Representation Of Vibrissae Research Articles

Acetylcholine release in rat frontal and somatosensory cortex is enhanced during tactile discrimination learning

We examined acetylcholine (ACh) release in the frontal cortex and in the vibrissae representation in the somatosensory cortex in rats during the acquisition of a tactile discrimination or during non-discrimination control procedures. Microdialysis samples were collected for 1 h in the home cage and then for the duration of 30 discrimination or control trials, across 5 consecutive days. Both groups showed significant testing-induced increases in ACh release in both cortical sites. However, rats in the discrimination training group showed even greater testing-induced increases in ACh release in the somatosensory cortex and had a proportionately greater increase in somatosensory cortex than in frontal cortex, relative to controls. The results suggest that, in addition to the widespread enhancement of ACh release associated with appetitive conditioning procedures in general, tactile discrimination training causes a regionally specific enhancement in ACh release in the somatosensory cortex that is related to discrimination performance.

Development and plasticity of local intracortical projections within the vibrissae representation of the rat primary somatosensory cortex.

Labelling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-A) was used to assess the development of projections within the primary somatosensory cortex (SI) of rats aged between postnatal day 2 and 8 (P-2 and P-8). 1,1'-Dioctadecyl-3,3,3,"3'-tetramethylindocarbocyanine perchlorate (Di-I) was used in these same animals to label thalamocortical afferents. Particular attention was paid to the emergence of lamina IV intracortical projections that form a pattern complementary to vibrissae-related thalamocortical afferents. A vibrissae-related pattern of Di-A-labelled cells and fibers that was restricted largely to the septa regions was not apparent in rats killed on P-2, but it was visible in animals killed on P-4 and later ages. Tracing with biotinylated dextran amine (BDA) was used to assess intra-SI projections of adult rats that sustained transection of the infraorbital nerve (ION) on P-0 or P-7 or implantation of a tetrodotoxin (TTX)-impregnated polymer chip over the cortex on P-0. Rats that sustained ION transection on P-7 or that had TTX implants demonstrated normal patterns of projections within SI. The patterns of labelling in the supra- and infragranular layers of the cortices of the rats that sustained ION transection on P-0 were generally similar to those in the other groups evaluated. However, in lamina IV, there was no organization that could be related to the distribution of the vibrissae. These results indicate that the vibrissae-related pattern of intracortical projections within SI develops shortly after birth and that two manipulations that alter cortical activity, but not the patterning of thalamocortical afferents (application of TTX and transection of the ION after thalamocortical afferent patterns are established), have no significant effect on it. However, a manipulation that alters thalamocortical development (transection of the ION on P-0) profoundly affects the patterning of intracortical connections.

Transient Increase of AMPA and NMDA Receptor Binding in the Barrel Cortex of Mice after Tactile Stimulation

The effects of sensory stimulation and sensory conditioning upon [3H]MK-801 [(+)-5-methyl-10,11-dihydro- 5H-dibenzo[a,d]cyclohepten-5,10-imine] binding toN-methyl-d-aspartate (NMDA) receptor sites and [3H]AMPA (α-amino-3-hydroxy-5-methylisoxasole-4-propionic acid) binding to AMPA receptor sites were examined in the primary somatosensory (SI) cortex of mice. Following short-lasting unilateral tactile stimulation of a selected row of vibrissae, and tactile stimulation paired with noxious stimulus (the pairing procedure was found to alter cortical representation of vibrissae), in vitro receptor binding autoradiography was performed on the sections of the barrel cortex of mice. One hour after the end of tactile stimulation or training procedure there was an increase of [3H]MK-801 and [3H]AMPA binding in the corresponding row of barrels in layer IV of the SI cortex of adult mice. These effects disappeared 24 h after the end of each experimental procedure. The results suggest that both subtypes of glutamate receptors are regulated in an activity-dependent way and that sensory stimulation transiently modifies local cortical processing.

Short-lasting classical conditioning induces reversible changes of representational maps of vibrissae in mouse SI cortex--a 2DG study.

It has been known for several years that receptive field properties of sensory cortical neurons can be altered by learning experiences. We attempted to visualize a global change of the cortical body map induced by learning. In order to do this a short-duration classical conditioning involving stimulation of a row of mystacial vibrissae in mice was followed with 2-deoxyglucose (2DG) mapping of functional activity. Three conditioning sessions that paired stimulation of a row of whiskers with a tail shock produced an increase of the functional representation in somatosensory cortex (SI) of a row of the whiskers stimulated during the training. This plastic change of vibrissal representation in SI was visualized with 2DG autoradiography a day after completion of training. The expansion of representation was the most pronounced in cortical layer IV, and to a lesser extent, in layer IIIb. The expansion was observed in conditioned but not in pseudoconditioned mice or in animals that received only the conditioned stimulus. If training was discontinued, the enlargement of vibrissal representation progressively faded. The reversal could be accelerated by a behavioral extinction procedure. This study gives the pictorial demonstration of rapid, transient, and extinguishable learning-dependent changes in SI cortical maps.

Remodelling in the array of cell aggregates in somatotopic representation of the facial vibrissae through the trigeminal sensory system of the mouse

The disposition of the facial vibrissae of the mouse is represented as a matrix-like array of cell aggregates in rows and columns at every station of the whisker-to-barrel pathway. In order to evaluate the role of each station in this pathway, lesions were made in the facial vibrissae of the mystacial group on P0–P3, and the animals were sacrificed on P8. The effects of the lesions on the cell aggregates in the array were analyzed by using cytochrome oxidase and gallocyanin cell-staining methods. Division of cell aggregates in the array was controlled by row basis interactions through the pathway up to the cerebral cortex. In this organization, affected cell aggregates which corresponded to the damaged vibrissae were eliminated and/or fused together in the array of the thalamic relay nucleus. On the basis of thalamic modification, the final array of cell aggregates was remodelled in the cerebral cortex. In contrast, affected cell aggregates remained degenerative spaces at the original sites in the array in relation to the damaged vibrissae in the brain stem trigeminal nuclear complex. These results indicate that a protoframework with row basis orientation for the division of cell aggregates is prepared in every station of the pathway at the time of lesioning, and adjustment of subcortical alterations in the thalamic relay nucleus is a decisive process to let the cerebral cortex remodel the topographic array of cell aggregates.

PS-24-3 Progressive changes in digital area of mammal motor cortex after forelimb nerve lesions

Using the model of infraorbital nerve (IoN) injury, we have studied the role IoN signals have on the developing vibrissal motor system. To this end, in ten rats, the IoN was severed on the day of birth: in five rats, the IoN was repaired to promote axon regeneration (Reinnervated group) while axon regeneration was prevented in the remaining five rats (Deafferented group). In another five rats, the isolated IoN was left intact (Sham group) and still another group of five rats was left untouched (Control group). After these rats had reached adulthood, the compound muscle action potential (MAP) was recorded from the vibrissa muscle and intracortical microstimulation (ICMS)-evoked movements were mapped in the frontal cortex contralateral to the operated side. We found: (i) no difference between Control, Sham and Reinnervated groups in the integrated MAPs and in the size and excitability of the M1 vibrissal representation. (ii) the Deafferented group showed a 42.9% decrease in the integrated MAP plus a 47.2% and 36.9% reduction, respectively, in the size and excitability of the M1 vibrissae representation. We conclude that, during perinatal life, IoN signals regulate the development of both the peripheral and central vibrissal motor system and that IoN reinnervation restores sensory signals able to stabilize normal development of the vibrissal motor system.

Recovery of vibrissae-dependent behavioral responses following barrelfield damage is not dependent upon the remaining somatosensory cortical tissue

Previous work has demonstrated that damage to the primary somatosensory cortex produces substantial deficits in a vibrissal cue-dependent discrimination task which recover gradually over the course of post-injury testing. The present study was designed to evaluate the possible site(s) and mechanisms underlying behavioral recovery in this task. Wistar rats were trained under red light in a T-maze to produce ipsilateral turns depending upon the presence of a vibrissal cue. Animals were then subjected to photothrombotic infractions of either the ipsilateral medial parietal cortex, the ipsilateral primary and secondary somatosensory cortex (SI/SII), the primary and secondary somatosensory cortices of both hemispheres (bilateral SI/SII) or sham surgical procedures. Behavioral testing resumed 24 hours following surgery, and continued for a total of 60 days. The performance of animals with infarcts restricted to the medial parietal cortex did not differ from that of sham-operated controls. Animals with either unilateral or bilateral SI/SII infarcts exhibited a significant decrease in percent correct responding as compared to shams and rats in the medial parietal group. These deficits recovered to pre-infarct levels over approximately 35–40 days. This rate of recovery was slower than the recovery exhibited by animals given medial parietal infarcts which spared the primary barrelfield cortex. The results of this study suggest that neither the contralateral somatosensory cortex nor the vibrissal representation within ipsilateral SII cortex play a critical role in the recovery process. The possibility that subcortical structures underlie the deficits observed following barrelfield cortical damage is discussed.

Patterning of local intracortical projections within the vibrissae representation of rat primary somatosensory cortex.

Anterograde and retrograde tracing with biotinylated dextran amine and Phaseolus vulgaris leukoagglutinin was used to assess projection patterns within the vibrissae representation of the rat's primary somatosensory cortex (S-I). Large and small injections of either tracer into the center of the vibrissae representation yielded dense anterograde and retrograde labelling throughout much of the tangential extent of the vibrissae representation within S-I. In all layers, the pattern and extent of retrograde and anterograde label was in rough congruence. The organization of this labelling varied across cortical layers. In layers II and III, labelled fibers extended away from injection sites in all directions and yielded a uniform pattern, which decreased in density with increasing distance from the tracer injection. There was a tendency for labelling to be more extensive along the representation of the row of vibrissae follicles that included the injection site than across rows. There was also a tendency for anterograde labelling to be more extensive in the direction of the representation of follicles more rostral on the face than that injected. In lamina IV, both labelled fibers and cells were restricted for the most part to the septa regions between the barrels. However, a small number of retrogradely labelled neurons were also located in the barrels (approximately one-ninth of the number found in the septa). The pattern observed in laminae II-III was repeated in layers V and VI. In these laminae, there was no evidence of a pattern of intracortical connections related to the vibrissae representation in overlying lamina IV.

Vibrissal motor cortex in the rat: connections with the barrel field.

The flow of information in the sensorimotor cortex may determine how somatic information modulates motor cortex neuronal activity during voluntary movement. Electrophysiological recordings and neuroanatomical tracing techniques were used to study the connections between the primary somatosensory cortex (SI) and the vibrissal representation of the primary motor cortex (MI) in rodents. Intracortical microstimulation (ICMS) was applied to the vibrissal region of the motor cortex to identify a site from which stimulation evoked movements of the vibrissae. Movements of only a single whisker were evoked by applying low-intensity stimulating current to particular locations within MI. A single injection of either horseradish peroxidase (HRP) or biocytin was made at the stimulus site in each animal, to retrogradely label cells in the somatosensory cortex. Receptive field (RF) responses were recorded from neurons in the barrel cortex to identify the sensory cortex representation of the same whisker that responded to ICMS. The site at which neurons responded predominately to manual stimulation of this particular vibrissa was marked by a small electrolytic lesion. The projection from the somatosensory cortex to the identified whisker representation in the motor cortex was determined by mapping the location of labeled neurons in tissue sections processed for either HRP or biocytin. The relationship of the labeled cells in SI to the barrel structures was determined from adjacent sections that were stained for cytochrome oxidase. In all cases, the barrel column associated with the relevant whisker contained labeled cells. Surrounding barrels also contained labeled cells, although fewer in number. Very few labeled cells were found in non-contiguous barrels. These results show that the SI to MI projection is somatotopically arranged, such that the sensory cortex representation of a whisker is morphologically connected to the motor cortex representation of the same whisker. Thus, sensory information is relayed to MI from the relevant whisker region in SI. Adjacent whisker regions also appear to relay somatic input, but presumably to a lesser degree. A second group of animals received single small injections of the anterograde tracer, Phaseolus vulgaris leucoagglutinin, to an electrophysiologically identified whisker representation in the sensory cortex. A single narrow column of labeled fibers was found in the motor cortex following such injections. Thus, the sensory cortex appears to relay somatic information from the vibrissae to restricted regions of the motor cortex in a somatotopically organized manner. Furthermore, the stimulus-evoked whisker movements suggest that certain features of the output map of the motor cortex are discretely organized.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of the somatosensory cortex of the star-nosed mole.

The nose of the star-nosed mole consists of a star-like array of 22 fleshy appendages that radiate from the nostrils and are moved about to explore the environment. The surface of each appendage, or ray, is densely packed with bulbous receptor organs (Eimer's organs) that are highly responsive to tactile stimulation. Here, we report that these rays have corresponding morphological specializations in somatosensory cortex. Using a stain for the metabolic enzyme, cytochrome oxidase (CO), to reveal subdivisions of cortex, we disclosed a complex pattern of CO-dense stripes or bands separated by sharp lines or septa of low CO staining. Multiunit microelectrode recordings of neural activity evoked by light tactile stimuli in somatosensory cortex of anesthetized moles allowed us to mark some of the bands and other CO-dark regions with small electrolytic lesions and later relate recording results to the CO pattern. The results suggest that the primary somatosensory cortex, S1, has an unusual ventrolateral location and orientation with representations of mouth, nose rays, facial vibrissae, forepaw, and trunk in a rostrocaudal sequence. Within this presumptive S1, the 11 rays of the contralateral nose are represented as a rostral-to-caudal cortical pinwheel of 11 stripes. Cortex ventral to the primary set of stripes contains a second rostrocaudal representation of the rays as a mirror image of the first. This second set of stripes may be part of the second somatosensory area, S2. A third pattern of CO stripes appears to merge partially with caudal stripes of the first two patterns, so that a full pattern of 11 stripes is not obvious. This representation may correspond to the ventral somatosensory area, VS, of other mammals. An extensive area of cortex separated from the nose by a large septum was responsive to stimulation of the forelimb. Auditory cortex is unusually caudal in this mole, and the presumptive primary visual area is relatively small. These specializations of somatosensory cortex in star-nosed moles may be more patent examples of the consequences of more general factors in brain development. The observations are consistent with the general rule that the terminations of sensory projections with discorrelated activity segregate.

Parvalbumin expression reveals a vibrissa-related pattern in rabbit SI cortex

The expression of parvalbumin-like immunoreactivity (PV-LIR) was examined in the mystacial representation within the primary somatosensory cortex (SI) of postnatal day 21 and adult rabbits. PV-LIR was expressed in a prominent vibrissa-like array of patches in layer IV despite the fact that barrels were indistinct in the cytoarchitecture. Each patch consisted of dense terminal-like PV-LIR and a preferential concentration of intensely labeled stellate neurons. Layer V contained scattered small and large intensely labeled basket cells. Layer Vb had a distinct layer of lightly labeled large pyramidal cells that received labeled basket cell terminations. Upper layer VI also contained patches of terminal-like PV-LIR that were in register with the overlying vibrissae pattern. These patches also contained a preferential distribution of labeled non-pyramidal cells as well as modified pyramidal cells. These results suggest that PV-LIR in rabbits delineates cortical modules composed of thalamorcotical afferents and inhibitory local circuits in the absence of a distinct barrel cytoarchitecure. In contrast, prior studies of rat SI cortex have revealed a distinct barrel cytoarchitecture but a uniform distribution of PV-LIR. The differences in PV-LIR between rodents and lagomorphs within the vibrissae representation in SI may be related to species differences in thalamic and local cortical circuits devoted to the whisker sense.

Reduction of GABAA receptor binding of [3H]muscimol in the barrel field of mice after peripheral denervation: transient and long-lasting effects.

The effect of peripheral sensory deprivation upon GABAA receptor binding of [3H]muscimol was investigated in the barrel cortex--cortical representation of mystacial vibrissae of mice--by means of in vitro quantitative autoradiography. Unilateral lesions of all vibrissae or selected rows of whiskers were performed neonatally or in adulthood. [3H]muscimol binding was examined after various survival times up to 60 days. Both types of lesions performed in adult mice resulted in a transient decrease (10-25%) of binding values in the deafferented areas of the barrel field as compared with the unoperated control side. Sixty days after denervation [3H]muscimol binding returned to control values. Similar results were found after neonatal removal of all vibrissae. Neonatal lesion of selected rows of vibrissae, however, resulted in a decrease of [3H]muscimol binding (by about 26%) lasting up to 60 days in corresponding rows of barrels. This last result was accompanied by severe cytoarchitectonic malformation of the barrel field. The results support the hypothesis that a decrease of inhibition plays a facilitatory role in the plastic reorganization of cortical circuitry.

Peripheral influences on the size and organization of somatotopic representations in the fetal rat cortex

Nerve lesions at different fetal ages and on the day of birth were used to determine the role of the periphery in establishing territories devoted to representations of different portions of the body surface in rat somatosensory cortex. Transection of the infraorbital nerve (ION), the trigeminal branch that supplies the whisker pad, resulted in a significant reduction in the area within the primary somatosensory cortex devoted to the representation of the mystacial vibrissae in fetal, but not newborn, rats. Such lesions in fetal, but not neonatal, rats also resulted in significant increases in the cortical area devoted to the representation of the lower lip and jaw. There was a significant positive correlation between the reduction in the vibrissae representation and the expansion of that of the lower lip and jaw. Damage to the ION in either neonatal or fetal rats failed to increase significantly the amount of cortex devoted to the representation of the forepaw. These results indicate that the primary afferent innervation of the periphery does influence the amounts of cortex devoted to representations of different parts of the body surface and that the representation of one region can expand significantly when that of another body part is reduced.

Characterization of functional organization within rat barrel cortex using intrinsic signal optical imaging through a thinned skull.

We used optical imaging of intrinsic signals to characterize the functional representations of mystacial vibrissae (whiskers) in rat somatosensory cortex. Stimulation of individual whiskers for 2 s at 5 Hz resulted in a discrete area of functional activity in the cortex. Images of whisker representations were collected both through the dura and through a thinned skull. We characterized the functional representation of a whisker both spatially and temporally with two-dimensional images and three-dimensional surface plots of intrinsic signal development in the cortex in response to whisker stimulation. Single unit recordings verified that the representation of the whisker obtained with optical imaging corresponded with the electrophysiological response area of that whisker in the cortex. Lesions in the center of the functional activity were found to be in the center of the dense cytochrome oxidase patch for the corresponding whisker. In addition, a 3 x 3 matrix of whiskers was stimulated and the distances between the centers of the imaged representations and the distances between the centers of the layer IV cytochrome oxidase staining of the nine whiskers were found to be highly correlated (r = 0.98). This study shows a striking correspondence among imaging, physiology, and anatomy in the rat somatosensory cortex. Furthermore, the ability to use optical imaging through a thinned skull should allow investigations into the long-term changes in a sensory representation within a single animal.

Barrelettes—architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. II. Normal post‐natal development

Vibrissal representations in the brainstem trigeminal complex (BTC) of rodents are manifested as architectural sub-units called barrelettes. The development of barrelettes was studied by using Nissl staining, cytochrome oxidase histochemistry, and Golgi-impregnation methods. On the day of birth (PND-1), barrelettes are manifested as longitudinal, histochemical cylinders in sub-nuclei principalis, interpolaris and caudalis of the BTC. One day later (PND-2), fully formed histochemical barrelette formations are seen in the three sub-nuclei. The development of cytoarchitectural barrelettes lags behind histochemical barrelettes by about two days. Between PND-2 and PND-3, longitudinal cytoarchitectonic cylinders begin to appear. By PND-3, BTC neurons segregate into five rows of barrelettes in the coronal plane. Segmentation of rows into individual barrelettes begins on PND-4, and complete cytoarchitectonic barrelette formations are seen by PND-5. Golgi-impregnation shows that on the day of birth, primary afferent terminals and dendritic arbors of second-order trigeminal neurons within the BTC are short and poorly ramified. Over the next five post-natal days, lengthening of these processes as well as elaboration into secondary and tertiary branches take place. Growth of these processes continues for two additional weeks, contributing to the increase in barrelette neuropils (hollows). As the neuropils expand, neuronal somata are pushed toward barrelette sides. Morphometric measurements show that there is a relatively constant rate of growth of barrelettes over the first three post-natal weeks. The growth rate of the barrelette formations is identical to that of BTC as a whole. Thus, at the time of birth, the volume of neural tissue in the brainstem allotted to vibrissae is fixed relative to that allotted to other sensory receptors. Several features of the early development of barrelettes are identified: (1) Chemoarchitectural barrelettes appear before cytoarchitectural barrelettes, suggesting that terminal arbors of primary trigeminal afferents are organized before their target neurons form barrelettes. (2) Early cytoarchitecture is manifested in the form of unsegmented rows, suggesting that rough, row-based topological maps are first formed, which are then fine-tuned into individual sub-units. Recent evidence shows that other vibrissal representations--thalamic barreloids and cortical barrels--also follow these "afferent-before-target" and "row-before-individual units" sequences of development. This gradual, afferent-dependent fine-tuning of topological organization is analogous to similar events during the early development of the visual system, and may be a general feature of developing sensory systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavioral and neurochemical indices of barrel cortex-basal ganglia interaction

Previous experiments from our laboratory have shown a wide variety of time-dependent lateralized changes in behavior and nigrostriatal function following unilateral manipulation of the mystacial vibrissae of rats. The present experiment investigated the effects of unilateral radiofrequency lesion of the cortical vibrissae representation (the barrel fields) in light of these results. We measured lateralized changes in behavior as well as tissue monoamines in neostriatum and substantia nigra, between 1 and 16 days post-lesion. Short-term asymmetries in exploratory behavior (thigmotactic scanning) and neostriatal serotonin metabolism that lasted up to day 6 were seen. In substantia nigra, time-related asymmetries in dopamine concentration were found with higher ipsilateral values on day 3 and higher contralateral values on day 6. After day 6, the animals had recovered from these acute effects and thereafter, neostriatal dopamine metabolism became asymmetrical. Also during this time, they showed a directional bias in spontaneous and apomorphine-induced turning. Finally, neostriatal serotonin was bilaterally elevated on day 16. These results parallel some of the effects previously seen following unilateral removal of the vibrissae, indicating that the barrel cortex is a critical link in the functional interaction between the vibrissae and basal ganglia.

Vibrissal roughness discrimination is barrelcortex-dependent

We have investigated the contribution of the neocortical vibrissal representation within the posterior medial barrel subfield (PMBSF) to the high performance levels obtained by rats in a complex roughness discrimination task mediated by vibrissal inputs. Nine binocularly occluded rats were trained in a two-choice roughness discrimination until they obtained the 85% correct response criteria. Subsequently, the PMBSF was localized by electrophysiological recordings and bilaterally ablated. The locus and extent of the cortical lesions were confirmed by histological analysis after additional training and testing. There was no evidence of task retention after the cortical lesion and barrelless rats were unable to obtain prelesion discriminative performance levels when stimulation was restricted solely to vibrissal cues. After extensive postlesion training, four of these rats were allowed to palpate the discriminanda with their forepaws. Under these conditions rats rapidly reached the 85% correct criterion once again. The present results indicate that the PMBSF is essential for complex tactile discrimination when sensory information is obtained through the vibrissae by active palpation. This deficit is specific for the vibrissal system, the PMBSF is not essential to solve the same tactile discrimination task when the source of the somatosensory information is provided by other non-vibrissal cutaneous sensory receptors.

Immediate and delayed changes of rat motor cortical output representation with new forelimb configurations.

These experiments examined motor cortical representation patterns after forelimb postural adjustments in rats. The experiments tested the hypothesis that postural adjustments that stretch muscles that are most strongly activated from the primary motor cortex (MI) enlarge their cortical representation. Intracortical electrical stimulation within MI, forelimb muscle activity and movements, and vibrissa movements were used to evaluate the border between the MI forelimb and vibrissa representations before and after forelimb position changes in anesthetized adult rats. The forelimb was originally maintained in retraction (wrist extension and elbow flexion) and then changed to protraction (wrist flexion and elbow extension). Movements and forelimb EMG evoked by electrical stimulation were evaluated during this period (up to 3 hr) through a set of four electrodes implanted in layer V of MI. Changing the forelimb configuration had both immediate and delayed effects on forelimb muscle activity evoked from MI. At some sites, the magnitude of evoked forelimb muscle activity immediately increased with forelimb protraction. At one-quarter of all sites, forelimb muscle activity was evoked where it was not previously detected following an average delay of 22-31 min after forelimb protraction. This change can be interpreted as an expansion of the forelimb area into the vibrissa representation. These data further support the hypothesis that motor cortical representations are flexible and show that sustained changes in somatic sensory input to MI are sufficient to reorganize MI output.

Effect of environmental complexity on the latency of cortical vibrissa potentials.

The effect of differential rearing on evoked potentials was studied in the vibrissal representation of the rat neocortex. Young rats were reared in either enriched, standard, or impoverished conditions from the age of 24-25 days. At 55-60 days of age, the epicortical responses to electric stimulation of a whisker follicle were analyzed under urethane anesthesia. Slight but significant shortening of the latency of initial positivity in the evoked potential was observed after rearing in the enriched condition as compared to the data obtained from the littermates that were reared in the standard or impoverished conditions.

Efferent neurons and suspected interneurons in second somatosensory cortex of the awake rabbit: receptive fields and axonal properties

1. Receptive-field properties of antidromically identified efferent neurons within the representation of vibrissae and sinus hairs above the mouth were examined in secondary somatosensory cortex (S-2) of fully awake adult rabbits. Efferent neurons studied included callosal neurons (CC neurons, n = 88), ipsilateral corticocortical neurons (C-IC neurons, n = 51) that project to primary somatosensory cortex (S-1), and corticofugal neurons of layer 5 (CF-5 neurons, n = 63) and layer 6 (CF-6 neurons, n = 42) that project to and/or beyond the thalamus. Appropriate collision tests demonstrated that substantial numbers of corticocortical efferent neurons (21 of 113 tested) project an axon to both the corpus callosum and to ipsilateral S-1. 2. Suspected interneurons (SINs, n = 62) were also studied. These neurons were not activated antidromically from any stimulus site but did respond synaptically to electrical stimulation of the ventrobasal (VB) thalamus with a burst of three or more spikes at frequencies of 600 to greater than 900 Hz. Most of these neurons also responded synaptically to stimulation of S-1 and the corpus callosum. The action potentials of these neurons were much shorter (mean, 0.49 ms) than those of efferent neurons (mean, 1.01 ms). 3. CF-5 neurons differed from CC, C-IC, and CF-6 neurons in their spontaneous firing rates, axonal properties, and receptive-field properties. Whereas CF-5 neurons had a mean spontaneous firing rate of 5.7 spikes/s, CC, C-IC, and CF-6 neurons all had mean values of less than 1/s. Axonal conduction velocities of CF-5 neurons were much higher (mean, 11.90 m/s) than either CC (mean, 2.63 m/s), C-IC (mean, 0.86 m/s), or CF-6 (mean, 1.73 m/s) neurons. A decrease in antidromic latency (the "supernormal" period), which was dependent on prior impulse activity, was seen in most CC, C-IC, and CF-6 neurons but was minimal or absent in CF-5 neurons of comparable conduction velocity. Although all CF-5 neurons responded to peripheral sensory stimulation, many CC (52%), C-IC (49%), and CF-6 (55%) neurons did not. CC and CF-6 neurons that did not respond to sensory stimulation had significantly lower axonal conduction velocities and spontaneous firing rates than those that responded to such stimulation. Whereas no CC, C-IC, or CF-6 neuron responded synaptically to callosal stimulation, 43% of CF-5 neurons (and 78% of SINs) did so respond. Similar differences in synaptic responsivity to stimulation of S-1 were seen in these populations.(ABSTRACT TRUNCATED AT 400 WORDS)