Reptilian complex

Reptilian Complex or R-Complex is a part of the triune brain model ('tri', as in 3 part) proposed by Paul D. MacLean. This theory seeks to explain brain function through the evolution of existing structures of the generic brain. The triune brain consists of:
 * 1) The R-complex
 * 2) The Limbic system and
 * 3) The neo-cortex

The brainstem and older, atavistic areas of the central nervous system control normal involuntary behavior that the conscious mind does not, such as the cardiac and respiratory functions. These are found in all vertebrates, but are not considered to be part of the "Triune Brain".

The R-complex is named for the most advanced part of the brain higher mammals share with reptiles. The R-complex consists of the following four parts of the human brain: the Cerebral Ventricles, the Thalamus, the Caudate Nucleus, and the Lenticular Nucleus (which contains the Putamen and the Globus Pallidus). Together, the Caudate Nucleus and the Lenticular Nucleus are called the "Basal Ganglia". (A "ganglion" is something that "bulges", and "basal" means at the base of something. Hence, since these two brain areas "bulge" from the Thalamus at the base of the brain, they are collectively called "the Basal Ganglia".)

The R-complex is responsible for rage and basic survival fight-or-flight responses. Often, the R-Complex can override the more rational function of the brain and result in unpredictable, primitive behavior in even the most sentient of creatures, humans included. A well developed and healthy neo-cortex can monitor R-Complex activity in sentient beings. The Reptilian complex is the most ancient part of a very successful brain scheme, evolutionarily speaking.

The theory, observable not only through fossil records and animal phylogeny, but notably during the stages of all mammalian and human prenatal development as well, explains that the evolution of the mammalian brain depended on and was enhanced in both its structure and function by a series of evolutionary plateaus. These evolutionary plateaus correspond closely with the phylogenic grouping of animals throughout the history of life on Earth.

Mammalian brain structure exists in the outer areas of the brain — the Limbic system and neo-cortex — which evolved more recently. The Limbic system, which was first introduced by MacLean in a paper in 1952, is similar to the brain of the more primitive mammals and is the source of emotions other than fear and anger, some aspects of personal identity, and some memory functions. The Limbic system is composed of the amygdala and the hippocampus. The neo-cortex, also known as the cerebral cortex, resembles the brain of more recent mammals in that it controls more highly evolved mentation such as reason and speech. Memory; the concepts of culture, art and literature; a prolonged childhood wherein learned behavior, vital to survival, is acquired along with generational recognition and care of family members are some of these more evolutionarily advanced brain activities which mammals have used to rise to such global prominence in so short a period of evolutionary time.