Limb Deafferentation Research Articles

A period of transient synaptic density unbalancing in the motor cortex after peripheral nerve injury and the involvement of microglial cells

Some types of peripheral nerve injury lead to limb deafferentation, which leads to remodeling of body representation areas in different parts of the brain, such as in the primary motor cortex and primary sensory cortex. This plasticity is a consequence of several cellular events, such as the emergence and elimination of synapses in these areas. Beside neurons, microglial cells are intimately involved in synapse plasticity, especially in synaptic pruning. In this study, we investigated the transient changes in synaptic density in the primary motor and sensory cortex after different types of peripheral nerve injury, as well as the behavior of microglial cells in each scenario. Male C57/B6 mice were divided into a control group (no injury), sciatic-crush group, and sciatic-transection group, and treated with PBS or minocycline daily for different time points. Both types of sciatic lesion led to a significant decrease of synaptophysin and PSD-95 positive puncta counts compared to control animals 4 days after lesion (DAL), which recovered at 7 DAL and was sustained until 14 DAL. The changes in synaptic puncta density were concomitant with changes in the density and morphology of microglial cells, which were significantly more ramified in the primary motor cortex of injured animals at 1 and 4 DAL. Although the decreased synaptic puncta density overlapped with an increased number of microglial cells, the number of lysosomes per microglial cell did not increase on day 4 after lesion. Surprisingly, daily administration of minocycline increased microglial cell number and PSD-95 positive puncta density by 14 DAL. Taken together, we found evidence for transient changes in synaptic density in the primary motor, related to peripheral injury with possible participation of microglia in this plasticity process.

Structural and functional organization of the lower jaw barrel subfield in rat primary somatosensory cortex.

Barrel subfields in rodent primary somatosensory cortex (SI) are important model systems for studying cortical organization and reorganization. During cortical reorganization that follows limb deafferentation, neurons in deafferented forelimb SI become responsive to previously unexpressed inputs from the lower jaw. Although the lower jaw barrel subfield (LJBSF) is a likely source of the input, this subfield has received little attention. Our aim was to describe the structural and functional organization of the normal LJBSF. To investigate LJBSF organization, a nomenclature for lower jaw skin surface was developed, cytochrome oxidase (CO) was used to label flattened-cut LJBSF sections, microelectrodes were used to map the lower jaw skin surface representation in SI, and electrolytic lesions, recovered from electrode penetrations, were used to align the physiological map to the underlying barrel map. LJBSF is a tear-shaped subfield containing approximately 24 barrels, arranged in eight mediolateral rows and a barrel-free zone capping the anterior border. The representation of the lower jaw skin consisting of chin vibrissae and microvibrissae embedded in common fur is somatotopically organized in a single map in the contralateral SI. This physiological map shows that the activity from the vibrissae aligns with the CO-staining of the underlying LJBSF. LJBSF barrels receive topographically ordered barrel-specific input from individual vibrissa and microvibrissae in the lower jaw but not from trident whiskers. The barrel-free zone receives topographically ordered input from the lower lip. These data demonstrating that the LJBSF is a highly organized subfield are essential for understanding its possible role in cortical reorganization.

Decreased Corticospinal Excitability after the Illusion of Missing Part of the Arm.

Previous studies on body ownership illusions have shown that under certain multimodal conditions, healthy people can experience artificial body-parts as if they were part of their own body, with direct physiological consequences for the real limb that gets ‘substituted.’ In this study we wanted to assess (a) whether healthy people can experience ‘missing’ a body-part through illusory ownership of an amputated virtual body, and (b) whether this would cause corticospinal excitability changes in muscles associated with the ‘missing’ body-part. Forty right-handed participants saw a virtual body from a first person perspective but for half of them the virtual body was missing a part of its right arm. Single pulse transcranial magnetic stimulation was applied before and after the experiment to left and right motor cortices. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) and the extensor digitorum communis (EDC) of each hand. We found that the stronger the illusion of amputation and arm ownership, the more the reduction of MEP amplitudes of the EDC muscle for the contralateral sensorimotor cortex. In contrast, no association was found for the EDC amplitudes in the ipsilateral cortex and for the FDI amplitudes in both contralateral and ipsilateral cortices. Our study provides evidence that a short-term illusory perception of missing a body-part can trigger inhibitory effects on corticospinal pathways and importantly in the absence of any limb deafferentation or disuse.

Interhemispheric neuroplasticity following limb deafferentation detected by resting-state functional connectivity magnetic resonance imaging (fcMRI) and functional magnetic resonance imaging (fMRI)

Functional connectivity magnetic resonance imaging (fcMRI) studies in rat brain show brain reorganization following peripheral nerve injury. Subacute neuroplasticity was observed 2 weeks following transection of the four major nerves of the brachial plexus. Direct stimulation of the intact radial nerve reveals a functional magnetic resonance imaging (fMRI) activation pattern in the forelimb regions of the sensory and motor cortices that is significantly different from that observed in normal rats. Results of this fMRI experiment were used to determine seed voxel regions for fcMRI analysis. Intrahemispheric connectivities in the sensorimotor forelimb representations in both hemispheres are largely unaffected by deafferentation, whereas substantial disruption of interhemispheric sensorimotor cortical connectivity occurs. In addition, significant intra- and interhemispheric changes in connectivities of thalamic nuclei were found. These are the central findings of the study. They could not have been obtained from fMRI studies alone—both fMRI and fcMRI are needed. The combination provides a general marker for brain plasticity. The rat visual system was studied in the same animals as a control. No neuroplastic changes in connectivities were found in the primary visual cortex upon forelimb deafferentation. Differences were noted in regions responsible for processing multisensory visual-motor information. This incidental discovery is considered to be significant. It may provide insight into phantom limb epiphenomena.

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.

Gray Matter Volume Loss after Upper Limb Amputation

Background: Brain plasticity occurs as a result of sensory and motor adaptation to increased or altered use and leads to reorganization of the respective representations [1]. Plasticity of the adult human brain following deafferentation has only been assessed functionally, revealing functional reorganization of somatosensory and motor cortices [2, 3]. Only a few reports exist reporting plasticity of groß brain structure [4]. In this study we explored whether structural brain plasticity occurs after amputation of the upper limb. Methods: We investigated a 33-year-old, right-handed male patient with an amputation of the right forearm following severe trauma. Fifteen weeks after amputation he received a myoelectric prosthesis which was regularly used. After removing signal uniformities, all scans were registered to the first scan using rigid-body transformation. Remaining local differences were then minimized by applying high-resolution deformations [5] and the local volume changes were calculated using the Jacobian determinant [6]. Results: Significant volume loss was detected in the primary motor cortex (M1), primary somatosensory cortex (S1), and in the superior parietal cortex contralateral to the amputated limb. Further volume loss was seen in the supplementary motor area (SMA), and the ipsilateral primary motor cortex and bilaterally in the cerebellum and the brain stem. Discussion: This case report is the first indication of structural brain plasticity in the adult human brain following limb deafferentation. The data suggest that loss of sensory input and lack of motor output leads not only to a functional reorganization, but may also cause a change of groß brain structure. The regions with volume loss cover most parts of the motor system such as M1, S1, SMA, superior parietal cortex, cerebellum, and brain stem. Thus, short-term changes occur not only in the functional but also in the structural organization of the motor and sensorimotor system. This is in line with current observations of short-term structural changes as a consequence of intensive training [7].

Effects of large-scale limb deafferentation on the morphological and physiological organization of the forepaw barrel subfield (FBS) in somatosensory cortex (SI) in adult and neonatal rats.

The physiological representation of the shoulder and surrounding body was examined in layer IV of somatosensory cortex (SI) in rats that had underground removal of the forelimb, either as newborns on postnatal day three (PND-3) or as adults (at least 8 weeks of age). Electrophysiological recordings were used to map the shoulder and body representations (physiological map), and the mitochondria marker, cytochrome oxidase (CO), was used to visualize recording sites in barrel and barrel-like structures (morphological map) in layer IV of deafferents and intact controls. The SI shoulder representation lies in a nebulously stained region that lies posterior to the forearm, wrist, and forepaw representations; the latter region is associated with the well-defined forepaw barrel subfield (FBS). The major findings are: (1) the shoulder is represented as a single zone located at the posterior extent of the SI body map in intact rats; (2) limb deafferentation in adult or neonatal rats that were physiologically mapped 6-16 weeks post-amputation resulted in two or more islets of "new" representation of the shoulder in the FBS in addition to the representation of the "original" shoulder in the posterior part of the body map; (3) deafferentations made in neonatal rats, physiologically mapped as adults, had a significantly greater (Mann-Whitney U) amount of "new" cortical representation within the FBS than did rats deafferented as adults; (4) fewer unresponsive sites in the FBS were found for neonate deafferents than for adult deafferents; (5) evoked response latencies following electrical stimulation of the shoulder were shortest for cortical sites within the "original" shoulder representation in intact controls, and latencies recorded at the "original" shoulder representation in deafferents were also shorter than latencies recorded in "new" shoulder representations in both groups of deafferents; and (6) morphological maps of the FBS were altered in neonate deafferents to the extent that the barrel structure was poorly formed, as exemplified by the absence of the four mediolateral running bands; however, the overall ovoid shape of the FBS was still apparent, but not as sharply defined as for intact controls or adult deafferents. Possible mechanisms for reorganization following large-scale deafferentation are discussed.

Serial kinematic analysis of the canine hindlimb joints after deafferentation and anterior cruciate ligament transection

Transection of the anterior cruciate ligament 2 weeks after ipsilateral hindlimb deafferentation leads to osteoarthritis of the knee joint within 3 weeks. We analyzed the gait of six dogs that underwent this procedure in order to identify kinematic changes that could account for this rapid joint degeneration. All animals were video taped, 1, 3, 6, 9 and 13 weeks after surgery while they trotted on a treadmill. In each dog, extension of the hip, knee and ankle joints of the unstable limb was increased, and the yield phase of the unstable knee was delayed or attenuated. When killed, five of six dogs showed a large full-thickness cartilage ulcer on the distal and/or anterior surface of the medial femoral condyle of the unstable knee; in the sixth dog, a smaller ulcer was observed. However, the severity of pathology in each individual was not obviously related to difference among the dogs in postoperative joint kinematics. These data, and results of prior studies in humans and dogs, suggest that knee hyperextension resulting from limb deafferentation, and knee instability resulting from anterior cruciate ligament transection, operate in concert to create a mechanical environment (i.e., increased tibiofemoral separation and changes in the loading of articular surfaces) that results in rapid joint breakdown.

Motor imagery: never in your wildest dream

The results from brain-lesioned patients might also contribute to our understanding of the function of the posterior parietal lobe. These new data suggest that the parietal cortex is a structure where the evaluation of motor performance, real and imaged, can occur based upon a comparison of corollary discharge and multimodal reafferent sensory signals with a stored internal representation of the motor plan as it normally unfolds. In line with this idea, it has been reported[21xDuhamel, J-R. et al. Brain. 1992; 115: 1387–1402Crossref | PubMed | Scopus (140)See all References, 22xHeide, W. et al. Ann. Neurol. 1995; 38: 739–748Crossref | PubMed | Scopus (158)See all References]that parietal-lesioned patients cannot complete the second of two, sequenced, saccadic eye movements without visual feedback of eye and target locations, which could be interpreted as a failure to use corollary discharge to evaluate movement. By contrast, the parietal-lesioned patients studied by Sirigu et al.[9xSirigu, A. et al. Science. 1996; 273: 1564–1568Crossref | PubMedSee all References[9]are able to execute properly but not image movement sequences, a finding that might indicate a dependence upon evaluating reafferent signals. It is possible that these different forms of motor-evaluation deficits result from lesions that disrupt separate subregions of the posterior parietal lobe and, consequently, separate components of the posterior parietal network for evaluating ongoing motor behavior. That the superior parietal lobe (SPL) contributes to decision-making processes by evaluating visuospatial information has been demonstrated recently in a magnetic resonance imaging (MRI) study of mental rotation[23xTagaris, G.A. et al. NeuroReport. 1996; 7: 773–776Crossref | PubMedSee all References[23]. The level of SPL activation was proportional to the number of errors made by subjects, who had to mentally rotate one of two visual images and decide whether the images were the same or mirror reflections of each other.Parallel data from physiological studies in monkeys also indicate that posterior parietal neurons, long known to have visuomotor activity unrelated to simple sensory or motor signals, might be organized into functional groups according to how each contributes to the process of movement evaluation. Within area 5, one population of cells, which discharges only following the onset of voluntary limb movement, becomes silent after peripheral deafferentation of the performing limb, while a second population, which discharged well in advance of the same limb movement, was unaffected by limb deafferentation[24xSeal, J., Gross, C., and Bioulac, B. Brain Res. 1982; 250: 229–243Crossref | PubMed | Scopus (63)See all References[24]. In addition, neuronal activity recorded from a visuomotor intraparietal cortical area prior to eye-movement onset, reflects the sensory signals that would be expected were the eyes already at the final target position[25xDuhamel, J-R., Colby, C.L., and Goldberg, M.E. Science. 1992; 255: 90–92Crossref | PubMedSee all References[25]; activity that could result only from a corollary discharge. Therefore, posterior parietal activity appears to reflect both reafferent signals and corollary discharge. Taken in conjunction with the finding of neuronal activity representing intended movements[16xKalaska, J.F. and Crammond, D.J. Cereb. Cortex. 1995; 5: 410–428Crossref | PubMed | Scopus (197)See all References[16](Fig. 3Fig. 3), the posterior parietal network has all the necessary components to transform visuospatial information[26xAndersen, R.A. Cereb. Cortex. 1995; 5: 457–469Crossref | PubMed | Scopus (230)See all References, 27xLacquaniti, F. et al. Cereb. Cortex. 1995; 5: 391–409Crossref | PubMed | Scopus (239)See all References]into the signals that are required to evaluate movements[28xSakata, H. et al. Cereb. Cortex. 1995; 5: 429–438Crossref | PubMed | Scopus (431)See all References[28]as they occur.Patients with parietal lesions can, despite their problems, mentally image movements and perform them. Thus, they do not have a failure of imagination, rather, unlike controls, the internal mental `movements' of patients with parietal lesions do not appear to obey physical laws or to be tied to physiological constraints. Perhaps this is because rather than imaging movements, they are free to imagine moving however they want to, as in a dream.

Changes in motor activity and biomechanics during balance recovery following cutaneous and muscular deafferentation

The effects of lower limb deafferentation were examined during execution of a balance recovery step following a forward fall induced by release of an initial inclined posture. The subjects were healthy control subjects and patients with a unilateral loss of the Achilles tendon reflex following S1 radiculitis. Deafferentation of healthy subjects was obtained by unilateral leg ischemia (four subjects) and by foot anesthesia (five subjects). The balance recovery step was characterized by the surface electromyographic (EMG) activity of right and left soleus and tibialis anterior muscles and the kinetics of the center of gravity and center of foot pressure. Experimentally induced and pathological deafferentation decreased the EMG activity of the ipsilateral soleus and lowered the vertical ground reaction force. The lower limb motor activity was more affected by loss of muscle proprioceptive afferents than by loss of plantar cutaneous afferents. Patients showed early and bilateral changes in soleus and tibialis activities, whichever side was affected. The step length of patients was also shorter than that of controls, but it remained similar before and after deafferentation in the healthy subjects. The results are discussed in terms of ipsilateral and crossed pathway connections and functional adaptive strategies.

Correlation between the kinematics of hindlimb movement and efferent activity in the decerebrate cat during scratching

Statistical dependence between parameters of hindlimb scratching movements and muscle electrical activity was studied in decerebrated cat during real scratching. Power of electrical signal to scratching and aiming muscles and duration of aiming muscle activity are determining factors of movement parameters (amplitude, velocity and phase duration). Experiments with limb deafferentation had shown that statistical dependences between movement parameters and muscle electrical activity reflect the peculiarities of the scratch generator functioning. Some investigated statistical dependences change under the afferent signal influence.

Kinematic effects of deafferentation and cerebellar ablation.

Trajectories of hand movements directed visually toward a stationary target were compared by kinematic methods in control monkeys, those with bilateral upper limb deafferentation, those with cerebellar ablation and those with both these lesions. The purpose of the investigation was to determine whether: (1) deafferented animals display kinematic abnormalities similar to those of decerebellate animals, and (2) a combination of these two lesions results in cumulative motor disorders. The decerebellate preparation had significant increases of the ratio of path length to target distance, mean deviation from a straight line path, segment angle (average change in direction between successive segments of the trajectory), target angle (average angle between the path taken by the hand and a straight line path to the target), peak velocity, average acceleration and peak acceleration. The deafferented preparation had increases of these parameters significantly greater than the decerebellate preparation. Neither preparation showed abnormalities of average linear velocity. The deafferented preparation rendered decerebellate had significant additional increases in all parameters except average segment angle, an index of angular velocity. Scatter plots of acceleration versus velocity and of segment angle versus target angle revealed increasing dispersion in the sequence: control, decerebellate, deafferented, deafferented with subsequent cerebellar ablation. The quantitative demonstration of cumulative worsening of motor performance when the deafferented animal is rendered decerebellate indicates that there are mechanisms of cerebellar dysmetria independent of the fusimotor efferent-spindle afferent reflex arc.

Cortical motor potential in monkeys before and after upper limb deafferentation

Four monkeys were trained to perform wrist extension at regular intervals. The motor potential (MP) and EMG associated with these movements were recorded before, and in two of the animals, after dorsal rhizotomy extending from C2 to T4. Although wrist extension of the deafferented limbs appeared superficially normal, the contractions developed more slowly and were more prolonged. There was also a substantial increase in the interval between onset of cortical and muscular activity (corticomuscular delay), as well as increased variability of CMD. These changes could be attributed in part to reduced spinal excitability due to deafferentation. Despite the marked changes in timing, the configuration of the MP was unaltered. It is suggested that kinesthetic feedback is not registered in motor cortex during the performance of skilled movements.