Overview of the brain

The brain is probably the most complex structure in the known universe; complex enough to coordinate the fingers of a concert pianist or to create a three-dimensional landscape from light that falls on a two-dimensional retina. While it is the product of many millions of years of evolution, some of the structures unique to the human species have only appeared relatively recently.

For example, only 100,000 years ago, the ancestors of modern man had a brain weighing only about one pound - roughly a third of the weight of the current version. Most of this increased weight is associated with the most striking feature of the human brain - the cortex - the two roughly symmetrical, corrugated and folded hemispheres which sit astride the central core.

Almost all the tasks that seem hard or difficult for human beings but that the present generation of computers can easily perform are associated with processing in parts of the relatively new cortex. Conversely, tasks that humans normally find easy but that are difficult for computers typically have a much longer evolutionary history. Although playing chess, doing higher mathematics and trouble-shooting electronic circuits may seem intellectually challenging for humans, current computers can cope very straightforwardly. However, a modern computer (even after much careful programming) is typically very poor at such simple tasks as sensing its environment or coordinating movements. A simple operation like recognizing someone's face, which we find rather straightforward, is a formidable problem for a computer. Indeed, a 2-year-old child will perform much better at these tasks! This observation is not so surprising, though, when one considers that the child is using multiple levels of processing that have evolved over many hundreds of thousands of years.

In evolutionary terms, all brains are extensions of the spinal cord. The distant ancestor of the human brain originated in the primordial seas some 500,000,000 years ago. Life and survival in those seas was relatively simple and in consequence these early brains consisted of just a few hundred nerve cells. As these initial sea-creatures evolved and became more complex, so too did the brain. A major change occurred when these early fish crawled out of the seas and onto the land. The enhanced difficulties of survival on land led to the creation of the "reptilian brain". This brain design is still visible in all modern reptiles and mammals and is a powerful clue to our common evolutionary ancestry.

The next major addition occurred with the mammalian brain in which a new structure emerged - the cerebrum or forebrain along with its covering, the cortex. By now, the brain consisted of literally hundreds of millions of nerve cells organized into separate regions of the brain and associated with different tasks. About 5,000,000 years ago, another type of cortex appeared in a new species - early man. In this brain, the surface of the cortex was organized into separate columnar regions less than one millimeter wide but each containing many millions of nerve cells or neurons. This new structure allowed much more complex processing to take place. Finally, about 100,000 years ago, this new cortex underwent rapid expansion with the advent of modern man. The present day cortex contains something like two-thirds of all neurons and weighs about three pounds - almost triple its weight only one hundred thousand years ago!

Thus the human brain consists of roughly three separate parts.

• The first segment in the lower section, sometimes called the brain stem, consisting of structures such as the medulla (controlling breathing, heart rate and digestion) and the cerebellum (coordinating senses and muscle movement). Much of these features are inherited "as is" from the reptilian brain.
• The second segment appears as a slight swelling in lower vertebrates and enlarges in the higher primates and ourselves into the midbrain. The structures contained here link the lower brain stem to the thalamus (for information relay) and to the hypothalamus (which is instrumental in regulating drives and actions). The latter is part of the limbic system.

  The limbic system, essentially alike in all mammals, lies above the brain stem and under the cortex and consists of a number of interconnected structures. Researchers have linked these structures to hormones, drives, temperature control, emotion, and one part, the hippocampus to memory formation. Neurons affecting heart rate and respiration appear concentrated in the hypothalamus and direct most of the physiological changes that accompany strong emotion. Aggressive behavior is linked to the action of the amygdala, which lies next to the hippocampus. The latter plays a crucial role in processing various forms of information as part of our long term memory. Damage to the hippocampus will produce global retrograde amnesia, or the inability to lay down new stores of information.

As we have seen, much of the lower and mid brain are relatively simple systems which are capable of registering experiences and regulating behavior largely outside of any conscious awareness (we don't have to think to remember to breathe!). In a sense, the human brain is like an archeological site with the outer layer composed of the most recent brain structure, and the deeper layers consisting of structures from our shared evolutionary history with the reptiles and mammals.

• Finally, the third section, the forebrain appears as a mere bump in the brain of the frog but balloons into the cerebrum of higher life forms and covers the brain stem like the head of a mushroom. It has further evolved in humans into the walnut-like configuration of left and right hemispheres. The highly convoluted surface of the hemispheres - the cortex - is about two millimeters thick and has a total surface area of about 1.5 square-meters (the size of a desktop).

  The structure of the cortex is extremely complicated. It is here that most of the "high-level" functions associated to the mind are implemented. Some of its regions are highly specialized - for example, the occipital lobes located near the rear of the brain are associated with the visual system. The motor cortex helps coordinate all voluntary muscle movements.

  

  The parietal lobes positioned in an arch over the center of the cortex contain a detailed map of whole body surface.

  More neurons may be dedicated to certain regions of the body than others - for example, the fingers have many more nerve endings than the toes. We can use this to construct a distorted map of the body which shows the emphasis given to certain regions of the body's surface.

  There is an approximate symmetry between left and right hemispheres - for example, there are two occipital lobes, two parietal lobes and there are two two frontal lobes.. However this symmetry is not exact - for example, the area associated with language appears only on the left hemisphere.

  The frontal lobes occupy the front part of the brain behind the forehead and compose the portion of the brain most closely associated with "control" of responses to input from the rest of the system. They are most closely linked with making decisions and judgments. 

  In most people, the left hemisphere is dominant over the right in deciding which response to make. Since the frontal lobes occupy 29 percent of the cortex in our species (as opposed to 3.5 percent in rats and 17 percent in chimpanzees), they are often regarded as an index of our evolutionary development. In individuals with normal hemispheric dominance, the left hemisphere, which manages the right side of the body, controls language and general cognitive functions. The right hemisphere, controlling the left half of the body, manages nonverbal processes, such as attention, pattern recognition, line orientation and the detection of complex auditory tones. Although the two hemispheres are in continual communication with each other, each acting as independent parallel processors with complementary functions, the dominant left-hemisphere appears most closely associated with a conscious self.

These structural features of the brain have been known for some time. In the section Brain components we will explore the nature of the cells themselves and later in Brain operation and processes try to understand how this set of intercommunicating complex structures we have described can possibly arise from the function and organization of the neurons themselves.