Thalamic projections to the primary and secondary somatosensory cortices in cat: Single and double retrograde tracer studies

Single and double retrograde tracer experiments were performed in cats in order to investigate the organization of thalamic neurons projecting to the primary (SI) and secondary (SII) somatosensory cortical areas. In one series of animals, horseradish peroxidase (HRP) was injected in either SI or SII, and the distribution of retrogradely labeled neurons was reconstructed in serial coronal and horizontal sections through the thalamus. In a second series of experiments, cats received injections of HRP in SI and tritiated, enzymatically inactive HRP (3H-apo-HRP) in SII of the same hemisphere. The results from these experiments provide more exact information than can be obtained in single tracer experiments with regard to (1) the distribution and number of neurons projecting to both SI and SII by way of axon collaterals and (2) the topographical relationship among populations of thalamic neurons projecting to SI, SII, or both targets. SI Single tracer experiments demonstrate, in agreement with previous findings, that, after injection of SI which are focused on the representations of the limbs, heavy retrograde labeling is present throughout VPL. Within this complex, densely and lightly labeled neurons are found consistently and show some preferential pattern of organization. Thus, while both types of neurons are uniformly distributed in VPLl, densely labeled neurons tend to be arranged in clusters, particularly in the ventral portion of VPLm. Outside VPL, moderate but unequivocal retrograde labeling is present in POm, even in cases in which the spread of injected tracer did not encroach upon area 5; labeling of intralaminar nuclei and of a transitional zone between VP and VL, known to receive ascending spinal afferents, is also a consistent feature of all cats with SI injections, although it cannot be excluded that this results from the spread of injected HRP into area 4. SII From cases with injection of HRP in the anterior ectosylvian gyrus, it appears that as the injection site is shifted from posterior to anterior, labeling of neurons in the thalamus shifts from the lateral portion of the posterior group (POl) and the caudal region of the medial portion of this same group (POm) to involve progressively more rostral portions of this nucleus and also VP. Within VP, SII-projecting neurons are confined primarily within the lateral portion (VPLl) and posterior cap, while in VPLm they are confined mainly to the periphery of this nuclear subdivision and are sparse within its core region. Labeled neurons are also present in the transitional VP-VL zone. Double tracer Simultaneous visualization of thalamic neurons projecting to SI, SII, or both targets shows that within VP these three neuronal populations are not distributed homogeneously. Rather, their differential distribution defines, on the basis of pattern of their cortical projections, three divisions of the thalamic somatosensory relay: (1) a central core region of VPLm, which contains predominantly neurons projecting to SI; (2) a shell of neurons within VPL, in which neurons projecting to SI are homogeneously distributed among neurons projecting to SII and neurons projecting to both SI and SII; and (3) a previously defined outer shell—outside VPL—which is characterized, as a whole, by widely divergent cortical connections. It is suggested that these three regions, distinguished from one another by their patterns of cortical projection, may correspond to similar differential sites of afferent projections, such that each zone-core, inner and outer shells—would be dominated by a different ascending pathway.