what is the reptile brain?

Question

As a Carl Sagan fan, I remember (in Cosmos TV shoe) "territoriality, aggression and ritual" are products of the reptile brain while higher-level stuff like loyalty, planning for future are mammalian, which implies different brain regions for each functions.

Even Sheldon Cooper has something to say about this:

Penny’s emotional responses originate from the primitive portion of the brain known as the Amygdala, while speech is centred in the much more recently developed Neocortex. The former can easily overpower the latter giving scientific credence to the notion of being rendered speechless.

And said by on-screen and off-screen neuroscientist and science consultant Mayim Bialik:

So says your prefrontal cortex. But meanwhile, the limbic system of your brain is calculating that if another woman is attracted to Leonard, it must be because he’s desirable.

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It implies that the brain is composed of three parts, with vast differences in development, can overpower each other and process stimuli independently.

What exactly is the reptile brain and do modern theories prove such a concept?

Answer 1

The reptilian brain is the oldest part of the triune brain. And the triune brain is a unified account of brain function and brain evolution.

The story goes like this. The brain can be divided into three sections:

• The reptilian brain, named so because it encompasses structures that did not change much from reptile to man. It includes the basal ganglia and cerebellum, and controls pretty much everything that a reptile can do: breathing, walking, appetite, sensation, simple motor reactions to the environment, etc.
• The mammalian brain (sometimes called paleomammalian), which encompasses structures that are unique to mammals but similar across mammalian species. These structures belong to the limbic system and are dedicated to processing emotion and memory. Anything your cat can do, should be either in the reptilian or the mammalian brain. Anything it can't do, is in the third part of the triune brain.
• The neocortex, found in primates, which is involved in higher order functions such as language.

Now, this is an oversimplification both structurally and functionally. For example, birds don't fit this theory very well because they don't have much of a limbic system, but do have some impressive problem solving skills. And functionally, these functions are interrelated in man, who can, for example, experience moral disgust which bridges higher-order and emotional functions. This is why the triune brain is the kind of thing you would find in an undergraduate psychology textbook, but it's not the kind of thing neuroscientists talk about among themselves.

Answer 2

I think the treatment of the “Reptile Brain's” power given by Big Bang really sells it short. Your question immediately brought to my mind a 2009 profile by the Washington Post's Gene Weingarten of parents who leave their children in the car to die, all by accident.

It's a long and difficult read, and it took me quite some time to dig it up again since I had forgotten the publication, the author, and the title: but it comes highly recommended from me (read “Fatal Distraction” on washingtonpost.com)

The first page that speaks directly to your question is this one, making reference to the basal ganglia in the brain. A brief excerpt:

Ordinarily, says [University of South Florida Cognitive Neuroscience Professor David] Diamond, this delegation of duty [between the so-called reptile brain and higher-order brain function] “works beautifully, like a symphony. But sometimes, it turns into the '1812 Overture.' The cannons take over and overwhelm.”

At a risk of doing a disservice by gross oversimplification of a fascinating and complex topic: the main thrust of the article's insight into cognition is that in rare circumstances, intelligent, emotionally healthy, and responsible parents may find themselves in circumstances where their basal ganglia take over their brain to such a great extent that they are capable of forgetting that their own children exist.

So if the reptile brain has such potential for destruction, albeit rarely to such a great extent, what is its purpose? Clearly in the majority of cases its functioning is very beautiful. When you walk you do not have to think analytically and poetically about every step you take. Your basal ganglia take care of this for you. This is Sagan's “ritual” reference.

If you need an emotional reason to accept that the reptile brain is real, read the Washington Post article. It'll stick with you. If you have more questions about the modern theories underpinning its machanisms, Dr. Diamond is not hard to get a hold of—he lists his contact information here.

Answer 3

The quote by Sheldon from Big Bang Theory is misguided, as the limbic system is not part of the "reptilian brain", it is part of the next development, the paleomammalian complex.

The Reptilian Complex

The R-complex consists of the brain stem and the cerebellum. Its purpose is closely related to actual physical survival and maintenance of the body. The cerebellum orchestrates movement. Digestion, reproduction, circulation, breathing, and the execution of the "fight or flight" response in stress are all housed in the brain stem. Because the reptilian brain is primarily concerned with physical survival, the behaviors it governs have much in common with the survival behaviors of animals. It plays a crucial role in establishing home territory, reproduction and social dominance. The overriding characteristics of R-complex behaviors are that they are automatic, have a ritualistic quality, and are highly resistant to change.

The Limbic System

The limbic system, the second brain to evolve, houses the primary centers of emotion. It includes the amygdala, which is important in the association of events with emotion, and the hippocampus, which is active in converting information into long term memory and in memory recall. Repeated use of specialized nerve networks in the hippocampus enhances memory storage, so this structure is involved in learning from both commonplace experiences and deliberate study. However, it is not necessary to retain every bit of information one learns. Some neuroscientists believe that the hippocampus helps select which memories are stored, perhaps by attaching an "emotion marker" to some events so that they are likely to be recalled. The amygdala comes into play in situations that arouse feelings such as fear, pity, anger, or outrage. Damage to the amygdala can abolish an emotion-charged memory. Because the limbic system links emotions with behavior, it serves to inhibit the R-complex and its preference for ritualistic, habitual ways of responding.

The limbic system is also involved in primal activities related to food and sex, particularly having to do with our sense of smell and bonding needs, and activities related to expression and mediation of emotions and feelings, including emotions linked to attachment. These protective, loving feelings become increasingly complex as the limbic system and the neocortex link up.

The Neocortex

Also called the cerebral cortex, the neocortex constitutes five-sixths of the human brain. It is the outer portion of our brain, and is approximately the size of a newspaper page crumpled together. The neocortex makes language, including speech and writing possible. It renders logical and formal operational thinking possible and allows us to see ahead and plan for the future. The neocortex also contains two specialized regions, one dedicated to voluntary movement and one to processing sensory information.

The Triune Brain Theory is a cross-species evolutionary study with the developmental processes underlying them (I borrowed a line from ₍₁₎ to use as a definition, as I thought it summed it up well). As mentioned in other answers, it is an oversimplification and doesn't match work across all species.

As opposed to the study of Embryology.

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References:

(1) Mammalian Brain Evolution
Weisbecker
Morphological Evo-Devo Group

Neural System Development
UNSW Embryology