Non-overlapping thalamocortical projections for separate forepaw digits before and after cortical reorganization in the raccoon

Malleability of the cortical hand map following a finger nerve block

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

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

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

Forelimb amputation-induced reorganization in the ventral posterior lateral nucleus (VPL) provides a substrate for large-scale cortical reorganization in rat forepaw barrel subfield (FBS)

In this study, we examined the role of the ventral posterior lateral nucleus (VPL) as a possible substrate for large-scale cortical reorganization in the forepaw barrel subfield (FBS) of primary somatosensory cortex (SI) that follows forelimb amputation. Previously, we reported that, 6 weeks after forelimb amputation in young adult rats, new input from the shoulder becomes expressed throughout the FBS that quite likely has a subcortical origin. Subsequent examination of the cuneate nucleus (CN) 1 to 30 weeks following forelimb amputation showed that CN played an insignificant role in cortical reorganization and led to the present investigation of VPL. As a first step, we used electrophysiological recordings in forelimb intact adult rats (n=8) to map the body representation in VPL with particular emphasis on the forepaw and shoulder representations and showed that VPL was somatotopically organized. We next used stimulation and recording techniques in forelimb intact rats (n=5) and examined the pattern of projection (a) from the forelimb and shoulder to SI, (b) from the forepaw and shoulder to VPL, and (c) from sites in the forepaw and shoulder representation in VPL to forelimb and shoulder sites in SI. The results showed that the projections were narrowly focused and homotopic. Electrophysiological recordings were then used to map the former forepaw representation in forelimb amputated young adult rats (n=5) at 7 to 24 weeks after amputation. At each time period, new input from the shoulder was observed in the deafferented forepaw region in VPL. To determine whether the new shoulder input in the deafferented forepaw VPL projected to a new shoulder site in the deafferented FBS, we examined the thalamocortical pathway in 2 forelimb-amputated rats. Stimulation of a new shoulder site in deafferented FBS antidromically-activated a cell in the former forepaw territory in VPL; however, similar stimulation from a site in the original shoulder representation, outside the deafferented region, in SI did not activate cells in the former forepaw VPL. These results suggest that the new shoulder input in deafferented FBS is relayed from cells in the former forepaw region in VPL. In the last step, we used anatomical tracing and stimulation and recording techniques in forelimb intact rats (n=9) to examine the cuneothalamic pathway from shoulder and forepaw receptive field zones in CN to determine whether projections from the shoulder zone might provide a possible source of shoulder input to forepaw VPL. Injection of biotinylated dextran amine (BDA) into physiologically identified shoulder responsive sites in CN densely labeled axon terminals in the shoulder representation in VPL, but also gave off small collateral branches into forepaw VPL. In addition, microstimulation delivered to forepaw VPL antidromically-activated cells in shoulder receptive field sites in CN. These results suggest that forepaw VPL also receives input from shoulder receptive sites in CN that are latent or subthreshold in forelimb intact rats. However, we speculate that following amputation these latent shoulder inputs become expressed, possibly as a down-regulation of GABA inhibition from the reticular nucleus (RTN). These results, taken together, suggest that VPL provides a substrate for large-scale cortical reorganization that follows forelimb amputation.

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.

Changes in corticothalamic modulation of receptive fields during peripheral injury-induced reorganization

The influence of corticothalamic projections on the thalamus during different stages of reorganization was determined in anesthetized raccoons that had undergone previous removal of a single forepaw digit. Single-unit recordings were made from 522 sites in the somatosensory nucleus of the thalamus (ventroposterior lateral nucleus) before and after lesioning parts of primary somatosensory cortex. In those parts of ventroposterior lateral nucleus that had intact input from the periphery, the cortical lesion resulted in an immediate 85% increase in receptive field (RF) size. In animals studied 2-6 weeks after digit amputation, peripherally denervated thalamic neurons had unique RFs that were larger than normal, and these were not further enlarged by cortical lesion. However, at longer periods of reorganization (>4 mo), when the new RFs of denervated neurons had decreased in size, cortical lesion again produced expansion of RF size. These data demonstrate that corticothalamic fibers modulate the spatial extent of thalamic RFs in intact animals, probably by controlling intrathalamic inhibition. This corticothalamic modulation is ineffective during the early stages of injury-induced reorganization when new RFs are being formed, but is reinstated after the new RFs have become stabilized. The fact that neurons in the denervated thalamic region retained their unique RFs after cortical lesion indicates that their new inputs are not being relayed from a reorganized cortex and support the view that some plasticity occurs in or below the thalamus.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

The neural basis of perceptual learning

Perceptual learning is a lifelong process. We begin by encoding information about the basic structure of the natural world and continue to assimilate information about specific patterns with which we become familiar. The specificity of the learning suggests that all areas of the cerebral cortex are plastic and can represent various aspects of learned information. The neural substrate of perceptual learning relates to the nature of the neural code itself, including changes in cortical maps, in the temporal characteristics of neuronal responses, and in modulation of contextual influences. Top-down control of these representations suggests that learning involves an interaction between multiple cortical areas.

Projections from the medial geniculate body to primary auditory cortex in neonatally deafened cats

In the present study, anatomical projections from the medial geniculate body (MGB) to primary auditory cortex (AI) were investigated in normal adult cats and in animals that were neonatally deafened with the ototoxic drug amikacin. Cochleotopic/tonotopic maps in AI (based on neural response characteristic frequency) were obtained with microelectrode recording techniques, and single or multiple injections of retrograde tracers (horseradish peroxidase and fluorescent dyes) were introduced into AI. The AI maps of the amikacin-treated cats had an abnormal cochleotopic organization, such that deprived cortical areas exhibited an expanded representation of intact regions of the damaged cochlea. However, retrograde tracer injections into different regions of AI produced a normal pattern of labeling in the ventral division of the medial geniculate body (MGBv). In both experimental and control animals, the main mass of labeled thalamic cells was found in the MGBv. Different isofrequency contours in AI receive input from different portions of the MGBv. Thus, cell arrays labeled by anterior AI injections were situated medially in MGBv, and injections into posterior AI labeled MGBv more laterally. Furthermore, the deafened cats did not develop a more divergent thalamocortical projection compared with normal control animals, indicating that an abnormal spread of the thalamocortical afferents across the frequency domain in AI (anterior-posterior axis) is not responsible for the altered cochleotopic map in these neonatally deafened animals. The relatively normal thalamocortical projection pattern suggests that, after neonatal cochlear lesions, the major reorganization of cochleotopic maps occurs at subthalamic levels.

Adult cortical dynamics

There are many influences on our perception of local features. What we see is not strictly a reflection of the physical characteristics of a scene but instead is highly dependent on the processes by which our brain attempts to interpret the scene. As a result, our percepts are shaped by the context within which local features are presented, by our previous visual experiences, operating over a wide range of time scales, and by our expectation of what is before us. The substrate for these influences is likely to be found in the lateral interactions operating within individual areas of the cerebral cortex and in the feedback from higher to lower order cortical areas. Even at early stages in the visual pathway, cells are far more flexible in their functional properties than previously thought. It had long been assumed that cells in primary visual cortex had fixed properties, passing along the product of a stereotyped operation to the next stage in the visual pathway. Any plasticity dependent on visual experience was thought to be restricted to a period early in the life of the animal, the critical period. Furthermore, the assembly of contours and surfaces into unified percepts was assumed to take place at high levels in the visual pathway, whereas the receptive fields of cells in primary visual cortex represented very small windows on the visual scene. These concepts of spatial integration and plasticity have been radically modified in the past few years. The emerging view is that even at the earliest stages in the cortical processing of visual information, cells are highly mutable in their functional properties and are capable of integrating information over a much larger part of visual space than originally believed.

Effect of unilateral partial cochlear lesions in adult cats on the representation of lesioned and unlesioned cochleas in primary auditory cortex

We examined the effect of unilateral restricted cochlear lesions in adult cats on the topographic representations ("maps") of the lesioned and unlesioned cochleas in the primary auditory cortex (AI) contralateral to the lesioned cochlea. Frequency (tonotopic) maps were derived by conventional multineuron mapping procedures in anesthetized animals. In confirmation of a study in adult guinea pigs (Robertson and Irvine [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months after the unilateral cochlear lesion the map of the lesioned cochlea in the contralateral AI was altered so that the AI region in which frequencies with lesion-induced elevations in cochlear neural sensitivity would have been represented was occupied by an enlarged representation of lesion-edge frequencies (i.e., frequencies adjacent to those with elevated cochlear neural sensitivity). Along the tonotopic axis of AI the total representation of lesion-edge frequencies could extend up to approximately 2.6 mm rostal to the area of normal representation of these frequencies. There was no topographic order within this enlarged representation. Examination of threshold sensitivity at the characteristic frequency (CF, frequency to which the neurons were most sensitive) in the reorganized regions of the map of the lesioned cochlea established that the changes in the map reflected a plastic reorganization rather than simply reflecting the residue of prelesion input. In contrast to the change in the map of the lesioned contralateral cochlea, the map of the unlesioned ipsilateral cochlea did not differ from those in normal animals. Thus, in contrast to the normal very good congruency between ipsilateral and contralateral AI maps, in the lesioned animals ipsilateral and contralateral maps differed in the region of AI in which there had been a reorganization of the map of the lesioned cochlea. Outside the region of contralateral map reorganization, ipsilateral and contralateral AI maps remained congruent within normal limits. The difference between the two maps in the region of contralateral map reorganization suggested, in light of the physiology of binaural interactions in the auditory pathway, that the cortical reorganization reflected subcortical changes. Finally, response properties of neuronal clusters within the reorganized map of the lesioned cochlea were compared to normative data with respect to threshold sensitivity at CF, the size of frequency "response areas," and response latencies. In the majority of cases, CF thresholds were similar to normative data. The frequency "response areas" were slightly less sharply tuned than normal, but not significantly. Response latencies were significantly shorter than normal in three animals and significantly longer in one animal.

The immediate effects of peripheral denervation on inhibitory mechanisms in the somatosensory thalamus

Multiunit recordings were made in the ventroposterior lateral nucleus of the thalamus in anesthetized raccoons. During recording from cells responding to cutaneous stimulation of a forepaw digit, the corresponding digit was denervated permanently (by cutting its four digital nerves) or temporarily (by injecting lidocaine into the base of the digit). Both procedures resulted in immediate increases in the inhibition that could be induced by stimulation of the adjacent digits when the original cutaneous receptive field was on the glabrous skin. In each case with temporary denervation, this enhanced off-focus inhibition decreased when the excitatory responses returned to normal. In contrast, temporary denervation of the digit during recording at sites in the hairy skin representation did not reveal this increased inhibition from adjacent digits. When capsaicin was applied to the digital nerves in two animals, the excitatory receptive fields of thalamic neurons increased in area, but were still restricted to the same part of the digit. These data indicate that the immediate unmasking of inhibitory responses, previously reported in primary somatosensory cortex of the raccoon, is also present in the thalamus. The capsaicin-induced expansion of excitatory receptive fields confirms previous experiments in other species, and suggests that C fibers play a role in modulating the size of cutaneous receptive fields. However, the enlargement of excitatory receptive fields by capsaicin is much less than the unmasking of inhibitory fields induced by digit denervation, and indicates that different mechanisms are involved in controlling these various inputs to thalamic neurons.

Neuronal response properties within subregions of raccoon somatosensory cortex 1 week after digit amputation

Multiple penetrations in the somatosensory cortex of three anesthetized raccoons 1 week following amputation of the fourth digit provided detailed information about somatotopy and neuronal responsiveness in the deafferented cortex. Recordings in a total of 601 penetrations (292 in deafferented cortex and 309 in the surrounding cortex) were compared with those from intact control animals described previously (Rasmusson et al., 1991). The level of spontaneous activity increased within the deafferented cortex, with 42% of the sites having high or moderate levels of spontaneous activity, in comparison with 18% in control animals. There was also an increase in the incidence of inhibitory responses to stimulation of adjacent digits (26% of the penetrations vs. 10% in control animals), confirming previous findings. These two variables, increased spontaneous activity and the presence of strong lateral inhibition, were highly correlated in individual penetrations. An unexpected finding was that the cortex representing the intact parts of forepaw was also disrupted with respect to these two measures, suggesting that amputation had an effect outside the deafferented region. In contrast, response properties that are more clearly a reflection of information processing in the dorsal column-medial lemniscal pathway (adaptation and threshold) were altered only within the deafferented region. The deafferented region was not homogeneous immediately after amputation, but consisted of a radically affected core region and a slightly affected fringe adjacent to the intact representations. This inhomogeneity had also been apparent with partial digit deafferentation, reported previously. The fringe, approximately 1 mm in width, may reflect overlapping projections from adjacent digits at one or more levels of the somatosensory pathway. Since the size of the fringe is similar to the maximum extent of reorganization found in other models of reorganization, the mechanisms of plasticity within this region may involve an unmasking of pre-existing synapses with slight modification in synaptic strength. However, the plasticity within the core region of the raccoon seen in these experiments, which may be 5 mm from nondeafferented cortex, requires more extensive changes, perhaps via polysynaptic pathways.

Chronic effects of total or partial digit denervation on raccoon somatosensory cortex

Electrophysiological recordings were made in the primary somatosensory cortex of anesthetized raccoons 14 to 169 days following digit amputation or 60 to 129 days after transection of the two nerves innervating the ventral surface of the fourth digit. The incidence of inhibitory responses decreased from 50% of the penetrations immediately after amputation to 35% over the first 3 weeks and to almost zero after 2 months. The number of sites with low-threshold excitatory responses increased from 4% to 14% to 50% during these same intervals. Initially, the excitatory fields were small and located over the nerve stumps, and were therefore probably due to direct stimulation of the damaged nerves. At 2 months after amputation, the excitatory receptive fields were large and diffuse. Although the size of receptive fields decreased during the later period (when the thresholds were also decreasing), there was no recovery of any precise somatotopic organization in the deafferented cortex. The reorganization process in the raccoon thus consists of at least two stages: The early stage is dominated by inhibitory connections, whereas the second involves a recovery and restructuring of excitatory inputs. From 2 to 4 months after partial digit denervation, there were only minor changes in response properties or somatotopic organization in the deafferented cortex as compared to immediately after nerve transection. Thus, few of the characteristics of reorganization induced by digit amputation were elicited by this treatment, which leaves some of the digit innervation intact. There was, however, an unexpected increase in the portion of the ventral digit that was able to activate the cortex, suggesting complexities in the peripheral innervation of the digit that need to be resolved.

Responsiveness of reorganized primary somatosensory (SI) cortex after local inactivation of normal SI cortex in chronic spinal cats

The cortical map of adult cats that sustained spinal cord transection at T12 when they were 2 weeks old is characterized by a clear duplication of the representation of the forelimb, rostral trunk, and neck. The novel representation is located in the cortical region that is, in nonoperated animals, normally devoted to the hindlimb representation. We have investigated the possibility that the reactivation of the deprived hindlimb cortex may be mediated by corticocortical projections from normal to reorganized cortex. The primary somatosensory (SI) cortex was initially mapped to determine the boundaries of the normal and reorganized cortical representations. Somatotopically corresponding regions in both normal and reorganized cortex representing the trunk, the web space, or the shoulder were more precisely mapped. Inactivation of normal cortex was achieved by the nanoinjection of a solution of lidocaine hydrochloride stained with Chicago sky blue. Two major findings are described. First, inactivation of a circumscribed region of normal cortex representing a given receptive field (RF) failed to reduce or inhibit the responsiveness of a somatotopically corresponding RF represented in reorganized cortex. Therefore, it is unlikely that intracortical connections between normal and reorganized cortex could account for the reorganizational processes observed in cats that sustained spinal cord transection at 2 weeks of age. Second, the chemical blockade of normal cortex provoked an increase of the responsiveness and of the size of the peripheral RFs represented in reorganized cortex. This finding suggests that there are corticocortical connections (possibly topographically organized) between normal and reorganized cortex, and that these connections are inhibitory.

Chronological observations of primary somatosensory cortical maps in kittens following low thoracic (T12) spinal cord transection at 2 weeks of age

The present study investigated the reorganization of the somatosensory cortex in kittens following T12 spinal cord transection at 2 weeks of age. Multiunit electrophysiological methods were used to map the somatosensory cortex of kittens at 3, 6, and 9 weeks after the transection. The entire reorganized cortical region was driven by substitute cutaneous inputs, primarily from the trunk, at 3 weeks after spinal cord transection. Although the level of cortical responsiveness remained the same throughout the 9 weeks studied, internal trunk representation changed, and there was an increase in shoulder girdle representation and emergence of forelimb representation. Poor somatotopic and topographic order was observed in the reorganized cortex, regardless of time posttransection. Finally, trunk receptive fields displayed a wide variety of shapes, sizes, and orientations not seen in the normal cortex.

Temporal progression of cortical reorganization following nerve injury

Damage to peripheral nerves of adult mammals causes reorganization of somatosensory maps in the cerebral cortex. An understanding of the temporal progression of cortical changes is important for understanding the underlying mechanisms. The present experiments utilized neurophysiological recordings to analyze the time course of reorganization in the S-I cortical hindpaw area in adult rats. Following loss of sciatic inputs, the cortical area responding to low threshold inputs from the hindpaw saphenous nerve expands. A brief, early onset period of rapid expansion is followed by a prolonged period of slow increase. The temporal progression suggests that early onset changes condition the central nervous system for later changes.

Connections between area 3b of the somatosensory cortex and subdivisions of the ventroposterior nuclear complex and the anterior pulvinar nucleus in squirrel monkeys

The goal of this study was to determine whether somatosensory thalamic nuclei other than the ventroposterior nucleus proper (VP) have connections with area 3b of the postcentral cortex in squirrel monkeys. Small injections of the anatomical tracers wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) or 3H-proline were placed in electrophysiologically identified representations of body parts. The results indicate that, besides the well-established somatotopically organized connections with VP, area 3b has connections with three other nuclei of the somatosensory thalamus: the ventroposterior superior nucleus (VPS ["shell" of VP]), the ventroposterior inferior nucleus (VPI), and the anterior pulvinar nucleus (Pa). Injections confined to area 3b or involving adjacent parts of area 3a or area 1 indicate that connections between VPS, VPI, and Pa and the postcentral cortex are somatotopically organized. In VPS, connections related to the hand were found medially, and connections related to the foot were lateral. In VPI, connections with the cortical representations of the mouth, hand, and foot were successively more lateral. In Pa, connections related to the mouth, hand, and foot were successively more ventral, lateral, and caudal, and the trunk region was caudomedial. The findings suggest that VPI contains a representation of all parts of the body, including the face. The connections of Pa with the primary somatosensory cortex, area 3b, the location of Pa relative to the ventroposterior nucleus, and the high degree of topographic order in the connections of Pa with the postcentral cortex suggest that Pa is an integral part of the somatosensory thalamus in monkeys and is homologous to the medial nucleus of the posterior group (Pom) in other mammals. Overall, the results contribute to the growing evidence that individual somatosensory cortical areas in monkeys receive inputs from multiple thalamic sources, and that a single thalamic nucleus has several cortical targets.

Acute effects of total or partial digit denervation on raccoon somatosensory cortex

The immediate effects of total or partial denervation of single digits (0-16 hr after nerve transection) on primary somatosensory cortex were studied electrophysiologically. Comparisons of response properties and cortical somatotopy were made between intact raccoons and four groups of raccoons with transection of some or all of the nerves innervating the fourth or fifth digit. Animals with all four digital nerves cut (amputation of the digit) were most different from normal. Approximately half of the penetrations in the affected cortical region showed inhibitory responses to stimulation of adjacent skin regions. These consisted of a strong response to stimulus offset and/or a suppression of spontaneous activity during indentation. Since these responses were substantially different from those recorded several months after digit amputation, additional changes in connectivity and synaptic strength must occur with chronic denervation. These inhibitory responses were not seen in animals with one, two, or three nerves cut per digit. In the animals with partial denervation of a digit, the greatest disruption occurred when both ventral nerves to the glabrous skin were transected. This yielded cell clusters with abnormally large receptive fields, disruptions in somatotopic organization, and a decreased occurrence of low-threshold responses. If only one nerve to glabrous skin was transected, there was less change, even if it was combined with transection of both nerves to hairy skin. These results suggest that the release of inhibitory responses in a cortical digital region by amputation is prevented by the retention of even one ventral nerve. None of the denervation conditions produced large nonresponsive areas of cortex, which would have indicated a loss of all inputs.

Projections of digit afferents to the cuneate nucleus in the raccoon before and after partial deafferentation

Within the cuneate nucleus of the raccoon, the representations of individual forepaw digits are anatomically separated by densely myelinated laminae. This unique arrangement was utilized to determine whether the terminations of cutaneous afferents from individual digits are precisely restricted to the appropriate region of the cuneate nucleus or overlap with afferents from adjacent digits. By using the transganglionic transport of horseradish peroxidase (HRP), it was found that, for each digit, the terminal labeling was restricted to the appropriate 150-250-micron-wide column that extended rostrocaudally throughout the nucleus. The topographical arrangement of digit input corresponded to the known electrophysiology, with the terminal column for the fifth digit located most medially within the nucleus and those for digits 4 to 1 successively more laterally. Within a column, the density of labeling was greater over cell clusters than between clusters. These results indicate that afferents from adjacent digits do not overlap in the cuneate nucleus. In six animals, the fifth digit was amputated, and 2-4 months later, HRP was injected into the nerves of the fourth digit to determine whether its afferents had sprouted into the denervated fifth-digit column. The projection pattern from the fourth digit in each of these animals was the same as in normal animals and the same as in the intact contralateral side. These results indicate that the reorganization seen in the cerebral cortex following peripheral deafferentation cannot be attributed to changes in the afferent fiber projections to the cuneate nucleus.

Nonoverlapping thalamocortical connections to normal and deprived primary somatosensory cortex for similar forelimb receptive fields in chronic spinal cats

The fluorescent dye retrograde tracing technique, using fast blue in combination with fluorogold, was used to examine thalamocortical projections from the ventrobasal complex to primary somatosensory cortex in chronic spinal cats that sustained T12 cord transection at 2 weeks of age. Following cord transection at this age, it has been shown that forelimb afferents can excite the deprived hindlimb projection zone, in addition to the region of somatosensory cortex that they normally occupy (McKinley et al., 1987). These two regions of cortex are separated by over 10 mm, thus facilitating the determination of whether the forelimb representation in "hindlimb cortex" is derived from the sector of the ventrobasal complex of the thalamus representing the forelimb, hindlimb, or both. Injections of the two dyes into separate regions of the cortex that were excited by the same peripheral forelimb receptive fields produced single labeling of two nonoverlapping clusters of thalamic neurons. This finding suggests that the projections for these two areas are independent and distinct, and indicates that altered thalamocortical projections do not contribute the critical component underlying reorganizational changes observed at the cortical level after spinal cord transection. It is hypothesized that the degree of reorganization required to achieve the magnitude of change observed in the cortex must occur below the level of the thalamocortical relay.

Somatotopically inappropriate projections from thalamocortical neurons to the SI cortex of the cat demonstrated by the use of intracortical microstimulation

Single thalamocortical neurons with receptive fields on the toes were antidromically activated by the passage of 300-microseconds, 0.5- to 10-microA pulses through glass micropipette electrodes placed within somatotopically identified regions of the digit representation of the cat first somatosensory (SI) cortex. The somatotopy of the cortex was determined using recordings from single cortical neurons (see "Methods"), and the positions of the all tracks were marked on an enlarged photograph of the postcruciate cortex. In two of the three protocols, a very precise map of the boundary between two adjacent toes was produced prior to attempting intracortical microstimulation. Slopes of the threshold-distance curves at the sites of the lowest recorded thresholds were on the order of 0.8 microA/10 micron. This value, together with information on the anatomy of the cortical arborizations of thalamocortical neurons (Landry and Deschenes, 1981), suggested that currents of 2 and 5 microA would not activate the cortical processes of thalamocortical neurons at distances greater than 50 and 90 microns, respectively. With currents below 5 microA, thalamocortical neurons could be antidromically activated at a number of sites at depths between 340 and 930 microns (layer IV and upper layer III) and between 1,050 and 1,460 microns (layer VI). A total of 13 thalamocortical neurons could be antidromically activated using current pulses of between 0.8 and 5.0 microA, from within tracks at tangential distances of 250-830 microns from the nearest track through the somatotopically appropriate region. Within somatotopically inappropriate regions, cortical neurons frequently had receptive fields on a toe adjacent to that bearing the receptive field of the thalamic neuron(s) under study. The possible relationship of somatotopically inappropriate projections to the reorganization of cortical somatotopy following digit amputation, paw amputation, and nerve section is discussed.