The fine structure of thoracic sympathetic neurons in the adult rat

Summary An investigation was made of the gross arrangement of the thoracic sympathetic rami, the histology and fine structure of their neurons, and of the light microscopy of thoracic spinal nerve roots in the rat. Sympathetic neurons were multipolar and were placed singly or in groups in the scanty stroma of collagen or among bundles of fine nerve fibers. Myelinated fibers in thoracic rami communicantes were either absent or occurred only in small numbers. Hence no white rami could be identified and thoracic preganglionic sympathetic fibers must have been unmyelinated. The few myelinated fibers in the sympathetic rami were probably somatic. Most sympathetic neurons were mononucleate and had a dense mottled nucleolus; a few binucleate neurons were observed. The nuclear envelope was always surrounded by a light perinuclear zone. The Nissl substance was usually arranged in distinct bodies which consisted of parallel, well-separated, and in some instances of closely packed layers of rough-surfaced cisternae; their membranes were occasionally fused. The sizes, shapes, texture, distribution and significance of dense bodies in the sympathetic perikaryon were described. A few whorls, onion or myelin-like structures were conjectured to be submicroscopic scars localizing presumptive minute areas of autolysis or necrosis. The satellite cell provided a fairly smooth and narrow coat around the sympathetic perikaryon, except where it contained the crenated nucleus or aggregates of cytoplasmic components. Axons and dendrites could not be classified according to the presence or absence of Nissl substance. Synaptic nerve endings, rarely placed as axo-somatic junctions at the sympathetic perikaryon, were usually observed at the neuronal processes, but their identification as axo-axonic or axo-dendritic endings could not be made. A comparison was made of the fine structure of sympathetic neurons in the rat, frog and man.This investigation was supported (in whole) by United States Public Health Service Grant NB-01879-07, Institute for Nervous Diseases and Blindness.