Tangential organization of thalamic projections to the neocortex in the mouse

Using the anterograde degeneration technique, we examine the tangential organization of a thalamofugal axon population (class I of Frost and Caviness, '80) whose terminations are preferentially distributed to the middle tier (located in layers III and/or IV) of three radially separated tiers of thalamic projections to the neocortex. Less extensive data are also presented on the tangential organization of thalamofugal axon populations (class II of Frost and Caviness, '80) that do not terminate preferentially in the middle tier, but that are otherwise heterogeneous with respect to their radial pattern of intracortical termination. The projections of class I axons are distributed to all neocortical fields with the possible exception of fields 13,25, and 35. The class I projections to a given cortical field (with the possible exception of the cortex of the second somatosensory representation) originate in only one thalamic nucleus. The class I projections of an individual thalamic nucleus form a cortical representation of the nucleus that constitutes a "first order line-to-line" (topologic) transformation of the nuclear volume. The ensemble of class I projections forms a cortical representation of the corresponding thalamic regions that constitutes a "second order line-to-line" (non-topologic) transformation of the thalamic volume. Class II axons project to all neocortical fields. Classs II and class I projections contrast in that the class II projections of multiple thalamic nuclei overlap in the tangential plane of any given sector of the cortex. While the class II projections of the intralaminar nuclei and the widely projecting ventromedial nucleus are known to be topologically organized, the tangential organization of class II projections arising in other nuclei is incompletely understood.