Biomedical subjects

G E Carvell

Publications and source records attributed to G E Carvell.

16 recordsLinked to original sources

PubMed1996

Abnormal tactile experience early in life disrupts active touch.

The importance of early tactile experience in the development of discriminative somatomotor function was assessed by examining the proficiency and movement strategies of rats raised without normal sensory inputs provided by their mystacial vibrissae. Infant-trimmed animals had their whiskers clipped daily from birth to 45 d of age, after which they were allowed to regrow for 60-70 d before initiation of behavioral training, which lasted as long as several months. Adult-trimmed animals had their whiskers trimmed for comparable periods during adulthood. Rats were tested on one of two tactile discriminations, rough versus smooth or rough versus rough, that differed with respect to the overall size of their surface features. Whisker movements during task performance were examined in detail using video-based motion analysis software. Infant-trimmed animals performed rough versus smooth discriminations as well as adult-trimmed rats or normally reared animals. Except for one subject, infant-trimmed rats were severely impaired in their ability to distinguish rough versus rough surfaces. Deficits persisted in spite of months of training with the regrown vibrissae. The animals that failed to master this task displayed whisking patterns that notably lacked frequencies in the normal range of 6-12 Hz. Thus, abnormal tactile experience early in life substantially, and perhaps permanently, impairs sensorimotor integration underlying active touch.

Animals↗

PubMed1996

Effects of baclofen and phaclofen on receptive field properties of rat whisker barrel neurons.

Extracellular single-unit recordings were made in somatosensory cortical barrels of fentanyl-sedated rats. Whiskers were deflected singly or in paired combinations. Iontophoretically-applied (-)-baclofen disproportionately reduced weak responses, and phaclofen disproportionately increased them, resulting in more tightly focused or more broadly focused receptive fields, respectively. Both drugs had only minor effects on surround inhibition. In light of previous findings, we conclude that GABAA and GABAB mechanisms both act to enhance spatial contrast, but that the former plays a much greater role in enhancing temporal resolution.

Animals↗

PubMed1996

Quantitative effects of GABA and bicuculline methiodide on receptive field properties of neurons in real and simulated whisker barrels.

1. Carbon fiber multibarrel glass microelectrodes were used to record extracellular single-unit activity during microiontophoretic application of gamma-aminobutyric acid (GABA) or bicuculline methiodide (BMI) onto layer IV barrel neurons in the somatosensory cortex of fentanyl-sedated rats. Excitatory and inhibitory aspects of the neurons' receptive fields were quantified with the use of controlled whisker stimuli. The principally activating whisker and one of its immediately adjacent neighbors were deflected alone or in paired combinations involving a condition-test paradigm. 2. Units were distinguished electrophysiologically on the basis of the time course of their action potential waveforms. Data were obtained from 26 regular-spike units (RSUs; presumed spiny stellate cells) and 7 fast-spike units (FSUs; presumed GABAergic neurons). An average of 15.0 nA of GABA produced a one-third to one-half reduction in RSU responses evoked by the maximally effective stimulus. An average of 8.7 nA of BMI was needed to counteract this reduction. This amount of BMI, in the absence of exogenous GABA, was found to increase average RSU and FSU responses by 98 and 53%, respectively, relative to predrug levels. 3. For RSUs, the BMI-induced twofold increase in responses evoked by moving the principal whisker at the neuron's best deflection angle was accompanied by an almost threefold increase in responses evoked by similarly moving an adjacent whisker. Disproportionately large percentage increases were also seen for responses to nonpreferred directions of principal and adjacent whisker movement. BMI thus effectively increased receptive field size and decreased angular tuning. Similarly, responses to stimulus offsets, which are normally smaller than ON responses, were increased proportionally more. 4. Predrug responses of FSUs were more vigorous than those of RSUs. However, FSUs showed a similar inverse relationship between percentage increase with BMI and initial response magnitude, although the proportional increases were less pronounced. 5. GABA, like BMI, had the greatest proportional effects on those responses that were initially smallest. It produced results opposite those of BMI, effectively decreasing receptive field size and sharpening angular tuning. 6. A previously described computational model of a barrel was tested for its ability to reproduce quantitatively the effects of BMI and GABA. The application of BMI was simulated by decreasing the strength of the inhibitory inputs onto the particular cell under study in the model network. GABA microiontophoresis was simulated by adding a constant hyperpolarizing voltage. The model RSUs and FSUs displayed proportional changes in response magnitude that were quantitatively similar to those of their biological counterparts. 7. Surround inhibition was greatly attenuated by BMI application, both for the real and simulated barrel neurons. Disinhibition was less pronounced for the former, perhaps because, unlike the simulated neurons, they also possess GABAB receptors, which are unaffected by BMI. 8. We conclude that the inhibitory receptive field properties of barrel neurons can be explained by intrabarrel inhibition and that the expansion of receptive field size and loss of angular tuning with BMI is due to an enhanced effectiveness of convergent, multi-whisker thalamocortical input. Examination of the model neurons' behavior suggests that the altered activity in response to GABA or BMI application, respectively, can be explained by the nonlinear effects of shifting somal membrane potential away from or toward the neuron's firing threshold.

Action Potentials↗

PubMed1996

The relationship of vibrissal motor cortex unit activity to whisking in the awake rat.

Rats actively sweep their whiskers back and forth to locate and palpate objects within their immediate environment. Microstimulation studies in anesthetized rats have demonstrated the presence of a large vibrissal motor representation in agranular cortex. However, the manner in which motor cortex neurons contribute to whisking behavior in the awake animal is unknown. This study represents an initial investigation of the relationship between the activity of task-related neurons in vibrissal motor cortex and the electromyographic (EMG) activity of the deep vibrissal pad muscles in the awake, freely whisking rat. Each animal was gently held in an experimenter's hands while the animal whisked the air. A spring-loaded, metal microelectrode mounted in a removable, miniature microdrive provided stabile recordings of motor cortex unit activity. Fine-wire electrodes implanted in the mystacial pad simultaneously recorded facial muscle activity. Results showed that the discharge of task-related neurons was correlated with changing levels of muscle output. Unit discharge was related in a tonic or phasic-tonic fashion to EMG activity. No units were found to discharge rhythmically in a 1:1 fashion with the periodicity of the whisking pattern. These findings support a role for vibrissal motor cortex in the initiation and modulation of the overall level of mystacial pad muscular output, but not in the generation of bursts of EMG activity responsible for individual whisking sweeps.

Animals↗

PubMed1995

Task- and subject-related differences in sensorimotor behavior during active touch.

Rats explore objects by rhythmically whisking them with their mystacial vibrissae. On two types of tactile discrimination tasks, macrogeometric and microgeometric, better performers palpated the discrimnanda for longer periods of time and used movement patterns that appeared to optimize whisking frequency bandwidth and the extent to which the vibrissae would be bent by object contact. On a task involving finely textured surfaces, good and poor performers differed in the temporal components of their whisking patterns, whereas the spatial domain was more important for animals palpating surfaces with widely separated features. These findings are consistent with increasing neurophysiological evidence that the central representation of the tactile periphery, in rodents and other mammals, is both integrative and dynamic.

Animals↗

PubMed1994

OFF response transformations in the whisker/barrel system.

1. Previous studies have demonstrated marked differences in the relative sizes of ON and OFF responses of neurons in the whisker/barrel system. In particular, OFF responses are unexpectedly large in thalamic neurons. Extracellular unit recordings were used to examine whether varying the time between stimulus onset and offset differently affects OFF responses of neurons in the trigeminal ganglion, ventrobasal thalamus, and somatosensory cortical layer IV. Controlled whisker stimuli were used to deflect individual vibrissal hairs in different directions. We hypothesized that, in part because of the gradual waning of central inhibition evoked by stimulus onset, OFF responses of thalamic and cortical neurons but not trigeminal ganglion cells would increase in size with longer duration stimuli, with relative changes being greatest in the cortex. 2. OFF response magnitudes for thalamic and cortical neuronal populations increased as the stimulus duration was increased from 200 to 1,400 ms. Increases were greater at nonoptimal deflection angles. Similarly, individual cells having smaller OFF responses for the short-duration stimulus tended to display proportionately greater increases when the stimulus was lengthened. OFF responses of trigeminal ganglion cells were largely unaffected by stimulus duration. 3. Barrel neurons were subclassified as regular-spike units (RSUs) or fast-spike units (FSUs) on the basis of the time course of their action potentials. ON and OFF responses were smaller in the former and, when the stimulus was lengthened, percentage increases in their OFF responses were greater than those in FSUs. Results illustrate nonlinear transformations of the thalamic input signal by RSUs, which are presumed to be excitatory barrel neurons, and extend previous findings of response similarities between thalamocortical units (TCUs) and FSUs, the latter of which are thought to be inhibitory. 4. The time course of OFF response suppression in cortical neurons suggests that stimulus onset evokes central inhibition having two components, a potent one lasting several tens of milliseconds and a weaker one lasting many hundreds of milliseconds. Background activity levels in cortex and thalamus were diminished for > or = 1,800 ms after whisker movement. 5. For TCUs, 200-ms stimuli were less likely than 1,400-ms stimuli to elicit an OFF response, but when responses occurred they consisted of a greater number of spikes timed closer together. By contrast, the 200-ms stimulus OFF responses of the RSUs and FSUs displayed longer interspike intervals than did their 1400-ms responses, with no change in the number of spikes per response.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1992

Responses of barrel cortex neurons in awake rats and effects of urethane anesthesia.

A "barrel" is an interconnected network of layer IV neurons that is an important component of a functional cortical column in the whisker area of the rodent primary somatosensory cortex. The present study was undertaken in order to resolve apparently conflicting findings from single-unit studies of barrel neurons conducted in rats maintained under different anesthetic conditions. Multiunit responses to controlled deflections of mystacial vibrissae were recorded from the whisker/barrel cortex of awake, undrugged rats, and responses at the same recording site were reexamined after the animal was anesthetized with urethane. In contrast to the awake condition, stimulus-evoked responses under urethane were characterized by a large late component. Such effects were more pronounced for deflections of noncolumnar or "adjacent" whiskers than for the columnar whisker. Latencies to peak responses were virtually identical for the columnar whisker in awake and urethane states (11.9 vs 11.8 ms) but were considerably longer for adjacent whisker deflections in urethane-anesthetized animals (15.5 vs 29.0 ms). The magnitudes of adjacent whisker responses, relative to the response evoked by the columnar whisker, varied with the laminar location of the recording site in awake but not in urethane-anesthetized animals; in awake rats, receptive fields were clearly smallest in the layer IV barrels. Results in the awake condition confirm those of previous studies conducted in unanesthetized or lightly sedated animals, and data obtained with urethane are comparable to others' results in urethane-anesthetized rats. The former have important implications for how barrel cortex processes information in behaving animals.

Anesthesia↗

PubMed1991

Functional asymmetries in the rodent barrel cortex.

Neurophysiological and 2-deoxyglucose (2DG) studies of the rodent whisker barrel cortex have demonstrated asymmetries in its functional organization. To examine the possibility that the activity gradients observed in metabolic studies can be attributed to subtle rostral-caudal and dorsal-ventral asymmetries in electrophysiologically measured surround or cross-whisker inhibition, we compared 2DG results with predictions generated from quantitative single-cell receptive field data. Despite differences in the two experimental approaches, there is remarkable agreement between the findings. (1) The distribution of 2DG activity declines across the barrel cortex of the behaving animal from anteromedial barrels to posterolateral barrels, and is qualitatively and quantitatively similar to the values predicted from neurophysiology. (2) The strength of surround inhibition in barrel neurons predicts the twofold increase in activation of the C3 barrel following acute clipping of adjacent whiskers. And (3) within a cortical column, the decrease in metabolic activity associated with adjacent whisker stimulation is greatest in layer IV and least in the infragranular layers; this corresponds to the laminar distribution of inhibitory interactions observed electrophysiologically.

Afferent Pathways↗

PubMed1991

Electromyographic activity of mystacial pad musculature during whisking behavior in the rat.

Cinematographic measurements of whisker movements generated by behaving rats were compared with electromyographic (EMG) activity recorded simultaneously from mystacial pad musculature. Muscle activity consisted of repetitive bursts, each of which initiated a "whisking" cycle consisting of a protraction followed by a retraction. Protraction amplitude and velocity were directly proportional to the amount of EMG activity during forward whisker movement. Overtime, the intensity of muscle discharge determined the set point about which the vibrissae moved; higher levels of muscle activity resulted in a greater degree of overall whisker protraction. These findings are consistent with the known anatomy of the facial musculature and underscore the importance of whisker protraction in the acquisition of tactile information by the vibrissae.

Animals↗

PubMed1990

Responses of rat trigeminal ganglion neurons to movements of vibrissae in different directions.

The response properties of 123 trigeminal ganglion neurons were studied, using controlled whisker deflections in different directions. When the distal end of the whisker was initially displaced 5.7 degrees (1 mm) from its neutral position, 81% of the cells responded with statistically more spikes/stimulus to movements in one to three of eight cardinal (45 degrees increment) directions than to the others. The more directionally selective the cell, the more vigorous was its response. On the basis of statistical criteria, 75% of the cells were classified as slowly adapting, 25% as rapidly adapting. A number of quantitative analyses indicated that slowly adapting units respond more selectively than rapidly adapting cells to the direction of whisker movement. Differences in directional sensitivities of rapidly and slowly adapting cells appear to parallel differences between their putative mechanoreceptive endings and the relationships between those endings and the vibrissa follicle's structure. Comparisons between the response properties of peripheral and central neurons in the vibrissa-lemniscal system indicate that the afferent neural signal is progressively and substantially transformed by mechanisms that function to integrate information from different peripheral receptors and from different, individual vibrissae.

Action Potentials↗

PubMed1990

Biometric analyses of vibrissal tactile discrimination in the rat.

Blindfolded rats were trained to stretch across a gap to palpate rough or smooth surfaces with their mystacial vibrissae. Animals learned to discriminate reliably a smooth surface from a rough surface having shallow (approximately 30 microns) grooves spaced at 90 microns intervals. Field-by-field video analyses confirmed that rats used only their vibrissae to contact the discriminanda. The whiskers swept across the surfaces at 696 degrees/sec during forward movements and 1106 degrees/sec for retracting movements. Mean amplitudes, which were 32 degrees, were considerably smaller than the total arc through which whiskers can move. Rats maintained whisker contact with discriminanda for several hundreds of msec, during which time the animals repetitively swept their vibrissae across the surface at a dominant frequency of 8 Hz. The range extended from 1 to 20 Hz, and the frequencies utilized varied within and among subjects. Whiskers contacted the discriminanda along the hair shaft, not at the whisker tips. The hair shafts were bent continually but to varying degrees as an animal palpated the surface, and more than one of the large caudal whiskers were almost always in contact with it. Thus, whiskers are not used independently as rigid levers. Results indicate that the capacity of the rodent whisker system to distinguish a smooth surface from a rough one is comparable to that of primates using their fingertips and suggest common strategies for active touch in the mammalian somatomotor system.

Animals↗

PubMed1989

Spatial organization of thalamocortical and corticothalamic projection systems in the rat SmI barrel cortex.

Axonal tracing techniques were used to examine the distribution of corticothalamic projection neurons in relation to the organization of the thalamocortical recipient zones in the whisker representation of the rat first somatic sensory cortex. Following injection of horseradish peroxidase into the physiologically defined vibrissa area in the ventrobasal complex of the thalamus, labeling in the cortex had a columnar appearance. Dense patches of anterograde labeling were located within the centers of the layer IV barrels and extended superficially through lamina III; the septa between barrels contained considerably less reaction product. Retrogradely labeled neurons were observed in lower layer V and layer VI where they were concentrated preferentially deep to the barrel centers. Regions deep to the septa displayed less overall labeling and a lower relative number of thalamic projecting neurons. Zones having the larger numbers of retrogradely labeled cells also contained terminallike labeling of either corticothalamic or thalamocortical origin. Following an injection that included the posterior group medial to the ventrobasal complex, anterograde labeling in layer IV was located largely in the septa. In conjunction with previous findings concerning the origin and termination of other projection systems in the barrel cortex, these results suggest that a vibrissal column contains a central core zone intimately linked with the ventrobasal thalamus that is bounded by narrower regions of more diverse inputs and outputs that form an interface between adjacent cortical columns.

Animals↗

PubMed1989

Thalamocortical response transformation in the rat vibrissa/barrel system.

1. Extracellular single-unit recordings and controlled whisker stimuli were used to compare response properties between cells in the "barreloids" of the thalamic ventrobasal complex and those in the cytochrome oxidase-rich centers of the "barrels" in the first somatic sensory cortex. Individual vibrissae were deflected alone or in paired combination involving the neuron's maximally excitatory whisker and an adjacent one in the same or neighboring whisker rows. Quantitative data were derived from 135 thalamocortical unit's (TCUs), 242 "regular-spike" barrel units (RSUs), and 16 "fast-spike" barrel units (FSUs) recorded in 26 normal adult rats. 2. Compared with TCUs, RSUs displayed lower rates of spontaneous activity and responded less vigorously to whisker stimuli. Proportionally, more than twice as many TCUs as RSUs responded in slowly adapting fashion to at least one angular direction of whisker displacement. Discharges of slowly adapting TCUs were approximately 3.5 times greater than those of slowly adapting RSUs. 3. Proportionally, about twice as many TCUs than RSUs responded selectively to whisker movements in different angular directions. 4. Cells in the thalamus responded more vigorously to a larger number of whiskers than RSUs in the cortex. Depending on the stimulus conditions, two to three times more TCUs than RSUs were excited by two or more whiskers. 5. Following displacement of an adjacent whisker, unit discharges to subsequent deflections of the maximally excitatory whisker were reduced in a time-dependent fashion. The time course of response suppression was similar in TCUs and RSUs, but inhibitory interactions between adjacent whiskers were observed much less often in the thalamus. A cyclic pattern of stimulus-evoked excitation/inhibition characterizes responses in the cortical barrels but is considerably less pronounced in the thalamic barreloids. 6. The presence and/or degree of response suppression depended on which adjacent whisker was stimulated and on the angular direction of that whisker's movement. For individual TCUs, some adjacent whiskers evoked inhibition, others did not. The vast majority of RSUs displayed response suppression to all adjacent whiskers. Unlike receptive fields of TCUs, those of RSUs have small--i.e., single-whisker--excitatory centers with potent and symmetrical inhibitory surrounds. 7. Fast-spike units in the barrels displayed the greatest spontaneous and stimulus-evoked activities and were the least selective for whisker movements at different angular directions. FSUs had the largest excitatory receptive fields; 100% responded to two or more vibrissae.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1988

Membrane potential changes in rat SmI cortical neurons evoked by controlled stimulation of mystacial vibrissae.

Intracellular recordings from rat somatic sensory vibrissa/barrel cortex demonstrate that whisker displacements evoke short latency excitatory postsynaptic potentials followed by longer lasting inhibitory potentials. The time course and whisker-related spatial distribution of the potentials represent synaptic correlates of the integration of whisker inputs observed in extracellular studies.

Animals↗

PubMed1987

Thalamic and corticocortical connections of the second somatic sensory area of the mouse.

Thalamic and corticocortical connections of the second somatic sensory area (SII) in the mouse cerebral cortex were investigated by means of the retrograde transport of horseradish peroxidase. Focal injections of the enzyme were made in physiologically determined locations within the parietal cortex. Results show that SII receives substantial inputs from topographically appropriate regions within the ipsilateral ventrobasal nucleus and from the ipsilateral posterior group. The limb representation, which was previously found to be responsive to auditory stimulation, received inputs also from the medial division of the medial geniculate body. The SII face representation, which is largely unresponsive to auditory stimuli, received little or no input from the medial geniculate body. SII injections yielded retrograde labeling in the topographically appropriate region in the first somatic sensory area (SI), and SI injections retrogradely labeled cells in SII in a pattern consistent with previous electrophysiological maps. Homotypical regions within SI and SII therefore appear to be reciprocally interconnected. SII also receives inputs from the ipsilateral motor cortex and from contralateral SI and SII. Finally, injections into the SI paw but not face regions yielded retrograde labeling in the thalamic ventrolateral nucleus. Thus, the distal limb representations in SI and SII each receive inputs from a third major relay nucleus (i.e., medial geniculate to SII, ventrolateral nucleus to SI) whereas the face representations do not. These results indicate a close functional interrelationship between homotypical areas in SI and SII, though the two areas differ in several important respects. It is proposed that SII in mice may complement the function of SI by helping to define the overall sensory context in which detailed tactile discriminations are made.

Animals↗

PubMed1986

Somatotopic organization of the second somatosensory area (SII) in the cerebral cortex of the mouse.

The somatotopic organization of the parietal cortex of barbiturate-anesthetized, adult mice was studied using tungsten microelectrodes. A complete representation of the contralateral face and body occupying approximately 4.0-4.5 mm2 was found immediately posterior and lateral to the representation of the face in the first somatosensory area (SI). Within this second somatosensory area (SII), the following findings were made: A relatively large region is devoted to representations of the paws and face, especially the sinus hairs associated with the anterior upper lip and mystacial vibrissae. Receptive fields on these body regions are among the smallest found in SII, though larger than corresponding receptive fields in SI. In particular, vibrissae receptive fields always include at least several adjacent whiskers, and paw receptive fields always include at least two adjacent digits. In regions representing proximal body parts, receptive fields are considerably larger, may include both contralateral and ipsilateral limb or trunk surfaces, and sometimes include the entire body and face. Responses to both somatosensory and auditory stimulation were consistently found in the body (i.e., trunk and limb) representation, but rarely found in the face region. The face is represented most anteriorly, and the hindlimb and tail most posteriorly. Forepaw and hindpaw digits and anterior aspects of the face (e.g., perioral sinus hairs and the incisors) are represented laterally, while the back, caudal head, and mystacial vibrissae are represented medially. Within SII, therefore, a "musculus" can be viewed as having an upright body orientation with the face area bordering the face representation within SI. By comparison with SI, SII is characterized by a less pronounced layer IV, which is of irregular thickness and packing density, and by less uniformity in the layering of pyramidal cells in lamina V. In addition, SII is generally thicker from pia to white matter than SI. These results are in general accord with earlier findings from evoked potential studies in mice, but are at variance with recent reports in mice and rats that the mystacial vibrissae have only a minimal, or no, representation within SII. Indeed, the present findings suggest that the representation of the whiskers in SII may have a specialized function paralleling that in SI.

Afferent Pathways↗