Fine structure of dorsal root terminals in the dorsal horn of the frog spinal cord.

Dorsal root fibers were labelled with cobalt and their terminals were identified in an oval-shaped area and in its lateral extension of the dorsal horn. These areas receive mainly cutaneous afferents. A number of labelled terminals exhibited either an electron lucent and swollen, or an electron dense and shrunken form of degeneration. The majority of the labelled terminals could be classified into three groups. Type A terminals were small, less than 2 microns in diameter, and lightly stained. Type B terminals were larger and neurofilaments and an electron dense material resulted in a dark staining of the bouton. Type C terminals contained medium size granulated vesicles. The type A and type B terminals were frequently found in postsynaptic positions. The presynaptic profiles contained spheric or elliptic vesicles, or they exhibited the structural characters of presynaptic dendrites. Preterminal dorsal root fibers, identified on account of their cobalt label, were also found in postsynaptic positions to the same kinds of presynaptic profiles. It is concluded that cobalt labelling is sufficiently selective for a reliable identification of dorsal root terminals. No correlation was found between the different forms of degeneration and the different types of terminal boutons. The possible functional significance of the postsynaptic relation of dorsal root fibers and terminals to a variety of presynaptic profiles is discussed.