Neonatal damage to a row of mystacial vibrissae in the mouse causes cytoarchitectonic alterations in the contralateral SmI barrel Cortex. The region for the appropriate row of barrels develops as a smaller homogeneous zone while barrels in adjacent rows are expanded. To investigate the effects of this phenomenon on the morphology of individual neurons, adult mice in which Row-C vibrissae (the middle row) had been cauterized on days 1--5 following birth were processes by the Golgi-Cox method. All neurons in layer IV of the Row-C zones, of the Row-C barrels of a control hemisphere, and some neurons in the adjacent enlarged Row-B barrels were measured with a computer-assisted microscope. Their location with respect to cytoarchitectonic boundaries was determined from a Nissl counterstain. Data from 239 cells are presented. For each cell, measures of dendritic length and branching were obtained. The orientation of the dendritic trees with respect to the barrel sides was also measured. The measures of dendritic lengths and branching did not show any differences between control and experimental animals or between animals damaged on different days. Measures of orientation did show changes related to the age at the time of damage. In animals damaged on postnatal day (PND)-3 or earlier, many cells in the Row-C zone were observed with dendrites orienting toward the adjacent Rows-B or -D. "Putative" Row-C cells in the expanded parts of Rows-B and -D were strongly oriented toward barrels in those rows. These results suggest that dendritic length and branching may be determined intrinsically but that the orientation of the dendritic trees appears to be strongly influenced by the pattern of extrinsic afferent inputs from the thalamus. In the case of the whisker-damaged animals, the orientation of the Row-C neuron dendritic trees toward the "functional" thalamocortical inputs in Rows-B and -D contributes strongly to the resultant cytoarchitectonic changes. The implications of these results for normal developmental processes and their relationship to functional studies of the cortex are considered.
Thalamocortical relay neurons from the rat ventrobasal nucleus were identified physiologically and injected intracellularly with horseradish peroxidase. The axons of these cells were followed through serial sections in order to determine if collaterals were given off within the ventrobasal nucleus or the thalamic reticular nucleus. No local collaterals were seen in the ventrobasal nucleus, thus indicating that interactions between relay cells in this nucleus are minimal. Of axons that could be followed into the internal capsule, 76% gave off visible collaterals in the thalamic reticular nucleus. Half of these axons had collaterals showing extensive branching with the potential of innervating a large number of thalamic reticular neurons. The other half had short, simple branches of restricted extent. No correlations were found between the physiological properties of a cell and the existence or extent of axon collaterals. These results describe the anatomical basis for the initial part of a feedback loop through the thalamic reticular nucleus that provides the major inhibitory influence on rat ventrobasal neurons.
The anatomical structure of physiologically identified neurons of the rat ventrobasal thalamus was studied in order to determine if there are morphologically distinct subsets of neurons that correlate with the somatosensory submodalities processed by these cells. Intracellular recordings were used to determine the modality and receptive field of a neuron, after which horseradish peroxidase was iontophoretically injected into the cell, allowing it to be histologically visualized. Computer-assisted measurements of the labeled cells were made to quantitatively analyze the dendritic structure. Cells were divided into physiological groups stimulated by whiskers, glabrous skin, furry skin, noxious stimulation, or joint rotation. Qualitatively, all cells appeared similar, with the same types of branching patterns. Dendritic spines and long, sinuous appendages were found on all distal dendrites. Quantitatively, no statistically significant differences in dendritic structure were found between functionally defined groups with the aid of a number of parameters, including a fitted dendritic ellipse. There was a weak correlation between somal cross-sectional area and receptive field size, suggesting larger cells processed larger receptive fields. In summary, the ventrobasal thalamus of the rat, in contrast to that of higher mammals, appears to contain only one major cell type and to have a very simple neuronal circuitry.
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