The olfactory nerve of the European pike (Esox lucius) contains 5.1 × 106axons with an average diameter of 0.20 ± 0,04 μm and a length of 5.5 cm in 1 meter long pike. Each axon contains an average of 4 microtubules as well as neurofilaments, smooth endoplasmic reticulum and about 500 mitochondria per centimeter. The number of neurofilaments ranges from zero in 15 of the cross sections to over 10 in 6. Neurofilaments generally occur in clusters located opposite to microtubule regions. Smooth ER can not be identified in 14 of the cross sections suggesting that this structure may not be continuous. Microtubules often display annular regions (halos) of low electron density ranging in size from 800 to 1300 Å. Halos from adjacent tubules usually merge into regional halos. The ratio of axoplasm to glial cytoplasm is 4.4:1, while the ratio of axonal plasma membrane to glial plasma membrane exceeds 7:1. A 4 cm nerve contains 1280 cm2of axolemma. This nerve represents an extreme in high density axonal packing and is therefore exceptionally well suited for biochemical, biophysical and physiological investigati
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