MODULE 13
The OLFACTORY SYSTEM
- - - The first order neurons of the olfactory system are the receptor cells. These cells have their cell bodies in the olfactory mucosa and send axons centrally, through small foramina in the cribriform plate, to enter the olfactory bulb. In the olfactory bulb synaptic relay occurs within a complex structure called a glomerulus. The major output cells of the bulb, conveying information centrally, are mitral and tufted cells. They form the olfactory tract, which lies on the ventral surface of the frontal lobe (Figure 2-13). The next view (Figure 1-29) shows how things look in the real brain. Blumenfeld shows this clearly in his Figure 18.5.
- - - Let's now trace the fibers within the olfactory tract back to the regions where they terminate. As we do so, you may want to refer to some of the figures in you lab manual. Figure 13-1 is a schematic side view of the brain that identifies most of the structures we will be mentioning and Figure 13-2 is a schematic view of this region from "above", with the diencephalon scooped out, so you can look right through it to see structures underneath - like the mammillary bodies.
- - - Because the anatomy of this part of the brain is so complex we have created three "call-up" views which we hope will help. Figure 13-3 is a lateral view, Figure 13-4 is an unlabeled version of Figure 13-1 and Figure 13-5 is an unlabeled version of Figure 13-2.
- - - The first frontal section we'll look at is Cu Slide 43. At this level, the olfactory tract is still a compact bundle of darkly staining fibers, but it has just fused with the orbital surface of the frontal lobe. Slides 42 and 41 look more or less the same, but with Cu Slide 40 we have reached the point where the tract divides into medial and lateral olfactory striae. This is a good place to pause and look at Figure 13-1 and Figure 13-2, both to see where we are and what lies ahead. The two striae will diverge and the surface enclosed by them is the anterior perforated substance. The lateral stria will run laterally far enough to reach the point where the temporal and frontal lobes connect, then turn medially with the fibers ending in the amygdala and the overlying periamygdaloid cortex. The medial stria will run medially and dorsally to cross the midline as one component (the anterior limb) of the anterior commissure. These fibers will then run forward in the contralateral olfactory tract. Now, look at the slides that show all this happening. Five slides are listed, but - as you know by now - the best way to go is to click on the first one, then use the "one slide caudal" button to step through the others.
- - - Some olfactory fibers may terminate in the primitive cortex on the surface of the region we call the anterior perforated substance. Most of them, however, terminate in the corticomedial division of the amygdala and the cortex overlying it.
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