The Afferent and Efferent Connections of the Ventromedial Thalamic Nucleus in the Rat MILES HERKENHAM Department of Psychology, Massachus,etts Institute of Technology, Cambridge, Massachusetts 02139
ABSTRACT The afferent and efferent connections of the ventromedial (VM) nucleus of the thalamus in the rat were studied by experiments using the methods of retrograde cell marking by horseraldish peroxidase (HRP) and anterograde fiber tracing by autoradiography. Tritiated amino acids deposited microelectrophoretically into VM label a cortical projection that is distributed to a sharply defined superficial portion of layer I of almost the entire extent of the ipsilateral neocortex. The labeling is most dense a t frontal cortical levels, where fibers radiate through the deeper layers to terminate in the outer onequarter of layer I throughout all neocortex rostral to the genu of the corpus callosum. A lesser number of labeled fibers extends caudally in a supracallosal location to innervate parieto-occipital cortical areas. Labeled collaterals ascend through the cortical layers t o reach layer I, where grains in the superficial portion are found in a gradually decreasing rostrocaudal gradient of density that reaches the caudal pole of the hemisphere. Coronal sections at most levels contain a band of labeling in layer I that extends uninterrupted from the callosal sulcus at the midline t o the banks of the rhinal sulcus laterally. Caudal retrosplenial and ventral temporal areas appear t o be the only
ABSTRACT The afferent and efferent connections of the ventromedial (VM) nucleus of the thalamus in the rat were studied by experiments using the methods of retrograde cell marking by horseraldish peroxidase (HRP) and anterograde fiber tracing by autoradiography. Tritiated amino acids deposited microelectrophoretically into VM label a cortical projection that is distributed to a sharply defined superficial portion of layer I of almost the entire extent of the ipsilateral neocortex. The labeling is most dense a t frontal cortical levels, where fibers radiate through the deeper layers to terminate in the outer onequarter of layer I throughout all neocortex rostral to the genu of the corpus callosum. A lesser number of labeled fibers extends caudally in a supracallosal location to innervate parieto-occipital cortical areas. Labeled collaterals ascend through the cortical layers t o reach layer I, where grains in the superficial portion are found in a gradually decreasing rostrocaudal gradient of density that reaches the caudal pole of the hemisphere. Coronal sections at most levels contain a band of labeling in layer I that extends uninterrupted from the callosal sulcus at the midline t o the banks of the rhinal sulcus laterally. Caudal retrosplenial and ventral temporal areas appear t o be the only
