Neomammalian brain

The neomammalian brain is one of three aspects of Paul McLean’s Triune Theory of the human brain. McLean was an American physician and neuroscientist that formulated his model in the 1960’s, which was published in his own 1990 book ‘The Triune Brain in Evolution’ ². McLean’s three-part theory explores how the human brain has evolved from ancestors, consisting of the reptilian, paleomammalian and neomammalian complexes. ^{2 5} McLean proposes that the neomammalian complex is only found in the higher order mammals, for example, the human brain, accounting for increased cognitive ability such as motor control, memory, improved reasoning and complex decision making. ¹

Paul McLean

Paul D McLean was an American physician and neuroscientist that was born in Phelps, New York in 1913 into a Presbyterian minister’s family, thus, ultimately becoming a religious man himself. McLean later passed away in Potomac, Maryland’s in 2007 ^{2 6}. McLean is famous for his significant contributions to brain research, psychiatry and physiology. McLean spent a large amount of time of his working life at Yale Medical School and the National Institutes of Health ⁶ where through his research he was able to publish Neuroscience texts such as reports, photographs and audio-visual material on his neurological findings. ² McLean spent 2 years during World War 2 serving as a medical officer for the Yale Unit, which later was known as the 39th General Hospital ⁶, this experience helped to shape McLean’s perspective on the impacts of Post-Traumatic Stress Disorder on fallen soldiers, which would ultimately shape his future studies into the way the human brain functions. Further achievements of McLean were coining the idea of the limbic system ⁴, the set of brain structures that surround the hypothalamus and are responsible for human emotions, memories and arousal 5, however, this notion of the limbic system has been questioned and debated by modern day neuroscience researchers, failing to conclude whether McLeans’ proposal is of accuracy ⁴.

Structure

The neomammalian brain consists of the cerebral neocortex, which is found in higher mammals, especially in the human brain, and is not found in birds or reptiles. The neomammalian brains structure is of great complexity, and has evolved over time allowing humans to reach the top of the food chain. The neocortex is made up of grey matter consisting of folds to increase the surface area and memory retention, these folds in humans are roughly 80% excitatory and 20% inhibitory. The arrangement of these folds differs from human to human, and is believed to account for the differing cogitative abilities of individual humans. It has been found by neuroscientists that the cerebral neocortex accounts for roughly 76% of the human brains total volume. The neocortex is predominately associated with high order brain functions such as motor control, sensory perception and cognition.

The neocortex can be divided into two sections; the proisocortex and the true isocortex. The Proisocortex is transitional between both the true isocotex and periallocortex, it can be found mainly in the cingulate gyrus, insula and the subcallosal areas of the brain. The true isocortex is a 6 layered cytoarchitecture that is predominately located in the frontal lobe, parietal lobe, temporal lobe and occipital lobe. Another individual feature of the neocortex is the way in which the matter is comprised together in columns. In the human brain, the six neocortex layers are 2.5mm thick, which contains thousands of different types of cells. Neuroscientists over the many years of research have struggled greatly to reach an agreed conclusion as to why the Neocortex is arranged in such a way; however, many suggest that the columns act as channels for intricate communication between cells and differing layers, this is believed to be another scientific explanation as to why higher order mammals have a high-level order of thinking in comparison to lower-order mammals, reptiles and birds.

Development

The neomammalian brain (Neocortex) is the newest addition to the Human Brain. McLean proposed that as animals evolved over the hundreds of millions of years, in order for an increased chance of survival, higher order animals developed an increased cognitive ability, which resulted in an increase in brain size. McLeans model is based on the larger the brain, the higher the order of thinking, thus, an increased cognitive ability. The neomammalian brain is in charge of ‘rational thinking’, his model follows Darwin’s natural selection idea of ’survival of the fittest’, where those mammals that developed features of the neomammalian brain survived and then passed this trait onto their offspring, until the majority of the population of higher order mammals attained the survival trait, this process occurred over millions of years. Archaeologists have discovered and are still discovering fossil records that allow comparative anatomy to occur between the modern-day Homo Sapien and primate ancestors. Due to the large percentage that the neomammalian brain takes up in the human brain (76%), comparative anatomy shows that the Homo Sapien has a much larger cranial size then early primate ancestors. It must be noted that many neuroscientists believe that McLean’s theory of the Triune Theory is false, however, what is a mutual belief between Neuroscientists, is that the features of the neomammalian brain is the reason as to why humans have such a high-level order of thinking.

Clinical Significance

Through comparing the three different sections of McLean’s Triune Theory, neuroscientists have been able to account for the complexity of the Human brain in comparison to Reptiles, Birds and other lower order mammals. Animal scientists have dissected a vast array of organism’s brains and through comparison ultimately concluded that the cerebral cortex (neomammalian Brain) has a different column structure to other organisms’ brain composition. The discovery of the six layered neomammalian brain has allowed neuroscientists to research into their differing roles. The six layers are broken down into three sections.

Layers one to three are referred to as the supragranular layers and play a vital role in the origin and termination of intercortical connections. Layer one is known as the molecular layer and is made up of very few nerve cells. Layer two is the external granular layer that is made up small, dense neurons. Layer three is the pyramidal layer and is made up larger pyramidal shaped neurons.

The second section of the neomammalian brain is the Internal Granular Layer, and has been labelled Layer Four by neuroscientists; this layer is responsible for receiving afferent signals from the hypothalamus and sends messages to the other layers, for example, Layer Four would receive messages about external temperature changes. The Internal Granular Layer acts as a medium which receives, processes and the sends signals to other parts of the brain, allowing the body to respond in such a way to combat the change in environment.

The final section is composed of layers five and six and is known as the infragranular layers; it connects the cerebral cortex with the subcortical regions of the brain, these regions are responsible for long-term memory, motor control and behavioural and emotional responses, damage to layers five and six can be detrimental to the overall fitness of the Homo Sapien.

These six layers of the neomammalian brain work interdependently to process neurological messages at a fast and high-quality level. These 6 layers are only found in the modern day human brain; however, other higher order mammals have features of these layer that give them higher order thinking.

References

1. Interaction Design Foundation. (2018). Our Three Brains. Available: https://www.interaction-design.org/literature/article/our-three-brains-the-rational-brain.

2. Koch, S. (2015). The Triune Brain Theory. Available: http://mybrainnotes.com/evolution-brain-maclean.html.

3. King, P. (2017). What is the current scientific status of the triune brain theory proposed by MacLean?. Available: https://www.quora.com/What-is-the-current-scientific-status-of-the-triune-brain-theory-proposed-by-MacLean.

4. Science Direct. (2016). Triune Brain. Available: https://www.sciencedirect.com/topics/neuroscience/triune-brain.

5. SciShow. (2018). No, You Dont Have a Reptilian Brain. Available: https://www.youtube.com/watch?v=InOr29Uxbvk.