Morphology of sympathetic preganglionic neurons in the neonatal rat spinal cord: An intracellular horseradish peroxidase study

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Abstract
Understanding the central neural control of autonomic functions requires a knowledge of the morphology of the preganglionic neurons, for the location of the dendritic arborizations of these neurons will indicate which central pathways may have access to them. In the present study, individual sympathetic preganglionic neurons in the neonatal rat spinal cord have been examined by the intracellular injection of horseradish peroxidase (HRP) in an in vitro preparation. Seventeen HRP-labeled preganglionic neurons in thoracic segments T1–T3 were examined in detail; of these, 12 somata were located in the intermediolateral cell column (IML), one in the lateral funiculus (LF), two in the intercalated nucleus (IC), and two at the border between IML and IC. All of the neurons had extensive dendritic arborizations arising from an average of six primary dendrites; the average total dendritic length for these cells was 2,343 μm. The morphology of preganglionic neurons differed depending on the location of their cell bodies. Preganglionic neurons located in the IML were essentially two-dimensional: the cells had some dendrites that coursed rostrocaudally for 300–500 μm within the IML and others that coursed mediolaterally, extending to the lateral surface of the cord and close to the central canal. Axons of these cells coursed ventrally from the cell body and exited from the spinal cord at the first ventral root caudal to the cell body. No intraspinal axon branches were observed. In contrast, preganglionic neurons with somata outside the IML had dendritic arborizations that radiated away from the cell body in three dimensions, extending into the IML, LF, and the dorsal and ventral horns of the spinal cord while traversing a rostrocaudal distance of about 400 μm. Two of these cells had axons that branched within the spinal gray. These results indicate that the dendrites of neonatal rat sympathetic preganglionic neurons extend outside the IML, where they may be contacted by inputs that do not terminate within IML. Moreover, the different dendritic arborizations of preganglionic neurons in different preganglionic nuclei may mean that the connectivity of preganglionic neurons varies according to cell body location. Finally, the intraspinal axon collaterals on some preganglionic neurons provide an anatomical substrate for local circuit interactions.

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