Distribution of Corticothalamic Projections from Various Somatotopic Representation Areas of the Primary Motor and Sensory Cortex

The locations and distribution of corticothalamic projections from various somatotopic representation areas of the primary motor (MI) and sensory (SI) areas of the cortex were studied in cats. Efferent fibers from MI neurons (fields 4y, 6ab) were found mainly to terminate in the ventral posterolateral and posteromedial (VPL, VPM), ventrolateral (VL), and reticular (R) nuclei, located in the rostral part of the thalamus, in contrast to the situation with the SI (fields 1, 2, 3a, 3b), which projected mainly to the caudal part of the thalamus, to the VPL, VPM, and R nuclei. A lateromedial organization was demonstrated for corticothalamic connections, with the cortical representation areas of the hindlimb being located mainly in the lateral part of the VPL nucleus, those of the forelimbs in the medial part, and those of the face and head being located not only in the VPL nucleus, but also in the VM and VPM nuclei. Quantitative comparison of the distributions of corticothalamic efferent fibers from different somatotopic representation areas in MI showed that the most extensive and massive connections with the thalamic nuclei (the VPL, VL, and R) bore the motor representation of the forelimbs, followed by the hindlimbs, trunk, and, finally, the face and head, which had the minimal level of representation. In contrast to the motor representation of the forelimbs and the face and head, which had a uniform distribution of fibers in the VPL, VL and R nuclei, the number of efferent fibers for the motor representation of the hindlimbs running to the VL nucleus was 2.5 times smaller than the numbers in the VPL and R nuclei, while the representation of the trunk projected mainly to the VL. The dominance of the corticothalamic connection is evidence for a greater level of involvement of the thalamic nuclei in supporting the functional specialization of particular somatotopic representations in the MI.

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