Morphological properties of respiratory intercostal motoneurons in cats as revealed by intracellular injection of horseradish peroxidase.

This study was undertaken to describe details of the location and cellular morphology of functionally identified (inspiratory or expiratory) external and internal intercostal motoneurons on the basis of intracellular injection of horseradish peroxidase (HRP). Sixty HRP-labeled motoneurons were examined; 44 in transverse, 16 in sagittal sections. In the upper thoracic segments (T3-T4), there was only a small overlap in the location of inspiratory external and internal intercostal motoneurons; the inspiratory external motoneurons were generally found more ventromedially within the ventral horn than either inspiratory or expiratory internal intercostal motoneurons. No major morphological differences were observed between the types of motoneurons studied. The number of primary dendrites ranged from 6 to 10. The dendrites projected mainly along the medial or the lateral border of the ventral horn, and rostrocaudally up to 1,760 micron from the cell body. The paths taken by dendrites to fill the territory occupied by the dendritic trees appeared to depend upon location of the cell body. Few dendrites penetrated the white matter. Axon diameters varied from 1.1 to 6.7 micron (mean 3.6 +/- 1.3 micron, n = 55). Collateral branches were identified in 78% of axons. The number of branches arising from a given axon varied from 1 to 4. It is concluded that the respiratory intercostal motoneurons form a morphologically homogeneous population, in spite of their functional differences.