Digit removal leads to discrepancies between the structural and functional organization of the forepaw barrel subfield in layer IV of rat primary somatosensory cortex

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.

Terminal Arbors of Callosal Axons Undergo Plastic Changes in Early-Amputated Rats

Sensory information is processed in specific brain regions, and shared between the cerebral hemispheres by axons that cross the midline through the corpus callosum. However, sensory deprivation usually causes sensory losses and/or functional changes. This is the case of people who suffered limb amputation and show changes of body map organization within the somatosensory cortex (S1) of the deafferented cerebral hemisphere (contralateral to the amputated limb), as well as in the afferented hemisphere (ipsilateral to the amputated limb). Although several studies have approached these functional changes, the possible finer morphological alterations, such as those occurring in callosal axons, still remain unknown. The present work combined histochemistry, single-axon tracing and 3D microscopy to analyze the fine morphological changes that occur in callosal axons of the forepaw representation in early amputated rats. We showed that the forepaw representation in S1 was reduced in the deafferented hemisphere and expanded in the afferented side. Accordingly, after amputation, callosal axons originating from the deafferented cortex undergo an expansion of their terminal arbors with increased number of terminal boutons within the homotopic representation at the afferented cerebral hemisphere. Similar microscale structural changes may underpin the macroscale morphological and functional phenomena that characterize limb amputation in humans.

Quantitative assessment of cortical somatosensory digit representations after median and ulnar nerve injury in rats

Incomplete recovery of sensory function is common after peripheral nerve injury (PNI). Despite reinnervation following injury, disorganized cortical representations persist and may contribute to functional deficits. There is a dearth of literature characterizing cortical responses after PNI in rodent models. Here we develop a quantitative electrophysiological method for mapping forepaw digit responses in primary somatosensory cortex (S1) of rats. We tested the hypothesis that PNI in the forelimb would generate significant, long lasting sensory deficits, and corresponding disorganization in S1. Rats underwent a transection of the proximal segment of the median and ulnar nerves in the forelimb followed by tubular repair. 4-12 months after nerve injury, we tested mechanosensory withdrawal thresholds and mapped S1 responses to mechanical stimulation of the digits. PNI produces persistent elevation of mechanical withdrawal thresholds, consistent with an impairment in sensory function. Assessment of cortical neurophysiology reveals a substantial disorganization of S1 somatotopy. Additionally, we document degraded timing and digit specificity of cortical responses. This quantitative measurement of long-term changes in S1 digit representations after forelimb nerve injury in rodents provides a framework for further studies focused on the development of therapeutic strategies to restore cortical and sensory function.

Cortical Plasticity After Surgical Tendon Transfer in Tetraplegics

Background: Developmental cortical plasticity with reorganization of cerebral cortex, has been known to occur in young and adult animals after permanent, restricted elimination of afferent (visual or somatosensory) input. In animals, cortical representation of unaffected muscles or sensory areas has been shown to invade the neighboring cortex when this is deprived of its normal sensory input or motor functions. Some studies indicate that similar cortical plasticity may take place in adult humans. Methods: In patients with a high cervical spinal cord injury leaving the patient without any movements of the fingers, we performed fMRI studies of the cortical representation of an elbow flexor muscle before and after a surgical procedure that changed its function to a thumb flexor, thus providing the patient with a useful grip. Results: Preoperatively, the elbow flexion movement was elicited from a cortical area corresponding with the "elbow area" in healthy individuals. Despite the fact that an elbow flexor was used for the post-operative key-grip, this movement in the tetraplegic patients was elicited from a similar brain region as in healthy controls (the "hand area"). This supports our hypothesis that control of that muscle shifts from a brain region typically associated with elbow movement, to one typically associated with wrist movements. Conclusion: The findings presented here show with fMRI that the human cortex is capable of reorganizing itself spatially after a relatively acute change in the periphery.

Somatosensory maps

Somatosensory areas containing topographic maps of the body surface are a major feature of parietal cortex. In primates, parietal cortex contains four somatosensory areas, each with its own map, with the primary cutaneous map in area 3b. Rodents have at least three parietal somatosensory areas. Maps are not isomorphic to the body surface, but magnify behaviorally important skin regions, which include the hands and face in primates, and the whiskers in rodents. Within each map, intracortical circuits process tactile information, mediate spatial integration, and support active sensation. Maps may also contain fine-scale representations of touch submodalities, or direction of tactile motion. Functional representations are more overlapping than suggested by textbook depictions of map topography. The whisker map in rodent somatosensory cortex is a canonic system for studying cortical microcircuits, sensory coding, and map plasticity. Somatosensory maps are plastic throughout life in response to altered use or injury. This chapter reviews basic principles and recent findings in primate, human, and rodent somatosensory maps.

Functional role of exercise-induced cortical organization of sensorimotor cortex after spinal transection

Spinal cord transection silences neuronal activity in the deafferented cortex to cutaneous stimulation of the body and untreated animals show no improvement in functional outcome (weight-supported stepping) with time after lesion. However, adult rats spinalized since neonates that receive exercise therapy exhibit greater functional recovery and exhibit more cortical reorganization. This suggests that the change in the somatotopic organization of the cortex may be functionally relevant. To address this issue, we chronically implanted arrays of microwire electrodes into the infragranular layers of the hindlimb somatosensory cortex of adult rats neonatally transected at T8/T9 that received exercise training (spinalized rats) and of normal adult rats. Multiple, single neuron activity was recorded during passive sensory stimulation, when the animals were anesthetized, and during active sensorimotor stimulation during treadmill-induced locomotion when the animal was awake and free to move. Our results demonstrate that cortical neurons recorded from the spinalized rats that received exercise 1) had higher spontaneous firing rates, 2) were more likely to respond to both sensory and sensorimotor stimulations of the forelimbs, and also 3) responded with more spikes per stimulus than those recorded from normal rats, suggesting expansion of the forelimb map into the hindlimb map. During treadmill locomotion the activity of neurons recorded from neonatally spinalized rats was greater during weight-supported steps on the treadmill compared with the neuronal activity during nonweight supported steps. We hypothesize that this increased activity is related to the ability of the animal to take weight supported steps and that, therefore, these changes in cortical organization after spinal cord injury are relevant for functional recovery.

Mapping cortical representations of the rodent forepaw and hindpaw with BOLD fMRI reveals two spatial boundaries

Electrical stimulation of the rat forepaw and hindpaw was employed to study the spatial distribution of BOLD fMRI. Averaging of multiple fMRI sessions significantly improved the spatial stability of the BOLD signal and enabled quantitative determination of the boundaries of the BOLD fMRI maps. The averaged BOLD fMRI signal was distributed unevenly over the extent of the map and the data at the boundaries could be modeled with major and minor spatial components. Comparison of three-dimensional echo-planar imaging (EPI) fMRI at isotropic 300 μm resolution demonstrated that the border locations of the major spatial component of BOLD signal did not overlap between the forepaw and hindpaw maps. Interestingly, the border positions of the minor BOLD fMRI spatial components extended significantly into neighboring representations. Similar results were found for cerebral blood volume (CBV) weighted fMRI obtained using iron oxide particles, suggesting that the minor spatial components may not be due to vascular mislocalization typically associated with BOLD fMRI. Comparison of the BOLD fMRI maps of the forepaw and hindpaw to histological determination of these representations using cytochrome oxidase (CO) staining demonstrated that the major spatial component of the BOLD fMRI activation maps accurately localizes the borders. Finally, 2-3 weeks following peripheral nerve denervation, cortical reorganization/plasticity at the boundaries of somatosensory limb representations in adult rat brain was studied. Denervation of the hindpaw caused a growth in the major component of forepaw representation into the adjacent border of hindpaw representation, such that fitting to two components no longer led to a better fit as compared to using one major component. The border of the representation after plasticity was the same as the border of its minor component in the absence of any plasticity. It is possible that the minor components represent either vascular effects that extend from the real neuronal representations or the neuronal communication between neighboring regions. Either way the results will be useful for studying mechanisms of plasticity that cause alterations in the boundaries of neuronal representations.

Bidirectional axonal plasticity during reorganization of adult rat primary somatosensory cortex

Cortical sensory maps contain discrete functional subregions that are separated by borders that restrict tangential activity flow. Interestingly, the functional organization of border regions remains labile in adults, changing in an activity-dependent manner. Here, we investigated if axon remodeling contributes to this reorganization. We located the border between the forepaw and lower jaw representation (forepaw/lower jaw border,(1) FP/LJ border) in SI of adult rats, and used a retrograde axonal tracer (cholera toxin subunit B(2), Ctb) to determine if horizontal axonal projections change after different durations of forelimb denervation or sham-denervation. In sham-denervated animals, neurons close to the border had axonal projections oriented away from the border (axonal bias). Forelimb denervation resulted in a sustained change in border location and a significant reduction in the axonal bias at the original border after 6 weeks of denervation, but not after 4 or 12 weeks. The change in axonal bias was due to an increase in axons that cross the border at 6 weeks, followed by an apparent loss of these axons by 12 weeks. This suggests that bidirectional axonal rearrangements are associated with relatively long durations of reorganization and could contribute transiently to the maintenance of cortical reorganization.

Mapping plasticity in the forepaw digit barrel subfield of rat brains using functional MRI

The topographic organization of the forepaw barrel subfield in layer IV of rat primary somatosensory cortex (S1) is a good model for studying neural function and plasticity. The goal of this study was to test the feasibility of functional MRI (fMRI) to map the forepaw digit representations in the S1 of the rat and its plasticity after digit amputation. Three dimensional echo-planar imaging with 300 micron isotropic resolution at 11.7 T was used to achieve high signal-to-noise ratios and laminar layer resolution. By alternating electrical stimulation of the 2nd (D2) and 4th (D4) digits, functional activation in layer IV of the barrel subfields could be distinguished using a differential analysis. Furthermore, 2 and a half months after the amputation of the 3rd digit in baby rats, the overlapping area between D2 and D4 representations was increased. This indicates that the forepaw barrel subfield previously associated with the ablated digit is now associated with the representation of nearby digits, which is consistent with studies using electrophysiology and cytochrome oxidase staining.

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.

Changes in intrinsic properties of pyramidal neurons in adult rat S1 during cortical reorganization

Peripheral denervation causes significant changes in the organization of developing or adult primary somatosensory cortex (S1). However, the basic mechanisms that underlie reorganization are not well understood. Most attention has been focused on possible synaptic mechanisms associated with reorganization. However, another important determinant of cortical circuit function is the intrinsic membrane properties of neurons in the circuit. Here we document changes in the intrinsic properties of pyramidal neurons in cortical layer 2/3 in adult rat primary somatosensory cortex (S1) after varying durations of forepaw denervation. Denervation of the forepaw induced a rapid and sustained shift in the location of the border between the forepaw and lower jaw representations of adult S1 (reorganization). Coronal slices from the reorganized region were maintained in vitro and the intrinsic properties of layer 2/3 pyramidal neurons of S1 were determined using whole cell recordings. In general, passive membrane properties were not affected by denervation; however, a variety of active properties were. The most robust changes were increases in the amplitudes of the fast and medium afterhyperpolarization (AHP) and an increase in the interval between action potentials (APs). Additional changes at some durations of denervation were observed for the AP threshold. These observations indicate that changes in intrinsic properties, mostly reflecting a decrease in overall excitation, may play a role in changes in cortical circuit properties during reorganization in adult S1, and suggest a possible role for AHPs in some of those changes.

Myelin stains reveal an anatomical framework for the representation of the digits in somatosensory area 3b of macaque monkeys

Brain sections cut parallel to the cortical surface revealed myelin-light septa that isolated representations of the digits and parts of the face, teeth, and tongue in area 3b of adult and infant macaque monkeys. The widths of the bands of cortex representing individual digits, as measured by the distances between isolating septa, were proportionally similar in infant (2-4 week) and adult monkeys. However, the bands for digits 1-3 were somewhat narrower in infant than adult monkeys. There was little variation in absolute widths across individuals in the infant or adult groups, or between left and right hemispheres of the same group. Widths for digits 1-4 progressively decreased. The results suggest that these isomorphs of digits emerge in prenatal or early postnatal development and typical variations in postnatal hand use have little impact on subsequent development. As the hand representation in somatosensory cortex of monkeys may be significantly altered after the partial loss of peripheral nerve inputs, the physiological representation is not completely constrained by the isolating septa. Instead, the septa may serve as a persistent marker of normal organization in studies of cortical reorganization.

Local circuit properties underlying cortical reorganization

Peripheral denervation has been shown to cause reorganization of the deafferented somatotopic region in primary somatosensory cortex (S1). However, the basic mechanisms that underlie reorganization are not well understood. In the experiments described in this paper, a novel in vivo/in vitro preparation of adult rat S1 was used to determine changes in local circuit properties associated with the denervation-induced plasticity of the cortical representation in rat S1. In the present studies, deafferentation of rat S1 was induced by cutting the radial and median nerves in the forelimb of adult rats, resulting in a rapid shift of the location of the forepaw/lower jaw border; the amount of the shift increased over the times assayed, through 28 days after denervation. The locations of both borders (i.e., original and reorganized) were marked with vital dyes, and slices from the marked region were used for whole-cell recording. Responses were evoked using electrical stimulation of supragranular S1 and recorded in supragranular neurons close to either the original or reorganized border. For each neuron, postsynaptic potentials (PSPs) were evoked by stimulation of fibers that crossed the border site (CB stim) and by equivalent stimulation that did not cross (NCB stim). Monosynaptic inhibitory postsynaptic potentials (IPSPs) were also examined after blocking excitatory transmission with 15 microM CNQX plus 100 microM DL-APV. The amplitudes of PSPs and IPSPs were compared between CB and NCB stimulation to quantify effects of the border sites on excitation and inhibition. Previous results using this preparation in the normal (i.e., without induced plasticity) rat S1 demonstrated that at a normal border both PSPs and IPSPs were smaller when evoked with CB stimulation than with NCB stimulation. For most durations of denervation, a similar bias (i.e., smaller responses with CB stimulation) for PSPs and IPSPs was observed at the site of the novel reorganized border, while no such bias was observed at the suppressed original border site. Thus changes in local circuit properties (excitation and inhibition) can reflect larger-scale changes in cortical organization. However, specific dissociations between these local circuit properties and the presence of the novel border at certain durations of denervation were also observed, suggesting that there are several intracortical processes contributing to cortical reorganization over time and that excitation and inhibition may contribute differentially to them.

Low grip strength, impaired tongue force and hyperactivity induced by overexpression of neurotrophin-3 in mouse skeletal muscle

Transgenic mice overexpressing neurotrophin-3 (NT-3) in skeletal muscle (mlc/NT-3 mice) develop abnormal muscle spindles in skeletal muscle and display abnormal motor function in the form of gait and locomotive disturbances. The purpose of this work was to characterize the functional consequences of NT-3 overexpression in skeletal muscle with further behavioral assessments that permitted inferences about muscle weakness in the tongue or forelimbs as well as potential central nervous system (CNS) abnormalities compared to wild-type controls. Wild-type (n=12) and mlc/NT-3 (n=12) male mice were tested in five procedures (in chronological order): lick dynamics, locomotor activity, grid ataxia, go-no-go discrimination procedure, and grip strength. Relative to wild-type mice, the mlc/NT-3 mice exhibited lower tongue force, hyperactivity, slowed limb retrieval in the grid ataxia test, similar discrimination performance, and lower grip strength. Overall, the data suggest that chronically elevated levels of NT-3 in mouse skeletal muscle cause muscle weakness in the mlc/NT-3 mice. Surprisingly, mlc/NT-3 mice also exhibited significant hyperactivity, suggesting that NT-3 overexpression in the periphery may have caused abnormalities in the CNS that are related to the cortical processing of proprioceptive afferent information.

Atropine prevents the changes in the hindlimb cortical area induced by hypodynamia-hypokinesia

It has been demonstrated that hypodynamia-hypokinesia (HH), a model of sensory disruption, induced a decrease in the cortical hindpaw representation and an enlargement of the cutaneous receptive fields (RFs). The present study was carried out to determine whether chronic application of atropine could prevent this reorganisation. The extent of the hindlimb representation on the somatosensory cortex was determined in control rats (C), rats submitted to HH (HH), and rats submitted to HH with a chronic cortical infusion of atropine (70 mM, HH-ATR). Our results show that the hindpaw cortical area was similar for the HH-ATR and C rats, and was smaller for the HH rats. The distribution of RFs was comparable for the C and HH-ATR groups with a high percentage of small RFs. In contrast, for the HH rats, the percentage of large RFs was higher. Atropine can thus prevent the reduction in the hindlimb cortical area induced by HH. These results suggest that cholinergic mechanisms contribute to cortical plasticity.

Single-cell correlates of a representational boundary in rat somatosensory cortex

In primary somatosensory cortex (S1), the transition from one representation to the next is typically abrupt when assayed physiologically. However, the extent of anatomical projections to and within the cortex do not strictly respect these physiologically defined transitions. Physiological properties, such as synaptic strengths or intracortical inhibition, have been hypothesized to account for the functionally defined precision of these representational borders. Because these representational borders can be translocated across the cortex by manipulations or behaviors that change the activity patterns of inputs to the cortex, understanding the physiological mechanisms that delimit representations is also an important starting point for understanding cortical plasticity. A novel in vivo and in vitro preparation has been developed to examine the cellular and synaptic mechanisms that underlie representational borders in the rat. In vivo, a short segment of the border between the forepaw-lower jaw representations in rat S1 was mapped using standard electrophysiological methods and was visibly marked using iontophoresis of pontamine sky blue dye. Slices were then obtained from this marked region and maintained in vitro. Intracellularly recorded responses to electrical stimulation of supragranular cortex were obtained from single neurons near the border in response to stimulation within the representational zone or across the border. Both excitatory and inhibitory responses were smaller when evoked by stimuli that activated projections that crossed borders, as compared with stimuli to projections that did not. These findings indicate that intracortical network properties are contributing to the expressions of representational discontinuities in the cortex.