Brain structure terminology consensus?

  • #1
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Main Question or Discussion Point

I am wondering if there is any consensus on the structure terminology of the brain.

1 - I've heard that the brain has the reptilian, mammalian(limbic) and neocortex parts that overall perform specific functions and evolved in that order.
- On that note, would reptiles have had any really poorly limbic or neocortex areas evolved, or would they have none of it? And also, do all animals (vertebrates or insects) have "reptilian" brains, or is what they have a brain "structure" that is pre-reptilian?

2 Secondly, I've also read about the separation terminology that divides the brain stem on one hand, and the cerebral cortex on the other.

3 - Finally I went to wikipedia and saw that there is another definition that the brain has three main parts: the cerebrum, the cerebellum and the brainstem. (this goes more along the second definition).
- But it also says that the cerebrum includes the limbic system (which clashes with the first defintion).
-It also says that all vertebrates have the cerebral cortex, which is the surface of the cerebrum (which would kind of clash with the reptilian brain only in reptiles, fish, etc, idea)
- It also says that all mammals have the neocortex (two new definitions).

Seems like the first definition seems to be a bit outdated, but I still hear many people talk about it as if it is part of the overall scientific agreement (or maybe I am mistaken?).

Is there a easily illustrated definition that gives the latest consensus on where things are at with the brain and which animals have which parts of the part, and what functionalities, and furthermore, which parts of the brains evolved in what "order" (if this is even possible to know).

Thanks for any thoughts!
 

Answers and Replies

  • #2
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It also says that all vertebrates have the cerebral cortex, which is the surface of the cerebrum (which would kind of clash with the reptilian brain only in reptiles, fish, etc, idea)
That's not accurate, only mammals have a cerebral cortex, or more commonly termed in the literature, "neocortex." You are correct in that serious confusion arises with the non-standardization in definitions. For example, the terms neocortex, cerebral cortex, and isocortex basically refer to the identical geographical definition of the mammalian brain. Why do we use 3 terms for the same thing? Mostly because the origins of these terms came from non-communicated distinct camps (before the internet) doing research on the problem and we want to keep each alive in our instruction on the subject so that when students go back and "mine" the literature in whatever pursuit they are interested in or required to do, they have the requisite language skills to read the more archaic and seminal publications. You have the same problem in physics and every other branch of human creation, say the calculus notation of Newton versus Liebnitz, the metric standards of the meter versus the yard, and I could list 100 others if you want to pay me to do so :oldsmile:
 
  • #3
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Thanks DiracPool,
Yes, it is all slightly confusing. Enough of "cross-overs" luckily. So, the "triune brain" system still holds as a concept generally speaking?
One of the main things that I wonder about is whether the brain stem, which clearly does very different functions than the cortex, evolved in all the animals?
And also, whether non-mammalian species have no cortex at all, or whether perhaps they have some really underdeveloped one?
And I also understand that the limbic system is exclusive in mammals? (or again, maybe it exists in many animals, just super underdeveloped?)
 
  • #4
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Thanks DiracPool,
Yes, it is all slightly confusing. Enough of "cross-overs" luckily. So, the "triune brain" system still holds as a concept generally speaking?
One of the main things that I wonder about is whether the brain stem, which clearly does very different functions than the cortex, evolved in all the animals?
And also, whether non-mammalian species have no cortex at all, or whether perhaps they have some really underdeveloped one?
And I also understand that the limbic system is exclusive in mammals? (or again, maybe it exists in many animals, just super underdeveloped?)
Well, if you're looking for an expert in evolutionary neurobiology you've come to right place, because that's one of my specialties.

To begin, I actually like the concept of MacLean's triune brain model, even though it is oversimplified. However, sometimes oversimplification is what is needed to get popular curiosity through the door. So, as you posed it, I would say that, yes, "the "triune brain" system still holds as a concept generally speaking"

One of the main things that I wonder about is whether the brain stem, which clearly does very different functions than the cortex, evolved in all the animals?
Well, be careful here on your definitions. "Animals" include insects, amphioxus, and nematodes. None of these creatures have a brain stem as we would think of them in relation to mammals. That said, we can trace distinctive homologies all the way down the line from modern primates to the even most primitive prokaryotes, at least as far as the primitive catecholamines are concerned.

As far as the evolutionary neuroanatomy of vertebrates are concerned, as a general broad scope we want to classify the evolution as fish, amphibian, reptile, and mammal, in that order.

And also, whether non-mammalian species have no cortex at all, or whether perhaps they have some really underdeveloped one?
By definition, non-mammalian species do not have have a neocortex. This is one of the qualifying attributes of being a mammal. That said, of course there is a "pre-structure" to the neocortex found in all vertebrates called the "pallium." In reptiles, the pallium composes a 3-layered structure that is almost exclusively sensory related in function, as it receives sensory information from the sense receptors and translates that information into action through basically direct connections to the motoric capacities of the basal ganglia structures.

In mammals, there's a bit of a change. In the mammalia brain we see the addition of three extra layers "stacked" on top of the more primitive reptilian cortex as they would stack more pancakes on your dish at iHop. So what happened in the transition from reptiles to mammals (or more appropriately the ancestral forms of each), is that the more recent layers stacked on top of the cortex took on a sensory integration role, and the more primitive deeper layers took on the more motoric role of engaging the motor effectors.
 
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  • #5
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Fantastic! Thanks so much for all this clarification!

I pretty much get the idea at this point, if it's ok, I'd like to confirm a few concepts, just to make sure I got it right:

- Even though mammals share the ancestral part of the brains of each fish, amphibians and reptiles, each of these species did spend millions of years developing their own "fish", "amphibian" and "reptile" brains which probably wouldn't resemble whatever those proto-species would have had.

- And similarly just like we contain ancient parts of fish, amphibian and reptile brains, each of these species including us, would contain whatever insects, amphioxus, and nematodes would have been evolving at the time, and yet again, these species spent their own times through millions of years evolving their brains further. And I imagine all of the different branches of brains would have developed brain levels that mammals wouldn't have at all. I wonder if there are brains out there that have extra structures built on top of the ancestral ones, completely different and "alien" to what mammals have? For example, I've read that cetaceans have an "extra lobar formation sandwiched between the limbic and supralimbic lobes, which is called the paralimbic" that no other species have.

- I really like your explanation of the pallium and the basal ganglia. That really clarified. Essentially, both reptiles and mammals then have their distinct sensory input and motor output domains. And I suppose reptiles don't have the limbic system, thus the direct connection between sensory-motor. Essentially, that could lead to a conclusion that they don't really "feel" the way mammals would. On the other hand, mammals have the limbic system, and that would be included in the "neo-cortex" part of the brain and also exclusive to mammals? I guess one of the evolved pancakes, between the neocortex and the basal ganglia would have been the limbic system.
 
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  • #6
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In reptiles, the pallium composes a 3-layered structure that is almost exclusively sensory related in function, as it receives sensory information from the sense receptors and translates that information into action through basically direct connections to the motoric capacities of the basal ganglia structures.
And another question related to the above info:
Would you say that human brains, despite having the limbic system and emotions, still have processes that directly connect sensory to motor? Would this essentially be instinct and reflexes? And perhaps aggression?
 
  • #7
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And to tie in an extra question/topic:
At which point do you think memory steps into the picture? I gather first there would be implicit memory (with explicit memory arising with the neocortex?), but would there be any of the procedural/conditioning/priming/conditioned reflex memories that would be associated with the brain-stem only and not with the limbic system? And any order of which one would arise with which part of the brain?
Would be great to know if there is any consensus on the classification of this too.
 
  • #8
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And similarly just like we contain ancient parts of fish, amphibian and reptile brains, each of these species including us, would contain whatever insects, amphioxus, and nematodes would have been evolving at the time, and yet again, these species spent their own times through millions of years evolving their brains further.
Well, as far as "retaining" certain features, I don't think that qualifies; every biological capacity in every animal undergoes natural selection and is subject to change.
 
  • #9
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Well, as far as "retaining" certain features, I don't think that qualifies; every biological capacity in every animal undergoes natural selection and is subject to change.
So saying "reptilian brain" merely refers to the brain structures that we share with the common "reptilian" ancestor, rather than it reflecting similarities with contemporary reptilian brains.
 
  • #10
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- I really like your explanation of the pallium and the basal ganglia. That really clarified. Essentially, both reptiles and mammals then have their distinct sensory input and motor output domains.
The evolution of the mammalian brain was a really spectacular event. It happened in two stages; the first stage was the evolution what you might call pre-placental or Pre-eutherian, to use a more technical term, mammals. This was the first stage and it happened about 200 million years ago. Pre-eutherian mammals include marsupials and monotremes such as the platypus, the kangaroo, and the echidna. What distinguishes their brain from the brain of the more recent eutherian version of the mammalian brain is that they have no distinguishable motor cortex. Instead, what they have is what we refer to as a "sensori-motor amalgam." SO one of the distinguishing features of the eutherian or placental-mammal brain is the feature of a distinguished motor strip.

Another distinguishing feature of eutherian mammals is the identification of a conspicuous cortical-pyramidal tract. This is tract of neurons that bypasses the basal ganglia structures and runs straight from the frontal motor cortex to mainly control the fine motor control of the hand, among other recently developed distal body parts that need fine coordination. In general, though, the appearance of the mammalian brain consolidated the 3 superficial layers of the cortex to an "input and integration" role of processing sensory stimuli arriving there via the sense receptors and dorsal thalamus relay, and consolidating the deeper 3 layers to a more "projection" role of taking the results of such processing and sending it out to various brain regions, most specifically the motor effectors.

And I suppose reptiles don't have the limbic system
That's not entirely correct; there are some reports that even amphibians have a homologue of the amygdala. The limbic system as we know it in mammals is basically the consolidation of features from the more primitive vertebrates into a subservient role to the neocortex in mammals. However, this is a complex issue I'm not going to elaborate on here.

Essentially, that could lead to a conclusion that they don't really "feel" the way mammals would.
I wouldn't read that much into it. I don't think that makes any difference as to whether or what a non-mammalian species experiences as emotion.

On the other hand, mammals have the limbic system, and that would be included in the "neo-cortex" part of the brain and also exclusive to mammals?
That would be incorrect, as I stated above. The limbic system is not considered a part of the neocortex and the limbic system structures are not exclusive to mammals. However, it is fair to say that we can define a unique set of structures in the mammalian brain called the "limbic system," and that this system includes the amygdala, the hippocampus, the septum, and a number of other brain regions, and that this system is unique to mammals in that composed collection as it exists.

I guess one of the evolved pancakes, between the neocortex and the basal ganglia would have been the limbic system.
Again, you're reading this wrong. All the evolved "pancakes" exist in the neocortex which is located right next to the skull bone, or what we call the "pial surface." The basal ganglia is what we call a "sub-cortical" structure, meaning that it exists below or beneath the cortex in the more central cavity of the brain. The limbic system is also a subcortical structure. These are two fundamental divisions of what we call the forebrain of mammals, to be distinguished from the midbrain and the hindbrain.
 
  • #11
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Origin
upload_2015-12-15_9-31-13.png

late Middle English: from Latin, literally ‘bark.’

Cortex as literally translated mean "bark," as in the bark of a tree. It exists superficially as the outside rim of the brain
 
  • #12
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I had to do some "homework" to catch up with terminology and read your input a few times.

I never realized that the cortex is so thin! We're talking few millimeters. That was one of the things tat was confusing me.
Pictures of the triune brain actually contributed to this confusion, as it made it seem like the cortex is very "large".

Plus the triune view makes it seem that:
- reptiles only have reptile brains, whereas the have the pallium, which is essentially their own "cortex". And they also have the homologue of the amygdala.
- mammals have only reptile+mammal brains, but that's not true, mammals actually have the cortex.
- primates have reptile+mammal+cortex brains, which is true, but again, it "suggests" that non-primate mammals don't have a cortex, because of the name "mammalian/limbic brain" distinction. Instead, primates simply have extra cortex functions that rest of the mammals don't have, like (I'm guessing) the pre-frontal functions and other general different cortex functionality.

I also found that the "forebrain - midbrain - hindbrain" visualisation really helped me understand things even more. Mainly because the forebrain actually includes both the cortex and the limbic system. And that each of those can have different terminology associated, depending on the species.

Perhaps there should be a separate "triune brain" drawings for all species, outlining each triune are names and their functionalities for each species?

Many many thanks for your input DiracPool.
 
  • #13
ElijahRockers
Gold Member
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I never realized that the cortex is so thin!
One of the predictors for neurological disorders in humans is actually the reduced volume of the neocortex in a particular area, very interesting.

I just started working in an academic neurology / hospital setting. As an image processing engineer, I have no prior experience with biology, but over the last few months I have been obliged to cram as much of the anatomy as I can manage (which is obviously not much) and we regularly refer to as the the neocortex as the gray matter, and the connective tissues in between as the white matter. However this is a huge oversimplification as the brain actually has many different cytoarchitectural characteristics that, I suppose along with functional characteristics, define particular regions of the white matter.

From what I've gathered, the neocortex, or gray matter, is supposedly where much of the computational proccesing happens, where the white matter is primarily for information connectivity, i.e. the information superhighways.

There cerebellum is very different in structure than the cortex, but also has a 'gray' and 'white' aspect to it, from what I've seen.

Anyone more knowledgeable, feel free to correct me, this is a learning opportunity
 
  • #14
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I was wondering about those areas right under the cortex!
I just watched this video today and it really helped understand many things about the cortex:
 
  • #15
ElijahRockers
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I don't have the inclination to watch an hour long lecture right about now, but I did notice it's from the Allen Istitute for Brain Science. One of my first assignments at work was to study and report on an academic paper that utilized the AIBS database, they have a really cool interactive tool that you can explore the correlation between genetic expression and functional connectivity in the mouse brain.
 
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I actually had dinner one evening with Christof Koch, the director the institute, and his sycophant girlfriend at the time, one night at a conference. I think he's mostly an incapable boob. Just my personal opinion. I don't have the greatest regard for Paul Allen either. Maybe it's just sour grapes because of the millions they've poured into this institute that produces basically nothing and is just a short drive a way from my house, and they never invite me up.
 
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Ah wow, sounds like quite an ivory tower :( I'd never heard of them and I just stumbled upon the video.
That is a shame to hear DiracPool, especially since you are in the same field, I've seen this happen too often in many environments, but it really hurts when it happens in the field of science.
I did find the video helpful though, although, I can imagine the information in it can be just resting on shoulders of other giants.
You've contributed to my posts on number of occasions and this specific thread has been incredible with your input. For what its worth, if you have editing power over this site, I can leave it in your discretion to edit the last bit of thread as you see fit. :)
 
  • #18
ElijahRockers
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millions they've poured into this institute that produces basically nothing
Granted I'm new to the field, but could you elaborate a bit? It seems to me like they offer a lot of open resources to further research in the field, more than any other entity I've stumbled across so far (with the probable exception of NIH), so it's quite surprising for me to hear that sentiment.
 
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It seems to me like they offer a lot of open resources to further research in the field, more than any other entity I've stumbled across so far (with the probable exception of NIH), so it's quite surprising for me to hear that sentiment.
Send me the link you're referring to and I'll check it out.
 
  • #20
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By definition, non-mammalian species do not have have a neocortex. This is one of the qualifying attributes of being a mammal. That said, of course there is a "pre-structure" to the neocortex found in all vertebrates called the "pallium." In reptiles, the pallium composes a 3-layered structure that is almost exclusively sensory related in function, as it receives sensory information from the sense receptors and translates that information into action through basically direct connections to the motoric capacities of the basal ganglia structures.
I was wondering if there is any research on how the different layers of the brain in different species contribute to memory formation?
Especially in the non-mammallian vertebrates with the brain structure that you described above.
I imagine they can only perform certain types of memories?
 
  • #21
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I was wondering if there is any research on how the different layers of the brain in different species contribute to memory formation?
Especially in the non-mammallian vertebrates with the brain structure that you described above.
I imagine they can only perform certain types of memories?
Well, this thread topic is mainly one related to evolutionary neuroanatomy, hence the "brain structure" title. Memory formation is a whole different topic. It involves complex chaotic dynamics which occur between different regions of the cortex. I actually have discussed the topic fairly extensively on this site and I'm guessing you could check my profile and mine my posts and view the threads on the subject. If you aren't able to find them, let me know and perhaps I can search.
 
  • #22
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That's true. Sounds good. Thanks!
 
  • #23
ElijahRockers
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Send me the link you're referring to and I'll check it out.
http://www.alleninstitute.org/our-science/brain-science/research/open-science-resources/ [Broken]
 
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  • #24
Laroxe
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Can I go back to the original post, though I suspect I'll just be adding to the confusion again. I think if your looking for terminology to describe the structure of the brain really what you should focus on is the anatomy about which there is a fair amount of consensus.
The triune brain is a way of thinking about how the brain evolved, its a useful model to help understand how the anatomy might have developed while at the same time being wrong in most respects. Most research now that looks at functioning is focussed on networks of communication and while some structures do appear to be significant in certain types of tasks the brain is a highly integrated system, that is always active and can use multiple pathways in achieving the same sort of outcome.
Remember the theory was developed in the 1960's and we know a lot more about how the brain functions and about evolution. Firstly basal ganglia, the core of the “reptilian brain” were present in the earliest jawed fish, well before reptiles and biology has a nasty habit of re-purposing structures, in fact basal structures play a role in action selection at all levels. In a similar way, the limbic system is not a distinct system at all, it has a number of anatomical structures with different functions and different connections. It is not wholly responsible for emotions nor is there any validity in trying to place emotions at a more primitive level of functioning to reasoning.
The cerebral cortex and the so-called "limbic system" have essentially the same structure in all mammals. There is no basis for saying that one is older than the other.
http://blogs.scientificamerican.com/guest-blog/revenge-of-the-lizard-brain/
 
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