Sensation, mechanoreceptor, and nerve fiber function after nerve regeneration

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Peripheral nerve fibers have the capacity to regenerate after Wallerian degeneration. Nevertheless, clinically meaningful sensory recovery is often unsatisfactory after surgical repair of severed nerves, which may be attributed to: (1) limited outgrowth of fibers,1 (2) disturbed function of regenerated fibers or reinnervated end organs,2 (3) incorrect reinnervation of receptors,4 and (4) disturbed information processing in the central nervous system (CNS).6 Microneurography has shown partial recovery of response characteristics of different types of rapidly and slowly adapting mechanoreceptors, though there was a discrepancy regarding which type of receptors became reinnervated. Reinnervation of Pacinian corpuscles was absent in some studies,8 whereas others showed reinnervation of all types of receptors.4 Moreover, reinnervated Pacinian corpuscles in rodents showed changes in the relation between terminal axons and the end organ.10 In nonhuman primates, histological studies showed misplaced axons within reinnervated Pacinian corpuscles, which could be a factor regarding abnormal physiological responses to tactile stimuli.12 The relation between recovery of functional sensibility and characteristics of mechanoreceptor and sensory nerve fiber function in patients with peripheral nerve lesions is, however, uncertain. Previous studies have raised the possibility that that sensation could recover because of cortical plasticity, despite poor peripheral nerve regeneration.13
The aim of the current study was to ascertain the relationships between clinical recovery and nerve fiber and mechanoreceptor function after surgical repair of ulnar or median nerve lesions by a collagen nerve guide or direct suture at the distal forearm level. Clinical recovery was measured by quantitative sensory hand function tests.14 Regeneration of nerve fibers and their functional characteristics were assessed by recording of the compound sensory nerve action potential (SNAP) using near‐nerve needle electrodes. However, electrical stimulation of digital nerves provides limited information about target reinnervation, because receptor function and the distal nerve fiber segment are excluded in these studies. To specifically assess target reinnervation, we have utilized a tactile probe to stimulate fast‐adapting mechanoreceptors at the fingertip.15 This method allows recording of a SNAP and analysis of the receptor‐associated responses16 to provide insight into recovery of mechanoreceptor function after reinnervation.
Our studies showed differential recovery of sensory modalities and that SNAPs remained extremely dispersed and polyphasic. These findings suggest that sensory fiber and receptor dysfunction influence the recovery of clinical perception. Clinical and sensory electrophysiological recovery were indistinguishable between repair types, supporting that regenerating nerve fibers in humans can traverse a short gap and correctly reinnervate the distal nerve stump and relevant mechanoreceptors.
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