Structure of the human sensorimotor system. II: Lateral symmetry.

We have evaluated the lateral symmetry of the human central sulcus, brainstem and spinal cord using quantitative histological and imaging techniques in specimens from 67 autopsy cases. Our purpose was to determine whether the preferred use of the right hand in the majority of humans is associated with grossly discernible asymmetries of the neural centers devoted to the upper extremities. In the accompanying report, we described a consistent set of morphological features in the depths of the central sulcus that localize the sensorimotor representation of the distal upper extremity. Measurements of the cortical surface in this region, and indeed throughout the entire central sulcus, showed no average lateral asymmetry. Cytoarchitectonic measurements of area 4 and area 3 confirmed this similarity between the left and right hemispheres. The medullary pyramids, which contain the corticospinal tracts, were also symmetrical, as were the cross-sectional areas of white and gray matter in the cervical and lumbar enlargements of the spinal cord. Finally, we found no lateral difference in the size and number of motor neurons in the ventral horns at these levels of the cord. Based on these several observations, we conclude that the preferred use of the right hand in humans occurs without a gross lateral asymmetry of the primary sensorimotor system.