Observations on the lateral olfactory tract of the rat.

The number and size of the axons in the later olfactory tract of the rat have been measured at several rostro-caudal levels, from material prepared for electron microscopy. Immediately caudal to the olfactory peduncle, an average of 42,000 +/- 3,000 axons were counted in the tract, while near the caudal limit of the tract an average of 32,000 +/- 2,800 axons were counted. The average internal cross-sectional area of axons measured at two levels of the tract was 1.6 +/- 1.3 mum2 and 1.1 +/- 0,9 mum2, corresponding to average internal diameters 1.4 +/- 1.3 mum and 1.2 +/- 1.1 mum, respectively. The axons in the lateral part of the tract were found to be significantly larger than those in the medial part of the tract; for one level the average cross-sectional area of axons in the lateral part of the tract was 1.6 +/- 1.0 mum2 (equivalent diameter 1.4 +/- 1,1 mum) while only 0.7 +/- 0.6 mum2 (equivalent diameter 0.9 +/- 0.9 mum) in the medial part of the tract. The thickness of the myelin sheath of the axons is generally related to axon diameter, increasing from 0.1-0.2 mum for axons 0.4 to 0.8 mum in diameter to 0.3-0.4 mum for axons greater than 2.0 mum in diameter. The ratio of the inside diameter to the outside diameter of the fiber (ratio "g") is between 0.7 and 0.8 for most axons in the lateral olfactory tract. The axons which leave the tract laterally and medially are substantially smaller than the axons within the tract (average cross-sectional area 0.55 +/- 0.35 mum2 on the lateral side) and probably are collaterals of the axons within the tract. Unmyelinated nerve processes, probably axons, were also found in the tract. Qualitative observations from light and electron microscopical material agree well with the quantitative data, and further suggest that mixing of axons from different parts of the olfactory bulb occurs in the rostral part of the tract. This is supported by experiments with injections of 3H-amino acids into localized portions of the olfactory bulb. These indicate that there is some degree of point-to-point organization in the most rostral part of the tract, but that this is lost in the caudal part of the olfactory peduncle.