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

TLDR: 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.

Abstract: The intracortical arborizations of neurons from the ventroposte- rolateral 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 cyto- architectonic 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; hair- driven 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 arboriza- tion 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 cyto- architectonic 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 sug- gested by Mountcastle ('57). He showed that cells recorded along a perpendicular tract in the primary somatic sensory (SI) cortex had over- lapping receptive fields and that they all re- sponded to the same modality of stimulation. Later, Hubel and Weisel ('62, '69) demon- strated 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 orienta- tion of axons of many cortical cells and sug- gested 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 dis- tinct 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 morpho- logical basis in the alternating patches of ter- minal ramifications of geniculocortical afferents (Huber and Weisel, '69; Ferster and LeVay, '78; Gilbert and Weisel '79). More re- cently, the patchy nature of the cortical projec- tion from the ventroposterolateral (VPL) nu- cleus of the thalamus has also been demon- strated by autoradiography (Friedman and Jones, '80). Since the topographic charac- teristics of cortical columns appear to be deter- mined by thalamocortical input, we injected single VPL afferents below SI after identifica- tion of their receptive fields. The aim of this