Why doesn't the olfactory nerve relay through the thalamus?

[Xalatan]

How come the olfactory bulb projects straight to the amygdala and the olfactory cortex without going through a tertiary thalamic nuclear relay (and cross-over)?

Edited January 4, 2015 by Xalatan

[StringJunky]

I don't know the answer but it might help to ask yourself "What would be the evolutionary advantage of having the olfactory pathway so short and direct?".

[iNow]

I believe part of the answer also related to how olfaction is a much more ancient sensation type in the vast epochs of evolution, beginning with basic chemical detection in simple organisms (and perhaps even sooner if you consider the chemistry involved in abiogenesis as applicable).

 

Basically, the thalamus and its related parts are relatively recent additions that were added on top of a foundation long ago set.

[Function]

If I'm not mistaken, there are several olfactory pathways that do run through the thalamus, to reach the orbitofrontal cortex. This last one is part of the neocortex, the most recent type of cortical structure in evolution, very typical for higher animal orders.

 

Here are the pathways I know from physiology:

• Anterior olfactory nucleus --> bilateral feedback to the olfactory tracts through the anterior commissure
• Olfactory tubercle --> thalamus --> orbitofrontal cortex | prefrontal cortex
• Amygdala --> hypothalamus, to induce feeding urge
• Piriform cortex --> thalamus --> orbitofrontal cortex | prefrontal cortex
• Entorhinal cortex --> hippocampal formation to form olfactory memory of the percepted smell

With piriform cortex being important for the true sensation of smell, and the olfactory tubercle important for emotional correlations with smell.

 

The other area's, as you can see, do not necessarily project through the thalamus. This is because the effects they induce are, as mentioned by the other users above, evolutionary conserved.

As far as I'm aware of, indeed, these tracts do not necessarily project through the thalamus and, in extenstion, the orbitofrontal cortex of the prefrontal cortex, but they might.

 

But I'm not aware of tracts leaving the piriform Cx and olfactory tubercle bypassing the thalamus.

 

-F.