Cells of origin and topographic organization of corticospinal neurons in the guinea pig by the retrograde HRP method

Abstract

In the guinea pig, horseradish peroxidase (HRP) was injected in the cervical or lumbar enlargements of the spinal cord in order to examine the origin and topographical organization of corticospinal (CS) neurons. The cortex was divided into granular and agranular regions to attempt correlations with the location of labeled CS neurons. These, of all sizes, are found only in pyramidal layer V of both kinds of cortical regions and could be seen as single, grouped or organized in clusters of 3–5 or more cells. The soma diameters of HRP CS neurons ranged from 13 to 49 μm. The largest labeled cells were present in the medial part of both agranular and granular cortices and included the giant pyramidal neurons which were found only in the rostromedial agranular cortex. The predominating intermediate-size cells were mostly present in lateral granular areas. The smaller cells were distributed in rostrolateral agranular and caudal granular regions. Morphological evidence suggests that an aggregation in clusters of labeled neurons and the different groups of CS neurons identified in different cortical areas may have additional differences in organization with respect to their precise topographical relations and functional properties. Neurons projecting to cervical levels were more abundant compared to projecting to the lumbar spinal cord, being distributed widely more lateral and anterior on the hemisphere to cortical neurons projecting to lumbar enlargement. These were located only medially on the dorsal surface of the brain. The two groups of labeled neurons were distributed in both agranular and granular cortices and occur adjacent to each other with only a narrow strip of overlap. The findings i.e. laminar organization, the pyramidal shape of CS neurons of all sizes, the presence of clusters and the somatotopic distribution of CS neurons seen in the guinea pig are discussed in comparison with equivalent findings on the corticospinal tract (CST) system of other species. The presence of labeled neurons within the cortex on both sides following unilateral HRP injections in the spinal cord points to a bilateral origin of the CST. This findings is discussed on the basis of the present study and previous autoradiographic findings.