SYNAPTIC ORGANIZATION OF THE INTERSTITIAL SUBDIVISION OF THE NUCLEUS-TRACTUS-SOLITARII AND OF ITS LARYNGEAL AFFERENTS IN THE RAT

The nucleus tractus solitarii, the first central relay for gustatory a nd a variety of visceral afferents, is also an integrative center for numerous functions. Its interstitial subdivision is involved in swallo wing and respiratory reflexes. The ultrastructural characteristics of this subdivision and of its laryngeal afferents were investigated in a dult rat by a serial-section study and by application of wheat germ ag glutinin-horseradish peroxidase conjugate to the peripheral afferent f ibers. The interstitial subnucleus contained scattered small neuronal cell bodies with such ultrastructural features as a large nucleus with deep indentations and an organelle-poor cytoplasm. On the basis of th eir size and vesicular content, the axon terminals were classified int o three categories. Group I and group II terminals were small or large , respectively, and contained mainly small, round, and clear synaptic vesicles. Group III terminals were also small but contained small, ple omorphic, and clear vesicles. Axodendritic synapses were the most nume rous. They were either asymmetrical, comprised of group I and II termi nals, or symmetrical, comprised of group III terminals. More than 50% were part of complex synaptic arrangements in the form of rosettes or glomeruli. Axosomatic contacts involved both group I and group III ter minals and were always symmetrical. A high frequency of axoaxonic syna pses was found. They were symmetrical, comprised of group III terminal s on group I or II terminals. Different types of symmetrical synaptic contacts made by dendrites were also found. This study indicates also that the ipsilateral interstitial subdivision constitutes the preferen tial site of termination for superior laryngeal afferents. The labeled axon terminals belonged exclusively to groups I and II and were invol ved in both axodendritic and axoaxonic synapses. Some of the axodendri tic synapses were part of rosettes or glomeruli. All these synaptic ar rangements may be considered a morphological substrate for important p rocessing of afferent information in the nucleus tractus solitarii. Th ey may account for some of the integrative functions of the interstiti al subnucleus such as physiological processes triggered from the super ior laryngeal nerve. (C) 1995 Wiley-Liss, Inc.