Light microscopic observations on the morphology of sympathetic preganglionic neurons in the pigeon, Columba livia.

Experiments were performed in anesthetized, immobilized, artificially respirated pigeons (Columba livia). Extracellular recordings from 56 antidromically activated and collided sympathetic preganglionic neurons were obtained. Eleven cells were intracellularly labeled with horseradish peroxidase and reconstructed at the light microscopic level. Electrophysiologically there were no statistical differences between labeled and unlabeled neurons. Four different somatic shapes were observed: fusiform, pyriform, multipolar and stellate. Nine of 11 cells were located within the principal preganglionic cell column (column of Terni), the other two were within nucleus intercalatus spinalis. Principal column neurons exhibited planar, horizontally aligned dendritic arbors with major extensions directed rostrocaudally. Unexpectedly, the majority of these cells also had dendritic branch projections which spanned the entire width of the ipsilateral zona intermedia. Contralateral dendritic terminal arborizations were evident in seven neurons. Intercalatus neurons were multipolar-shaped and exhibited a notably different dendritic arrangement from principal column preganglionic cells. The dendrites of intercalated cells coursed obliquely within the transverse spinal cord axis, giving rise to major dendritic extensions into the base of the dorsal horn, the dorsolateral funiculus, and the dorsal aspects of the ventral horn. Irrespective of somatic subnuclear location, the morphology of preganglionic dendrites was similar: (1) Largely primary, secondary, and tertiary processes were smooth. (2) Fine caliber proximal and distal elements appeared beaded or "varicose." (3) Distal processes gave rise to thin-stalked, spine-like appendages. The axons of preganglionic neurons arose from cell bodies as well as primary and secondary dendrites. The axons of two cells branched intraspinally. The present findings provide detailed descriptions of the somatic structures and accompanying dendritic trees of preganglionic neurons within nucleus intercalatus. The observations also include anatomical evidence showing the intraspinal collateralization of sympathetic preganglionic axons. In general, avian sympathetic preganglionic neurons located within the principal cell column appear to be structurally homologous to their mammalian counterparts within the intermediolateral cell column of thoracic spinal cord.