The mouse olfactory peduncle. 2.The anterior limb of the anterior commissure

Abstract

The central core of the olfactory peduncle [the tissue connecting the olfactory bulb (OB) to the forebrain] includes a white matter tract that extends caudally to the anterior commissure (AC). The purpose of the present study was to examine this “anterior limb of the anterior commissure” (ALAC) to determine if the axons that progress through it segregate on the basis of their point of origin, neurotransmitter type, size, or shape. While local differences in axon density were observed in the ALAC, they were not consistent between samples of the anterior and posterior peduncle, and no other compartmentalization within the tract was observed. The innervation of the caudal olfactory peduncle by neuromodulatory fibers was examined to determine if they enter the region via the ALAC. Cholinergic fibers (CHAT) densely filled the peduncle, followed in order by serotonergic, noradrenergic, histaminergic, and orexinergic processes. Differences in the distribution of the fibers were noted for each system. While each axon type could be observed in the ALAC, it is probable that they enter the peduncle though several routes. Data for axon caliber in the ALAC was compared to information previously collected on the peduncle's other white matter region, the lateral olfactory tract (LOT). Axons in the ALAC were smaller, suggesting that the olfactory system is organized with a fast system for distributing incoming sensory information and a more economical, distributed system for subsequent processing.

Myelination of the developing lateral olfactory tract and anterior commissure

“…First, they may reflect sampling differences between groups. While no regional differences in axon caliber have been reported in the AC (Brunjes, 2013), it is possible that the random sampling methods employed provided proportionally more small caliber axons in the experimental group. This explanation seems possible as the g-ratios (the proportion of myelin relative to axon size) did not differ between groups: thus, when controlling for axon size, unilateral naris occlusion did not affect myelin thickness.…”

“…6m). At P30, there was substantial variation in different areas of the AC (from 56.60 to 173.62 myelinated fibers/100μm 2 ), although no consistent patterns were found (Brunjes, 2013). Again, calculations of percent myelinated, promyelinated, and unmyelinated confirmed density measures (Fig.…”

“… E = embryonic day; P = postnatal day; 1 Walz et al, 2006; 2 Silver et al, 1982; 3 Logan et al, 2012; 4 Kucharski and Hall, 1987; 5 Brunjes et al, 2011; 6 Brunjes, 2013. …”

The mouse olfactory peduncle. 3. Development of neurons, glia, and centrifugal afferents

“…Most studies examined the same area of the caudal peduncle (the region where AONpP completely encircles the SVZ/ALAC core and Layer 2 is still separate from the overlying cerebral cortex, e.g., Figures 1C, I–L ) used in previous work (Brunjes, 2013). In studies of GABAergic interneurons, anterior sections containing the entire lateral arc of AONpE were also selected.…”

The mouse olfactory peduncle 4: Development of synapses, perineuronal nets, and capillaries

“…Axon collaterals descend from the LOT into the superficial portion of Layer 1 (Layer 1a) where they contact the apical dendrites of pyramidal cells (Figure ). Although many regions innervate AONpP (including hypothalamic, amygdaloid, thalamic, and neocortical regions and neuromodulators including acetylcholine, histamine, norepinephrine, orexin, and serotonin; Brunjes, ; Brunjes et al, , ), Luskin and Price (, ) defined two major associational fibers systems serving the area. The first is a topographically organized input from the APC and posterior piriform cortex (PPC) into the deep portion of Layer 1 (Layer 1b; Figure ).…”