A quantitative comparison of the hominoid thalamus: III. A motor substrate—the ventrolateral complex
Nuclear volumes, nerve cell densities, numbers of neurons, and volumes of nerve cell perikarya of the thalamic ventrolateral complex (VL), a neural substrate for movement, were measured in specimens from two gibbons, one gorilla, one chimpanzee, and three humans, and the values were compared. The human VL had about one-and-a-half times as many neurons as did those of the great apes. The relative frequencies of the sizes of nerve cell perikarya differed slightly in the ventrolateral segment of VL; no differences were noted in the rest of VL. Compared with findings from other parts of the thalamus, the differences in the volumes of VL were greater than those found in the thalamic sensory nuclei, similar to those of rest of the thalamus, and less than those found in the whole brain. The increased number of neurons in human VL was similar to that of the somatosensory relay complex, but greater than those of the auditory and visual nuclei and less than those of the limbic and association nuclei. In human evolution, the numbers of neurons in the VL appeared to increase at a faster rate than did neurons of the pyramidal tract, whereas the motor cortex apparently increased at a rate greater than VL.