On certain fluorescent axon terminals containing granular synaptic vesicles in the cerebellar nucleus lateralis

Summary

Thin, unmyelinated, green fluorescent fibers bearing fine beads or varicosities have been found in the neuropil and near blood vessels of the nucleus lateralis. In electron micrographs these fibers are identifiable as a class of axons, the CAT fibers, which contain large and small granular synaptic vesicles and agranular vesicles in their varicosities. There are two types of CAT fiber. 1) The CAT₁ terminals contain many large and elongated vesicles, 700–1700 Å in size, with dark, homogeneously dense centers; a few small granular vesicles each with an intensely osmiophilic particle, and small agranular vesicles. These terminals have not been seen in synaptic contact with other elements of the neuropil. 2) The CAT₂ terminals have a very thin unmyelinated connecting thread between small varicosities. The varicosities contain small agranular synaptic vesicles and small granular ones containing either a single dense particle, or an elliptical, intensely osmiophilic droplet flanked by lighter semicircular particles. Large granular vesicles, 750–950 Å each, with a variably dense center, are also found. These terminals form conventional axodendritic synapses with Gray's type 1 synaptic junctions and the subsynaptic specialization of Taxi, as well as synapses on thorns of spiny neurons.

It is suggested that the CAT₁ and CAT₂ fibers may be the electron microscope equivalents of norepinephrine- and 5-hydroxytryptamine-containing fluorescent axons. These fibers probably have extrinsic origins since no fluorescent cells or perikarya with small or large granular vesicles have been found in the lateral nucleus. Their origins, however, are unknown. The proximity of these fluorescent fibers to blood vessels is discussed, and their functions are the subject of some speculation.