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

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.

Frequency Shapes the Quality of Tactile Percepts Evoked through Electrical Stimulation of the Nerves

Electrical stimulation of the peripheral nerves of human participants provides a unique opportunity to study the neural determinants of perceptual quality using a causal manipulation. A major challenge in the study of neural coding of touch has been to isolate the role of spike timing-at the scale of milliseconds or tens of milliseconds-in shaping the sensory experience. In the present study, we address this question by systematically varying the pulse frequency (PF) of electrical stimulation pulse trains delivered to the peripheral nerves of seven participants with upper and lower extremity limb loss via chronically implanted neural interfaces. We find that increases in PF lead to systematic increases in perceived frequency, up to ∼50 Hz, at which point further changes in PF have little to no impact on sensory quality. Above this transition frequency, ratings of perceived frequency level off, the ability to discriminate changes in PF is abolished, and verbal descriptors selected to characterize the sensation change abruptly. We conclude that sensation quality is shaped by temporal patterns of neural activation, even if these patterns are imposed on a fixed neural population, but this temporal patterning can only be resolved up to ∼50 Hz. These findings highlight the importance of spike timing in shaping the quality of a sensation and will contribute to the development of encoding strategies for conveying touch feedback through bionic hands and feet.SIGNIFICANCE STATEMENT A major challenge in the study of neural coding of touch has been to understand how temporal patterns in neuronal responses shape the sensory experience. We address this question by varying the pulse frequency (PF) of electrical pulse trains delivered through implanted nerve interfaces in seven amputees. We concomitantly vary pulse width to separate the effect of changing PF on sensory quality from its effect on perceived magnitude. We find that increases in PF lead to increases in perceived frequency, a qualitative dimension, up to ∼50 Hz, beyond which changes in PF have little impact on quality. We conclude that temporal patterning in the neuronal response can shape quality and discuss the implications for restoring touch via neural interfaces.

Sensory feedback for limb prostheses in amputees

Commercial prosthetic devices currently do not provide natural sensory information on the interaction with objects or movements. The subsequent disadvantages include unphysiological walking with a prosthetic leg and difficulty in controlling the force exerted with a prosthetic hand, thus creating health issues. Restoring natural sensory feedback from the prosthesis to amputees is an unmet clinical need. An optimal device should be able to elicit natural sensations of touch or proprioception, by delivering the complex signals to the nervous system that would be produced by skin, muscles and joints receptors. This Review covers the various neurotechnological approaches that have been proposed for the development of the optimal sensory feedback restoration device for arm and leg amputees.

Intrafascicular stimulation of monkey arm nerves evokes coordinated grasp and sensory responses

High-count microelectrode arrays implanted in peripheral nerves could restore motor function after spinal cord injury or sensory function after limb loss. In this study, we implanted Utah Slanted Electrode Arrays (USEAs) intrafascicularly at the elbow or shoulder in arm nerves of rhesus monkeys (n = 4) under isoflurane anesthesia. Input-output curves indicated that pulse-width-modulated single-electrode stimulation in each arm nerve could recruit single muscles with little or no recruitment of other muscles. Stimulus trains evoked specific, natural, hand movements, which could be combined via multielectrode stimulation to elicit coordinated power or pinch grasp. Stimulation also elicited short-latency evoked potentials (EPs) in primary somatosensory cortex, which might be used to provide sensory feedback from a prosthetic limb. These results demonstrate a high-resolution, high-channel-count interface to the peripheral nervous system for restoring hand function after neural injury or disruption or for examining nerve structure.

Long term assessment of axonal regeneration through polyimide regenerative electrodes to interface the peripheral nerve

Polyimide sieve electrodes were implanted between the severed ends of the sciatic nerve in rats. The degree of axonal regeneration through the electrode was examined by physiological and histological methods from 2 to 12 months postimplantation. Regeneration was successful in the 30 animals implanted. Functional reinnervation of hindlimb targets progressed to reach maximal levels at 6 months. Comparatively, the reinnervation of distal plantar muscles was lower than that of proximal muscles and of digital nerves. The number of regenerated myelinated fibers increased from 2 to 6 months, when it was similar to control values. The majority of myelinated fibers crossing the via holes and regenerated through the distal nerve had a normal appearance. However, in a few cases decline of target reinnervation and loss of regenerated nerve fibers was found from 6 to 12 months postimplantation. Motor axons labeled by ChAT immunoreactivity regenerated scattered within minifascicles, although they were found at higher density at the periphery of the regenerated nerve. The number of ChAT-positive axons was markedly lower distally than proximally to the sieve electrode.

Management of infant brachial plexus injuries

Management of brachial plexus injuries is geared toward normalization of limb function, primarily through optimization of nerve regeneration and mechanical increase in elbow flexion and shoulder stabilization. Changes in the skeletal muscles and the osteous structures of the upper extremity are ongoing throughout the course of treatment, mandating continual assessment and aggressive rehabilitation. In patients who present too late for microsurgical intervention, irreversible changes take place in skeletal muscles, highlighting the importance of early referral. However, secondary procedures have been shown to be beneficial in older patients and in those whose primary procedures failed. Further advances in bionics and stem cell therapy may help replace the dynamic functional deficits of obstetric brachial plexus palsy.

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.

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

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.

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.