THE JOURNALOF COMPARATIVE NEUROLOGY 199:345-371(1981) Intracortical Arborizations and Receptive Fields of ldent

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 tissues 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 rostra1 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 111. None terminated in layers I and 11. 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 pm in diameter. Some fibers generated two such bushes with an uninvaded region of about 300 pm between them. It is proposed that this patchy organization underlies in part the columnar organization of area 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 ofthe 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. The cortical column is a concept that has proved useful in understanding the cerebral cortex. The existence of columns was first suggested by Mountcastle ('57). He showed that cells recorded along a perpendicular tract in the primary somatic sensory (SI) cortex had overlapping receptive fields and that they all responded to the same modality of stimulation. Later, Hubel and Weisel ('62, '69) demonstrated the existence of orientation and ocular dominance columns in the primary visual area. The radial arrangement of nerve cells in many cortical regions often suggests columnar arrangements. Lorente de NO ('38) pointed out that the cortical unit is a narrow cylinder of cells stretching from layer I1 to layer V1 and having roughly columnar shape. Colonnier ('66) stressed the preferential vertical orientation of axons of many cortical cells and suggested that they are for processing inputs into vertical chains of neurons. He also emphasized that if the cortical circuitry has the necessary wiring for supporting the existence of cortical columns "the anatomical columns are not distinct separate morphological entities." This implicitly suggested that the territorial limits of the functional columns were determined by the connectivity of thalamocortical afferents and by the tangential spread of inhibition within the cortex. In the primary visual area, ocular dominance columns have their morphological basis in the alternating patches of terminal ramifications of geniculocortical afferents (Huber and Weisel, '69; Ferster and LeVay, '78; Gilbert and Weisel '79). More recently, the patchy nature of the cortical projection from the ventroposterolateral (VPL) nucleus of the thalamus has also been demonstrated by autoradiography (Friedman and Jones, '80). Since the topographic characteristics of cortical columns appear to be determined by thalamocortical input, we injected single VPL afferents below SI after identification of their receptive fields. The aim of this 0021-996718111993-0345$07.50 0 ALAN R. LISS, INC. 346 PIERRE LANDRY AND MARTIN DESCHENES study was to determine the areal distribution, laminar ramification, and topographic characteristics of single thalamocortical fibers having different receptive field modalities. MATERIALS AND METHODS