigan Hand Outcomes Questionnaire.

RESULTS

Forty-eight transected digital nerves (25 control and 23 PEG) across 22 patients were analyzed. PEG-fused nerves demonstrated significantly higher MRCC scores at 2 weeks (OR, 16.95; 95% CI, 1.79 to 160.38; P = 0.008) and 1 month (OR, 13.40; 95% CI, 1.64 to 109.77; P = 0.009). Participants in the PEG cohort also had significantly higher average Michigan Hand Outcomes Questionnaire scores at 2 weeks (Hodge g , 1.28; 95% CI, 0.23 to 2.30; P = 0.0163) and 1 month (Hodge g , 1.02; 95% CI, 0.04 to 1.99; P = 0.049). No participants had adverse events related to the study drug.

CONCLUSION

PEG fusion promotes early sensory recovery and improved patient well-being following peripheral nerve repair of digital nerves.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, II.

Restoration of sensory information via bionic hands

Individuals who have lost the use of their hands because of amputation or spinal cord injury can use prosthetic hands to restore their independence. A dexterous prosthesis requires the acquisition of control signals that drive the movements of the robotic hand, and the transmission of sensory signals to convey information to the user about the consequences of these movements. In this Review, we describe non-invasive and invasive technologies for conveying artificial sensory feedback through bionic hands, and evaluate the technologies' long-term prospects.

Intraneural Topography of Rat Sciatic Axons: Implications for Polyethylene Glycol Fusion Peripheral Nerve Repair

Peripheral nerve injuries are the most common type of nerve trauma. We have been working with a novel repair technique using a plasmalemmal fusogen, polyethylene glycol (PEG), to re-fuse the membranes of severed axons. PEG-fusion repair allows for immediate re-innervation of distal targets, prevents axonal degeneration, and improves behavioral recovery. PEG-fusion of severed axons is non-specific, and we have previously reported that following injury and PEG-fusion misconnections between spinal motoneurons and their distal targets were present. Surprisingly, appropriately paired proximal and distal motor axons were observed in all PEG-fused animals. We hypothesized that a topographic organization of axons contributing to the sciatic nerve could explain the incidence of appropriate connections. We traced the course of specific axon populations contributing to the sciatic nerve in young adult male and female rats. Following intraneural injection of Fast Blue into the tibial branch, labeled axons were confined to a discrete location throughout the course of the nerve. Following intramuscular injection of cholera toxin-conjugated horseradish peroxidase into the anterior tibialis, labeled axons were confined to a smaller but still discrete location throughout the nerve. In both cases, the relative locations of labeled axons were consistent bilaterally within animals, as well as across animals and sexes. Thus, the relatively consistent location of specific axon populations could allow for realignment of appropriate populations of axons, and enhanced behavioral recovery seen in PEG-fused animals. Knowing the organization of axons within the sciatic nerve promotes accurate territory realignment during repair, therefore aiding in recovery outcomes.

Specializations of somatosensory innervation in the skin of humpback whales (Megaptera novaeangliae)

Cetacean behavior and life history imply a role for somatosensory detection of critical signals unique to their marine environment. As the sensory anatomy of cetacean glabrous skin has not been fully explored, skin biopsy samples of the flank skin of humpback whales were prepared for general histological and immunohistochemical (IHC) analyses of innervation in this study. Histology revealed an exceptionally thick epidermis interdigitated by numerous, closely spaced long, thin diameter penicillate dermal papillae (PDP). The dermis had a stratified organization including a deep neural plexus (DNP) stratum intermingled with small arteries that was the source of intermingled nerves and arterioles forming a more superficial subepidermal neural plexus (SNP) stratum. The patterns of nerves branching through the DNP and SNP that distribute extensive innervation to arteries and arterioles and to the upper dermis and PDP provide a dense innervation associated through the whole epidermis. Some NF-H+ fibers terminated at the base of the epidermis and as encapsulated endings in dermal papillae similar to Merkel innervation and encapsulated endings seen in terrestrial mammals. However, unlike in all mammalian species assessed to date, an unusual acellular gap was present between the perineural sheaths and the central core of axons in all the cutaneous nerves perhaps as mechanism to prevent high hydrostatic pressure from compressing and interfering with axonal conductance. Altogether the whale skin has an exceptionally dense low-threshold mechanosensory system innervation most likely adapted for sensing hydrodynamic stimuli, as well as nerves that can likely withstand high pressure experienced during deep dives.

Biomimetic encoding model for restoring touch in bionic hands through a nerve interface

OBJECTIVE

Hand function can be restored in upper-limb amputees by equipping them with anthropomorphic prostheses controlled with signals from residual muscles. The dexterity of these bionic hands is severely limited in large part by the absence of tactile feedback about interactions with objects. We propose that, to the extent that artificial touch mimics its natural counterpart, these sensory signals will be more easily integrated into the motor plan for object manipulation.

APPROACH

We describe an approach to convey tactile feedback through electrical stimulation of the residual somatosensory nerves that mimics the aggregate activity of tactile fibers that would be produced in the nerve of a native hand during object interactions. Specifically, we build a parsimonious model that maps the stimulus-described as time-varying indentation depth, indentation rate, and acceleration-into continuous estimates of the time-varying population firing rate and of the size of the recruited afferent population.

MAIN RESULTS

The simple model can reconstruct aggregate afferent responses to a wide range of stimuli, including those experienced during activities of daily living.

SIGNIFICANCE

We discuss how the proposed model can be implemented with a peripheral nerve interface and anticipate it will lead to improved dexterity for prosthetic hands.

Topographical distribution of motor fascicles in the sciatic-tibial nerve of the rat

Knowledge of the intraneural topography of peripheral nerves may help to improve nerve repair after injuries and the selectivity of neural interfaces. We studied the fascicular pattern of motor fibers of the rat sciatic-tibial nerve. We carried out an anatomical dissection of the muscular tributaries of the tibial nerve in the leg. Immunohistochemistry against choline acetyltransferase was used to identify motor axons. Retrograde tracing allowed localization of the muscular fascicles at proximal levels of the sciatic trunk. The distribution of motor fibers in transverse section of the tibial nerve is not homogeneous; two clusters were identified, each one containing fibers of functionally related muscles. Retrograde tracing allowed for the identification of motor fascicles, each one well localized along the sciatic nerve. In the rat there is a somatotopic organization of the sciatic nerve, with muscular fascicles maintaining the same relative position along the entire nerve.

Peripheral nerve fascicles: Anatomy and clinical relevance

Within a peripheral nerve, the individual nerve fibers are grouped together in fascicles. Whether there is somatotopic organization within these fascicles has long been of interest, the subject of many investigations, and somewhat controversial. Evidence from diverse sources now points to important somatotopic clustering of nerve fibers within most of the length of the nerve. Information is lacking regarding proximal segments, particularly the plexus and spinal nerve root levels. As a result of this somatotopic arrangement, partial focal nerve lesions can produce restricted clinical deficits that defy the classic rules of localization. Examples of such restricted nerve lesions are provided in this review. Recognition of fascicle somatotopy is also important in the surgical approach to disorders of peripheral nerves.

Population behaviour of human cutaneous mechanoreceptive units

Groups of fibres rather than single afferents may be responsible for encoding various intensity aspects of tactile skin stimulation. Reconstruction of population responses of primary afferent fibres to skin displacement provided data in support of this idea, but evidence from direct recordings that demonstrated multifibre activity deriving from groups of single units firing in response to defined skin stimuli were not reported. Procedures are summarised which allow identification and sampling of such recordings in man. For SAII units it was demonstrated how different directions of skin stretch engaging a particular cutaneous area produced different responses of a unit population innervating that site. In response to localised vibratory stimuli synchronous discharges of several co-activated PC afferents were recorded at each vibratory cycle, which is a previously not described pattern of peripheral PC encoding. Population projection of activity within modality segregated clusters of afferents supplying the same skin area might serve as basic projection units and constitute the peripheral counterparts to sensory columns, believed to be the central cognitive correlates, in the cortex. Thus, it is tempting to postulate fibre population projection as a peripheral basis for somatosensory processing in man.

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.

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

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

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.

Sympathetic skin responses of the hand in normal subjects: shorter latency at distal phalanx

We studied the conduction of the sympathetic skin response (SSR) in the hands of 35 normal subjects by simultaneous recording at five sites. The mean latency of the SSR in all subjects increased from the wrist (W) to the middle phalanx (M), but the SSR latency at the distal phalanx (D) was shorter than that at the middle phalanx. The mean conduction time and conduction velocity from W to M was 197.5 ms and 0.87 m/s, respectively, and that from W to D was 48.8 ms and 2.34 m/s, respectively. For evaluation of the cause of the shorter latency at D, digital nerve blocking was performed in two subjects. The blocking of the volar digital nerves at the proximal phalanx abolished SSR at M and D. It was postulated that the initiation or conduction of sudomotor nerve impulse to the distal phalanx would be facilitated compared with those to the other proximal sites in the hand. The SSR conduction time between W and M may be a means of detecting alteration of sympathetic sudomotor nerve activity.

Peripheral afferents with common function cluster in the median nerve and somatotopically innervate the human palm

Concentric needle electrodes with a central core diameter of 20-30 microm were used to explore median nerve fascicles in man. Such electrodes can simultaneously monitor subtle electrophysiological and topographical features even within parts of a fascicle. Single-unit recordings from myelinated fibres were more easily obtained at some intrafascicular sites than others. Typically, groups of identified myelinated fibres in these regions, possibly corresponding to a cluster of Ranvier nodes, tended to be fibres responding to stimuli of the same modality. These afferents innervated the glabrous skin of the human hand and fingers in a somatotopic manner. In particular, the somatotopy even seemed to be present at the receptor level in the skin. This novel aspect of peripheral nerve organisation is probably of fundamental importance for the interplay between peripheral and central processes involved in somatosensation both under normal conditions and in disease. Some clinical implications of the findings are discussed.

Multiple action potential waveforms of single units in man as signs of variability in conductivity of their myelinated fibres

Percutaneous microneurography was performed with concentric needle electrodes to record neural activity from myelinated fibres in human peripheral nerves. Template matching techniques were used together with interspike interval analysis and studies on functional class, receptive field characteristics, conduction velocities and other single fibre properties to classify single units. Sometimes the same fibres exhibited different action potentials at the same time. The potentials had some common features, but differed either in their waveform types or only in duration. There was a correlation between the occurrence of the different potential shapes and firing frequency of the studied unit. The outcome of the studies suggested that there was a common denominator which could explain the observations. Most likely, momentary fluctuations in excitability of the myelinated fibres occurring during the relative refractory period or the supernormal period were responsible for the variations in complexity of the studied units due to a partial block of fibre propagation probably caused by the recording electrode. Thus, action potentials deriving from the same axon may not always have the same shapes. Methods for unit classification, such as template matching, are discussed in the light of our findings.

Electrophysiological evidence of Ranvier node clustering in human sensory nerve fascicles

Thin concentric needle electrodes were used to explore intact median nerve fascicles in human subjects. In particular, the presence of single units, probably recorded from nodes of Ranvier, was studied in different parts of a fascicle. Single-unit activity in myelinated fibers was rarely found at numerous sites. In many other intrafascicular areas, a substantial number of single units could be discriminated in the same or nearby recording sites with the same technique. To account for the neurophysiological results, stochastic models and statistical tests were developed to test various hypotheses concerning intrafascicular nerve fiber arrangements. The acquired data suggested both an intrafascicular modality grouping of nerve fibers and a simultaneous clustering of the Ranvier nodes of these fibers within very restricted areas of a fascicle. It was further concluded that the yield when searching for units in different types of nerve preparations may depend upon the ultrastructure of the explored nerve segments.