Cells of origin of corticothalamic projections upon the centromedian and parafascicular nuclei in the cat.

The horseradish peroxidase method was used to study the cortical projections upon the centromedian and parafascicular (CM-Pf) thalamic complex in the cat. Following HRP injections into the CM-Pf complex, labeled corticothalamic neurons are found predominantly in cortical layer V, and rarely in layer VI. They are medium in size, pyramidal in shape, and usually occur singly, though occasionally in small clusters. These retrogradely labeled neurons have an extremely widespread distribution, being scattered throughout approximately the rostral four-fifths of the cat's neocortex. They are found in all neocortical gyri, except the posterior lateral and posterior suprasylvian gyri, but are more numerous in the rostral neocortical regions, especially within the depths of the cruciate and presylvian sulci. The pattern of distribution of these corticothalamic neurons, as revealed herein, bears a remarkable resemblance to the organization of the corticostriatal projection, as shown in another study. The similarity between the corticothalamic and corticostriatal projections is discussed. Also discussed are the present results regarding the corticothalamic projection with regard to other studies using neurophysiological and neuroanatomical methods.