Successive depth variations in microvascular distribution of rat somatosensory cortex

Although hemodynamic-based functional brain imaging techniques are powerful tools to explore the brain functions noninvasively, hemodynamic-based signal is strongly affected by spatial configuration of microvessels. Understanding the quantitative relations between microvascular structure and functional activity is therefore significant to make a valid signal interpretation for the imaging techniques. In the present study, we evaluated depth profiles of microvascular distributions in rat somatosensory subfields (barrel field, forelimb region, trunk region and hindlimb region) and characterized depth variations in microvascular structures, such as locations, lengths and directions of microvessels, throughout the cortical layers (I-VI). To obtain the accurate microvascular structure, we made a customized casting method by using confocal laser scanning microscope. We observed that microvascular distribution successively varied throughout the cortical layers (I-VI) and that the maximum number density of microvessels was consistently found in middle layers (III-V). In addition, superficial layers had relatively long microvessels, almost perpendicular to the cortical surface, whereas middle layers had short microvessels propagating in all directions. These regional differences in microvascular structures were closely related to the somatosensory subfields, e.g., barrel field was the greatest number density of microvessels among the investigated subfields. Based on these observations, we compared microvascular profiles with previously reported distribution patterns of tissue partial pressure of oxygen (pO2). The results showed that tissue pO2 was correlated with microvascular distribution in some but not all of the subfields. This finding shows that detailed microvascular profiles are helpful to investigate causal relationships between microvascular structure and functional activities in cerebral cortex.     

Somatotopic astrocytic activity in the somatosensory cortex

Astrocytes play a crucial role in maintaining neuronal function and monitoring their activity. According to neuronal activity maps, the body is represented topographically in the somatosensory cortex. In rats, neighboring cortical areas receive forelimb (FL) and hindlimb (HL) sensory inputs. Whether astrocytic activity is also restricted to the cortical area receiving the respective peripheral sensory inputs is not known. Using wide field optical imaging we measured changes in the concentration of astrocytic calcium within the FL and HL sensorimotor cortex in response to peripheral sensory inputs. Mapping the calcium signals upon electrical stimulation of the forepaw and hindpaw we found activity largely restricted to the FL and HL area, respectively. In comparison to neuronal activity the time course of the astrocytic calcium activity was considerably slower. The signal took 6 s to peak after the onset of a 2 Hz and 2 s long electrical stimulation of the hindpaw and 8 s for a 4 s stimulation. The astrocytic signals were delayed relative to cerebral blood flow measured using laser speckle imaging. The intensity of both the astrocytic and neuronal signals in the HL sensorimotor cortex declined with increase in stimulation frequency. Moreover, blocking neuronal input by tetrodotoxin abolished astrocytic calcium signals. We suggest that the topographical representation of the body is not only true for cortical neurons but also for astrocytes. The maps and the frequency‐dependent activations reflect strong reciprocal neuroglial communication and provide a new experimental approach to explore the role of astrocytes in health and disease. © 2013 Wiley Periodicals, Inc.     

Digit tapping model of functional activation in the rat somatosensory cortex

To establish a non-invasive model for functional activation of the rat somatosensory cortex, the forepaw digits of halothane-anesthetized rats were tapped while the blood flow (laser-Doppler flow, LDF) and somatosensory evoked potential (SSEP) responses in the forelimb area of the somatosensory cortex (S1FL) were measured. The distal phalanges of the forepaw digits were lightly tapped for 10s with an aluminum bar at frequencies between 1 and 40 Hz, with 0.4 cm total bar displacement. The LDF signal was normalized to the baseline preceding each stimulus block and averaged. The LDF response to digit tapping in the contralateral, but not ipsilateral S1FL, commenced within 1s, peaked at 11+/-0.5% (S.E.M.) above baseline within 2-3s, decreased to a plateau of 5+/-0.3% for the duration of the stimulation, and returned to baseline within 5-10s following tapping cessation. The LDF peak and plateau were not significantly different at different tapping frequencies. In the contralateral, but not ipsilateral, S1FLs, tapping produced an SSEP with positive (P1) and negative (N1) peaks at 27+/-0.5 and 47+/-0.2m s, respectively, after onset of the tap stimulation. As the tapping frequency increased from 1 to 20 Hz, the P1-N1 peak-to-peak amplitude decreased. At 30 and 40 Hz, the shortened interstimulus interval entrained the individual SSEPs into a steady-state evoked response. This study demonstrates that a robust functional activation of the forelimb region of primary somatosensory cortex of halothane-anesthetized rats can be produced by non-invasively tapping the forepaw digits and quantified with LDF and SSEP.     

Organization of somatosensory cortex in the laboratory rat (Rattus norvegicus): Evidence for two lateral areas joined at the representation of the teeth

Lateral somatosensory areas have not been explored in detail in rats, and theories on the organization of this region are based largely on anatomical tracing experiments. We investigated the topography of this region by using microelectrode recordings, which were related to flattened cortical sections processed for cytochrome oxidase (CO). Two lateral somatosensory areas were identified, each containing a complete representation of the body. A larger, more medial representation formed a mirror image of S1 along the rostrocaudal axis of the head region corresponding to the previously identified secondary somatosensory area (S2). A smaller, more lateral representation formed a mirror image of S2 along the rostrocaudal axis of the forelimb and hindlimb regions and likely corresponds to the parietal ventral area (PV) identified in other mammals. We also investigated the representation of the dentition and identified regions of cortex responsive to tooth stimulation. The lower incisor representation was rostral to the lower lip region of S1, and the upper incisor representation was lateral to the buccal pad region of S1. The upper and lower incisors flanked the tongue representation. An additional large region of far lateral cortex responded to both incisors. Finally, five CO-dense modules were consistently identified rostral and lateral to the S1 face representation, which we refer to as OM1, OM2, OM3, FM, and HM. These modules closely correspond to the physiologically identified areas representing the lower incisor (OM1) and tongue (OM2) regions of S1 and the mixed tooth (OM3), forelimb (FM1), and hindlimb (HM) representations of S2 and PV.     

The organization and mutability of the forepaw and hindpaw representations in the somatosensory cortex of the neonatal rat

The present study demonstrates that the primary somatosensory cortex of the rat contains a map of the entire body surface that is discernible with a routine anatomical staining technique, the succinic dehydrogenase reaction. The overall proportions of this map are relatively constant from rat to rat and very similar to those reported in previous physiological investigations (Welker: Brain Res. 26:259-275, '71, J. Comp. Neurol. 166:173-190, '76). We found 67% of the map to be related to the head of the rat, 15% to the forelimb, 14% to the trunk, and 4% to the hindlimb. Within the forelimb and hindlimb representations, there is a consistent internal organization that can be related to specific peripheral structures (digits or palm pads). Further, damage to either the periphery or the nerves innervating these regions on the day of birth produces disruptions in the normal pattern, but damage on day 6 or later does not. We interpret these results as indicating that the role of the periphery in organizing central neuronal structures during development previously demonstrated for the trigeminal system extends to the entire rat somatosensory system. Comparison of the present results with physiological studies of adult cortical maps after peripheral damage suggests to us that different substrates underlie the changes reported in the adult.     

Age-dependent differences in reorganization of primary somatosensory cortex following low thoracic (T12) spinal cord transection in cats

The organization of primary somatosensory cortex was examined in chronic spinal cats that had sustained cord transection at T12 at 3 ages: 2 and 6 weeks of age, and as adults. Five months to 1 yr following transection, the deprived cortex was mapped electrophysiologically (multiunit recordings). The topographical organization found at each age was compared to that present in normal adults to study effects of developmental age on the ability of the somatosensory system to adjust to changes in afferent input. Cortical responses to deprivation of somatosensory input were age dependent. In animals cord transected at 2 weeks of age, the remaining somatic afferent input excited both its normal cortical area and the area normally reserved for the hindlimb. This resulted in 2 somatotopic maps of the rostral trunk and forelimb. In contrast, in cats spinalized at 6 weeks of age, there was only 1 map for the remaining somatosensory input that was distributed across the mediolateral axis of the primary somatosensory cortex. As a result, the remaining somatosensory input was shifted medially from its normal position and was narrower with respect to the rostrocaudal area driven by light tactile input. The amount of cortex that each body region could excite was essentially the same as in normal animals. In adults, a third response was observed; regions normally devoted to forelimb and trunk appeared to be unchanged, and the region previously serving the hindlimb responded only to a limited extent, and only to tactile stimulation of the trunk. In all cases, however, some sites in the cortex could be excited by parts of the body that in normal animals were served by cortical regions from 3 to 10 mm away, a distance much in excess of the maximum extent of reported thalamocortical overlap. We suggest that the various patterns of cortical organization observed at different ages reflect different developmental processes that are active at the time of transection. Further, we hypothesize that often, in major denervations such as spinal cord transection, a significant component of the reorganization occurs at synaptic levels below the cortex in young animals.     

Comparison of reorganization of the somatosensory system in rats that sustained forelimb removal as neonates and as adults

Studies of sensory pathways in several species indicate that the extent and form of reorganization resulting from deafferentation early in life vs. adulthood are not the same. The reasons for such differences are not well understood. To gain further insight into age-dependent mechanisms of reorganization, this study compared the consequences of neonatal vs. adult forelimb amputation in rats at multiple levels of the sensory pathway, including primary somatosensory cortex, brainstem, and dorsal root ganglia. At the cortical level, the average area of the functional forelimb-stump representation from rats amputated as adults was significantly smaller (P < 0.05) than that of neonatally amputated rats (4.3 +/- 1.3 mm(2) vs. 6.6 +/- 1.5 mm(2), respectively). At the brainstem level, neonatally amputated rat cuneate neurons possessed the following responsivities: 20% stump responsive, 40% responsive to both stump and hindlimb, 30% responsive to another body region, and 10% unresponsive. In contrast, cuneate neurons of adult amputated rats were 70% stump responsive, 2% responsive to both stump and hindlimb, and 30% unresponsive. A significantly (P < 0.001) greater percentage of the C(6)-C(8) dorsal root ganglia neurons of adult amputated rats were unresponsive to peripheral stimulation vs. neurons from neonatally amputated rats (48% vs. 16%, respectively). These results indicate that the reorganization that occurs in response to forelimb amputation at birth vs. adulthood is distinctly different at each of these levels of the dorsal column-medial lemniscal pathway. Possible mechanisms to account for these differences are considered.     

Age-dependent capacity for somatosensory cortex reorganization in chronic spinal cats

Primary somatosensory cortex was mapped in chronic spinal cats that were spinalized (T12) at two weeks and 6 weeks of age. The magnitude of cortical reorganization is age-dependent. In cats spinalized at two weeks, extensive reorganization of the deafferented hindlimb region resulted in a second complete map of intact tactile input from the trunk and forelimb, while in cats transected at 6 weeks of age, trunk afferent input only partially activated the deafferented hindlimb region.     

Calcium-dependent and ouabain-resistant oxygen consumption in the rat neurohypophysis

To analyze rapid changes in energy metabolism in the neurohypophysis, pO2 was measured in the tissue in vitro with a miniature O2 electrode (tip diameter less than 100 microns, 90% response time less than 3 s). Electrical stimulation (20 Hz, 5 s) evoked immediate pO2 decreases by 93.4 +/- 10.5 mm Hg (mean +/- S.E.M., n = 12) which lasted for about 1 min and were blocked by tetrodotoxin (1 microM) or sodium cyanide (1 mM). Replacement of Ca2+ in the perifusing medium with Mn2+ reduced the pO2 decreases to 23.1 +/- 4.9% (n = 5) of the value before the replacement. In normal medium, ouabain application (1 mM, 3 min) suppressed the electrically evoked pO2 decreases only slightly to 82.6 +/- 6.5% (n = 5). In the Mn2+ medium, the same ouabain application suppressed the pO2 changes to 28.8 +/- 1.4%. High K+ (70 mM) evoked pO2 decreases by 175.8 +/- 14.9 mm Hg (n = 5) within 1-2 min. These pO2 changes were reduced to 35.6 +/- 3.8% in an Mn2+ medium. Veratridine (100 microM) evoked pO2 decreases by 204.8 +/- 36.3 mm Hg (n = 5). During the pO2 decreases, the effects of electrical or high K+ stimulation on pO2 were blocked. These results indicate that O2 consumption was evoked by electrical stimulation, and probably that high K+ or veratridine application in the neurohypophysis is mainly dependent on extracellular calcium and resistant to ouabain. The relationship between O2 consumption and exocytotic release is discussed.     

Callosal axon arbors in the limb representations of the somatosensory cortex (SI) in the agouti (Dasyprocta primnolopha)

The present report compares the morphology of callosal axon arbors projecting from and to the hind- or forelimb representations in the primary somatosensory cortex (SI) of the agouti (Dasyprocta primnolopha), a large, lisencephlic Brazilian rodent that uses forelimb coordination for feeding. Callosal axons were labeled after single pressure (n = 6) or iontophoretic injections (n = 2) of the neuronal tracer biotinylated dextran amine (BDA, 10 kD), either into the hind- (n = 4) or forelimb (n = 4) representations of SI, as identified by electrophysiological recording. Sixty-nine labeled axon fragments located across all layers of contralateral SI representations of the hindlimb (n = 35) and forelimb (n = 34) were analyzed. Quantitative morphometric features such as densities of branching points and boutons, segments length, branching angles, and terminal field areas were measured. Cluster analysis of these values revealed the existence of two types of axon terminals: Type I (46.4%), less branched and more widespread, and Type II (53.6%), more branched and compact. Both axon types were asymmetrically distributed; Type I axonal fragments being more frequent in hindlimb (71.9%) vs. forelimb (28.13%) representation, while most of Type II axonal arbors were found in the forelimb representation (67.56%). We concluded that the sets of callosal axon connecting fore- and hindlimb regions in SI are morphometrically distinct from each other. As callosal projections in somatosensory and motor cortices seem to be essential for bimanual interaction, we suggest that the morphological specialization of callosal axons in SI of the agouti may be correlated with this particular function.     

A comparative topographical analysis of dorsal column nuclear and cerebral cortical projections to the basilar pontine gray in rats

The somatotopic distribution of dorsal column nuclear projections within the basilar pontine gray was examined in relation to the massive corticopontine projection system that emanates most heavily from motor and somatosensory cortex. The distribution patterns of these two systems were compared by combining autoradiographic and degeneration axonal tracing methods within individual animals. Stereotaxic injections of tritiated leucine (50 microCi/microliter) and lesions by aspiration were made in animals under ketamine hydrochloride anesthesia. The forelimb cortical injections (0.1-0.3 microliter) were centered in either sensory or motor cortical regions as determined by intracortical microstimulation and multiunit recording techniques. Because sensory and motor hindlimb cortical areas overlap extensively in the rat, hindlimb cortical injections (0.1-0.3 microliter) were limited to a single hindlimb sensorimotor cortical region. The corresponding contralateral dorsal column nucleus, cuneatus or gracilis, was then aspirated. A somatotopic distribution of fore- and hindlimb corticopontine fibers were found in discrete regions of the ipsilateral pontine gray. Hindlimb sensorimotor corticopontine fibers distributed caudal to forelimb projections. Similarly, pontine afferents from the dorsal column nuclei terminated somatotopically in the caudal half of the contralateral pontine gray in that gracilopontine fibers distributed caudal to cuneopontine fibers. Within individual animals, partially overlapping terminations were seen from nucleus cuneatus and the forelimb sensory cortical area as well as from nucleus gracilis and the hindlimb sensorimotor cortical area. No overlap existed in the pontine terminations from nucleus cuneatus and the forelimb motor cortical area.     

Movement representation in the dorsal and ventral premotor areas of owl monkeys: A microstimulation study

We used intracortical microstimulation to investigate the lateral premotor cortex and neighboring areas in 14 hemispheres of owl monkeys, focusing on the somatotopic distribution of evoked movements, thresholds for forelimb movements, and the relative representation of proximal and distal forelimb movements. We elicited movements from the dorsal and ventral premotor areas (PMD, PMV), the caudal and rostral divisions of primary motor cortex (Mlc, Mlr), the frontal eye field (FEF), the dorsal oculomotor area (OMD; area 8b), the supplementary motor area (SMA), and somatosensory cortex (areas 3a and 3b). Area PMD was composed of architectonically distinguishable caudal and rostral subdivisions (PMDc, PMDr). Stimulation of PMD elicited movements of the hindlimb, forelimb, neck and upper trunk, face, and eyes. Hindlimb and forelimb movements were represented in the caudalmost part of PMDc. Face, neck, and eye movements were represented in the lateral and rostral parts of PMDc and in PMDr. Stimulation of PMV elicited forelimb and orofacial movements, but not hindlimb movements. Both proximal and distal forelimb movements were elicited from PMDc and PMV, although PMD stimulation elicited mainly shoulder and elbow movements, while PMV stimulation evoked primarily wrist and digit movements. Distal movements were evoked more frequently from PMV than from Mlr or Mlc. Across cases, the median forelimb thresholds for PMDc and PMV were 60 and 36 μA, respectively, values that differ significantly from each other and from the value of 11 μA obtained for Mlr. Our observations indicate that premotor cortex is much more responsive to electrical stimulation than commonly thought, and contains a large territory from which eye movements can be elicited. These results suggest that in humans, much of the electrically excitable cortex located on the precentral gyrus, including cortex sometimes considered part of the frontal eye field, is probably homologous to the premotor cortex of nonhuman primates. © 1996 Wiley-Liss, Inc.     

Effect of adequate stimulation of the vestibular apparatus on the locomotor activity of the muscles in the fore- and hindlimbs of the guinea pig. Rotation relative to the lateral axis

Influence of adequate vestibular stimulation by tilting about a transverse axis on the locomotor activity of fore- and hindlimb muscles was investigated in precollicularly decerebrated guinea pigs. The locomotor activity was evoked by electrical stimulation of the mesencephalic locomotor region. An increase in the forelimb extensors activity and a decrease in the hindlimb extensors activity in the support phase of the locomotor cycle were observed during head downward position, opposite changes of these activities were observed during head upward position. A decrease in the forelimb flexor activity in swing phase during head downward position and an increase in this activity during head upward position was registered. Phase shifts of the locomotor activity changes of forelimb extensors altered from 60 to -30 degrees, hindlimb extensors--from -150 to -220 degrees, forelimb flexors--from -140 to -220 degrees during sinusoidal tilting in frequency range of 0.02-0.4 Hz and amplitude +/- 20 degrees. Mechanisms of observed changes in the locomotor activity of muscles are discussed.     

Neurofunctional disturbances as related to cortical ischemia and white matter ischemia

We evaluated regional cerebral blood flow (rCBF) by means of hydrogen clearance method as well as [14C]-iodoantipyrine autoradiographic method, cortical auditory evoked potentials (AEP), somatosensory evoked potentials (SEP) induced by forelimb (median nerve) stimulation (SEP-F), and SEP induced by hindlimb (tibial nerve) stimulation (SEP-H) in cats after occlusion of the left middle cerebral artery (MCA) under alpha-chloralose anesthesia. According to the degree of ischemia, the experimental animals were divided into two groups. One was the critical ischemia which was defined as permanent total suppression of AEP, and low residual blood flow in the auditory cortex. And the other was the non-critical ischemia which included transient suppression and spontaneous recovery of the cortical sensory evoked potentials, and high residual blood flow (greater than 15 ml/100 g/min). In one cat with transient suppression of three kinds of sensory evoked potentials, the [14C]-iodoantipyrine (IAP) autoradiograph revealed only a limited ischemic area of subcortical white matter. In the critical ischemia group, ischemia of the primary sensory cortex ranged from the mostly affected primary auditory cortex (supplied by the MCA) to the least affected hindlimb projection area within primary somatosensory cortex (supplied by the ACA). The forelimb projection area of the primary somatosensory cortex (supplied by both ACA and MCA) showed a mild or moderate reduction of rCBF after occlusion. The [14C]-IAP autoradiograph showed severe reduction of the white matter including the somatosensory pathway in the wide range. However, rCBF in the thalamus and hindlimb projection area within somatosensory cortex was almost intact in the cat with ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Numb rats walk - a behavioural and fMRI comparison of mild and moderate spinal cord injury

Assessment of sensory function serves as a sensitive measure for predicting the functional outcome following spinal cord injury in patients. However, little is known about loss and recovery of sensory function in rodent spinal cord injury models as most tests of sensory functions rely on behaviour and thus motor function. We used functional magnetic resonance imaging (fMRI) to investigate cortical and thalamic BOLD-signal changes in response to limb stimulation following mild or moderate thoracic spinal cord weight drop injury in Sprague-Dawley rats. While there was recovery of close to normal hindlimb motor function as determined by open field locomotor testing following both degrees of injury, recovery of hindlimb sensory function as determined by fMRI and hot plate testing was only seen following mild injury and not following moderate injury. Thus, moderate injury can lead to near normal hindlimb motor function in animals with major sensory deficits. Recovered fMRI signals following mild injury had a partly altered cortical distribution engaging also ipsilateral somatosensory cortex and the cingulate gyrus. Importantly, thoracic spinal cord injury also affected sensory representation of the upper nonaffected limbs. Thus, cortical and thalamic activation in response to forelimb stimulation was significantly increased 16 weeks after spinal cord injury compared to control animals. We conclude that both forelimb and hindlimb cortical sensory representation is altered following thoracic spinal cord injury. Furthermore tests of sensory function that are independent of motor behaviour are needed in rodent spinal cord injury research.     

Metabolic mapping of rat striatum: somatotopic organization of sensorimotor activity

Diseases that affect the striatum produce movement disorders, for which rats have been a useful model. To determine the organization of functional, neural activity in the rat striatum related to motor activity, we used electrical stimulation of the motor cortex and [¹⁴C]deoxyglucose autoradiography. The stimulation produced movements of each of three body regions. Both the motor and somatosensory cortex were activated. Image analysis was used to objectively localize peak activation and to provide a map for further stereotaxic and localization studies. In the anterior striatum, in the dorsolateral sector, region of peak activation were well separated for each body region: the hindlimb peak activation was dorsomedial, the forelimb ventrolateral and vibrissae medial. Also, the activation fields were larger in anterior than in posterior striatum. Furthermore, activation ipsilateral to movement was present and the peak localization was offset from peaks contralateral to movement. In addition, there were activation regions in lateral striatum where body region representations may overlap. This is the first demonstration of a global striatal somatotopy that separates the limbs and vibrissae in rats. The functional average revealed by the deoxyglucose autoradiography showed a predominant isotropic or rod-like representation of sensorimotor activity for the limbs in striatum during movement and confirms aspects of the anatomy known for the corticostriate system in primates: metabolism was ‘patchy,’ and extended throughout long anteroposterior domains in striatum. These extensive and patchy arrangements suggest integrative, combinational and/or associative networks.     

Stimulation of D2-receptors in rat nucleus accumbens slices inhibits dopamine and acetylcholine release but not cyclic AMP formation

We compared some functional responses of D1- and D2-receptor stimulation in tissue slices of rat neostriatum with those in slices of the nucleus accumbens. In both brain regions D2-receptor stimulation inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine. In both brain regions D1-receptor stimulation and forskolin increased the cyclic AMP formation. Only in the neostriatum, stimulation of D2-receptors inhibited the formation of cyclic AMP, brought about by forskolin or by D1-receptor stimulation. It is concluded from these experiments that, although functional responses of D2-receptor stimulation can be demonstrated in the nucleus accumbens, D2-receptors in this brain region are apparently uncoupled to adenylate cyclase.     

Evolution of the dynamic changes in functional cerebral oxidative metabolism from tissue mitochondria to blood oxygen

The dynamic properties of the cerebral metabolic rate of oxygen consumption (CMR(O2)) during changes in brain activity remain unclear. Therefore, the spatial and temporal evolution of functional increases in CMR(O2) was investigated in the rat somato-sensory cortex during forelimb stimulation under a suppressed blood flow response condition. Temporally, stimulation elicited a fast increase in tissue mitochondria CMR(O2) described by a time constant of ~1 second measured using flavoprotein autofluorescence imaging. CMR(O2)-driven changes in the tissue oxygen tension measured using an oxygen electrode and blood oxygenation measured using optical imaging of intrinsic signal followed; however, these changes were slow with time constants of ~5 and ~10 seconds, respectively. This slow change in CMR(O2)-driven blood oxygenation partly explains the commonly observed post-stimulus blood oxygen level-dependent (BOLD) undershoot. Spatially, the changes in mitochondria CMR(O2) were similar to the changes in blood oxygenation. Finally, the increases in CMR(O2) were well correlated with the evoked multi-unit spiking activity. These findings show that dynamic CMR(O2) calculations made using only blood oxygenation data (e.g., BOLD functional magnetic resonance imaging (fMRI)) do not directly reflect the temporal changes in the tissue's mitochondria metabolic rate; however, the findings presented can bridge the gap between the changes in cellular oxidative rate and blood oxygenation.     

Organization of adult motor cortex representation patterns following neonatal forelimb nerve injury in rats

Somatotopic representation patterns in the motor cortex (MI) of rats that had a unilateral forelimb amputation on the first postnatal day were examined after 2-4 months of survival. Intracortical electrical stimulation and recording techniques were used to map the somatic representation in MI and in the somatic sensory cortex (SI). In normal rats, vibrissa, forelimb, and hindlimb areas comprise the bulk of the MI representation. Stimulation within the forelimb area elicits elbow, wrist, or digit movements at the lowest current intensities. The proximal limb representation appears to be contained within the distal forelimb area, since shoulder movements are nearly always evoked by stimulating at higher current intensities at some distal forelimb sites. In agreement with previous studies, the distal forelimb representation overlapped the adjacent part of the granular SI cortex. Following removal of the forelimb at birth, 3 novel features of MI organization were observed. First, the areas from which stimulation evoked movements of the vibrissa or the shoulder musculature were larger than normal. Stimulation thresholds were lower than those required for comparable movements in normal rats throughout these areas, suggesting that nerve section had not simply unmasked a high-threshold representation. Second, vibrissa movements were more commonly paired with movements of the proximal forelimb muscles at the same site. Third, stimulation in the adjacent granular SI cortex failed to evoke shoulder or trunk movements, although receptive-field mapping in this region showed that cells were responsive to cutaneous stimulation of the trunk and shoulder region. These results indicate that several organizational features develop differently in MI following perinatal nerve injury: certain remaining muscle groups have enlarged cortical representations, there is a strengthening of some normally weak connections from MI to the proximal musculature, and muscles are grouped in unusual combinations. These data demonstrate that the formation of MI representation patterns is strongly influenced by nerve injury during the perinatal period.