Progressive transneuronal changes in the brainstem and thalamus after long-term dorsal rhizotomies in adult macaque monkeys

Regressive changes in sizes of somatosensory cuneate nucleus after sensory loss in primates

Neurons in the early stages of processing sensory information suffer transneuronal atrophy when deprived of their activating inputs. For over 40 y, members of our laboratory have studied the reorganization of the somatosensory cortex during and after recovering from different types of sensory loss. Here, we took advantage of the preserved histological material from these studies of the cortical effects of sensory loss to evaluate the histological consequences in the cuneate nucleus of the lower brainstem and the adjoining spinal cord. The neurons in the cuneate nucleus are activated by touch on the hand and arm, and relay this activation to the contralateral thalamus, and from the thalamus to the primary somatosensory cortex. Neurons deprived of activating inputs tend to shrink and sometimes die. We considered the effects of differences in species, type and extent of sensory loss, recovery time after injury, and age at the time of injury on the histology of the cuneate nucleus. The results indicate that all injuries that deprived part or all of the cuneate nucleus of sensory activation result in some atrophy of neurons as reflected by a decrease in nucleus size. The extent of the atrophy is greater with greater sensory loss and with longer recovery times. Based on supporting research, atrophy appears to involve a reduction in neuron size and neuropil, with little or no neuron loss. Thus, the potential exists for restoring the hand to cortex pathway with brain-machine interfaces, for bionic prosthetics, or biologically with hand replacement surgery.

Greater Reduction in Contralesional Hand Use After Frontoparietal Than Frontal Motor Cortex Lesions in Macaca mulatta

We previously reported that rhesus monkeys recover spontaneous use of the more impaired (contralesional) hand following neurosurgical lesions to the arm/hand representations of primary motor cortex (M1) and lateral premotor cortex (LPMC) (F2 lesion) when tested for reduced use (RU) in a fine motor task allowing use of either hand. Recovery occurred without constraint of the less impaired hand and with occasional forced use of the more impaired hand, which was the preferred hand for use in fine motor tasks before the lesion. Here, we compared recovery of five F2 lesion cases in the same RU test to recovery after unilateral lesions of M1, LPMC, S1 and anterior portion of parietal cortex (F2P2 lesion - four cases). Average and highest %use of the contralesional hand in the RU task in F2 cases were twice that in F2P2 cases (p < 0.05). Recovery in the RU task was closely associated with volume and percentage of lesion to caudal (new) M1 (M1c) in both F2 and F2P2 lesion cases. One F2P2 case, with the largest M1c lesion and a large rostral somatosensory cortex (S1r) lesion developed severe contralesional hand non-use despite exhibiting some recovery of fine motor function initially. We conclude that the degree of reduced use of the contralesional hand is primarily related to the volume of M1c injury and that severe non-use requires extensive injury to M1c and S1r. Thus, assessing peri-Rolandic injury extent in stroke patients may have prognostic value for predicting susceptibility to RU and non-use in rehabilitation.

Transfer of the peroneal component of the sciatic nerve in total brachial plexus lesion: An anatomical feasibility study

Closed brachial plexus lesions (BPLs) are generally associated with a traumatic mechanism of forced traction between the neck and the shoulder-arm complex. For brachial plexus reconstruction different techniques have been proposed with donor motor nerves like intercostal nerves, or the ipsilateral cervical plexus, the phrenic nerve, the contralateral C7 root, and many others. Despite all these surgical possibilities, the overall recovery is generally poor and not satisfactory. The principal drawback is linked to the loss of upper limb proprioception, in a way that dramatically influences even a good motor recovery, so in complete BPLs the sensory loss still represents a debilitating problem. In this anatomical feasibility study, the possibility to transfer the peroneal component of the sciatic nerve as a donor for complete BPLs has been evaluated. This technique would conceptually bring an important motor and sensory contribution to the upper limb using pure motor and sensory branches of the sciatic nerve. Performing immediate tendon transfer for foot drop palsy could significantly decrease the morbidity of the surgical procedure.

Reorganization of Higher-Order Somatosensory Cortex After Sensory Loss from Hand in Squirrel Monkeys

Unilateral dorsal column lesions (DCL) at the cervical spinal cord deprive the hand regions of somatosensory cortex of tactile activation. However, considerable cortical reactivation occurs over weeks to months of recovery. While most studies focused on the reactivation of primary somatosensory area 3b, here, for the first time, we address how the higher-order somatosensory cortex reactivates in the same monkeys after DCL that vary across cases in completeness, post-lesion recovery times, and types of treatments. We recorded neural responses to tactile stimulation in areas 3a, 3b, 1, secondary somatosensory cortex (S2), parietal ventral (PV), and occasionally areas 2/5. Our analysis emphasized comparisons of the responsiveness, somatotopy, and receptive field size between areas 3b, 1, and S2/PV across DCL conditions and recovery times. The results indicate that the extents of the reactivation in higher-order somatosensory areas 1 and S2/PV closely reflect the reactivation in primary somatosensory cortex. Responses in higher-order areas S2 and PV can be stronger than those in area 3b, thus suggesting converging or alternative sources of inputs. The results also provide evidence that both primary and higher-order fields are effectively activated after long recovery times as well as after behavioral and electrocutaneous stimulation interventions.

Neurodegeneration in the Spinal Ventral Horn Prior to Motor Impairment in Cervical Spondylotic Myelopathy

Remote gray matter pathology has been suggested rostral to the compression site in cervical spondylotic myelopathy (CSM). We therefore assessed neurodegeneration in the gray matter ventral and dorsal horns. Twenty patients with CSM and 18 healthy subjects underwent a high-resolution structural and diffusion magnetic resonance imaging protocol at vertebra C2/C3. Patients received comprehensive clinical assessments. T2*-weighted data provided cross-sectional area measurements of gray matter ventral and dorsal horns to identify atrophy. At the identical location, mean diffusivity (MD) and fractional anisotropy (FA) determined the microstructural integrity. Finally, the relationships between neurodegeneration occurring in the gray and white matter and clinical impairment were investigated. Patients suffered from mild-to-moderate CSM with mainly sensory impairment. In the ventral horns, cross-sectional area was not reduced (p = 0.863) but MD was increased (p = 0.045). The magnitude of MD changes within the ventral horn was associated with white matter diffusivity changes (MD: p = 0.013; FA: p = 0.028) within the lateral corticospinal tract. In contrast, dorsal horn cross-sectional area was reduced by 16.0% (p < 0.001) without alterations in diffusivity indices, compared with controls. No associations between the magnitude of ventral and dorsal horn neurodegeneration and clinical impairment were evident. Focal cord gray matter pathology is evident remote to the compression site in vivo in CSM patients. Microstructural changes in the ventral horns (i.e., motoneurons) related to corticospinal tract integrity in the absence of atrophy and marked motor impairment. Dorsal horn atrophy corresponded to main clinical representation of sensory impairment. Thus, neuroimaging biomarkers of cord gray matter integrity reveal focal neurodegeneration prior to marked clinical impairment and thus could serve as predictors of ensuing impairment in CSM patients.

Human Thalamic Somatosensory Nucleus (Ventral Caudal, Vc) as a Locus for Stimulation by INPUTS from Tactile, Noxious and Thermal Sensors on an Active Prosthesis

The forebrain somatic sensory locus for input from sensors on the surface of an active prosthesis is an important component of the Brain Machine Interface. We now review the neuronal responses to controlled cutaneous stimuli and the sensations produced by Threshold Stimulation at Microampere current levels (TMIS) in such a locus, the human thalamic Ventral Caudal nucleus (Vc). The responses of these neurons to tactile stimuli mirror those for the corresponding class of tactile mechanoreceptor fiber in the peripheral nerve, and TMIS can evoke sensations like those produced by the stimuli that optimally activate each class. These neuronal responses show a somatotopic arrangement from lateral to medial in the sequence: leg, arm, face and intraoral structures. TMIS evoked sensations show a much more detailed organization into anterior posteriorly oriented rods, approximately 300 microns diameter, that represent smaller parts of the body, such as parts of individual digits. Neurons responding to painful and thermal stimuli are most dense around the posterior inferior border of Vc, and TMIS evoked pain sensations occur in one of two patterns: (i) pain evoked regardless of the frequency or number of spikes in a burst of TMIS; and (ii) the description and intensity of the sensation changes with increasing frequencies and numbers. In patients with major injuries leading to loss of somatic sensory input, TMIS often evokes sensations in the representation of parts of the body with loss of sensory input, e.g., the phantom after amputation. Some patients with these injuries have ongoing pain and pain evoked by TMIS of the representation in those parts of the body. Therefore, thalamic TMIS may produce useful patterned somatotopic feedback to the CNS from sensors on an active prosthesis that is sometimes complicated by TMIS evoked pain in the representation of those parts of the body.

Neuronal responses to tactile stimuli and tactile sensations evoked by microstimulation in the human thalamic principal somatic sensory nucleus (ventral caudal)

The normal organization and plasticity of the cutaneous core of the thalamic principal somatosensory nucleus (ventral caudal, Vc) have been studied by single-neuron recordings and microstimulation in patients undergoing awake stereotactic operations for essential tremor (ET) without apparent somatic sensory abnormality and in patients with dystonia or chronic pain secondary to major nervous system injury. In patients with ET, most Vc neurons responded to one of the four stimuli, each of which optimally activates one mechanoreceptor type. Sensations evoked by microstimulation were similar to those evoked by the optimal stimulus only among rapidly adapting neurons. In patients with ET, Vc was highly segmented somatotopically, and vibration, movement, pressure, and sharp sensations were usually evoked by microstimulation at separate sites in Vc. In patients with conditions including spinal cord transection, amputation, or dystonia, RFs were mismatched with projected fields more commonly than in patients with ET. The representation of the border of the anesthetic area (e.g., stump) or of the dystonic limb was much larger than that of the same part of the body in patients with ET. This review describes the organization and reorganization of human Vc neuronal activity in nervous system injury and dystonia and then proposes basic mechanisms.

The functional significance of cortical reorganization and the parallel development of CI therapy

For the nineteenth and the better part of the twentieth centuries two correlative beliefs were strongly held by almost all neuroscientists and practitioners in the field of neurorehabilitation. The first was that after maturity the adult CNS was hardwired and fixed, and second that in the chronic phase after CNS injury no substantial recovery of function could take place no matter what intervention was employed. However, in the last part of the twentieth century evidence began to accumulate that neither belief was correct. First, in the 1960s and 1970s, in research with primates given a surgical abolition of somatic sensation from a single forelimb, which rendered the extremity useless, it was found that behavioral techniques could convert the limb into an extremity that could be used extensively. Beginning in the late 1980s, the techniques employed with deafferented monkeys were translated into a rehabilitation treatment, termed Constraint Induced Movement therapy or CI therapy, for substantially improving the motor deficit in humans of the upper and lower extremities in the chronic phase after stroke. CI therapy has been applied successfully to other types of damage to the CNS such as traumatic brain injury, cerebral palsy, multiple sclerosis, and spinal cord injury, and it has also been used to improve function in focal hand dystonia and for aphasia after stroke. As this work was proceeding, it was being shown during the 1980s and 1990s that sustained modulation of afferent input could alter the structure of the CNS and that this topographic reorganization could have relevance to the function of the individual. The alteration in these once fundamental beliefs has given rise to important recent developments in neuroscience and neurorehabilitation and holds promise for further increasing our understanding of CNS function and extending the boundaries of what is possible in neurorehabilitation.

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.

Outcome evaluation with signal activation of functional MRI in spinal cord injury

OBJECTIVE

The authors investigated the changes of cortical sensorimotor activity in functional MRI (fMRI) and functional recovery in spinal cord injury (SCI) patients who had been treated by bone marrow cell transplantation.

METHODS

Nineteen patients with SCI were included in this study; ten patients with clinical improvement and nine without. The cortical sensorimotor activations were studied using the proprioceptive stimulation during the fMRI.

RESULTS

Diagnostic accuracy of fMRI with neurological improvement was 70.0% and 44.4% for sensitivity and specificity, respectively. Signal activation in the ipsilateral motor cortex in fMRI was commonly observed in the clinically neurological improved group (p-value=0.002). Signal activation in the contralateral temporal lobe and basal ganglia was more commonly found in the neurological unimproved group (p-value<0.001). Signal activation in other locations was not statistically different.

CONCLUSION

In patients with SCI, activation patterns of fMRI between patients with neurologic recovery and those without varied. Such plasticity should be considered in evaluating SCI interventions based on behavioral and neurological measurements.

Stereology of cerebral cortex after traumatic brain injury matched to the Glasgow outcome score

Magnetic resonance imaging provides evidence for loss of both white and grey matter, in terms of tissue volume, from the cerebral hemispheres after traumatic brain injury. However, quantitative histopathological data are lacking. From the archive of the Department of Neuropathology at Glasgow, the cerebral cortex of 48 patients was investigated using stereology. Patients had survived 3 months after traumatic brain injury and were classified using the Glasgow Outcome Scale as follows: moderately disabled (n = 13), severely disabled (n = 12) and vegetative state (n = 12); and controls. Some patients from the archive were diagnosed with diffuse axonal injury post-mortem. Comparisons of changes in cortical neuron population across Glasgow Outcome Scale groups between diffuse axonal injury and non-diffuse axonal injury patients were undertaken using effect size analyses. The hypotheses tested were that (i) thinning of the cerebral cortex occurred after traumatic brain injury; (ii) changes in thickness of cortical layers in Brodmann areas 11, 10, 24a and 4 differed; and (iii) different changes occurred for neuronal number, their size and nearest neighbour index across Glasgow Outcome Scale groups. There was a greater loss of large pyramidal and large non-pyramidal neurons with a more severe score on the Glasgow Outcome Scale from all four cortical regions, with the greatest loss of neurons from the prefrontal cortex of patients with diffuse axonal injury. There were differences in the changes of number of medium and small pyramidal and non-pyramidal neurons between different cortical regions, and between patients with and without diffuse axonal injury. Generally, a decrease in the somatic diameter of pyramidal and non-pyramidal neurons was associated with a more severe clinical outcome. However, in the motor cortex a more severe Glasgow Outcome Scale was associated with an increased diameter of medium pyramidal neurons and small non-pyramidal cells. Pyramidal and non-pyramidal neurons did not follow a Poisson distribution within the neuropil of control patients. Pyramidal neurons were usually scattered while medium and small non-pyramidal neurons were clustered. An increased spacing between remaining neurons usually occurred across Glasgow Outcome Scale groups. It is concluded that loss of neurons resulted in reduced executive and integrative capability in patients after traumatic head injury.

Cortical Changes Following Spinal Cord Injury with Emphasis on the Nogo Signaling System

After spinal cord injury, structural as well as functional modifications occur in the adult CNS. Sites of plastic changes include the injured spinal cord itself as well as cortical and subcortical structures. Previously, cortical reorganization in response to sensory deprivation has mainly been studied using peripheral nerve injury models, and has led to a degree of understanding of mechanisms underlying reorganization and plastic changes. Deprivation or damage-induced CNS plasticity is not always beneficial for patients, and may underlie the development of conditions such as neuropathic pain and phantom sensations. Therefore, efforts not only to enhance, but also to control the capacity of plastic changes in the CNS, are of clinical relevance. Novel methods to stimulate plasticity as well as to monitor it, such as transcranial magnetic stimulation and functional magnetic resonance imaging, respectively, may be useful in diverse clinical situations such as spinal cord injury and stroke. Here, human and animal studies of spinal cord injury are reviewed, with special emphasis on the contribution of the Nogo signaling system to cortical plasticity.

Synaptic plasticity, neurogenesis, and functional recovery after spinal cord injury

Spinal cord injury research has greatly expanded in recent years, but our understanding of the mechanisms that underlie the functional recovery that can occur over the weeks and months following the initial injury, is far from complete. To grasp the scope of the problem, it is important to begin by defining the sensorimotor pathways that might be involved by a spinal injury. This is done in the rodent and nonhuman primate, which are two of the most commonly used animal models in basic and translational spinal injury research. Many of the better known experimentally induced models are then reviewed in terms of the pathways they involve and the reorganization and recovery that have been shown to follow. The better understood neuronal mechanisms mediating such post-injury plasticity, including dendritic spine growth and axonal sprouting, are then examined.

Chapter 27: Neural plasticity after nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory, and autonomic functions in the denervated segments of the body due to the interruption of axons, degeneration of distal nerve fibers, and eventual death of axotomized neurons. Functional deficits caused by nerve injuries can be compensated by reinnervation of denervated targets by regenerating injured axons or by collateral branching of undamaged axons, and remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of adequate target reinnervation; however, plasticity has limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain and hyperreflexia. After axotomy, neurons shift from a transmitter to a regenerative phenotype, activating molecular pathways that promote neuronal survival and axonal regeneration. Peripheral nerve injuries also induce a cascade of events, at the molecular, cellular, and system levels, initiated by the injury and progressing throughout plastic changes at the spinal cord, brainstem nuclei, thalamus, and brain cortex. Mechanisms involved in these changes include neurochemical changes, functional alterations of excitatory and inhibitory synaptic connections, sprouting of new connections, and reorganization of sensory and motor central maps. An important direction for research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, and are also able to modulate central nervous system reorganization, amplifying positive adaptive changes that improve functional recovery and also reducing undesirable effects.

Two cases of improvement of smooth pursuit eye movements after selective posterior rhizotomy

OBJECTIVE

Selective posterior rhizotomy (SPR) represents a standard neurosurgical approach in the treatment of spasticity in children with cerebral palsy (CP). Beside the reduction of spasticity in lower limbs, SPR may have suprasegmental effects, considerably above the surgery site. In this communication, we report on the improvement of smooth pursuit eye movements (SPEM) in two children after SPR.

MATERIAL AND METHODS

Four children with CP underwent SPR. Eye movements were registered by infrared video-oculography before and after the surgery.

RESULTS

The analysis of SPEM showed the improvement of the correlation coefficient of the eye response to the stimulus after SPR in two subjects. Improvement of SPEM performance was largely due to suppression of spontaneous fixation nystagmus.

CONCLUSION

SPR may lead to the improvement of SPEM in children with CP. The influence of SPEM improvement on quality of life in a group of severely disabled nonambulant children with CP remains to be assessed.

Experience-dependent changes in spatiotemporal properties of cutaneous inputs remodel somatosensory cortical maps following skin flap rotation

Contiguous skin surfaces that tend to be synchronously stimulated are represented in neighbouring sectors of primary somatosensory maps. Moreover, neuronal receptive fields (RFs) are reshaped through ongoing competitive/cooperative interactions that segregate/desegregate inputs converging onto cortical neuronal targets. The present study was designed to evaluate the influence of spatio-temporal constraints on somatotopic map organization. A vascularized and innervated pedicle flap of the ventrum skin bearing nipples was rotated by 180 degrees . Electrophysiological maps of ventrum skin were elaborated in the same rats at 24 h after surgery and 2 weeks after parturition. Neurones with split RFs resulting from the surgical separation of formerly adjoining skin surfaces were more numerous in non-nursing than nursing rats. RFs that included newly adjacent skin surfaces on both sides of the scar line emerged in nursing rats, suggesting that the spatial contiguity of formerly separated skin surfaces induced a fusion of their cortical representations through nursing-induced stimulation. In addition, nursing-dependent inputs were found to reincorporate the rotated skin flap representation in an updated topographical organization of the cortical map. A skin territory including recipient and translocated skin areas was costimulated for 7 h, using a brushing device. Neural responses evoked by a piezoelectric-induced skin indentation before and after skin brushing confirmed the emergence of RFs crossing the scar line and contraction of non-brushed components of split RFs. Our findings provide further evidence that the spatiotemporal structure of sensory inputs changing rapidly or evolving in a natural context is critical for experience-dependent reorganization of cortical map topography.

Cortical and subcortical plasticity in the brains of humans, primates, and rats after damage to sensory afferents in the dorsal columns of the spinal cord

The failure of injured axons to regenerate following spinal cord injury deprives brain neurons of their normal sources of activation. These injuries also result in the reorganization of affected areas of the central nervous system that is thought to drive both the ensuing recovery of function and the formation of maladaptive neuronal circuitry. Better understanding of the physiological consequences of novel synaptic connections produced by injury and the mechanisms that control their formation are important to the development of new successful strategies for the treatment of patients with spinal cord injuries. Here we discuss the anatomical, physiological and behavioral changes that take place in response to injury-induced plasticity after damage to the dorsal column pathway in rats and monkeys. Complete section of the dorsal columns of the spinal cord at a high cervical level in monkeys and rats interrupts the ascending axon branches of low threshold mechanoreceptor afferents subserving the forelimb and the rest of the lower body. Such lesions render the corresponding part of the somatotopic representation of primary somatosensory cortex totally unresponsive to tactile stimuli. There are also behavioral consequences of the sensory loss, including an impaired use of the hand/forelimb in manipulating small objects. In monkeys, if some of the afferents from the hand remain intact after dorsal column lesions, these remaining afferents extensively reactivate portions of somatosensory cortex formerly representing the hand. This functional reorganization develops over a postoperative period of 1 month, during which hand use rapidly improves. These recoveries appear to be mediated, at least in part, by the sprouting of preserved afferents within the cuneate nucleus of the dorsal column-trigeminal complex. In rats, such functional collateral sprouting has been promoted by the post-lesion digestion of the perineuronal net in the cuneate nucleus. Thus, this and other therapeutic strategies have the potential of enhancing sensorimotor recoveries after spinal cord injuries in humans.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Monkey Models of Recovery of Voluntary Hand Movement After Spinal Cord and Dorsal Root Injury

The hand is unique to the primate and manual dexterity is at its finest in the human (Napier 1980), so it is not surprising that cervical spinal injuries that even partially block sensorimotor innervation of the hand are frequently debilitating (Anderson 2004). Despite the clinical need to understand the neuronal bases of hand function recovery after spinal and/or nerve injuries, relatively few groups have systematically related subtle changes in voluntary hand use following injury to neuronal mechanisms in the monkey. Human and macaque hand anatomy and function are strikingly similar, which makes the macaque the favored nonhuman primate model for the study of postinjury dexterity. In this review of monkey models of cervical spinal injury that have successfully related voluntary hand use to neuronal responses during the early postinjury months, the focus is on the dorsal rhizotomy (or dorsal rootlet lesion) model developed and used in our laboratory over the last several years. The review also describes macaque monkey models of injuries to the more central cervical spine (e.g., hemisection, dorsal column) that illustrate methods to assess postlesion hand function and that relate it to neurophysiological and neuroanatomical changes. Such models are particularly important for understanding what the sensorimotor pathways are capable of, and for assessing the outcome of therapeutic interventions as they are developed.

Cuneate nucleus reorganization following cervical dorsal rhizotomy in the macaque monkey: its role in the recovery of manual dexterity

Immediately following a dorsal rhizotomy that removes input from the thumb, index, and middle fingers, the macaque is unable to execute movements that require controlled apposition of these digits. We have previously shown that within the early weeks and months following one of these lesions, there is 1) a re-emergence of part or all of the cortical hand map; 2) central axonal sprouting of spared primary afferents into the dorsal horn and cuneate nucleus; and 3) substantial although incomplete recovery of hand function (Darian-Smith [204] J. Comp. Neurol. 470:134-150; Darian-Smith and Ciferri [2005] J. Comp. Neurol. 491:27-45). In this study we asked: What neuronal reorganization occurs in the cuneate nucleus during this "recovery" period? And, does it contribute to the recovery of manual dexterity? To address these questions, the representation of the hand was electrophysiologically mapped (by unitary receptive field [RF] recordings) in the pars rotunda of the cuneate nucleus at either 1-2 weeks (short term) or 16-32 weeks (long term) post-rhizotomy. In short-term monkeys, the region deprived of input from the thumb, index, and middle finger was found to be unresponsive to cutaneous stimulation. However, at 16-32 weeks later, when dexterity had largely recovered, RFs of cuneate neurons could again be mapped within the cuneate nucleus, primarily in a region bordering the deprived zone. We conclude that the cuneate pre- and postsynaptic reorganization that occurs following dorsal rhizotomy plays a key role in the recovery of hand function.

The calcium binding proteins calbindin, parvalbumin, and calretinin have specific patterns of expression in the gray matter of cat spinal cord

Calcium binding proteins (CBPs) regulate intracellular levels of calcium (Ca(2+)) ions. CBPs are particularly interesting from a morphological standpoint, because they are differentially expressed in certain sub-populations of cells in the nervous system of various species of vertebrate animals. However, knowledge on the cellular regulation governing such cell-specific CBP expression is still incomplete. In this work on the L7 segment of the cat spinal cord, we analyzed the localization and morphology of neurons expressing the CBPs calbindin-28 KD (CB), parvalbumin (PV), and calretinin (CR), and co-expressing CB and PV, CB and CR, and PV and CR. Single CBP-positive ((+)) neurons showed specific distributions: (1) CB was present in small neurons localized in laminae I, II, III and X, in small to medium size neurons in laminae III-VI, and in medium to large neurons in laminae VI-VIII; (2) PV was present in small size neurons in laminae III and IV and in medial portions of laminae V and VI, medium neurons and in lamina X at the border with lamina VII, in medium to large neurons in laminae VII and VIII; (3) CR labeling was detected in small size neurons in laminae I, II, III and VIII, in medium to large size neurons in laminae I and III-VII, and in small to medium size neurons in lamina X. Double labeled neurons were a small minority of the CBP(+) cells. Co-expression of CB and PV was seen in 1 to 2% of the CBP(+) cells, and they were detected in the ventral and intermediate portions of lamina VII and in lamina X. Co-localization of CB and CR was present in 0.3% of the cells and these cells were localized in lamina II. Double labeling for PV and CR occurred in 6% of the cells, and the cells were localized in ventral part of lamina VII and in lamina VIII. Overall, these results revealed distinct and reproducible patterns of localization of the neurons expressing single CBPs and co-expressing two of them. Distinct differences of CBP expression between cat and other species are discussed. Possible relations between the cat L7 neurons expressing different CBPs with the neurons previously analyzed in cat and other animals are suggested.

Decrease of thalamic gray matter following limb amputation

Modern neuroscience has elucidated general mechanisms underlying the functional plasticity of the adult mammalian brain after limb deafferentation. However, little is known about possible structural alterations following amputation and chronic loss of afferent input in humans. Using voxel-based morphometry (VBM), based on high-resolution magnetic resonance images, we investigated the brain structure of 28 volunteers with unilateral limb amputation and compared them to healthy controls. Subjects with limb amputation exhibited a decrease in gray matter of the posterolateral thalamus contralateral to the side of the amputation. The thalamic gray matter differences were positively correlated with the time span after the amputation but not with the frequency or magnitude of coexisting phantom pain. Phantom limb pain was unrelated to thalamic structural variations, but was positively correlated to a decrease in brain areas related to the processing of pain. No gray matter increase was detected. The unilateral thalamic differences may reflect a structural correlate of the loss of afferent input as a secondary change following deafferentation.

Role of cochlear integrity in cochlear nucleus glucose metabolism and neuron number after cochlea removal in aging broiler chickens

In the chicken auditory system, cochlear nucleus (nucleus magnocellularis, NM) neurons receive their only excitatory input from the ipsilateral cochlea. Cochlea removal (CR) results in an immediate decrease in NM neuron electrical activity, followed by death of approximately 30% of NM neurons. Previous work showed a decrease in NM activity and subsequent loss of NM neurons in all chicks. Egg layer adults showed NM neuron loss after CR, while neuron number remained stable in broiler adults. This suggested that effects of CR on NM were age- and breed-dependent. We now know that most aging egg layer chickens maintain largely normal cochleae throughout adulthood. Some exhibit cochlear damage with age. The converse is true of broiler chickens. Most aging broiler chickens display cochlear degeneration, with some maintaining normal cochlear anatomy throughout adulthood. The presence of extensive cochlear damage may alter the effect of CR on NM, leading to the described differences. Here, we examine the effect of unilateral CR on NM glucose metabolism and neuron number in 2, 30, 39, and 52 week-old broiler chickens found to have normal cochleae. Chickens with damaged cochleae were excluded. Using 2-deoxyglucose uptake to evaluate bilateral NM glucose metabolism, we found significantly decreased uptake ipsilateral to CR at each age examined. Bilateral cell counts revealed significant neuron loss ipsilateral to CR at each age examined. This suggests that NM glucose metabolism decreases and subsequent neuron death occurs in aging broiler chickens when a normal cochlea is removed. The status of the cochlea must play a role in the effect of deafferentation on NM glucose metabolism and neuron survival. The effect of CR appears to be dependent upon neither age nor breed, but upon cochlear integrity instead.

The function of microglia through purinergic receptors: neuropathic pain and cytokine release

Microglia play an important role as immune cells in the central nervous system (CNS). Microglia are activated in threatened physiological homeostasis, including CNS trauma, apoptosis, ischemia, inflammation, and infection. Activated microglia show a stereotypic, progressive series of changes in morphology, gene expression, function, and number and produce and release various chemical mediators, including proinflammatory cytokines that can produce immunological actions and can also act on neurons to alter their function. Recently, a great deal of attention is focusing on the relation between activated microglia through adenosine 5'-triphosphate (ATP) receptors and neuropathic pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes, or infection. This type of pain can be so severe that even light touching can be intensely painful and it is generally resistant to currently available treatments. There is abundant evidence that extracellular ATP and microglia have an important role in neuropathic pain. The expression of P2X4 receptor, a subtype of ATP receptors, is enhanced in spinal microglia after peripheral nerve injury model, and blocking pharmacologically and suppressing molecularly P2X4 receptors produce a reduction of the neuropathic pain. Several cytokines such as interleukin-1beta (IL-1beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) in the dorsal horn are increased after nerve lesion and have been implicated in contributing to nerve-injury pain, presumably by altering synaptic transmission in the CNS, including the spinal cord. Nerve injury also leads to persistent activation of p38 mitogen-activated protein kinase (MAPK) in microglia. An inhibitor of this enzyme reverses mechanical allodynia following spinal nerve ligation (SNL). ATP is able to activate MAPK, leading to the release of bioactive substances, including cytokines, from microglia. Thus, diffusible factors released from activated microglia by the stimulation of purinergic receptors may have an important role in the development of neuropathic pain. Understanding the key roles of ATP receptors, including P2X4 receptors, in the microglia may lead to new strategies for the management of neuropathic pain.

Reduction of the hand representation in the ipsilateral primary motor cortex following unilateral section of the corticospinal tract at cervical level in monkeys

BACKGROUND

After sub-total hemi-section of cervical cord at level C7/C8 in monkeys, the ipsilesional hand exhibited a paralysis for a couple of weeks, followed by incomplete recovery of manual dexterity, reaching a plateau after 40-50 days. Recently, we demonstrated that the level of the plateau was related to the size of the lesion and that progressive plastic changes of the motor map in the contralesional motor cortex, particularly the hand representation, took place following a comparable time course. The goal of the present study was to assess, in three macaque monkeys, whether the hand representation in the ipsilesional primary motor cortex (M1) was also affected by the cervical hemi-section.

RESULTS

Unexpectedly, based on the minor contribution of the ipsilesional hemisphere to the transected corticospinal (CS) tract, a considerable reduction of the hand representation was also observed in the ipsilesional M1. Mapping control experiments ruled out the possibility that changes of motor maps are due to variability of the intracortical microstimulation mapping technique. The extent of the size reduction of the hand area was nearly as large as in the contralesional hemisphere in two of the three monkeys. In the third monkey, it represented a reduction by a factor of half the change observed in the contralesional hemisphere. Although the hand representation was modified in the ipsilesional hemisphere, such changes were not correlated with a contribution of this hemisphere to the incomplete recovery of the manual dexterity for the hand affected by the lesion, as demonstrated by reversible inactivation experiments (in contrast to the contralesional hemisphere). Moreover, despite the size reduction of M1 hand area in the ipsilesional hemisphere, no deficit of manual dexterity for the hand opposite to the cervical hemi-section was detected.

CONCLUSION

After cervical hemi-section, the ipsilesional motor cortex exhibited substantial reduction of the hand representation, whose extent did not match the small number of axotomized CS neurons. We hypothesized that the paradoxical reduction of hand representation in the ipsilesional hemisphere is secondary to the changes taking place in the contralesional hemisphere, possibly corresponding to postural adjustments and/or re-establishing a balance between the two hemispheres.

Activity-dependent maintenance and growth of dendrites in adult cortex

Whereas it is widely accepted that the adult cortex is capable of a remarkable degree of functional plasticity, demonstrations of accompanying structural changes have been limited. We examined the basal dendritic field morphology of dye-filled neurons in layers III and IV of the mature barrel cortex after vibrissal-deafferentation in adult rats. Eight weeks later, the tendency for these neurons to orient their dendritic arbors toward the center of their home barrel was found to be disrupted by the resultant reduced activity of thalamocortical innervation. Measures of spine density and total dendritic length were normal, indicating that the loss of dendritic bias was accompanied by growth of dendrites directed away from the barrel center. This finding suggests that in the mature cortex, the apparently static structural attributes of the normal adult cortex depend on maintenance of patterns of afferent activity; with the corollary that changes in these patterns can induce structural plasticity.

Absence of localized grey matter volume changes in the motor cortex following spinal cord injury

The consequences of spinal cord injury (SCI) have considerable effects on motor function, typically resulting in functional impairment. Pathological changes have been studied at the site of trauma, rostrocaudally within the cord, and in the periphery. Few studies, however, have investigated the consequences of SCI at the cortical level. Magnetic resonance imaging (MRI) was used to explore the morphological changes in the grey and white matter within the primary motor (M1) cortex of individuals with cervical SCI. The "precentral knob," a landmark of M1 cortex dedicated to hand function, was selected for regionally specific measurements of change. Thirty-one hemispheres of SCI subjects and 28 hemispheres of control subjects were compared using a manual measurement after the images were segmented into grey matter, white matter, and cerebral spinal fluid (CSF). No significant differences in grey matter area measured at the precentral knob were found with the manual approach. An automated voxel-based morphometric analysis was also performed and demonstrated no significant differences in grey or white matter volume within an M1 region of interest. These data suggest that there is no gross anatomical change within M1 following cervical SCI. Our previously reported findings of reorganization of cortical motor output maps following SCI therefore likely result from changes in functional organization rather than anatomical changes.

Primary afferent terminal sprouting after a cervical dorsal rootlet section in the macaque monkey

We examined the role of primary afferent neurons in the somatosensory cortical "reactivation" that occurs after a localized cervical dorsal root lesion (Darian-Smith and Brown [2000] Nat. Neurosci. 3:476-481). After section of the dorsal rootlets that enervate the macaque's thumb and index finger (segments C6-C8), the cortical representation of these digits was initially silenced but then re-emerged for these same digits over 2-4 postlesion months. Cortical reactivation was accompanied by the emergence of physiologically detectable input from these same digits within dorsal rootlets bordering the lesion site. We investigated whether central axonal sprouting of primary afferents spared by the rhizotomy could mediate this cortical reactivation. The cortical representation of the hand was mapped electrophysiologically 15-25 weeks after the dorsal rootlet section to define this reactivation. Cholera toxin subunit B conjugated to horseradish peroxidase was then injected into the thumb and index finger pads bilaterally to label the central terminals of any neurons that innervated these digits. Primary afferent terminal proliferation was assessed in the spinal dorsal horn and cuneate nucleus at 7 days and 15-25 postlesion weeks. Labeled terminal bouton distributions were reconstructed and the "lesion" and control sides compared within each monkey. Distributions were significantly larger on the side of the lesion in the dorsal horn and cuneate nucleus at 15-25 weeks after the dorsal rootlet section, than those mapped only 7 days postlesion. Our results provide direct evidence for localized sprouting of spared (uninjured) primary afferent terminals in the dorsal horn and cuneate nucleus after a restricted dorsal root injury.

Bilateral effects of spinal overhemisections on the development of the somatosensory system in rats

Connections of the forepaw regions of somatosensory cortex (S1) were determined in rats reared to maturity after spinal cord overhemisections at cervical level C3 on postnatal day 3. Overhemisections cut all ascending and descending pathways and intervening gray on one side of the spinal cord and the pathways of the dorsal funiculus contralaterally. Bilateral lesions of the dorsal columns reduced the size of the brainstem nuclei by 41%, and the ventroposterior lateral subnucleus (VPL) of the thalamus by 20%. Bilateral lesions also prevented the emergence of the normal cytochrome oxidase barrel pattern in forepaw and hindpaw regions of S1. Injections of wheat germ agglutinin conjugated to horseradish peroxidase were placed in the forepaw region of granular S1 and surrounding dysgranular S1 contralateral to the hemisection. The VPL nucleus was densely labeled, whereas the adjoining ventroposterior medial subnucleus, VPM, representing the head, was unlabeled. Thus, there was no evidence of abnormal connections of VPM to forepaw cortex. Foci of transported label in the ipsilateral hemisphere appeared to be in normal locations and of normal extents, but connections in the opposite hemisphere were broadly and nearly uniformly distributed in sensorimotor cortex in a pattern similar to that in postnatal rats. Rats with incomplete lesions that spared the dorsal column pathway on the left side but not the right demonstrated surprisingly normal distributions of callosal connections in the nondeprived right hemisphere, even though the injected left hemisphere was deprived. Thus, the development of the normal pattern of callosal connections depends on dorsal column input and not on normal interhemsipheric interactions.

Delayed reorganization of the shoulder representation in forepaw barrel subfield (FBS) in first somatosensory cortex (SI) following forelimb deafferentation in adult rats

We previously reported that 6-16 weeks after forelimb amputation in adult rats, neurons in layer IV of rat first somatosensory cortex (SI) in the forepaw barrel subfield (FBS) associated with the representation of the forepaw became responsive to new input from the shoulder (Pearson et al. 1999). These new shoulder-responsive sites in deafferented FBS had longer evoked response latencies than did sites in the shoulder representation located in the posterior part of the trunk subfield, hereafter referred to as the original shoulder representation. Furthermore, projection neurons in the original shoulder representation in both intact and deafferented adults did not extend their axons into the FBS, and ablation of the original shoulder representation cortex and/or the second somatosensory cortex (SII) failed to eliminate new shoulder input in the deafferented FBS (Pearson et al. 2001). These results led us to conclude that large-scale reorganization in FBS quite likely involved a subcortical substrate. In addition, the time course for large-scale cortical reorganization following forelimb amputation was unknown, and this information could shed light on potential mechanisms for large-scale cortical reorganization. In the present study, we extended our previous findings of large-scale cortical reorganization in the FBS by investigating the time course for reorganization following forelimb amputation. The major findings are: a) deafferented forelimb cortex remained unresponsive to shoulder stimulation during the 1st week following forelimb amputation; b) new responses to shoulder stimulation were first observed in deafferented forelimb cortex 2-3 weeks after forelimb amputation; however, the new shoulder input was restricted to locations in the former forearm cortex; c) islet(s) of new shoulder representation were first observed in deafferented FBS 4 weeks after amputation; these islets occupied a larger percentage of FBS in subsequent weeks; d) portions of FBS remained unresponsive as many as 4 months after deafferentation (maximum time examined between amputation and recording); and e) the increase in total size of the shoulder representation appeared to result from the establishment of new shoulder representations that were often discontinuous from the original shoulder representation. These findings provide evidence that forelimb amputation results in delayed reorganization of the FBS and we describe possible mechanisms and substrates underlying the reorganization.

Effects of age and cochlear damage on the metabolic activity of the avian cochlear nucleus

Most aging commercially raised broiler chickens display a progressive loss of cochlear hair cells in a pattern similar to the cochlear degeneration found in aging humans: basal (high frequency) hair cells are affected first, followed by apical (low frequency) hair cells [Durham et al., Hear. Res. 166 (2002) 82-95]. Here, cochlear anatomy was assessed from scanning electron micrographs. Then, the metabolic activity of cochlear nucleus (nucleus magnocellularis, NM) neurons in 15-19, 30, 39, 40, and 65-66 week old broiler chickens was examined using cytochrome oxidase histochemistry and compared to the degree of cochlear abnormality. Cochleae of 15-19 week old birds are largely normal; therefore the level of NM metabolic activity is considered the baseline. Cochleae of the 30 week old group display mild damage and hair cell regeneration in the base. Metabolic activity in rostral (high frequency) NM is increased relative to the baseline, while activity remains unchanged in caudal (low frequency) NM. The 39 and 65-66 week old groups display severe and total damage extending into the apex of the cochlea. Metabolic activity is decreased in rostral and caudal NM at these ages. These results suggest that auditory central nervous system metabolism (cytochrome oxidase activity) is affected by changes in the aging chicken cochlea.

Human brain plasticity: an emerging view of the multiple substrates and mechanisms that cause cortical changes and related sensory dysfunctions after injuries of sensory inputs from the body

Injuries of peripheral inputs from the body cause sensory dysfunctions that are thought to be attributable to functional changes in cerebral cortical maps of the body. Prevalent theories propose that these cortical changes are explained by mechanisms that preeminently operate within cortex. This paper reviews findings from humans and other primates that point to a very different explanation, i.e. that injury triggers an immediately initiated, and subsequently continuing, progression of mechanisms that alter substrates at multiple subcortical as well as cortical locations. As part of this progression, peripheral injuries cause surprisingly rapid neurochemical/molecular, functional, and structural changes in peripheral, spinal, and brainstem substrates. Moreover, recent comparisons of extents of subcortical and cortical map changes indicate that initial subcortical changes can be more extensive than cortical changes, and that over time cortical and subcortical extents of change reach new balances. Mechanisms for these changes are ubiquitous in subcortical and cortical substrates and include neurochemical/molecular changes that cause functional alterations of normal excitation and inhibition, atrophy and degeneration of normal substrates, and sprouting of new connections. The result is that injuries that begin in the body become rapidly further embodied in reorganizational make-overs of the entire core of the somatosensory brain, from peripheral sensory neurons to cortex. We suggest that sensory dysfunctions after nerve, root, dorsal column (spinal), and amputation injuries can be viewed as diseases of reorganization in this core.

Adaptive responses of monkey somatosensory cortex to peripheral and central deafferentation

This study deals with two kinds of activity-dependent phenomena in the somatosensory cortex of adult monkeys, both of which may be related: (1) mutability of representational maps, as defined electrophysiologically; (2) alterations in expression of genes important in the inhibitory and excitatory neurotransmitter systems. Area 3b of the cerebral cortex was mapped physiologically and mRNA levels or numbers of immunocytochemically stained neurons quantified after disrupting afferent input peripherally by section of peripheral nerves, or centrally by making lesions of increasing size in the somatosensory thalamus. Survival times ranged from a few weeks to many months. Mapping studies after peripheral nerve lesions replicated results of previous studies in showing the contraction of representations deprived of sensory input and expansion of adjacent representations. However, these changes in representational maps were in most cases unaccompanied by significant alterations in gene expression for calcium calmodulin-dependent protein kinase isoforms, for glutamic acid decarboxylase, GABA(A) receptor subunits, GABA(B) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or N-methyl-D-aspartate (NMDA) receptor subunits. Mapping studies after lesions in the ventral posterior lateral nucleus (VPL) of the thalamus revealed no changes in cortical representations of the hand or fingers until >15% of the thalamic representation was destroyed, and only slight changes until approximately 45% of the representation was destroyed, at which point the cortical representation of the finger at the center of a lesion began to shrink. Lesions destroying >60% of VPL resulted in silencing of the hand representation. Although all lesions were associated with a loss of parvalbumin-immunoreactive thalamocortical fiber terminations, and of cytochrome oxidase staining in a focal zone of area 3b, no changes in gene expression could be detected in the affected zone until >40-50% of VPL was destroyed, and even after that changes in mRNA levels or in numbers of GABA-immunoreactive neurons in the affected zone were remarkably small. The results of these studies differ markedly from the robust changes in gene expression detectable in the visual cortex of monkeys deprived of vision in one eye. The results confirm the view that divergence of the afferent somatosensory pathways from periphery to cerebral cortex is sufficiently great that many fibers can be lost before neuronal activity is totally silenced in area 3b. This divergence is capable of maintaining a high degree of cortical function in the face of diminishing inputs from the periphery and is probably an important element in promoting representational plasticity in response to altered patterns of afferent input.

Long-term trans-synaptic glial responses in the human thalamus after peripheral nerve injury

Limb denervation leads to reorganization of the representational zones of the somatosensory cortex. Using [11C](R)-PK11195, a sensitive in vivo marker of glial cell activation, and PET, we provide first evidence that limb denervation induces a trans-synaptic increase in [11C](R)-PK11195 binding in the human thalamus but not somatosensory cortex: these brain structures appeared morphologically normal on magnetic resonance imaging (MRI). The increased thalamic signal was detectable many years after nerve injury, indicating persistent reorganization of the thalamus. This glial activation, beyond the first-order projection area of the injured neurons, may reflect continually altered afferent activity. Our findings support the view that long-term rearrangement of cortical representational maps is significantly determined within the thalamus.

Somatotopic reorganization in the brainstem and thalamus following peripheral nerve injury in adult primates

Injury-induced reorganization of central somatotopic maps is a phenomenon that has proven to be useful for elucidating the mechanisms and time course of neural plasticity. To date, the overwhelming majority of this line of research has focused on such plastic events in cortical areas, at the expense of subcortical structures. In this study, we used multi-unit electrophysiological recording techniques to assess the somatotopic organization of brainstem and thalamic areas following chronic survival from paired median and ulnar nerve section in adult squirrel monkeys. We report that the extent of cutaneously-driven reorganization in both the cuneate nucleus of the brainstem and the ventroposterior lateral nucleus of the thalamus is comparable to that previously documented for area 3b of cortex. These observations are consistent with those previously reported in thalamus, and are unique for brainstem.

Remodelling of sensorimotor maps in paraplegia: a functional magnetic resonance imaging study after a surgical nerve transfer

The adult mammalian brain has the capacity of reorganising its neural connections in response to lesions/modifications of the peripheral and central nervous system. We show in vivo, using functional magnetic resonance imaging (fMRI), that in paraplegics the lower-limb sensorimotor cortex is invaded by the arm representation. This functional reshaping appears to be reversible. Indeed, surgical transfer of the ulnar nerve to the ipsilateral quadriceps and hip muscles allowed their contraction in a paraplegic patient. During fMRI, these voluntary movements activated the hip and thigh representation in sensorimotor cortex. We suggest that the functional recovery of the lower-limb functional maps might have been driven by the restored somatosensory inputs from the reactivated periphery. The voluntary movements of the lower-limbs are regained through the 're-awakening' of the corresponding sensorimotor cortex.

Referred phantom sensations and cortical reorganization after spinal cord injury in humans

To test the hypothesis that cortical remapping supports phantom sensations, we examined referred phantom sensations and cortical activation in humans after spinal-cord injury (SCI) at the thoracic level (T3-T12). Of 12 SCI subjects, 9 reported phantom sensations, and 2 reported referred phantom sensations. In both of these subjects, referred phantom sensations were evoked by contact in reference zones (RZ) that were not adjacent in the periphery and were not predicted to be adjacent in the postcentral gyrus (PoCG), suggesting that representations separated by centimeters of cortical space were simultaneously engaged. This finding was supported by functional MRI (fMRI). In a subject with a T6-level complete SCI, contact in RZ on the left or right forearm projected referred phantom sensations to the ipsilateral chest. During fMRI, contact in either forearm RZ evoked activity in the central PoCG (the position of the forearm representation) and the medial PoCG (the position of the chest representation) with >/=1.6 cm of nonresponsive cortex intervening. In contrast, stimulation in non-RZ forearm and palm regions in this subject and in lesion-matched SCI subjects evoked central but not medial PoCG activation. Our findings support a relation between PoCG activation and the percept of referred phantom sensations. These results, however, present an alternative to somatotopic cortical reorganization, namely, cortical plasticity expressed in coactivation of nonadjacent representations. The observed pattern suggests that somatotopic subcortical remapping, projected to the cortex, can support perceptual and cortical reorganization after deafferentation in humans.

Spinal cord atrophy and reorganization of motoneuron connections following long-standing limb loss in primates

Primates with long-standing therapeutic amputations of a limb at a young age were used to investigate the possibility that deefferented motor nerves sprout to new muscle targets. Injections of anatomical tracers into the muscles proximal to the amputated stump labeled a larger extent of motoneurons than matched injections on the intact side or in normal animals, including motoneurons that would normally supply only the missing limb muscles. Although the total numbers of distal limb motoneurons remained normal, some distal limb motoneurons on the amputated side were smaller in size and simpler in form. These results suggest that deprived motoneurons survive and retain function by reinnervating new muscle targets. The sprouted motor efferents may account for some of the reorganization of primary motor cortex that follows long-standing amputation.