Disinhibition of Human Primary Somatosensory Cortex After Median Nerve Transection and Reinnervation

Abstract

Despite state-of-the-art surgical and postoperative treatment, median nerve transection causes lasting impaired hand function due to limitations in the nerve's reinnervation ability. The defective innervation and thus controllability of the affected hand can shape the brain's control of manual behaviors. Earlier studies of changes in the processing of tactile stimuli have focused mainly on stimulation of the reinnervated hand and lack sufficient control over the brain's use of the tactile input in perceptual terms. Here we used fMRI to measure brain activity (BOLD-signal) in 11 people with median nerve injury and healthy controls (N = 11) when performing demanding tactile tasks using the tip of either the index or little finger of either hand. For the nerve-injured group, the left median nerve had been traumatically transected in the distal forearm and surgically repaired on average 8 years before the study. The hand representation of their contralesional (right) primary somatosensory cortex (S1) showed greater activity compared to controls when the left reinnervated index finger was used, but also when the left-hand little finger and the fingers of the right hand innervated by uninjured nerves were used. We argue that the overall increase in activity reflects a general disinhibition of contralesional S1 consistent with an augmented functional reorganizational plasticity being an ongoing feature of chronic recovery from nerve injury. Also, the nerve-injured showed increased activity within three prefrontal cortical areas implicated in higher-level behavioral processing (dorsal anterior cingulate cortex, left ventrolateral prefrontal and right dorsolateral prefrontal cortex), suggesting that processes supporting decisionmaking and response-selection were computationally more demanding due to the compromised tactile sensibility.

The Phantom Satiation Hypothesis of Bariatric Surgery

“…Cortical plasticity has been studied in the context of injuries to somatosensory rather than viscerosensory nerves ( Mogilner et al, 1993 ; Nordmark and Johansson, 2020 ). Nonetheless, structural and functional changes are likely to occur after bariatric surgery in viscerosensory cortical areas involved in evoking satiation.…”

Carpal tunnel surgery dampens thalamocortical and normalizes corticocortical functional connectivity

“…Alternately, stronger rsFC could reflect cortical disinhibition, a compensatory mechanism to magnify weaker sensory input from a damaged nerve to maintain sensory perception in the hand, 22 as proposed to occur in patients whose median nerves were transected and subsequently repaired. 29 A rodent model of chronic nerve constriction found enhanced thalamic-S1 FC during the development of neuropathic pain, 73 and stronger thalamic-S1 rsFC compared with HCs has been reported in other chronic pain conditions 74–76 including upper extremity neuropathic pain, where stronger thalamic-S1 FC was associated with lower tactile acuity and weaker FC was linked with higher pain scores. 77 Conversely, diabetic-neuropathy patients show reduced thalamic-S1 FC, particularly those with mechanical hyperalgesia.…”

“… 17 , 18 In carpal tunnel syndrome, the S1 shows blurring of the digit representations of the affected hand 19–22 and abnormal responses to hand stimulation, 21 , 23 , 24 which can normalize after acupuncture 25 , 26 and surgical 24 , 27 treatment. Patients whose median nerve was transected and subsequently repaired exhibit abnormal responses to vibrotactile hand stimulation 28 and tactile tasks 29 in S1 30 and also in the secondary somatosensory, lateral parietal, premotor, inferior temporal, and prefrontal cortices. In addition to the S1, 31–34 it is well known that the thalamus 35–39 is impacted by chronic pain, 40 and thalamocortical dysrhythmia 41 is associated with chronic neuropathic pain.…”

Changes in primary somatosensory cortex following allogeneic hand transplantation or autogenic hand replantation

“…Humans also appear to show interhemispheric transfer of plasticity, such as in patients with median nerve injury (Chemnitz et al, 2015;Nordmark and Johansson, 2020), unilateral amputation (Bogdanov et al, 2012;Valyear et al, 2020), and hand transplantation (Frey et al, 2008), whose contralesional S1 becomes more responsive to ipsilateral tactile stimulation. These changes may be associated with gray matter changes that may arise from decreased manual repertoire and increased reliance on vision for control of manual dexterity (Nordmark et al, 2018).…”