Intracortical arborizations and receptive fields of identified ventrobasal thalamocortical afferents to the primary somatic sensory cortex in the cat.

The intracortical arborizations of neurons from the ventroposterolateral thalamic nucleus (VPL) in the cat were studied by intraaxonal injections of horseradish peroxidase (HRP) following identification of their receptive fields. In the primary somatic sensory cortex (SI) VPL cells terminated in different cytoarchitectonic areas according to their receptive field modality. Fibers excited by deep tissue or joint rotation arborized preferentially in area 3a. Those responding tonically to cutaneous stimuli were located in the anterior part of area 3b; hairdriven cells terminated in area 3b and in the rostral pole of area 1. All fibers had a similar laminar distribution within SI. Axons terminated mostly in layers VI, iV, and the lower part of layer III. None terminated in layers I and II. Most terminal arbors were oriented along the mediolateral axis of the brain. The main arborization of a single VPL cell formed a bush of about 500 micrometers in diameter. some fibers generated two such bushes with an uninvaded region of about 300 micrometer between them. It is proposed that this patchy organization underlies in part the columnar organization of areas SI. Many VPL cells had secondary projection sites in SI. These were issued from smaller-sized collaterals and were located in a different cytoarchitectonic area than that of the main terminal plexuses. A significant number of these collaterals projected to area 4, Insufficient filling of the collaterals by HRP prevented a more complete characterization of the secondary arbors.