Immediate and simultaneous sensory reorganization at cortical and subcortical levels of the somatosensory system

Aberrant cortical activity, functional connectivity, and neural assembly architecture after photothrombotic stroke in mice

Despite substantial progress in mapping the trajectory of network plasticity resulting from focal ischemic stroke, the extent and nature of changes in neuronal excitability and activity within the peri-infarct cortex of mice remains poorly defined. Most of the available data have been acquired from anesthetized animals, acute tissue slices, or infer changes in excitability from immunoassays on extracted tissue, and thus may not reflect cortical activity dynamics in the intact cortex of an awake animal. Here, in vivo two-photon calcium imaging in awake, behaving mice was used to longitudinally track cortical activity, network functional connectivity, and neural assembly architecture for 2 months following photothrombotic stroke targeting the forelimb somatosensory cortex. Sensorimotor recovery was tracked over the weeks following stroke, allowing us to relate network changes to behavior. Our data revealed spatially restricted but long-lasting alterations in somatosensory neural network function and connectivity. Specifically, we demonstrate significant and long-lasting disruptions in neural assembly architecture concurrent with a deficit in functional connectivity between individual neurons. Reductions in neuronal spiking in peri-infarct cortex were transient but predictive of impairment in skilled locomotion measured in the tapered beam task. Notably, altered neural networks were highly localized, with assembly architecture and neural connectivity relatively unaltered a short distance from the peri-infarct cortex, even in regions within 'remapped' forelimb functional representations identified using mesoscale imaging with anaesthetized preparations 8 weeks after stroke. Thus, using longitudinal two-photon microscopy in awake animals, these data show a complex spatiotemporal relationship between peri-infarct neuronal network function and behavioral recovery. Moreover, the data highlight an apparent disconnect between dramatic functional remapping identified using strong sensory stimulation in anaesthetized mice compared to more subtle and spatially restricted changes in individual neuron and local network function in awake mice during stroke recovery.

Lower jaw-to-forepaw rapid and delayed reorganization in the rat forepaw barrel subfield in primary somatosensory cortex

We used the forepaw barrel subfield (FBS), that normally receives input from the forepaw skin surface, in rat primary somatosensory cortex as a model system to study rapid and delayed lower jaw-to-forepaw cortical reorganization. Single and multi-unit recording from FBS neurons was used to examine the FBS for the presence of "new" lower jaw input following deafferentations that include forelimb amputation, brachial plexus nerve cut, and brachial plexus anesthesia. The major findings are as follows: (1) immediately following forelimb deafferentations, new input from the lower jaw becomes expressed in the anterior FBS; (2) 7-27 weeks after forelimb amputation, new input from the lower jaw is expressed in both anterior and posterior FBS; (3) evoked response latencies recorded in the deafferented FBS following electrical stimulation of the lower jaw skin surface are significantly longer in both rapid and delayed deafferents compared to control latencies for input from the forepaw to reach the FBS or for input from lower jaw to reach the LJBSF; (4) the longer latencies suggest that an additional relay site is imposed along the somatosensory pathway for lower jaw input to access the deafferented FBS. We conclude that different sources of input and different mechanisms underlie rapid and delayed reorganization in the FBS and suggest that these findings are relevant, as an initial step, for developing a rodent animal model to investigate phantom limb phenomena.

Artificial gravity during a spaceflight analog alters brain sensory connectivity

Spaceflight has numerous untoward effects on human physiology. Various countermeasures are under investigation including artificial gravity (AG). Here, we investigated whether AG alters resting-state brain functional connectivity changes during head-down tilt bed rest (HDBR), a spaceflight analog. Participants underwent 60 days of HDBR. Two groups received daily AG administered either continuously (cAG) or intermittently (iAG). A control group received no AG. We assessed resting-state functional connectivity before, during, and after HDBR. We also measured balance and mobility changes from pre- to post-HDBR. We examined how functional connectivity changes throughout HDBR and whether AG is associated with differential effects. We found differential connectivity changes by group between posterior parietal cortex and multiple somatosensory regions. The control group exhibited increased functional connectivity between these regions throughout HDBR whereas the cAG group showed decreased functional connectivity. This finding suggests that AG alters somatosensory reweighting during HDBR. We also observed brain-behavioral correlations that differed significantly by group. Control group participants who showed increased connectivity between the putamen and somatosensory cortex exhibited greater mobility declines post-HDBR. For the cAG group, increased connectivity between these regions was associated with little to no mobility declines post-HDBR. This suggests that when somatosensory stimulation is provided via AG, functional connectivity increases between the putamen and somatosensory cortex are compensatory in nature, resulting in reduced mobility declines. Given these findings, AG may be an effective countermeasure for the reduced somatosensory stimulation that occurs in both microgravity and HDBR.

Adult neuroplasticity employs developmental mechanisms

Although neural plasticity is now widely studied, there was a time when the idea of adult plasticity was antithetical to the mainstream. The essential stumbling block arose from the seminal experiments of Hubel and Wiesel who presented convincing evidence that there existed a critical period for plasticity during development after which the brain lost its ability to change in accordance to shifts in sensory input. Despite the zeitgeist that mature brain is relatively immutable to change, there were a number of examples of adult neural plasticity emerging in the scientific literature. Interestingly, some of the earliest of these studies involved visual plasticity in the adult cat. Even earlier, there were reports of what appeared to be functional reorganization in adult rat somatosensory thalamus after dorsal column lesions, a finding that was confirmed and extended with additional experimentation. To demonstrate that these findings reflected more than a response to central injury, and to gain greater control of the extent of the sensory loss, peripheral nerve injuries were used that eliminated ascending sensory information while leaving central pathways intact. Merzenich, Kaas, and colleagues used peripheral nerve transections to reveal unambiguous reorganization in primate somatosensory cortex. Moreover, these same researchers showed that this plasticity proceeded in no less than two stages, one immediate, and one more protracted. These findings were confirmed and extended to more expansive cortical deprivations, and further extended to the thalamus and brainstem. There then began a series of experiments to reveal the physiological, morphological and neurochemical mechanisms that permitted this plasticity. Ultimately, Mowery and colleagues conducted a series of experiments that carefully tracked the levels of expression of several subunits of glutamate (AMPA and NMDA) and GABA (GABAA and GABAB) receptor complexes in primate somatosensory cortex at several time points after peripheral nerve injury. These receptor subunit mapping experiments revealed that membrane expression levels came to reflect those seen in early phases of critical period development. This suggested that under conditions of prolonged sensory deprivation the adult cells were returning to critical period like plastic states, i.e., developmental recapitulation. Here we outline the heuristics that drive this phenomenon.

Malleability of the cortical hand map following a finger nerve block

Electrophysiological studies in monkeys show that finger amputation triggers local remapping within the deprived primary somatosensory cortex (S1). Human neuroimaging research, however, shows persistent S1 representation of the missing hand's fingers, even decades after amputation. Here, we explore whether this apparent contradiction stems from underestimating the distributed peripheral and central representation of fingers in the hand map. Using pharmacological single-finger nerve block and 7-tesla neuroimaging, we first replicated previous accounts (electrophysiological and other) of local S1 remapping. Local blocking also triggered activity changes to nonblocked fingers across the entire hand area. Using methods exploiting interfinger representational overlap, however, we also show that the blocked finger representation remained persistent despite input loss. Computational modeling suggests that both local stability and global reorganization are driven by distributed processing underlying the topographic map, combined with homeostatic mechanisms. Our findings reveal complex interfinger representational features that play a key role in brain (re)organization, beyond (re)mapping.

General principles of brain electromagnetic rhythmic oscillations and implications for neuroplasticity

Neuro-plasticity describes the ability of the brain in achieving novel functions, either by transforming its internal connectivity, or by changing the elements of which it is made, meaning that, only those changes, that affect both structural and functional aspects of the system, can be defined as "plastic." The concept of plasticity can be applied to molecular as well as to environmental events that can be recognized as the basic mechanism by which our brain reacts to the internal and external stimuli. When considering brain plasticity within a clinical context-that is the process linked with changes of brain functions following a lesion- the term "reorganization" is somewhat synonymous, referring to the specific types of structural/functional modifications observed as axonal sprouting, long-term synaptic potentiation/inhibition or to the plasticity related genomic responses. Furthermore, brain rewires during maturation, and aging thus maintaining a remarkable learning capacity, allowing it to acquire a wide range of skills, from motor actions to complex abstract reasoning, in a lifelong expression. In this review, the contribution on the "neuroplasticity" topic coming from advanced analysis of EEG rhythms is put forward.

fNIRS brain measures of ongoing nociception during surgical incisions under anesthesia

Significance: Functional near-infrared spectroscopy (fNIRS) has evaluated pain in awake and anesthetized states. Aim: We evaluated fNIRS signals under general anesthesia in patients undergoing knee surgery for anterior cruciate ligament repair. Approach: Patients were split into groups: those with regional nerve block (NB) and those without (non-NB). Continuous fNIRS measures came from three regions: the primary somatosensory cortex (S1), known to be involved in evaluation of nociception, the lateral prefrontal cortex (BA9), and the polar frontal cortex (BA10), both involved in higher cortical functions (such as cognition and emotion). Results: Our results show three significant differences in fNIRS signals to incision procedures between groups: (1) NB compared with non-NB was associated with a greater net positive hemodynamic response to pain procedures in S1; (2) dynamic correlation between the prefrontal cortex (PreFC) and S1 within 1 min of painful procedures are anticorrelated in NB while positively correlated in non-NB; and (3) hemodynamic measures of activation were similar at two separate time points during surgery (i.e., first and last incisions) in PreFC and S1 but showed significant differences in their overlap. Comparing pain levels immediately after surgery and during discharge from postoperative care revealed no significant differences in the pain levels between NB and non-NB. Conclusion: Our data suggest multiple pain events that occur during surgery using devised algorithms could potentially give a measure of "pain load." This may allow for evaluation of central sensitization (i.e., a heightened state of the nervous system where noxious and non-noxious stimuli is perceived as painful) to postoperative pain levels and the resulting analgesic consumption. This evaluation could potentially predict postsurgical chronic neuropathic pain.

Microgravity effects on the human brain and behavior: Dysfunction and adaptive plasticity

Emerging plans for travel to Mars and other deep space destinations make it critical for us to understand how spaceflight affects the human brain and behavior. Research over the past decade has demonstrated two co-occurring patterns of spaceflight effects on the brain and behavior: dysfunction and adaptive plasticity. Evidence indicates the spaceflight environment induces adverse effects on the brain, including intracranial fluid shifts, gray matter changes, and white matter declines. Past work also suggests that the spaceflight environment induces adaptive neural effects such as sensory reweighting and neural compensation. Here, we introduce a new conceptual framework to synthesize spaceflight effects on the brain, Spaceflight Perturbation Adaptation Coupled with Dysfunction (SPACeD). We review the literature implicating neurobehavioral dysfunction and adaptation in response to spaceflight and microgravity analogues, and we consider pre-, during-, and post-flight factors that may interact with these processes. We draw several instructive parallels with the aging literature which also suggests co-occurring neurobehavioral dysfunction and adaptive processes. We close with recommendations for future spaceflight research, including: 1) increased efforts to distinguish between dysfunctional versus adaptive effects by testing brain-behavioral correlations, and 2) greater focus on tracking recovery time courses.

Congenitally decorticate children's potential and rights

This article is the first indepth ethical analysis of empirical studies that support the claim that children born without major parts of their cerebral cortex are capable of conscious experiences and have a rudimentary capacity for agency. Congenitally decorticate children have commonly been classified as persistently vegetative, with serious consequences for their well-being and opportunities to flourish. The paper begins with an explication of the rights-based normative framework of the argument, including conceptual analysis of the terms 'agency', 'potentiality for agency' and 'gradual approach of agency'. It critically examines Alan Gewirth's account of the criteria for being a rights bearer and principles for settling rights conflicts between agents and potential agents. It then applies the rights-based normative framework to the ethical challenges associated with care for congenitally decorticate children. It argues that recent empirical studies support the claim that the concepts 'potential for agency' and 'capacity for rudimentary agency' apply to children who are born without major parts of their cerebral cortex. The article finally discusses important medical ethical implications of these results. It specifically focuses on congenitally decorticate children's preparatory rights to a stimulating intellectual and social environment.

Remapping in Cerebral and Cerebellar Cortices Is Not Restricted by Somatotopy

A fundamental organizing principle in the somatosensory and motor systems is somatotopy, where specific body parts are represented separately and adjacently to other body parts, resulting in a body map. Different terminals of the sensorimotor network show varied somatotopic layouts, in which the relative position, distance, and overlap between body-part representations differ. Since somatotopy is best characterized in the primary somatosensory (S1) and motor (M1) cortices, these terminals have been the main focus of research on somatotopic remapping following loss of sensory input (e.g., arm amputation). Cortical remapping is generally considered to be driven by the layout of the underlying somatotopy, such that neighboring body-part representations tend to activate the deprived brain region. Here, we challenge the assumption that somatotopic layout restricts remapping, by comparing patterns of remapping in humans born without one hand (hereafter, one-handers, n = 26) across multiple terminals of the sensorimotor pathway. We first report that, in the cerebellum of one-handers, the deprived hand region represents multiple body parts. Importantly, the native representations of some of these body parts do not neighbor the deprived hand region. We further replicate our previous findings, showing a similar pattern of remapping in the deprived hand region of the cerebral cortex in one-handers. Finally, we report preliminary results of a similar remapping pattern in the putamen of one-handers. Since these three sensorimotor terminals (cerebellum, cerebrum, putamen) contain different somatotopic layouts, the parallel remapping they undergo demonstrates that the mere spatial layout of body-part representations may not exclusively dictate remapping in the sensorimotor systems.

SIGNIFICANCE STATEMENT When a hand is missing, the brain region that typically processes information from that hand may instead process information from other body parts, a phenomenon termed remapping. It is commonly thought that only body parts whose information is processed in regions neighboring the hand region could “take up” the resources of this now deprived region. Here we demonstrate that information from multiple body parts is processed in the hand regions of both the cerebral cortex and cerebellum. The native brain regions of these body parts have varying levels of overlap with the hand regions of the cerebral cortex and cerebellum, and do not necessarily neighbor the hand regions. We therefore propose that proximity between brain regions does not limit brain remapping.

Blocking tactile input to one finger using anaesthetic enhances touch perception and learning in other fingers

Brain plasticity is a key mechanism for learning and recovery. A striking example of plasticity in the adult brain occurs following input loss, for example, following amputation, whereby the deprived zone is “invaded” by new representations. Although it has long been assumed that such reorganization leads to functional benefits for the invading representation, the behavioral evidence is controversial. Here, we investigate whether a temporary period of somatosensory input loss to one finger, induced by anesthetic block, is sufficient to cause improvements in touch perception (“direct” effects of deafferentation). Further, we determine whether this deprivation can improve touch perception by enhancing sensory learning processes, for example, by training (“interactive” effects). Importantly, we explore whether direct and interactive effects of deprivation are dissociable by directly comparing their effects on touch perception. Using psychophysical thresholds, we found brief deprivation alone caused improvements in tactile perception of a finger adjacent to the blocked finger but not to non-neighboring fingers. Two additional groups underwent minimal tactile training to one finger either during anesthetic block of the neighboring finger or a sham block with saline. Deprivation significantly enhanced the effects of tactile perceptual training, causing greater learning transfer compared with sham block. That is, following deafferentation and training, learning gains were seen in fingers normally outside the boundaries of topographic transfer of tactile perceptual learning. Our results demonstrate that sensory deprivation can improve perceptual abilities, both directly and interactively, when combined with sensory learning. This dissociation provides novel opportunities for future clinical interventions to improve sensation.

Why Does the Cortex Reorganize after Sensory Loss?

A growing body of evidence demonstrates that the brain can reorganize dramatically following sensory loss. Although the existence of such neuroplastic crossmodal changes is not in doubt, the functional significance of these changes remains unclear. The dominant belief is that reorganization is compensatory. However, results thus far do not unequivocally indicate that sensory deprivation results in markedly enhanced abilities in other senses. Here, we consider alternative reasons besides sensory compensation that might drive the brain to reorganize after sensory loss. One such possibility is that the cortex reorganizes not to confer functional benefits, but to avoid undesirable physiological consequences of sensory deafferentation. Empirical assessment of the validity of this and other possibilities defines a rich program for future research.

Quantitative sensory testing after macroreplantation: evidence for a specific somatosensory profile

A comprehensive functional recovery is one of the criteria for successful replantation of an amputated limb. Functionality of a replanted limb is strongly dependent on its regained sensibility. In previous studies concerning the sensibility of replanted limbs, only a few somatosensory submodalities were examined in small samples. The purpose of this study is to provide a full pattern of somatosensory symptoms after replantation. Quantitative sensory testing was performed according to a standardized protocol in a sample of 15 patients who underwent replantation of their upper limb proximal to the radiocarpal joint (macroreplantation). Results indicate that most of these patients showed a specific somatosensory profile characterized by thermal and mechanical hypoesthesia and hyperalgesia in response to pressure pain, whereas no single case of hyperalgesia to heat pain occurred. This distinct profile of impaired somatosensation shares some features of the somatosensory profile of neuropathic pain syndromes. Patients' limbs that were replanted many years before the present quantitative sensory testing showed more sensory deficits than patients with more recent replantations. This knowledge might be helpful in the development of more specific and more successful rehabilitation programs with replanted patients and improves the behavioral function of the replanted limb.

Passive cycling in neurorehabilitation after spinal cord injury: A review

CONTEXT/OBJECTIVE

Passive cycling (PC) may represent a potential alternative neurorehabilitation program for patients who are too weak or medically unstable to repeatedly practice active movements. We review here the most important animal and human studies addressing PC after spinal cord injury (SCI).

METHODS

A MEDLINE search was performed using following terms: "passive", "cycling", "pedaling", "pedalling","spinal cord injury".

RESULTS

Experimental studies revealed that PC modulated spinal reflex and reduced spasticity. PC also reduced autonomic dysreflexia and elicited cardio-protective effects. Increased levels of mRNA for brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor and neurotrophin-4 were found. In contrast, human studies failed to show an effect of PC on spasticity reduction and did not support its application for prevention of cardiovascular disease-related secondary complications.

CONCLUSION

Available evidence to support the use of PC as standard treatment in patients with SCI is still rather limited. Since it is conceivable that PC motion could elicit sensory inputs to activate cortical structures and induce cortical plasticity changes leading to improved lower limb motor performance, further carefully designed prospective studies in subjects with SCI are needed.

Inhibitory masking controls the threshold sensitivity of retinal ganglion cells

KEY POINTS

Retinal ganglion cells (RGCs) in dark-adapted retinas show a range of threshold sensitivities spanning ∼3 log units of illuminance. Here, we show that the different threshold sensitivities of RGCs reflect an inhibitory mechanism that masks inputs from certain rod pathways. The masking inhibition is subserved by GABA_C receptors, probably on bipolar cell axon terminals. The GABAergic masking inhibition appears independent of dopaminergic circuitry that has been shown also to affect RGC sensitivity. The results indicate a novel mechanism whereby inhibition controls the sensitivity of different cohorts of RGCs. This can limit and thereby ensure that appropriate signals are carried centrally in scotopic conditions when sensitivity rather than acuity is crucial.

ABSTRACT

The responses of rod photoreceptors, which subserve dim light vision, are carried through the retina by three independent pathways. These pathways carry signals with largely different sensitivities. Retinal ganglion cells (RGCs), the output neurons of the retina, show a wide range of sensitivities in the same dark-adapted conditions, suggesting a divergence of the rod pathways. However, this organization is not supported by the known synaptic morphology of the retina. Here, we tested an alternative idea that the rod pathways converge onto single RGCs, but inhibitory circuits selectively mask signals so that one pathway predominates. Indeed, we found that application of GABA receptor blockers increased the sensitivity of most RGCs by unmasking rod signals, which were suppressed. Our results indicate that inhibition controls the threshold responses of RGCs under dim ambient light. This mechanism can ensure that appropriate signals cross the bottleneck of the optic nerve in changing stimulus conditions.

Transcriptome response to infraorbital nerve transection in the gonadally intact male rat barrel cortex: RNA-seq

The effects of infraorbital nerve (ION) transection on gene expression in the adult male rat barrel cortex were investigated using RNA sequencing. After a 24-hour survival duration, 98 genes were differentially regulated by ION transection. Differentially expressed genes suggest changes in neuronal activity, excitability, and morphology. The production of mRNA for neurotrophins, including brain-derived neurotrophin factor (BNDF), was decreased following ION transection. Several potassium channels showed decreased mRNA production, whereas a sodium channel (Na(V)β4) associated with burst firing showed increased mRNA production. The results may have important implications for phantom-limb pain and complex regional pain syndrome. Future experiments should determine the extent to which changes in RNA result in changes in protein expression, in addition to utilizing laser capture microdissection techniques to differentiate between neuronal and glial cells.

Intracortical and Thalamocortical Connections of the Hand and Face Representations in Somatosensory Area 3b of Macaque Monkeys and Effects of Chronic Spinal Cord Injuries

Brains of adult monkeys with chronic lesions of dorsal columns of spinal cord at cervical levels undergo large-scale reorganization. Reorganization results in expansion of intact chin inputs, which reactivate neurons in the deafferented hand representation in the primary somatosensory cortex (area 3b), ventroposterior nucleus of the thalamus and cuneate nucleus of the brainstem. A likely contributing mechanism for this large-scale plasticity is sprouting of axons across the hand-face border. Here we determined whether such sprouting takes place in area 3b. We first determined the extent of intrinsic corticocortical connectivity between the hand and the face representations in normal area 3b. Small amounts of neuroanatomical tracers were injected in these representations close to the electrophysiologically determined hand-face border. Locations of the labeled neurons were mapped with respect to the detailed electrophysiological somatotopic maps and histologically determined hand-face border revealed in sections of the flattened cortex stained for myelin. Results show that intracortical projections across the hand-face border are few. In monkeys with chronic unilateral lesions of the dorsal columns and expanded chin representation, connections across the hand-face border were not different compared with normal monkeys. Thalamocortical connections from the hand and face representations in the ventroposterior nucleus to area 3b also remained unaltered after injury. The results show that sprouting of intrinsic connections in area 3b or the thalamocortical inputs does not contribute to large-scale cortical plasticity. Significance statement: Long-term injuries to dorsal spinal cord in adult primates result in large-scale somatotopic reorganization due to which chin inputs expand into the deafferented hand region. Reorganization takes place in multiple cortical areas, and thalamic and medullary nuclei. To what extent this brain reorganization due to dorsal column injuries is related to axonal sprouting is not known. Here we show that reorganization of primary somatosensory area 3b is not accompanied with either an increase in intrinsic cortical connections between the hand and face representations, or any change in thalamocortical inputs to these areas. Axonal sprouting that causes reorganization likely takes place at subthalamic levels.

Modulation of pain via expectation of its location

BACKGROUND

The spatial precision of expectancy effects on pain is unclear. We hypothesized that expecting nociceptive stimuli at particular skin sites would have an analgesic effect on nociceptive stimuli presented between them (middle zone).

METHODS

Laser stimuli (evoking pin-prick pain) were delivered to three discrete skin zones on the forearm, under two conditions. During 'Localization', participants' expectation of stimuli was spatially divided between two locations (expected stimuli in only the outer two skin zones): pain intensity and stimulus location were judged. During 'No-localization' (control condition), participants had no expectation concerning stimulus location; only pain intensity was rated. Additional experiments assessed the importance of the actual location on the forearm by: shifting all skin zones proximally towards the elbow (control for joint proximity, Experiment 2); adding a fourth zone distally (control for interaction between joint proximity and enhanced distal inhibition, Experiment 3).

RESULTS

All experiments demonstrated spatially specific pain modulation, but only Experiment 2 (near elbow) supported our hypothesis: middle zone pain intensity was significantly lower (p = 0.02) during Localization than No-localization. Experiment 1 (near wrist) found reduced pain intensity during Localization only for the distal zone (p = 0.04). Experiment 3 confirmed this effect: reduced pain during Localization occurred only for the most distal zone (p = 0.046).

CONCLUSION

Expecting a painful stimulus in non-adjacent skin sites has spatially specific effects on pain modulation, but this reflects an interaction between the expected location of stimulation and the actual location. This suggests a more complex connection between somatotopic maps and nociceptive modulation than previously thought; several distinct mechanisms likely contribute.

Effect of deafferentation from spinal anesthesia on pain sensitivity and resting-state functional brain connectivity in healthy male volunteers

Patients may perceive paradoxical heat sensation during spinal anesthesia. This could be due to deafferentation-related functional changes at cortical, subcortical, or spinal levels. In the current study, the effect of spinal deafferentation on sensory (pain) sensitivity was studied and linked to whole-brain functional connectivity as assessed by resting-state functional magnetic resonance imaging (RS-fMRI) imaging. Deafferentation was induced by sham or spinal anesthesia (15 mg bupivacaine injected at L3-4) in 12 male volunteers. RS-fMRI brain connectivity was determined in relation to eight predefined and seven thalamic resting-state networks (RSNs) and measured before, and 1 and 2 h after spinal/sham injection. To measure the effect of deafferentation on pain sensitivity, responses to heat pain were measured at 15-min intervals on nondeafferented skin and correlated to RS-fMRI connectivity data. Spinal anesthesia altered functional brain connectivity within brain regions involved in the sensory discriminative (i.e., pain intensity related) and affective dimensions of pain perception in relation to somatosensory and thalamic RSNs. A significant enhancement of pain sensitivity on nondeafferented skin was observed after spinal anesthesia compared to sham (area-under-the-curve [mean (SEM)]: 190.4 [33.8] versus 13.7 [7.2]; p<0.001), which significantly correlated to functional connectivity changes observed within the thalamus in relation to the thalamo-prefrontal network, and in the anterior cingulate cortex and insula in relation to the thalamo-parietal network. Enhanced pain sensitivity from spinal deafferentation correlated with functional connectivity changes within brain regions involved in affective and sensory pain processing and areas involved in descending control of pain.

Reconciling homeostatic and use-dependent plasticity in the context of somatosensory deprivation

The concept of homeostatic plasticity postulates that neurons maintain relatively stable rates of firing despite changing inputs. Homeostatic and use-dependent plasticity mechanisms operate concurrently, although they have different requirements for induction. Depriving central somatosensory neurons of their primary activating inputs reduces activity and results in compensatory changes that favor excitation. Both a reduction of GABAergic inhibition and increase in glutamatergic excitatory transmission are observed in input-deprived cortex. Topographic reorganization of the adult somatosensory cortex is likely driven by both homeostatic and use-dependent mechanisms. Plasticity is induced by changes in the strengths of synaptic inputs, as well as changes in temporal correlation of neuronal activity. However, there is less certainty regarding the in vivo contribution of homeostatic mechanisms as in vitro experiments rely on manipulations that create states that do not normally occur in the living nervous system. Homeostatic plasticity seems to occur, but more in vivo research is needed to determine mechanisms. In vitro research is also needed but should better conform to conditions that might occur naturally in vivo.

Facial transplantation: the first 9 years

Since the first facial transplantation in 2005, 28 have been done worldwide with encouraging immunological, functional, psychological, and aesthetic outcomes. Unlike solid organ transplantation, which is potentially life-saving, facial transplantation is life-changing. This difference has generated ethical concerns about the exposure of otherwise young and healthy individuals to the sequelae of lifelong, high-dose, multidrug immunosuppression. Nevertheless, advances in immunomodulatory and immunosuppressive protocols, microsurgical techniques, and computer-aided surgical planning have enabled broader clinical application of this procedure to patients. Although episodes of acute skin rejection continue to pose a serious threat to face transplant recipients, all cases have been controlled with conventional immunosuppressive regimens, and no cases of chronic rejection have been reported.

Increased cortical responses to forepaw stimuli immediately after peripheral deafferentation of hindpaw inputs

Both central and peripheral injuries of the nervous system induce dramatic reorganization of the primary somatosensory cortex. We recently showed that spinal cord injuries at thoracic level in anesthetized rats can immediately increase the responses evoked in the forepaw cortex by forepaw stimuli (above the level of the lesion), suggesting that the immediate cortical reorganization after deafferentation can extend across cortical representations of different paws. Here we show that a complete deafferentation of inputs from the hindpaw induced by injury or pharmacological block of the peripheral nerves in anesthetized rats also increases the responses evoked in the forepaw cortex by forepaw stimuli. This increase of cortical responses after peripheral deafferentation is not associated with gross alterations in the state of cortical spontaneous activity. The results of the present study, together with our previous works on spinal cord injury, suggest that the forepaw somatosensory cortex is critically involved in the reorganization that starts immediately after central or peripheral deafferentation of hindpaw inputs.

Peripheral nervous system origin of phantom limb pain

Nearly all amputees continue to feel their missing limb as if it still existed, and many experience chronic phantom limb pain (PLP). What is the origin of these sensations? There is currently a broad consensus among investigators that PLP is a top-down phenomenon, triggered by loss of sensory input and caused by maladaptive cortical plasticity. We tested the alternative hypothesis that PLP is primarily a bottom-up process, due not to the loss of input but rather to exaggerated input, generated ectopically in axotomized primary afferent neurons in the dorsal root ganglia (DRGs) that used to innervate the limb. In 31 amputees, the local anesthetic lidocaine was applied intrathecally and/or to the DRG surface (intraforaminal epidural block). This rapidly and reversibly extinguished PLP and also nonpainful phantom limb sensation (npPLS). Control injections were ineffective. For intraforaminal block, the effect was topographically appropriate. The suppression of PLP and npPLS could also be demonstrated using dilute lidocaine concentrations that are sufficient to suppress DRG ectopia but not to block the propagation of impulses generated further distally in the nerve. PLP is driven primarily by activity generated within the DRG. We recommend the DRG as a target for treatment of PLP and perhaps also other types of regional neuropathic pain.

The reactivation of somatosensory cortex and behavioral recovery after sensory loss in mature primates

In our experiments, we removed a major source of activation of somatosensory cortex in mature monkeys by unilaterally sectioning the sensory afferents in the dorsal columns of the spinal cord at a high cervical level. At this level, the ascending branches of tactile afferents from the hand are cut, while other branches of these afferents remain intact to terminate on neurons in the dorsal horn of the spinal cord. Immediately after such a lesion, the monkeys seem relatively unimpaired in locomotion and often use the forelimb, but further inspection reveals that they prefer to use the unaffected hand in reaching for food. In addition, systematic testing indicates that they make more errors in retrieving pieces of food, and start using visual inspection of the rotated hand to confirm the success of the grasping of the food. Such difficulties are not surprising as a complete dorsal column lesion totally deactivates the contralateral hand representation in primary somatosensory cortex (area 3b). However, hand use rapidly improves over the first post-lesion weeks, and much of the hand representational territory in contralateral area 3b is reactivated by inputs from the hand in roughly a normal somatotopic pattern. Quantitative measures of single neuron response properties reveal that reactivated neurons respond to tactile stimulation on the hand with high firing rates and only slightly longer latencies. We conclude that preserved dorsal column afferents after nearly complete lesions contribute to the reactivation of cortex and the recovery of the behavior, but second-order sensory pathways in the spinal cord may also play an important role. Our microelectrode recordings indicate that these preserved first-order, and second-order pathways are initially weak and largely ineffective in activating cortex, but they are potentiated during the recovery process. Therapies that would promote this potentiation could usefully enhance recovery after spinal cord injury.

Cortical reorganization after macroreplantation at the upper extremity: a magnetoencephalographic study

With the development of microsurgical techniques, replantation has become a feasible alternative to stump treatment after the amputation of an extremity. It is known that amputation often induces phantom limb pain and cortical reorganization within the corresponding somatosensory areas. However, whether replantation reduces the risk of comparable persisting pain phenomena as well as reorganization of the primary somatosensory cortex is still widely unknown. Therefore, the present study aimed to investigate the potential development of persistent pain and cortical reorganization of the hand and lip areas within the sensory cortex by means of magnetoencephalographic dipole analyses after replantation of a traumatically amputated upper limb proximal to the radiocarpal joint. Cortical reorganization was investigated in 13 patients with limb replantation using air puff stimulation of the phalanges of both thumbs and both corners of the lower lip. Displacement of the centre of gravity of lip and thumb representations and increased cortical activity were found in the limb and face areas of the primary somatosensory cortex contralateral to the replanted arm when compared to the ipsilateral hemisphere. Thus, cortical reorganization in the primary somatosensory cortex also occurs after replantation of the upper extremity. Patients' reports of pain in the replanted body part were negatively correlated with the amount of cortical reorganization, i.e. the more pain the patients reported, the less reorganization of the subjects' hand representation within the primary somatosensory cortex was observed. Longitudinal studies in patients after macroreplantation are necessary to assess whether the observed reorganization in the primary somatosensory cortex is a result of changes within the representation of the replanted arm and/or neighbouring representations and to assess the relationship between the development of persistent pain and reorganization.

Promoting plasticity in the somatosensory cortex to alter motor physiology

Somatosensory pathways and cortices contribute to the control of human movement. In humans, non-invasive transcranial magnetic stimulation techniques to promote plasticity within somatosensory pathways and cortices have revealed potent effects on the neurophysiology within motor cortices. In this mini-review, we present evidence to indicate that somatosensory cortex is positioned to influence motor cortical circuits and as such, is an ideal target for plasticity approaches that aim to alter motor physiology and behavior in clinical populations.

AMPA and GABA(A/B) receptor subunit expression in the cuneate nucleus of adult squirrel monkeys during peripheral nerve regeneration

The primate somatosensory neuroaxis provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for AMPA and GABAA/B receptor subunits in the cuneate nucleus of adult squirrel monkeys 1 and 5 months after median nerve compression. This method of nerve injury allowed the investigation of the way in which patterns of receptor correlates change during peripheral nerve regeneration. These results are compared to cortical data collected within the same animals. As observed in the cortex, the pattern of subunit staining in the brainstem 1 month after nerve compression suggests that the sensory deprived nucleus enters a state of reorganization. That is, the expression of GluR2/3 AMPA receptor subunits is significantly increased, while GABA α1 and GABABR1b receptor subunits are significantly decreased. Five months after nerve injury, the pattern of subunit expression is again very similar to that observed in the infragranular layers of cortex. At this later time we observe a significant increase in GluR2/3 and GABABR1a, with no change in GABAAα1, and a significant decrease in GABABR1b. Together these results suggest that during reorganization and recovery from injury the brainstem and cortex are governed by homogeneous mechanisms of plasticity.

Reorganization of the intact somatosensory cortex immediately after spinal cord injury

Sensory deafferentation produces extensive reorganization of the corresponding deafferented cortex. Little is known, however, about the role of the adjacent intact cortex in this reorganization. Here we show that a complete thoracic transection of the spinal cord immediately increases the responses of the intact forepaw cortex to forepaw stimuli (above the level of the lesion) in anesthetized rats. These increased forepaw responses were independent of the global changes in cortical state induced by the spinal cord transection described in our previous work (Aguilar et al., J Neurosci 2010), as the responses increased both when the cortex was in a silent state (down-state) or in an active state (up-state). The increased responses in the intact forepaw cortex correlated with increased responses in the deafferented hindpaw cortex, suggesting that they could represent different points of view of the same immediate state-independent functional reorganization of the primary somatosensory cortex after spinal cord injury. Collectively, the results of the present study and of our previous study suggest that both state-dependent and state-independent mechanisms can jointly contribute to cortical reorganization immediately after spinal cord injury.

Passive exercise of the hind limbs after complete thoracic transection of the spinal cord promotes cortical reorganization

Physical exercise promotes neural plasticity in the brain of healthy subjects and modulates pathophysiological neural plasticity after sensorimotor loss, but the mechanisms of this action are not fully understood. After spinal cord injury, cortical reorganization can be maximized by exercising the non-affected body or the residual functions of the affected body. However, exercise per se also produces systemic changes – such as increased cardiovascular fitness, improved circulation and neuroendocrine changes – that have a great impact on brain function and plasticity. It is therefore possible that passive exercise therapies typically applied below the level of the lesion in patients with spinal cord injury could put the brain in a more plastic state and promote cortical reorganization. To directly test this hypothesis, we applied passive hindlimb bike exercise after complete thoracic transection of the spinal cord in adult rats. Using western blot analysis, we found that the level of proteins associated with plasticity – specifically ADCY1 and BDNF – increased in the somatosensory cortex of transected animals that received passive bike exercise compared to transected animals that received sham exercise. Using electrophysiological techniques, we then verified that neurons in the deafferented hindlimb cortex increased their responsiveness to tactile stimuli delivered to the forelimb in transected animals that received passive bike exercise compared to transected animals that received sham exercise. Passive exercise below the level of the lesion, therefore, promotes cortical reorganization after spinal cord injury, uncovering a brain-body interaction that does not rely on intact sensorimotor pathways connecting the exercised body parts and the brain.

Nerve injury-induced changes in GABA(A) and GABA(B) sub-unit expression in area 3b and cuneate nucleus of adult squirrel monkeys: further evidence of developmental recapitulation

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for the GABA(A) α1, GABA(B)R1a, and GABA(B)R1b receptor subunits in somatosensory area 3b, and cuneate nucleus one week after median nerve compression in adult squirrel monkeys. We find a significant decrease in GABA(A) α1 subunit staining across all cortical layers and within both soma and neuropil of the deprived cortical and brainstem regions. The GABA(B) staining showed an opposing shift in deprived regions, with a significant increase in presynaptic GABA(B)R1a staining, and a significant decrease in postsynaptic GABA(B)R1b staining in deprived regions of the cortex and brainstem. These changes in receptor subunit expression generate patterns that are very similar to those reported in the neonate. Furthermore, the similarities between brainstem and cortical expression suggest conserved forms of adult plasticity in these two regions. Taken together these results, along with the results from our previous paper investigating AMPA subunit expression in these same animals, support the hypothesis that deprived neurons enter a previously hidden state of developmental recapitulation that serves to prime the brain for NMDA receptor mediated receptive field reorganization.

Simultaneous recording of rat auditory cortex and thalamus via a titanium-based, microfabricated, microelectrode device

Direct recording from sequential processing stations within the brain has provided opportunity for enhancing understanding of important neural circuits, such as the corticothalamic loops underlying auditory, visual, and somatosensory processing. However, the common reliance upon microwire-based electrodes to perform such recordings often necessitates complex surgeries and increases trauma to neural tissues. This paper reports the development of titanium-based, microfabricated, microelectrode devices designed to address these limitations by allowing acute recording from the thalamic nuclei and associated cortical sites simultaneously in a minimally invasive manner. In particular, devices were designed to simultaneously probe rat auditory cortex and auditory thalamus, with the intent of recording auditory response latencies and isolated action potentials within the separate anatomical sites. Details regarding the design, fabrication, and characterization of these devices are presented, as are preliminary results from acute in vivo recording.

Masked excitatory crosstalk between the ON and OFF visual pathways in the mammalian retina

A fundamental organizing feature of the visual system is the segregation of ON and OFF responses into parallel streams to signal light increment and decrement. However, we found that blockade of GABAergic inhibition unmasks robust ON responses in OFF α-ganglion cells (α-GCs). These ON responses had the same centre-mediated structure as the classic OFF responses of OFF α-GCs, but were abolished following disruption of the ON pathway with L-AP4. Experiments showed that both GABA(A) and GABA(C) receptors are involved in the masking inhibition of this ON response, located at presynaptic inhibitory synapses on bipolar cell axon terminals and possibly amacrine cell dendrites. Since the dendrites of OFF α-GCs are not positioned to receive excitatory inputs from ON bipolar cell axon terminals in sublamina-b of the inner plexiform layer (IPL), we investigated the possibility that gap junction-mediated electrical synapses made with neighbouring amacrine cells form the avenue for reception of ON signals. We found that the application of gap junction blockers eliminated the unmasked ON responses in OFF α-GCs, while the classic OFF responses remained. Furthermore, we found that amacrine cells coupled to OFF α-GCs display processes in both sublaminae of the IPL, thus forming a plausible substrate for the reception and delivery of ON signals to OFF α-GCs. Finally, using a multielectrode array, we found that masked ON and OFF signals are displayed by over one-third of ganglion cells in the rabbit and mouse retinas, suggesting that masked crossover excitation is a widespread phenomenon in the inner mammalian retina.

Sensory retraining: a cognitive behavioral therapy for altered sensation

Sensory retraining teaches the patient to ignore or blot out postinjury unpleasant orofacial sensations to optimally tune into and decipher the weakened and damaged signals from the tissues. Sensory retraining is a simple, inexpensive, noninvasive exercise program, which initiated shortly after injury, can lessen the objectionable impression of orofacial altered sensations. Sensory retraining exercises are most effective on decreasing the perceived burden associated with hypoesthetic orofacial altered sensations.

Targeted mini-strokes produce changes in interhemispheric sensory signal processing that are indicative of disinhibition within minutes

Most processing of sensation involves the cortical hemisphere opposite (contralateral) to the stimulated limb. Stroke patients can exhibit changes in the interhemispheric balance of sensory signal processing. It is unclear whether these changes are the result of poststroke rewiring and experience, or whether they could result from the immediate effect of circuit loss. We evaluated the effect of mini-strokes over short timescales (<2 h) where cortical rewiring is unlikely by monitoring sensory-evoked activity throughout much of both cortical hemispheres using voltage-sensitive dye imaging. Blockade of a single pial arteriole within the C57BL6J mouse forelimb somatosensory cortex reduced the response evoked by stimulation of the limb contralateral to the stroke. However, after stroke, the ipsilateral (uncrossed) forelimb response within the unaffected hemisphere was spared and became independent of the contralateral forelimb cortex. Within the unaffected hemisphere, mini-strokes in the opposite hemisphere significantly enhanced sensory responses produced by stimulation of either contralateral or ipsilateral pathways within 30-50 min of stroke onset. Stroke-induced enhancement of responses within the spared hemisphere was not reproduced by inhibition of either cortex or thalamus using pharmacological agents in nonischemic animals. I/LnJ acallosal mice showed similar rapid interhemispheric redistribution of sensory processing after stroke, suggesting that subcortical connections and not transcallosal projections were mediating the novel activation patterns. Thalamic inactivation before stroke prevented the bilateral rearrangement of sensory responses. These findings suggest that acute stroke, and not merely loss of activity, activates unique pathways that can rapidly redistribute function within the spared cortical hemisphere.

Changes in CNS structures after spinal cord lesions implications for BMI

It is well established that a spinal circuitry can generate locomotor movements of the hindlimbs in absence of descending supraspinal inputs. This is based, among others, on the observation that after a complete spinalization, cats can walk with the hindlimbs on a treadmill. Does this spinal pattern generator (CPG) also participate in the recovery of locomotion after a partial spinal cord lesion (SCI)? After such SCI, functional reorganization can occur spontaneously along the whole neuraxis, namely the spinal cord circuitry below the lesion (CPG) and in supraspinal structures still partially connected to the spinal cord. This review focuses mainly on the capacity of the spinal and supraspinal structures to reorganize spontaneously after incomplete SCI in animals (rats and cats). BMI approaches to foster recovery of functions after various types of SCI should take into account these changes at the various levels of the CNS.

Spinal cord injury immediately changes the state of the brain

Spinal cord injury can produce extensive long-term reorganization of the cerebral cortex. Little is known, however, about the sequence of cortical events starting immediately after the lesion. Here we show that a complete thoracic transection of the spinal cord produces immediate functional reorganization in the primary somatosensory cortex of anesthetized rats. Besides the obvious loss of cortical responses to hindpaw stimuli (below the level of the lesion), cortical responses evoked by forepaw stimuli (above the level of the lesion) markedly increase. Importantly, these increased responses correlate with a slower and overall more silent cortical spontaneous activity, representing a switch to a network state of slow-wave activity similar to that observed during slow-wave sleep. The same immediate cortical changes are observed after reversible pharmacological block of spinal cord conduction, but not after sham. We conclude that the deafferentation due to spinal cord injury can immediately (within minutes) change the state of large cortical networks, and that this state change plays a critical role in the early cortical reorganization after spinal cord injury.

Changes in S1 neural responses during tactile discrimination learning

In freely moving rats that are actively performing a discrimination task, single-unit responses in primary somatosensory cortex (S1) are strikingly different from responses to comparable tactile stimuli in immobile rats. For example, in the active discrimination context prestimulus response modulations are common, responses are longer in duration and more likely to be inhibited. To determine whether these differences emerge as rats learned a whisker-dependent discrimination task, we recorded single-unit S1 activity while rats learned to discriminate aperture-widths using their whiskers. Even before discrimination training began, S1 responses in freely moving rats showed many of the signatures of active responses, such as increased duration of response and prestimulus response modulations. As rats subsequently learned the discrimination task, single unit responses changed: more cortical units responded to the stimuli, neuronal sensory responses grew in duration, and individual neurons better predicted aperture-width. In summary, the operant behavioral context changes S1 tactile responses even in the absence of tactile discrimination, whereas subsequent width discrimination learning refines the S1 representation of aperture-width.

Differential tactile and motor recovery and cortical map alteration after C4-C5 spinal hemisection

After incomplete spinal cord injury (SCI), the adult central nervous system is spontaneously capable of substantial reorganizations that can underlie functional recovery. Most studies have focused on intraspinal reorganizations after SCI and not on the correlative cortical remodeling. Yet, differential studies of neural correlates of the recovery of sensory and motor abilities may be conducted by segregating motor and somatosensory representations in distinct and topologically organized primary cortical areas. This study was aimed at evaluating the effects of a cervical (C4-C5) spinal cord hemisection on sensorimotor performances and electrophysiological maps in primary somatosensory (S1) and motor (M1) cortices in adult rats. After SCI, an enduring loss of the affected forepaw tactile sensitivity was paralleled by the abolishment of somatosensory evoked responses in the deprived forepaw area within the S1 cortex. In contrast, severe motor deficits in unilateral forelimb were partially restored over the first postoperative month, despite remnant deficits in distal movement. The overall M1 map size was drastically reduced in SCI rats relative to intact rats. In the remaining M1 map, the shoulder and elbow movements were over-represented, consistent with the behavioral recovery of proximal joint movements in almost all rats. By contrast, residual wrist representations were observed in M1 maps of half of the rats that did not systematically correlate with a behavioral recovery of these joint movements. This study highlights the differential potential of ascending and descending pathways to reorganize after SCI.

Large-scale expansion of the face representation in somatosensory areas of the lateral sulcus after spinal cord injuries in monkeys

Transection of dorsal columns of the spinal cord in adult monkeys results in large-scale expansion of the face inputs into the deafferented hand region in the primary somatosensory cortex (area 3b) and the ventroposterior nucleus of thalamus. Here, we determined whether the upstream cortical areas, secondary somatosensory (S2) and parietal ventral (PV) areas, also undergo reorganization after lesions of the dorsal columns. Areas S2, PV, and 3b were mapped after long-term unilateral lesions of the dorsal columns at cervical levels in adult macaque monkeys. In areas S2 and PV, we found neurons responding to touch on the face in regions in which responses to touch on the hand and other body parts are normally seen. In the reorganized parts of S2 and PV, inputs from the chin as well as other parts of the face were observed, whereas in area 3b only the chin inputs expand into the deafferented regions. The results show that deafferentations lead to a more widespread brain reorganization than previously known. The data also show that reorganization in areas S2 and PV shares a common substrate with area 3b, but there are specific features that emerge in S2 and PV.

Computational role of large receptive fields in the primary somatosensory cortex

Computational studies are challenging the intuitive view that neurons with broad tuning curves are necessarily less discriminative than neurons with sharp tuning curves. In the context of somatosensory processing, broad tuning curves are equivalent to large receptive fields. To clarify the computational role of large receptive fields for cortical processing of somatosensory information, we recorded ensembles of single neurons from the infragranular forelimb/forepaw region of the rat primary somatosensory cortex while tactile stimuli were separately delivered to different locations on the forelimbs/forepaws under light anesthesia. We specifically adopted the perspective of individual columns/segregates receiving inputs from multiple body location. Using single-trial analyses of many single-neuron responses, we obtained two main results. 1) The responses of even small populations of neurons recorded from within the same estimated column/segregate can be used to discriminate between stimuli delivered to different surround locations in the excitatory receptive fields. 2) The temporal precision of surround responses is sufficiently high for spike timing to add information over spike count in the discrimination between surround locations. This surround spike-timing code (i) is particularly informative when spike count is ambiguous, e.g., in the discrimination between close locations or when receptive fields are large, (ii) becomes progressively more informative as the number of neurons increases, (iii) is a first-spike code, and (iv) is not limited by the assumption that the time of stimulus onset is known. These results suggest that even though large receptive fields result in a loss of spatial selectivity of single neurons, they can provide as a counterpart a sophisticated temporal code based on latency differences in large populations of neurons without necessarily sacrificing basic information about stimulus location.