In our last lesson we talked about brain basics: our brain as a sensory organ, how it has evolved over time, and what its prime directives are. This time we'll delve into how our brains are organized, and talk about some of the key functions of those major areas.
In order to perform all of the key actions we discussed in Lesson 1, the brain has evolved into a wonderful and highly functional structure. As you have already learned, the brain is not just one homogeneous mass of tissue, but a complex and hierarchical organization. Parallel systems exist to mediate various distinct functions. In general, the complexity of brain structure and the functions that these different structures mediate are organized in a bottom-to-top organization.
The bottom line is that the brain has lots of parts. For the purposes of this course, I will divide the brain into four major areas: brainstem, diencephalon, limbic, and neocortical. This, of course, is not the only way to divide the brain. Before examining the four major areas in our chosen model, let's spend a few moments exploring some of the other ways that the brain can be broken into component parts.
MacLean's Triune Brain Model
In one of the most original and useful ways of understanding the human brain, Paul MacLean, a pioneer of modern neuroscience, has defined three distinct systems within the brain that correspond to key evolutionary systems that have developed across various species. This model is known as the Triune Brain model.
The Triune Brain model defines the lower, less complex areas of the brain as being similar in structure and function to the reptilian brain (hence his term the R-complex). Take a minute to look at the table below to see how he has organized the four areas of the brain that we will focus on in this lesson. These areas are uniquely organized in primates.
MacLean's Triune Brain
The other more common approaches to the division of the human brain are outlined in the table below. In the most simple, there are three divisions: hindbrain, midbrain, and forebrain. The developmental style of dividing is based upon the developmental heritage of the given constituent parts. (You'll find the four-part model of division that we're using in this course at the far left-hand column of the table.)
The key observation in organizational process is that, in all cases, the brain has a hierarchical organization, with the lowest complexity at the bottom and highest on top. The most complex part of the brain is the cortex. When examining genetic homology across species, the frontal cortex (part of the neocortex) is the most "uniquely" human.
Complexity of brain function, in ascending order. Image courtesy of Bruce D. Perry, M.D., Ph.D.
Matching the physical hierarchical structure is a hierarchy of function. The lower brainstem areas mediate the simplest regulatory functions, while the neocortex mediates the most complex. The key thing to remember here is that different brain areas and systems mediate different functions. This will be important when trying to understand the changes in emotional, behavioral, and cognitive functioning that take place when someone is threatened.
Now let's return to the organization model of the brain, which I had told you we'll use in this course. Do you remember the four major areas? I'll refresh your memory! They are the neocortical, brainstem, diencephalon, and limbic regions.