Thalamic input to areas 3a and 2 in monkeys.

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Abstract
Portions of the thalamic VPLc [ventral posterior lateral nucleus, caudal part] nucleus receiving deep and cutaneous inputs and the adjacent VPLo [ventral posterior lateral nucleus, oral part] nucleus were defined by single or multiunit recording in anesthetized monkeys [Macaca fascicularis and M. mulatta]. After marking the border between deep and cutaneous zones with a microlesion, tritiated amino acids were injected in 1 or other zone of VLPc or in VPLo, and the differential distributions of thalamocortical fibers emanating from the different regions were later demonstrated autoradiographically. The VPLc nucleus projects to areas 3a, 3b, 1 and 2. The VPLo nucleus and its extension, VLc [ventral lateral nucleus caudal part], project only to area 4. In the floor of the central sulcus and in other regions where area 4 and the somatosensory cortex come together, there is a cytoarchitectonic delineation of area 3a as a region with an attenuated layer IV receiving inputs from VPLc. Area 3a defined in this way is usually distinct from any zone containing giant pyramidal cells extending posteriorly from area 4. The giant pyramidal cell zone receives inputs from VPLo and where there is a small amount of overlap between giant cells and the attenuated layer IV of area 3a, there may be a little overlap of the VPLo and VPLc projections. Both area 3a and area 2 receive thalamocortical input from a shell region of VPLc extending up the anterior aspect and through the anteroposterior extent of the dorsal aspect of the nucleus. In this region, neurons are activated by deep stimuli, including joint movement and manipulation of tendons and muscle bellies. Area 3a receives inputs from all parts of this zone and not from any obvious transition zone between VPLc and VPLo. Area 2 may receive inputs only from posterior parts of the zone. A larger, central core of VPLc contains neurons activated by light tactile stimulation and projecting only to areas 3b and 1. Area 3a is apparently an integral part of the somatosensory cortex that shares deep thalamic inputs with area 2. Areas 3a and 2 surround a central cutaneous core consisting of areas 3b and 1, each of which appears to contain a separate representation of the body surface.