Ultrastructure of region of a low safety factor in inhomogeneous giant axon of the cockroach.

1. The structure of the ventral giant axons of the cockroach at the level of ganglion T3 was studied by means of light and electron microscopy. 2. From serial sections and cobalt injections, the axons diameter was found to range between 40 and 60 mum at the caudal end of ganglion T3; toward the center of T3 they narrow to 20-40 mum, and again expand to 30-45 mum anteriorly in ganglion T3. 3. Each giant axon sends off several branches, 1-15 mum in diameter, into the neuropil. The giant axons and the bases of their branches are enveloped by cytoplasmic processes of glial cells. The periaxonal space is about 100-200 A. 4. Distally the branches are devoid of glial envelopes and the extracellular space between the branches and other axonal profiles is about 200 A. Terminals with presumptive chemical synapses on the giant axon branches were found. Clear vesicles, 300-400 A in diameter, are seen clustered together. The width of the supposedly synaptic gap is about 100 A. 5. In some areas the branches and other axonal profiles form close appositions.