#9 - Olfactory Lobes

I've included a diagram of the salamander's brain as seen from above. I hope you're able to print it out OK. If there's a problem, let me know and I'll bring a hard copy to the meeting. My reference for the diagram is C. Judson Herrick's, The Brain of the Tiger Salamander, University of Chicago Press, 1965, one of my all time favorite books. The format for other references to this book will be [H:page number]. As you can see from [K&W:52-53], the human brainstem is a fairly straight forward elaboration of the salamander's brainstem. It's when we get to the salamander's olfactory lobes and the human structures derived from them that we find major evolutionary changes.

The olfactory lobes are particularly important, since it is they which eventually evolve into the cerebral hemispheres which bear the cerebral cortex on their exterior. K&W discusses the human olfactory lobes and provides some pictures on [K&W:59] and mentions them briefly on [K&W:138]. However, as you will see, what we now call the olfactory lobes in humans is just a small part of the olfactory lobes in our precortical ancestors. Since the human cerebral hemispheres evolved from the precortical olfactory lobes, many of the structures of the precortical olfactory lobes are now spoken of as being part of the cerebral hemispheres.

The salamander's olfactory lobes are roughly cylindrical in shape. At the very front of each lobe is the olfactory nucleus, #1 in the diagram. Although there are both medial (towards the midline) and lateral (towards the outside) tracts which convey olfactory information away from the olfactory nucleus, the lateral tract to the basal ganglia and amygdala seems to be the more important.

The hippocampus and septal nuclei, #'s 2 & 3 in the diagram, form the medial walls of the precortical olfactory lobes, while the basal ganglia and the amygdala, #'s 4 & 5, form the lateral walls. The top or roof of the olfactory lobes (known to neuroanatomists as the "dorsal pallium"), #6, provides a path of communication between the medial and lateral walls of the olfactory lobes. It is here, in the dorsal pallium, that the cerebral cortex will eventually evolve.

This section has taken longer than the others because it turned out that much of what I thought I knew about the olfactory lobes was mistaken. The biggest surprise to me was that there are not one but three different kinds of memory. This is discussed on [K&W:490-511]. At the time my neuroanatomy book was published in 1983, only the association between explicit memory and the hippocampus was widely recognized.

K&W's diagrams for explicit memory [K&W:503,508], implicit memory [K&W:508] and emotional memory [K&W:510-511] all include cortical connections. Yet it is clear that precortical animals, which don't have a cortex, still have at least some memory.

Many fish and amphibians return to the place where they were born in order to breed. They leave when they mature because the breeding ground doesn't have adequate food supplies to support the adult population. They return because they evolved in fresh water and their eggs and offspring still require a fresh water environment. Salamanders return from the wooded hills searching for the pond in which they were born, and 95% of the salmon who return from the sea find the stream of their birth. Both the salmon and the salamander recognize the place of their birth by its unique odor.

I don't know how important it is for us to guess which subcortical structures are responsible for precortical memory, but the amygdala receives strong input not only from the ascending activating systems described on [K&W:174] but also from both the lateral olfactory tract and the hypothalamus [C:634-638], which is the center of our sexual responses. Therefore, the amygdala is probably an important component of the subcortical memory circuit which helps salamanders and salmon find their breeding grounds. However, there is no reason to believe that the hippocampus, which is implicated in human explicit memory, and the basal ganglia, which is implicated in human implicit memory, don't also play a part.

K&W provides further discussion of the amygdala as part of the limbic system on [K&W:58-59,418-419], as related to hunger on [K&W:424], as related to sex on [K&W:429], as related to fear on [K&W:491-492], and as related to emotion in general on [K&W:434-435]. As you can see, all these are aspects of the emotional sides of our lives. With the evolution of the cerebral cortex, the amygdala's subcortical connections remain essentially unchanged [H:266].

The precortical basal ganglia is a relatively simple mass of neurons in the lateral wall of the cylindrical olfactory lobe. In contrast, the human basal ganglia is a complex structure which is usually subdivided into the caudate nucleus, the putamen, the globus pallidus and the nucleus accumbens septi. Of these, only the nucleus accumbens septi, which is sometimes known as the "pleasure center" and is briefly mentioned in [K&W:438-441], is clearly differentiated in the subcortical brain [H:266].

Once the cerebral cortex has evolved, the major input/output of the basal ganglia is from/to the cerebral cortex. The human basal ganglia is briefly discussed with pictures on [K&W:55-58,375-377] and mentioned, again with pictures, on [K&W:502-503,508-511]. The human basal ganglia has been pulled into its curvilinear ram's-horn-like shape by the expansion of the cerebral cortex. We will revisit the basal ganglia after we have discussed the evolution of the cerebral cortex. It's role in human behavior is much more important than K&W let on.

In addition to memory, your book discusses the human hippocampus as it relates to the limbic system on [K&W:58-59,418], as it relates to learning on [K&W:180-189], and as it relates to sex on [K&W:427,519].

K&W don't mention the septal nuclei. Like the hippocampus, they receive input from the olfactory nucleus [H:267]. Unlike the hippocampus which has grown remarkably during evolution, the septal nuclei have remained relatively small. However, they are an important part of the limbic system with reciprocal connections to and from the amygdala, the hypothalamus, the hippocampus and the habenula [C:618, 640]. These connections did not change with the evolution of the cerebral cortex [H:266].

In the precortical brain, olfactory and memory information are relayed to the superior colliculus in the midbrain where they help to determine the animal's behavior. Once the cortex evolves, it provides massive input to all other parts of the nervous system, and the superior colliculus relinquishes its position as the highest center of sensory motor integration to the cortex.

This ends our discussion of the precortical brain except for the endocrine system. Although the endocrine system probably doesn't change much with the evolution of the cortex, as I admitted earlier, I don't know much about the endocrine system, so I want to complete this review of the nervous system first. We will review the evolution of the cerebral cortex next.