The structure of the nucleus of the lateral olfactory tract

The nucleus of the lateral olfactory tract (NLOT) was studied in rats with the rapid Golgi method. The nucleus is in the rostral part of the amygdala and has three distinct layers. Layer I is a superficial, fibrous layer; layer II, an intermediate, cell‐dense layer; and layer III, a loosely textured cell and fiber layer. The commissural component of the stria terminalis forms at the apex of layer III. Layer II contains pyramidal and stellate cells; the former is more abundant. The apical dendrites of the pyramidal cells bow outward, point ventrally, and extend through layer I to the pial surface. The apical and basilar dendrites weave a dendrite capsule around layer II, except along its border with layer I. Most pyramidal cell axons go dorsally through layer III into the commissural component of the stria terminalis. The axons normally give off long, thin collaterals that travel rostrally into the forebrain. Other, shorter collaterals remain near the parent cell. Stellate cells have spine‐poor dendrites that radiate throughout layers I and II. Their axons generate a dense terminal field that is confined to layer II. A special group of neurons, the border neurons, occur along the junction between layers I and II. Many of them look like modified pyramidal cells, and some look like horizontal cells. The axons of the latter ramify among the pyramidal cell apical bouquet dendrites in layer I. Along the perimeter of layer I, near the pial surface, are rounded cell bodies that have moderately spiny dendrites and axons that project dorsally. Layer III neurons are the largest cells in the NLOT. Three types of large cells were identified: large spiny neurons, large nonspiny neurons, and pyramidal cells, which were least common. The dendrites of all three neuron types reach beyond layer III into layer II or the adjacent anterior amygdala. Their axons were not followed far. They travel dorsally and give off a few collaterals, some of which enter layer II. Also in layer III, mainly in its apical region, are small nonspiny cells. Their dendrites and axons appear to be limited to layer III. The afferent fibers in layer I generally run parallel to the pial surface. They have a few short collaterals and boutons en passant. One afferent group in layer I is made up of thick axons that enter via the rostral edge of the layer. It looks as though they make repetitive synaptic contacts on pyramidal cell dendrites in layer I. Layer II afferents enter either from the perimeter of the layer or through layer III. The former axons go across the layer, giving off several short collaterals and boutons en passant. The axons that descend into layer II also emit collaterals as they continue ventrally into layer I. A special group of layer II afferent fibers have innumerable varicosities and boutons en passant. Layer III afferents are mostly axons that traverse the layer. They have short collaterals that end in the layer and longer ones that go ventrally through layer II into layer I. We have attempted to correlate our Golgi data about NLOT afferents and efferents with the results obtained by other investigators who have used experimental tract‐tracing methods. We conclude by suggesting that the NLOT may have a significant role in localizing olfactory stimuli to one side of the midline.