Laminar Organization of Thalamic Projections to the Rat Neocortex

Nerve fibers transmitting information from the thalamus to the cerebral cortex may be classified according to their major cortical layers of termination. (i) One class consists of inputs from thalamic relay nuclei for vision, audition, and somesthesis to layer IV, layer III, or both. In contrast, autoradiographic studies of projections from other thalamic nuclei reveal strikingly different patterns of termination: (ii) layer VI (or layer V, or both) is the target of fibers from the intralaminar nuclei, and (iii) layer I is the target for fibers from the ventromedial and magnocellular medial geniculate nuclei. (iv) The remaining class is typified by termination both in layer I and in additional layers that depend on the cortical area in which the terminations are found. The data demonstrate that convergent thalamic inputs to a given cortical area are usually not confluent within a layer and provide a new frame-work for categorizing thalamic nuclei.