3D Reconstructions of Axons

by Gordon M. G. Shepherd

Shown here is the microscopic end of the scale: the ultrastructural appearance of individual unmyelinated axonal branches from an intracortical axonal arbor. Short portions of several axons (the tubular, colored structures) are seen passing through a slab of cerebral cortex, from stratum radiatum of area CA1 in the adult rat hippocampus. These axons were three-dimensionally reconstructed from a stack of ~100 electron micrographs.

The axon segments are ~10 micrometers in length. The synapses (not shown) occur at the swellings, also called varicosities or synaptic boutons. The tight spacing of varicosities -- one every 3 to 5 micrometers along the axon, on average -- enables a single axonal arbor to make tens of thousands of synapses. Curiously, mitochondria populate only 50% of the varicosities, raising the possibility of differences in the regulation of Ca2+ and ATP. The axonal shafts between the varicosities are extremely thin, with diameters averaging 170 nanometers -- just enough to contain a cytoskeletal scaffolding of one to several microtubules. Action potentials appear to propagate along these branches with a high degree of reliability, but at rates far slower than larger-diameter axons.