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

Capsaicin-Induced Skin Desensitization Differentially Affects A-Delta and C-Fiber-Mediated Heat Sensitivity

Localized neuropathic pain can be relieved following the topical application of high-concentration capsaicin. This clinical effect is thought to be related to the temporary desensitization of capsaicin- and heat-sensitive epidermal nociceptors. The objective of the present study was to examine whether the changes in thermal sensitivity induced by high-concentration topical capsaicin can be explained entirely by desensitization of capsaicin-sensitive afferents. For this purpose, we characterized, in 20 healthy human volunteers, the time course and spatial extent of the changes in sensitivity to thermal stimuli preferentially activating heat-sensitive A-fiber nociceptors, heat-sensitive C-fiber afferents, and cool-sensitive A-fiber afferents. The volar forearm was treated with a high-concentration capsaicin patch for 1 h. Transient heat, warm and cold stimuli designed to activate Aδ- and C-fiber thermonociceptors, C-fiber warm receptors, and Aδ-fiber cold receptors were applied to the skin before and after treatment at days 1, 3, and 7. Reaction times, intensity ratings, and quality descriptors were collected. The stimuli were applied both within the capsaicin-treated skin and around the capsaicin-treated skin to map the changes in thermal sensitivity. We found that topical capsaicin selectively impairs heat sensitivity without any concomitant changes in cold sensitivity. Most interestingly, we observed a differential effect on the sensitivity to thermal inputs conveyed by Aδ- and C-fibers. Reduced sensitivity to Aδ-fiber-mediated heat was restricted to the capsaicin-treated skin, whereas reduced sensitivity to C-fiber-mediated heat extended well beyond the treated skin. Moreover, the time course of the reduced sensitivity to C-fiber-mediated input was more prolonged than the reduced sensitivity to Aδ-fiber-mediated input.

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

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

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.

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.

Interactions between inputs from adjacent digits in somatosensory thalamus and cortex of the raccoon

Interactions between somatosensory afferents arriving from different points in the periphery play an important role in sensory discrimination and also provide the substrate for plasticity following peripheral injury. To examine the extent and time course of such interactions, extracellular recordings were made from neurons in the primary somatosensory cortex and the ventroposterior lateral thalamus of anesthetized raccoons. Interactions between adjacent digits were studied using the conditioning-test paradigm in which a test pulse was delivered to the digit containing the neuron's receptive field (the on-focus digit) at various intervals following conditioning stimulation of an adjacent, off-focus digit. Off-focus stimulation produced predominantly inhibition of the test response with a maximum effect at 20-40 ms in both cortex and thalamus. The mean inhibition was approximately twice as large in the thalamus as in the cortex. Recordings were made in other animals after unmyelinated C fibers had been destroyed in the on-focus digit by subcutaneous injection of capsaicin. This resulted in a doubling of the responses evoked by the test stimulus in both regions, but the spontaneous discharge rate was not changed. The amount of inhibition produced in the cortex was unchanged by capsaicin treatment, but was reduced in the thalamus compared to control animals. This indicates that capsaicin-sensitive peripheral afferents provide a tonic control over interdigit inhibition in the thalamus.

Mechanisms for acute changes in sensory maps

Many studies have examined changes in the topographic representations of the special senses in cerebral cortex following partial peripheral deafferentations. This approach has demonstrated the short- medium- and long-term aspects of plasticity. However, the extensive capacity for immediate plasticity, while first demonstrated more than 15 years ago, still challenges explanation. What such studies indicate is that each locus in sensory cortex receives viable input from a far wider area of the sensory epithelium than is represented in the normal receptive field, with the implication that much of this input is normally inhibited. Consideration of the geometric and temporal aspects of receptive field plasticity suggests that this inhibition must be tonic and must derive its driving input from a tonically active periphery.

Functional plasticity in the human primary somatosensory cortex following acute lesion of the anterior lateral spinal cord: neurophysiological evidence of short-term cross-modal plasticity

The primary somatosensory cortex (S1) in adult animals and humans is capable of rapid modification after deafferentation. These plastic changes may account for a loss of tonic control by nociceptive inputs over inhibitory mechanisms within structures of the dorsal column-medial lemniscal system. Most studies, however, have been performed under conditions where deafferentation of C and A delta fibres coexists with large-diameter fibres deafferentation. In this study the effect of the acute lesion of one ascending anterior lateral column on neuronal activity within the dorsal column-medial lemniscal system was assessed by recording somatosensory evoked potentials (SEPs) in seven patients who underwent unilateral percutaneous cervical cordotomy (PCC) as treatment for drug-resistant malignant pain.Spinal, brainstem and cortical SEPs were recorded 2h before and 3h after PCC by stimulating the posterior tibial nerve at both ankles. Amplitudes of cortical potentials obtained by stimulation of the leg contralateral to PCC were significantly increased after PCC. No significant changes in spinal or brainstem potentials were observed. PCC did not affect SEP components obtained by stimulation of the leg ipsilateral to PCC. Our results suggest that nociceptive deafferentation may induce a rapid modulation of cortical neuronal activity along the lemniscal pathway, thus providing the first evidence in humans of short-term cortical plasticity across the spinothalamic and lemniscal systems.

Effects of temporary deafferentation on raccoon post-synaptic dorsal column neurons

The effects of temporary deafferentation were studied in 54 post-synaptic dorsal column (PSDC) neurons in the cervical spinal cord of the raccoon. Deafferentation was induced by the injection of lidocaine into the base of the digit containing the receptive field of the neuron. These neurons all had receptive fields on a single digit of the forepaw and in no case did a new receptive field appear following lidocaine injection. High intensity electrical stimulation of an off-focus digit (adjacent to the one with the receptive field) produced responses in 92% of the neurons prior to lidocaine injection. The strength of these off-focus responses was not increased by deafferentation of the on-focus digit, as might be expected if they were being suppressed by the major input; rather it was decreased. These results argue against a convergence of multi-digit inputs onto these PSDC neurons that is masked under normal circumstances. As previously described for neurons in the cuneate nucleus, deafferentation produced a significant decrease in the spontaneous activity of PSDC neurons, indicating that they receive a tonic excitatory input from the periphery. The importance of these results in understanding the starting point for injury-induced reorganization is discussed.

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.

Long-term changes in behavior and regional cerebral blood flow associated with painful peripheral mononeuropathy in the rat

We identified long-term (up to 12 weeks), bilateral changes in spontaneous and evoked pain behavior and baseline forebrain activity following a chronic constriction injury (CCI) of the sciatic nerve. The long-term changes in basal forebrain activation following CCI were region-specific and can be divided into forebrain structures that showed either: (1) no change, (2) an increase, or (3) a decrease in activity with regard to the short-term (2 weeks) changes we previously reported. All the rats showed spontaneous pain behaviors that persisted throughout the 12-week observation period, resembling the pattern of change found in four limbic system structures: the anterior dorsal thalamus, habenular complex, and the cingulate and retrosplenial cortices. In contrast, heat hyperalgesia was delayed in onset until 4 weeks following CCI, but then persisted, showing a nearly constant level of increased responsiveness. The forebrain activation that resembles this behavioral pattern of change is found in somatosensory cortex, and in the hypothalamic paraventricular nucleus and the basolateral amygdala. Finally, mechanical allodynia, which was maximal during the first 2 weeks following nerve injury and gradually recovered by the seventh post-operative week uniquely matches the time course of changes in ventrolateral and ventroposterolateral thalamic activity. Our results indicate that peripheral nerve damage results in persistent changes in behavior and resting forebrain systems that modulate pain perception. The persistent abnormalities in the somatosensory cortex and thalamus suggest that the sensory thalamocortical axis is functionally deranged in certain chronic pain states.

Sensory enrichment after peripheral nerve injury restores cortical, not thalamic, receptive field organization

Sensory perception can be severely degraded after peripheral injuries that disrupt the functional organization of the sensory maps in somatosensory cortex, even after nerve regeneration has occurred. Rehabilitation involving sensory retraining can improve perceptual function, presumably through plasticity mechanisms in the somatosensory processing network. However, virtually nothing is known about the effects of rehabilitation strategies on brain organization, or where the effects are mediated. In this study, five macaque monkeys received months of enriched sensory experience after median nerve cut and repair early in life. Subsequently, the sensory representation of the hand in primary somatosensory cortex was mapped using multiunit microelectrodes. Additionally, the primary somatosensory relay in the thalamus, the ventroposterior nucleus, was studied to determine whether the effects of the enrichment were initiated subcortically or cortically. Age-matched controls included six monkeys with no sensory manipulation after median nerve cut and regeneration, and one monkey that had restricted sensory experience after the injury. The most substantial effect of the sensory environment was on receptive field sizes in cortical area 3b. Significantly greater proportions of cortical receptive fields in the enriched monkeys were small and well localized compared to the controls, which showed higher proportions of abnormally large or disorganized fields. The refinements in receptive field size and extent in somatosensory cortex likely provide better resolution in the sensory map and may explain the improved functional outcomes after rehabilitation in humans.

Acute spiral ganglion lesions change the tuning and tonotopic organization of cat inferior colliculus neurons

Many studies have reported plastic changes in central auditory frequency organization after chronic cochlear lesions. These studies employed mechanical, acoustic or drug-induced disruptions of restricted regions of the organ of Corti that permanently alter its tuning and sensitivity and require an extended recovery period before central effects can be measured. In this study, mechanical lesions were made to 1 mm sectors of the spiral ganglion (SG). These lesions remove a restricted portion of the cochlear output, but leave the organ of Corti and basilar membrane intact. Multiunit mapping assessed the pre- and post-lesion tonotopic organization of the inferior colliculus (IC). Immediately after SG lesions, IC neurons previously tuned to the lesion frequencies became less sensitive to those frequencies but more sensitive to lesion edge frequencies, resulting in a shift in their characteristic frequencies (CFs). Notches in the excitatory response areas at frequencies corresponding to the lesion frequencies and expansion of spatial tuning curves were also observed. CFs of neurons tuned to unlesioned frequencies were unchanged. These results suggest that 'plastic' changes similar to those observed after long survival times in previous studies require little or no experience and occur within minutes to hours following the lesion.

Evidence for brainstem and supra-brainstem contributions to rapid cortical plasticity in adult monkeys

Cortical maps can undergo amazingly rapid changes after injury of the body. These changes involve functional alterations in normal substrates, but the cortical and/or subcortical location(s) of these alterations, and the relationships of alterations in different substrates, remain controversial. The present study used neurophysiological approaches in adult monkeys to evaluate how brainstem organization of tactile inputs in the cuneate nucleus (CN) changes after acute injury of hand nerves. These data were then compared with analogous data from our earlier cortical area 3b studies, which used the same approaches and acute injury, to assess relationships of cuneate and cortical changes. The results indicate that cuneate tactile responsiveness, receptive field locations, somatotopic organization, and spatial properties of representations (i.e., location, continuity, size) change during the first minutes to hours after injury. The comparisons of cuneate and area 3b organization further show that some cuneate changes are preserved in area 3b, whereas other cuneate changes are transformed before being expressed in area 3b. The findings provide evidence that rapid reorganization in area 3b, in part, reflects mechanisms that operate from a distance in the cuneate nucleus and, in part, reflects supracuneate mechanisms that modify brainstem changes.

Simultaneous reorganization in thalamocortical ensembles evolves over several hours after perioral capsaicin injections

Reorganization of the somatosensory system was quantified by simultaneously recording from single-unit neural ensembles in the whisker regions of the ventral posterior medial (VPM) nucleus of the thalamus and the primary somatosensory (SI) cortex in anesthetized rats before, during, and after injecting capsaicin under the skin of the lip. Capsaicin, a compound that excites and then inactivates a subset of peripheral C and Adelta fibers, triggered increases in spontaneous firing of thalamocortical neurons (10-15 min after injection), as well as rapid reorganization of the whisker representations in both the VPM and SI. During the first hour after capsaicin injection, 57% of the 139 recorded neurons either gained or lost at least one whisker response in their receptive fields (RFs). Capsaicin-related changes continued to emerge for >/=6 h after the injection: Fifty percent of the single-neuron RFs changed between 1-2 and 5-6 h after capsaicin injection. Most (79%) of these late changes represented neural responses that had remained unchanged in the first postcapsaicin mapping; just under 20% of these late changes appeared in neurons that had previously shown no plasticity of response. The majority of the changes (55% immediately after injection, 66% 6 h later) involved "unmasking" of new tactile responses. RF change rates were comparable in SI and VPM (57-49%). Population analysis indicated that the reorganization was associated with a lessening of the "spatial coupling" between cortical neurons-a significant reduction in firing covariance that could be related to distances between neurons. This general loss of spatial coupling, in conjunction with increases in spontaneous firing, may create a situation that is favorable for the induction of synaptic plasticity. Our results indicate that the selective inactivation of a peripheral nociceptor subpopulation can induce rapid and long-evolving (>/=6 h) shifts in the balance of inhibition and excitation in the somatosensory system. The time course of these processes suggest that thalamic and cortical plasticity is not a linear reflection of spinal and brainstem changes that occur following the application of capsaicin.

Perceptual distortions of the human body image produced by local anaesthesia, pain and cutaneous stimulation

1. Knowledge of the size and orientation of the hand is essential if it is to be moved accurately in space. We used two psychophysical methods to determine whether the perceived size of a body part changes when its sensory input is changed: first, the selection of scaled drawings which matched the apparent size of a body part, and second, a motor task in which the subject drew the body part to depict its perceived size. 2. Complete anaesthesia of the thumb (with a digital nerve block) significantly increased its perceived size by 60-70% when assessed with both psychophysical methods. During this anaesthesia, the perceived size of the adjacent index finger or digits on the contralateral side was unaltered. However, the size of the unanaesthetized lips increased (by approximately 50%). 3. Marked sensory loss for the lips (produced by topical anaesthetics) significantly increased their perceived size when assessed with both methods of measurement. There was a small increase in apparent size of the thumb. 4. To determine whether changes in perceived size could also be produced by an elevation of peripheral inputs, innocuous electrical stimulation of the digital nerves and also painful cooling of the digit were used. Both procedures produced small but significant increases in perceived size of the stimulated part. 5. The results highlight lability in the perceived size of parts of the body and how this affects motor output. The data may reveal perceptual consequences of acute changes in central somatosensory maps, changes which are known to occur with deafferentation.

Receptor organ damage causes loss of cortical surround inhibition without topographic map plasticity

Following restricted peripheral damage, reorganization of adult sensory or motor cortex is believed to depend on loss of surround inhibition, which unmasks latent inputs to the deprived cortex. Here I demonstrate that limited damage to auditory receptors causes loss of functional surround inhibition in the cortex, unmasking of latent inputs and significantly altered neural coding. However, these changes do not lead to plasticity of the cortical map, defined by the most sensitive input from the receptor surface to each cortical location. Thus, in sensory cortex, loss of surround inhibition as a consequence of receptor organ damage does not necessarily result in cortical map plasticity.

A focal zone of thalamic plasticity

In this study, sensory maps in the thalamus were investigated by examining their volume and shape. We determined the forelimb representation in adult rats after the removal of hindlimb input by nucleus gracilis lesions. Three-dimensional reconstructions of thalamic sensory maps were obtained from a grid of electrode penetrations. We found that the volume of the shoulder sensory map contracted >50% at an acute time interval (n = 6), followed by a robust volumetric sensory map expansion of 25% at 1 week (n = 8) and 1 month (n = 8) after lesion relative to controls (n = 8). The topology of the volumetric increase was scrutinized by slicing functional maps in the coronal, sagittal, and horizontal planes. The equivalence of such slices from each animal was established by virtue of their distance from either a functional or neuroanatomical landmark. Surprisingly, all of the volumetric increase unequivocally occurred in a circumscribed coronal slice 300 micron thick. This focal zone was located toward the rostral pole of the thalamic tactile relay, the ventroposterolateral nucleus. Analysis in the sagittal plane revealed that, unexpectedly, the shoulder map volume expanded by superimposing its representation on that of the forepaw, via an advancement of the shoulder representation by 0.6 mm medially. We propose a "hot spot" hypothesis in which focal zones of plasticity may not be specific to the thalamus but may have manifestations elsewhere in the nervous system, such as the cerebral cortex or dorsal column nuclei.

Quantitative analysis of cuneate neurone responsiveness in the cat in association with reversible, partial deafferentation

1. Partial deafferentation, based on peripheral nerve section or local anaesthetic blockade, has been reported to induce both immediate loss of responsiveness and/or immediate reorganization in receptive fields of neurones in the somatosensory system. In the present study, in anaesthetized cats, we have used a rapid, reversible deafferentation procedure based on cold block of the median nerve in order to evaluate quantitatively the response characteristics of cuneate neurones (n = 39) before, during and after partial deafferentation. 2. The first hypothesis tested was that cuneate neurones with input from ulnar or superficial radial nerve fields in the vicinity of the median nerve field should undergo, in association with median nerve blockade, an increased level of responsiveness to tactile stimuli within the ulnar or radial nerve zone, and an expansion of their cutaneous receptive fields. However, among eighteen cuneate neurones of this type, there was no evidence for any systematic enhancement of responsiveness nor, in at least sixteen of the eighteen neurones, any evidence for receptive field expansion. 3. The second hypothesis tested was that cuneate neurones whose input came from both the median nerve and another peripheral nerve source should undergo, in association with median nerve blockade, an increase in responsiveness to the remaining input and an expansion of the receptive field into the field of that remaining nerve source. However, in none of thirteen neurones of this type tested was there evidence of such a change. 4. The third hypothesis was that cuneate neurones whose control' receptive fields were within the median nerve zone of deafferentation should show an emergence of novel receptive fields and responsiveness from areas around the field of innervation of the median nerve. However, in none of eight neurones of this type was there evidence for such changes in adjacent skin areas. 5. In conclusion, with the use of cold block of the median nerve for partial deafferentation, the present study has confirmed previous findings of denervation-related loss of responsiveness in dorsal column nuclei neurones. The conflicting findings in studies of central nervous system plasticity indicate the need to understand better factors that do and do not lead to acute central changes.

Reorganization of the raccoon cuneate nucleus after peripheral denervation

The effects of peripheral nerve transection on the cuneate nucleus were studied in anesthetized raccoons using extracellular, single-unit recordings. The somatotopic organization of the cuneate nucleus first was examined in intact, control animals. The cuneate nucleus in the raccoon is organized with the digits represented in separate cell clusters. The dorsal cap region of the cuneate nucleus contains a representation of the claws and hairy skin of the digits. Within the representation of the glabrous skin, neurons with rapidly adapting properties tended to be segregated from those with slowly adapting properties. The representations of the distal and proximal pads on a digit also were segregated. Electrical stimulation of two adjacent digits provided a detailed description of the responses originating from the digit that contains the tactile receptive field (the on-focus digit) and from the adjacent (off-focus) digit. Stimulation of the on-focus digit produced a short latency excitation in all 99 neurons tested, with a mean of 10.5 ms. These responses had a low threshold (426 microA). Stimulation of an off-focus digit activated 65% of these neurons. These responses had a significantly longer latency (15.3 ms) than on-focus responses and the threshold was more than twice as large. Two to five months after amputation of digit 4, 97 cells were tested with stimulation of digits 3 and 5. A total of 44 were in the intact regions of the cuneate nucleus. They had small receptive fields on intact digits and their responses to electrical stimulation did not differ from the control neurons. The remaining 53 neurons were judged to be deafferented and in the fourth digit region on the basis of their location with respect to intact neurons. All but two of these cells had receptive fields that were much larger than normal, often including more than one digit and part of the palm. When compared with the off-focus control neurons, their responses to electrical stimulation had lower thresholds and an increased response probability and magnitude. The latencies of these cells did not decrease, however, and were the same as the off-focus control values. The enhanced responses of the deafferented neurons to adjacent digit stimulation indicate that there is a strengthening of synapses that were previously ineffective. The increased proportion of neurons that could be activated after amputation suggests that there is also a growth of new connections. This experiment demonstrates that reorganization in the adult somatotopic system does occur at the level of the dorsal column nuclei. As a consequence, many of the changes reported at the cortex and thalamus may be due to the changes occurring at this first synapse in the somatosensory pathway.

Electrophysiological effects of temporary deafferentation on two characterized cell types in the nucleus gracilis of the rat

Single- and multiunit recordings were made in the nucleus gracilis of anaesthetized rats in order to study the characteristics of the responses to natural cutaneous stimulation before and during local anaesthetic-induced deafferentation. Two types of cells were found which exhibited different electrophysiological features at rest and in response to stimulation of their receptive fields (RFs). Low-frequency (LF) neurons (77%) had very low spontaneous activity, and most could be antidromically activated from the medial lemniscus. High-frequency (HF) cells (23%) had a much higher spontaneous discharge rate, with shorter spike duration, and did not project through the lemniscus. Both cell types generated phasic responses with similar latencies following cutaneous stimulation. Recordings of lemniscal axons had similar characteristics to those of LF neurons. Within minutes after anaesthetizing the functional centre of the RF, the LF and HF cells displayed new RFs, and enhanced responses to stimuli delivered at the periphery of the original fields. Firing rates increased during anaesthesia, but only in LF cells. Using a paired-stimulation paradigm, many LF neurons displayed during anaesthesia a decrease of the normal inhibition that the conditioning stimulus evoked on the responses to the test stimulus; the opposite effect was observed in all HF cells. These results suggest that (i) LF neurons correspond to thalamic projection cells, and HF neurons may be inhibitory interneurons; (ii) by disinhibiting LF (but not HF) cells, temporary deafferentation may increase neuronal responsiveness to peripheral stimulation, and thus contribute to reveal new RFs, and (iii) these changes in the nucleus gracilis may partly account for the reorganization of representational maps at higher levels of the somatosensory pathways.

Short-term plasticity in primary somatosensory cortex of the rat: rapid changes in magnitudes and latencies of neuronal responses following digit denervation

Recordings were made from neurons in primary somatosensory (SmI) forepaw cortex of rats to study the time course of changes in responses beginning immediately following denervation (ligation) of a single digit. Before denervation, neuronal receptive fields (RFs) defined by tactile stimulation varied in size from small regions of one digit to larger areas covering several digits and palmar pads. With electrical stimulation, most neurons responded best to one (on-focus) digit and less to other (off-focus) digits; on-focus stimulation yielded more spikes per stimulus and shorter spike latencies (Lmin) than did off-focus stimulation. After ligation of the on-focus digit, most neurons showed increased responsiveness to stimulating one or several off-focus digits and palmar regions of the forepaw: (1) tactile stimulation showed that the RFs of all but one neuron expanded to include previously "ineffective" skin regions, such as digits or palmar pads adjoining the original RF; (2) electrical stimulation usually evoked stronger responses from neighboring off-focus digits and sometimes elicited novel responses from previously ineffective digits--seven of ten neurons showed increases in spikes per stimulus, which tended to approach stable values within 60-90 min after denervation; three of ten neurons showed decreases in Lmin with time, but most revealed no significant changes. These results suggest that dynamic response properties, as well as RFs, of SmI cortical neurons can be modified rapidly by blocking afferent input from dominant on-focus skin regions. RFs expand and novel responses appear, with concomitant increases in response magnitude and, in some cases, decreases in response latency over time. These findings seem to reflect a rapid increase in synaptic efficacy of weak or previously ineffective inputs from cutaneous afferent nerve fibers.

Transneuronal changes of the inhibitory circuitry in the macaque somatosensory thalamus following lesions of the dorsal column nuclei

The inhibitory circuitry of the ventroposterolateral nucleus (VPL) of the macaque somatosensory thalamus was analyzed in normal animals and in those surviving for a few days or several weeks following a unilateral lesion of the cuneate nucleus, the source of medial lemniscal (ML) axons carrying information from the contralateral upper extremity. Inhibitory synaptic terminals in the VPL were defined as those that contain flattened or pleomorphic synaptic vesicles and that can be shown to be immunoreactive for gamma-aminobutyric acid (GABA). There are two types of these profiles: F axon terminals that arise from neurons of the thalamic reticular nucleus, and perhaps from VPL local circuit neurons (LCNs); and the dendritic appendages of LCNs that form presynaptic dendrites (PSDs). ML terminals normally have extensive synaptic interactions with PSDs but not with F axon terminals. Electron microscopic analyses revealed that cuneatus lesions resulted in a rapid loss of ML terminals and a statistically significant reduction in both F and PSD synaptic profiles. Confocal scanning microscopy also demonstrated a profound loss of GABA immunoreactivity in the deafferented VPL. These changes persisted for more than 20 weeks, without any evidence of reactive synaptogenesis of surviving sensory afferents or of inhibitory synapses. The changes in GABA circuitry are transneuronal, and the possible mechanisms that may underlie them are discussed. It is suggested that the altered GABAergic circuitry of the VPL in the monkey may serve as a model for understanding changes in somatic sensation in the human following peripheral or central deafferentation.

Initial cortical reactions to injury of the median and radial nerves to the hands of adult primates

The area 3b hand cortex of adult squirrel monkeys was mapped during the first minutes to hours after transecting the radial and median nerves to the hand. The objective was to evaluate initial cortical reactions to this injury and to determine whether patterns and extents of cortical change are similar in different individuals. There are 5 main findings. First, cortical aggregates related to ulnar nerve inputs from the hand rapidly expanded to occupy an additional 21% of the cortical hand map. Second, face and forearm inputs, which normally activate areas adjacent to hand cortex, rapidly expanded into areas of 4% and 1% of the hand cortex respectively. Third, cortical changes involved shifts in receptive field locations that were initiated within minutes after injury. Fourth, the spatial patterns and extents of cortical change were similar in different individuals. Finally, the pattern of cortical change produced after this injury differed from the pattern seen after injury of the median and ulnar nerves. These rapid expansions are a beginning point from which further changes must progress; however, in contrast to changes accompanying chronic hand injuries, these initial cortical reactions do not appear dictated by use of uninjured inputs.

The immediate effects of peripheral deafferentation on neurons of the cuneate nucleus in raccoons

Single-unit recordings were obtained from 42 neurons in the cuneate nucleus of 12 anesthetized raccoons. All neurons had receptive fields on the glabrous skin of a forepaw digit. Temporary removal of the dominant excitatory input to a neuron, by injection of lidocaine into the base of the digit, did not result in any expansion of the excitatory receptive field onto adjacent, "off-focus" digits. Similarly, the responses evoked from the off-focus digits by electrical stimulation, which had a longer latency and a higher threshold, were not improved during the lidocaine block. Inhibition was produced in the majority of neurons by high-intensity mechanical stimulation of the off-focus digits, but this was also unchanged when the dominant excitatory input to the neurons was blocked. Since this from of inhibition is not apparent in the somatosensory thalamus before denervation, the spontaneous activity of thalamic neurons must be controlled by inputs other than the cuneate nucleus. These results also indicate that the long-term reorganization seen in the thalamus and cortex is not attributable to a simple unmasking of connections from the adjacent digits within the cuneate nucleus, but may involve strengthening of the connections responsible for longer-latency responses. The only significant change induced in cuneate neurons by temporary denervation was a decrease in the firing rates of 69% of the neurons that had spontaneous activity. Since it is unlikely that any of the large-diameter afferents from touch receptors can account for this finding, mechanically insensitive afferent fibers from the digit may contribute to the spontaneous activity of cuneate neurons, either directly or via a relay in the spinal cord.

Changes in the organization of the ventroposterior lateral thalamic nucleus after digit removal in adult raccoon

The ventroposterior lateral nucleus of the thalamus was studied in seven raccoons that had undergone amputation of the fourth digit between 2 and 5 months previously. Extracellular recordings were made in a series of closely spaced penetrations through the thalamus in chloralose anesthetized animals. The responses to cutaneous stimulation of the forepaw were used to reconstruct the somatotopic organization of the thalamus and to identify recording sites believed to be located in the digit zone that had lost its peripheral input. Twelve penetrations that passed through both of the adjacent fifth and third digit regions were analyzed in detail to delineate this deafferented region. None of the recording sites in this region were completely silent, indicating that the deafferented thalamus had undergone significant reorganization of its inputs. At most sites, the neurons had receptive fields on the skin surrounding the amputation wound and including one of the adjacent digits. Approximately half of the sites had low thresholds in the range of normal thalamic neurons. These results indicate that the ventroposterior thalamus is capable of substantial reorganization, which may account for much of the reorganization seen in somatosensory cortex.

Adaptive changes in the somatotopic properties of individual thalamic neurons immediately following microlesions in connected regions of the nucleus cuneatus

We examined the ability of thalamic neurons in the ventrobasal complex to show adaptive changes in receptive field properties following the loss of projections from the nucleus cuneatus. Thalamic responses to air jet stimulation were tested at multiple peripheral sites before and after making discrete microlesions in topographically-matched regions of the nucleus cuneatus. Prior to making a lesion, crosscorrelation analysis and orthodromic microstimulation were used to confirm the source of cuneate neurons projecting to the thalamic recording site. A total of 69 thalamic neurons were recorded from 29 rats. Following placement of a microlesion (100-200 microns diameter) in the nucleus cuneatus, 34 thalamic neurons did not show significant changes in stimulus-induced responses, possibly because the lesion was too small or because critical sites in the receptive field were not tested. The remaining 35 neurons were affected by cuneate microlesions, but the change in responsiveness varied according to stimulation site. When the most responsive site in the receptive field was examined, 24 neurons exhibited significant decreases and three neurons showed significant increases in responsiveness. Cuneate microlesions produced decreases at moderately responsive sites, but the reduction in response magnitude was smaller than at the most responsive site. When responses near the receptive field boundary were examined, 11 neurons displayed significant increases and only four neurons showed significant decreases. For two neurons without well-defined receptive field boundaries, cuneate microlesions caused new excitatory responses to emerge from sites that had formerly caused a slight inhibition of spontaneous activity. In all cases of increased responsiveness, the changes appeared on only one side of a neuron's receptive field. This asymmetry may account for the fact that the probability of detecting receptive field expansion increased from 27% (six of 22 experiments) to 71% (five of seven experiments) when the number of stimulation sites located throughout the receptive field was increased. These results indicate that the receptive field structure of individual neurons shows adaptive properties immediately after loss of the predominant ascending inputs.

Reorganisation of primary afferent nerve terminals in the brainstem after peripheral nerve injury. An anatomical study in cats

A pure sensory nerve (the superficial branch of the radial nerve) in adult cats was cut to investigate the changes in the nerve endings (terminals) on the neurons of the nucleus cuneatus of the brainstem. In one group of cats (n = 22) the ends of the cut nerve were approximated immediately by epineural suturing to promote optimum regeneration. In another group (n = 11) the proximals tump of the nerve was enclosed in a capsule to prevent regeneration. Four to 17 months later the same nerve was re-exposed. The sutured nerves were cut and nerve-tracer was exhibited to the proximal end of the cut nerves and to the proximal stump of the nerves which had been encapsulated. The purpose was to investigate the labelling of nerve terminals in the cuneate nucleus, because it receives an input of primary afferents from the front leg. The nerve and the cuneate nucleus of the opposite side served as controls. Labelled terminals were distributed throughout the dorsal part of the entire rostrocaudal extent of the cuneate nucleus. The distribution was patchy and was superimposed on clusters of nerve cells. The quantity of labelled nerve terminals on the experimental and control sides was compared: 60% of the labelling observed on the control side was in the sutured nerves while the encapsulated nerves exhibited only 32%. This difference was apparent 4 months after transection of the nerve. Up to 17 months after the nerve was cut, however, there was some increase in the quantity of labelled nerve terminals and this was most apparent in cats in which the nerves had been sutured.

Decrease and long-term recovery of choline acetyltransferase immunoreactivity in adult cat somatosensory cortex after peripheral nerve transections

The functional reorganization of cerebral cortex following peripheral deafferentation is associated with changes in a number of neurotransmitters and related molecules. Acetylcholine (ACh) enhances neuronal responsiveness and could play a role in activity-dependent cortical plasticity. In this study, choline acetyltransferase (ChAT) immunohistochemistry was used to investigate ACh innervation of the primary somatosensory cortex in cats sustaining complete unilateral forearm and paw denervations. Survival times of 2-52 weeks were examined. The deafferented contralateral cortex was defined electrophysiologically, and quantitative estimates of ChAT-immunoreactive fiber density were obtained from the forelimb and hindlimb sectors of area 3b in both hemispheres. In the 3b forelimb sector contralateral to the deafferentation, a decrease in density of ChAT-positive fibers relative to the ipsilateral hemisphere was apparent at 2 weeks and most pronounced at 13 weeks, involving all cortical layers except layer I. There was no such decrease in the hindlimb sector, but the loss of ChAT immunoreactivity extended to sectors representing proximal forelimb and trunk. Changes in ChAT immunoreactivity were no longer found after 1 year of survival. This long-lasting but reversible lowering of ChAT immunoreactivity could result from a loss of afferent activity in basalis neurons and/or trophic influences retrogradely exerted by cortex on these cells. Reduced ACh transmission might then contribute to the loss of gamma aminobutyric acid (GABA) inhibition in the deafferented cortex by decreasing the activation of inhibitory interneurons. The long-term recovery of a normal ChAT immunoreactivity in cortex could be a consequence of its functional reorganization.

Control variables and proprioceptive feedback in fast single-joint movement

Sensorimotor mechanisms were studied on the basis of kinematic and electromyographic data as well as the static torque developed by the muscles as a function of joint angle. The latter relationship is known as the torque/angle characteristic. Fast single-joint movement may result from a shift in this characteristic and a change in its slope. Such movements were studied at the wrist in 9 normal and 1 deafferented subject. After training to flex the wrist to a target, subjects repeated the same movements but in random test trials movements were opposed by the load generated by linear position feedback to a torque motor. At the end of the loaded trials, the load was suddenly removed. In the second experiment, subjects made wrist movements to the target that were opposed by the load and, on random test trials, the movements were not loaded. In these test trials, the wrist arrived in a static position outside the target zone. In both experiments, subjects were instructed not to correct errors. The final torque/angle characteristics specified in the movements were reconstructed on the basis of the static wrist positions and torques before and after unloading. Normal subjects made movements by shifting the position of the torque/angle characteristic and by increasing its slope. If subjects indeed maintained the same pattern of control variables (descending commands), the same final position of the characteristic would be reproduced from trial to trial regardless of load perturbations. This assumption of equifinality was tested by comparing the final position of the wrist in nonloaded movements with that after removal of the load in loaded movements. Equifinality was observed in normal subjects. Movements in the deafferented subject were also associated with a shift of the torque/angle characteristic and a change in its slope. However, she was unable to consistently reproduce its final position. In spite of muscle coactivation, her maximal stiffness was lower than in normal subjects. In the absence of vision, the subject made movements with the load by increasing the slope of the characteristic instead of by shifting its position far enough. Load perturbation affected her final wrist position (inequifinality), which may reflect the presence of a significant hysteresis of the characteristic as a result of the absence of stretch reflexes. The deficits following deafferentation presumably result from the destruction of biomechanical and sensorimotor mechanisms including the ability of control variables to specify the positional frame of reference for afferent and descending systems.

Development, critical period plasticity, and adult reorganizations of mammalian somatosensory systems

This review covers recent progress in three major areas of investigation in somatosensory systems: development, developmental plasticity and functional reorganization. Important findings relate to the development of periphery-related patterning in thalamic afferents to somatosensory cortex, the controversial role of neural activity in the development and plasticity of periphery-related afferent patterning in the brainstem and cortex, experience-dependent reorganizations in adult somatosensory cortex, and the locus of these changes.