The organization of thalamocortical relay neurons in the rat ventrobasal complex studied by the retrograde transport of horseradish peroxidase

The organization of thalamocortical relay neurons in the thalamic ventrobasal complex (VB) of the rat was investigated by the use of the retrograde axonal transport of horseradish peroxidase (HRP).

Abstract

Abstract The organization of thalamocortical relay neurons in the thalamic ventrobasal complex (VB) of the rat was investigated by the use of the retrograde axonal transport of horseradish peroxidase (HRP). Injections of HRP into somatosensory cortex (SI) resulted in a distinctive gradient of neuronal and non‐neuronal HRP reaction product. Electrophysiologically characterized points of SI injected with small volumes of HRP labeled a sector of neurons in VB ipsilateral to the injection. This zone of labeled neurons consisted of a complex curvilinear array or lamina with a rostral hollow or solid expansion of densely packed HRP positive neurons which continues caudally as a less dense tapering wing. Despite this complex arrangement, an approximate pattern of somatotopy was determined indicating the direction of shifts in peripheral receptive fields moving in any axis of VB. The number of labeled neurons projecting upon a point in SI was also determined. Injections of the cortical vibrissae, face and forepaw representations labeled a greater number of neurons in VB per unit area of cortex than did injections in the hindlimb or body representations. The total number of HRP positive neurons in VB increased proportionately with areal increase of the HRP injections into overlapping cortical representations. The number of HRP positive neurons in the rostral half of the laminae increased almost linearly with the area of injection, while the number of HRP positive neurons in the caudal half of the laminae showed relatively smaller increases.