Axonal trajectories between mouse somatosensory thalamus and cortex

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Abstract
An in vitro brain slice preparation has been used to label fibers connecting the somatosensory thalamus and cortex of the mouse. In 400–800-μm brain slices, the pathway between the ventrobasal complex and somatosensory cortex was labeled under direct vision with horseradish peroxidase crystals (HRP), HRP-Nonidet P-40 (NP40) detergent chips, or a solution of HRP/dimethylsulfoxide. Thalamocortical and corticofugal fibers are organized into a plexiform system of bundles that appears to be fairly constant from animal to animal. Bundles of fibers projecting from the ventrobasal complex course between regularly spaced groups of thalamic neurons. Thalamocortical axons do not invariably leave the thalamus via the fiber bundle closest to the perikarya. Thus, nearest-neighbor relationships are abolished before these axons have even left the thalamus. The axon bundles traverse the thalamic reticular nucleus lateral to the complex. The axons then rotate about one another, analogous to the coiling of strands in rope about a central axis. This accounts for the well known 180° rotation in the mediolateral direction between thalamic and cortical maps. Laterally, fiber bundles converge and diverge within the internal capsule so that nearest-neighbor relationships are lost. Individual thalamocortical axons do not bifurcate proximal to the subcortical white matter. After single bundles of fibers reach a point just below the subcortical white matter, their individual fibers diverge widely. Within the subcortical white matter most afferent fibers make a small dorsally concave loop prior to taking one of two possible courses: Some of the fibers ascend directly into the overlying cortex usually angled towards the dorsal surface of the brain; other fibers run in the subcortical white matter for variable distances prior to ascending into cortex. Within somatosensory cortex, smooth axons branch near their terminals in layers IV and VI. Axonal terminal and branching patterns of these axons within somatosensory cortex are similar to those found in in vivo preparations. Most axons are smooth, but other axons are beaded. Some beaded axons project to layer I. Corticofugal fibers are labeled. Fibers leaving somatosensory cortex have an angle of descent opposite to the angle of ascent for afferent fibers, and are often fasciculated in the cortex and subcortical white matter. Within the subcortical white matter efferent fibers often loop in a direction opposite to that of afferent fibers. Corticofugal fibers occasionally give off a collateral corticostriatal branch within the internal capsule. It is otherwise difficult to differentiate afferent from efferent fibers within the internal capsule. Efferent fibers course through the point of rotation; some fibers continue in the cerebral peduncle, while others enter the vertrobasal complex. Corticothalamic terminals have been identified within the ventrobasal complex. The principal findings of this study are that nearest-neighbor relationships of third-order afferent Fibers are abolished at many points along the thalamocortical pathway (i.e., as axons leave the ventrobasal complex, within the internal capsule, and within the subcortical white matter). Third-order afferents “seek out” appropriate cortical targets to re-establish nearestneighbor relationships that exist in the ventrobasal thalamus. The point of rotation, where the mediolateral inversion between thalamic and cortical maps takes place, has been demonstrated. Significant differences exist between thalamocortical and corticofugal fibers, at both ends of the pathway, which generally permits discrimination between the two fiber types.