A Very Brief Tour of the Parts of the Brain

A Very Brief Tour of the Parts of the Brain

By Dr. Rob DeSalle

It goes without saying that the brain is a complex organ, so it is natural to try to break it down component by component. Unfortunately, this is not so easy, as the components themselves are complex. Making the task even harder is the fact that regions have multiple purposes and kind of meld into each other. The simplest approach to understanding the brain’s organization is to summarize each of the major areas and their primary functions.

Neuroscientists have used three general methods to establish the different parts of the brain.

The first is by comparison to other animals. For instance, since the cortex of more advanced mammalian brains is absent or very rudimentary in fish, the mammalian cortex becomes something “new” to define.
The second way is much more morbid. Many people suffer brain damage, and some have become famous neurological case histories. They include HM, who lost a part of his brain called the hippocampus during surgery, and Phineas Gage, a 19th century railroad worker whose prefrontal cortex was injured during a work accident. People with these and other head injuries have enabled neuroscientists to isolate and study regions of the brain involved in specific functions. Neurologists observe the person during his lifetime for clinical or behavioral differences, dissect the brain after death, and correlate the injured areas with the behavioral changes. (Brain scanning technology has since made it possible to observe affected regions in living patients.)
The third method requires surgery, which can be done under local anesthesia. During the surgery, the surgeon stimulates a specific region of the brain with a probe. The surgeon then asks the patient what sensation he or she feels, or observes which parts of the body twitch. This made it possible to map specific parts of the surface of the brain to specific functions.

Finally we need to realize that the brain has an outer anatomy and a deeper inner anatomy—a bit like a hamburger. The outer “bun” has a specific function, but to ignore the burger itself would be to ignore a substantial part of the hamburger’s makeup. Indeed, there are brain regions deep below the surface of the brain that have very important and far-reaching functions. Three simple diagrams below define basic parts of the brain we’ll need to know for this course.

Let’s start with the most basic way of describing the brain – by its two hemispheres. Figure 1 shows a diagram of the brain seen from the top (known as a superior view) that shows the left and righthemispheres. The line down the middle is called themidsagital lineand it separates the two hemispheres. The “hills and valleys” made by the folds of the brain are calledgyri(hills) andsulci(valleys).

Figure 1. Birds-eye view

The two hemispheres, divided by the midsagital line. ©AMNH

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Next we move on to the cortex, or outer layer of the brain, which has classically been divided in to four areas. Starting at the top of the head(Figure 2)and going clockwise, we have theparietal, occipital, temporalandfrontallobes, or POTF, which is a good way to remember them. Add two of the more primitive areas of the brain, thecerebellumand thebrain stem, and we have the basic map of the bulk of the brain.

Figure 2. The major lobes of the brain

These are the main sections of the brain at the surface. ©AMNH/Richard Tibbits

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Some regions within these major lobes have been mapped to specific functions(Figure 3). We’ll start withBroca’s areaandWernicke’s area, which affect speech. People with impairments in these regions can develop aphasia, a disorder in speaking or understanding language. Broca’s region controls how we make the sounds we call speech. Wenicke’s area controls how we interpret those sounds. The regions that process information for sight and for hearing are called thevisual cortexand theauditory cortexrespectively. Not all of our visual and auditory functions reside in these areas, but they seem to be responsible for a large portion of visual and auditory processing. The so-calledEinstein area, technically theangular gyrus, is where abstract thinking and metaphorical interpretation take place. The prefrontal cortex, located at the very front of the brain, is conspicuously enlarged in humans relative to other mammals. Finally, two extremely interesting strips of the outer layer of the brain control our sensory and motor perception – thesensory cortexand themotor cortex.

Figure 3. Primary areas of the brain

Regions of the brain that have been mapped to specific functions. ©AMNH/Richard Tibbits

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These regions of the brain were mapped during surgery. That’s how we know, for instance, that specific regions of the sensory cortex control sensation in the hands, while another region controls sensation in the tongue. In fact, the entire region has been correlated with parts of the body, as shown in Figure 4.

Because we now know how much of the sensory cortex and how much of the motor cortex are dedicated to each part of the body, we can create what’s called a homunculus: the crazy-looking figure shown in Figure 5.

Note that a lot of the sensory and motor cortex is devoted to monitoring the hands and lips and ears and nose, but very little is dedicated to the abdomen or legs.

Finally we need to discuss some of the inner parts of the brain, starting with one very important region called the limbic system. (See pages 157-159 in your textbook for some nuances about its name.) The parts of the limbic system are shown in the diagram below.

Figure 4. Sensory map

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Figure 5. The homonculus

This vastly distorted human model is proportioned to reveal the amount of brain space dedicated to various touch centers in the body. ©AMNH/ Denis Finnin

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Figure 6. The limbic system

The regions highlighted above (all located towards the center of the brain rather than the surface) comprise the limbic system. ©AMNH/Richard Tibbits

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We’ll discuss only the very basic components of the limbic system, which has several subparts. These include the hippocampus, the amygdala, and the hypothalamus. Perhaps the best way to remember them is to start with the hippocampus, which is involved inmemory. It’s the structure that hangs down and looked a little like a seahorse to early anatomists, who gave it the Greek name for seahorse. At the tip of the hippocampus lies the amygdala, a small region important in our expression of emotional responses. The hippocampus kind of winds its way on top of the thalamus, which is an important relay station for sensory and motor functions. Just below that is the hypothalamus, which has a large number of functions, the most important of which is involved in hormone regulation. Hormones are small molecules that interact with proteins to regulate body processes such as growth, the maturation and activity of the reproductive system, digestion, and how our bodies metabolize other chemicals and molecules. Hormones are produced in specific organs in the body and then moved around via the bloodstream to influence other organs.

We hope that this whirlwind tour of the brain isn’t too overwhelming. We’ll be throwing other anatomical terms at you throughout the course, but if you learn these basic brain regions, you’ll do fine.