Primary afferent terminals in the nucleus of the solitary tract of the frog: an electron microscopic study.

In the frog solitarius nucleus, primary afferent terminals of the facial and glossopharyngeal-vagal nerves were identified with cobalt labelling and electron microscopy. The labelled terminals were grouped in two main categories, one with small (1-2 micron) and pale terminals, and another with large (3-5 micron) and dark terminals. The small terminals greatly outnumbered the large ones. In addition many terminals intermediate in size and staining reactions were found. All kinds of labelled boutons contained medium-size clear synaptic vesicles, among which dense-core vesicles of the smaller type frequently occurred. The labelled primary afferent terminals established axo-dendritic contacts of the asymmetric type. Close to these contact sites they were themselves very frequently contacted by a profile interpreted as presynaptic in relation to them. Such profiles contained spherical, pleomorphic (including dense-core) or flattened vesicles; a fourth kind was interpreted as presynaptic dendrites. It is concluded that viscerosensory fibres, as opposed to somatosensory fibres, predominantly generate small and lightly stained terminals. It is likely that the effect of synaptic transmission at the solitarius tract terminals is modulated in a very versatile manner by the various presynaptic profiles converging on these terminals.