The Amazing Human Brain and Human Development
 

   
 
Lesson 2: Brain Organization and Function

Looking At The Brain

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.

It's Complex

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

Name Part(s)
Neomammalian   

Neocortex and key thalamic nuclei

Paleomammalian
Limbic cortex and associated limbic system including amygala and hippocampus
Protoreptilian or R-Complex
Caudate nucleus, putamen, globus pallidus and associated brainstem inputs

More Models

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.
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.



 


Do You Remember the Brain's Key Actions?

Remember: the brain senses, processes, stores, and acts.

What Does Homologous Mean?

Two things that are homologous correspond to each other, or are similar in position, value, structure, or function. In biological terms, homologous parts (organs, limbs, etc.) are similar in structure and evolutionary origin, although not necessarily in function. For instance, the flippers of a seal and the hands of a human being are homologous.


The Neocortex

The neocortex is made up of several sections. Use the figure 3 to reference these sections as you read more about them.

Functional
Division

Constituent Parts Developmental Division Primary Division
Neocortex  

Cerebral cortex
Frontal Lobes
Temporal Lobes
Parietal Lobes
Occipital Lobes
Corpus Callosum

Telencephalon
Cerebral
Hemispheres
Forebrain
Limbic
Cingulate Cortex
Amygdala
Hippocampus
Septum
Amygdala
Hippocampus

Basal ganglia
Caudate Nucleus
Putamen
Globus Pallidus
Diencephalon
Thalamus
Hypothalamus
Diencephalon
Diencephalon
Brainstem
Midbrain
Superior Colliculus
Inferior Colliculus

Cerebellum
Pons
Medulla Oblongata
Mesencephalon
Brainstem
Midbrain
Hindbrain
Spinal Cord
Spinal Cord
Spinal Cord

Cerebral Cortex

The largest part of the brain is called the cerebrum, which comprises about 90 percent. The word cerebrum, in fact, comes from the Latin word for brain. The cerebral cortex is actually the gray, wrinkled surface encompassing the cerebrum. The cerebral cortex looks wrinkly because of its many folds.

Although it is about as thick as corrugated cardboard, if you laid it out flat, the cerebral cortex could almost cover the top of your kitchen table. There are 10-to-14 billion neurons contained within the cerebral cortex, a whopping number when you consider that the world's human population is less than that!

Cerebral Lobes

Nestled beneath the cerebral cortex are the central lobes of the brain. There are four of them, and they are responsible for critical daily activities such as hearing, vision, speech, and executive function.

The frontal lobe is divided into four functional areas:

  1. The primary motor cortex is involved in the initiation of voluntary movements. It is composed of the precentral gyrus.
  2. The premotor area is made up of the remainder of the precentral gyrus and is also important in the initiation of voluntary movements.
  3. Broca's area, located primarily in the left cerebral hemisphere, is important in the production of speech and written language.
  4. The prefrontal cortex comprises the remainder of the frontal lobe. It is involved in what may be described as personality, insight, and foresight.
The parietal lobe is associated with three functions:
  1. The postcentral gyrus is concerned with the initial cortical processing of tactile and proprioceptive (sense of position) information.
  2. Much of the interior parietal lobule of one hemisphere (generally the left), together with portions of the temporal lobe, is involved in the comprehension of language (Wernicke's Area).
  3. The remainder of the parietal cortex subserves complex aspects of orientation of the individual in space and time.
The temporal lobe is associated in general with three functions:
  1. A small area of the temporal lobe is the primary auditory cortex.
  2. The parahippocampal gyrus and hippocampus, as part of the limbic system, are involved in emotional and visceral responses.
  3. The temporal lobe is involved in complex aspects of learning and memory recall.

The occipital lobe is more or less exclusively concerned with visual functions. Primary visual cortex is contained in the walls of the calcarine sulcus and some of the nearby cortex. The remainder of the lobe is referred to as a visual association cortex that is involved in higher-order processing of visual information.

Still More Cerebral Cortex

The cerebral cortex also contains major internal structures (some of which you've probably heard of) such as the:

  • Forebrain: credited with the highest intellectual functions of thinking, planning, and problem solving
  • Hippocampus: involved more directly in memory formation and retrieval
  • Thalamus: serves as a relay station for virtually all the information coming into the brain
  • Hypothalamus: contains neurons that serve as relay stations for internal regulatory systems, monitoring information coming in from the autonomic nervous system


 


Another Brainy Factoid

The sperm whale may triumph by having the biggest brain, but who wins the pea-brain competition? Stegosaurus, also known as the "plated lizard," roamed the United States 150 million years ago and weighed in at a mighty two tons. And how large was the massive brain that this creature needed to orchestrate its existence? Smaller than a ping-pong ball!


The Diencephalon

The diencephalon has four main substructures: thalamus, hypothalamus, epithalamus, and subthalamus. We'll focus on the first two.

In Latin, thalamus means little room. In the brain, the thalamus is located deep inside and between the two cerebral hemispheres, so it is indeed a little room. The thalamus is a nuclear mass of great importance in both sensory and motor systems. No sensory information, with the exception of olfactory information, reaches the cerebral cortex without first passing through and being processed by thalamic nuclei.

The thalamus functions as a way station between the brain and the spinal cord. If you experience sensations such as pain, pressure, or temperature, you have your thalamus to thank! Senses such as taste, sight, sound, and touch also must pass through the thalamus as they make their first stops in the brain.

The prefix hypo means under, so consider the hypothalamus as being located under the thalamus. In medical lingo, the hypothalamus is referred to as inferior to the thalamus. The lower, or inferior, surface of the hypothalamus is actually one of the very few parts of the diencephalon visible on an intact brain. (Remember, the thalamus is the inner room.)

The hypothalamus is tiny, about the size of a small bean, and comprises about 1/300th of the brain's total weight. Despite its unimpressive size, the hypothalamus is the major visceral control center of the brain. It is your hypothalamus that regulates your body temperature. And it is your hypothalamus that sends you a signal to let you know that you're hungry, thirsty, tired, mad, or sad. The hypothalamus is involved in limbic system function as well.



 


Thalamus or Hypothalamus?

Experiencing sensations such as pain or pressure? That's the job of your thalamus. Feeling hungry, tired, or irritable? Your hypothalamus is busy sending you those types of signals.


The Brainstem

The brainstem is located below the thalamus and the hypothalamus. The brainstem is about three inches in length and is about as big around as a thumb. The brainstem is divided into the midbrain, the pons, and the medulla.

At the top of the brainstem is the midbrain. The midbrain is the portion of the brain that adjusts the sensitivities of your ears to noises and your eyes to light.

Beneath the midbrain is the pons, a term that means bridge. The pons functions as a conduit between the brain and the spinal cord and is composed mainly of bunches of nerve fibers that connect the cerebral cortex with the cerebellum and the spinal cord. Your pons controls sleeping and dreaming functions.

The medulla is only about an inch long and connects the brain to the spinal cord. It is the medulla that regulates many of the things you do without thought that are critical to your existence, such as heartbeat, breathing, and swallowing. Just think how complicated life would be if every time you had to swallow, you had to focus on the act and will it to happen! The medulla also controls your brain's vomiting center.

Coming Up

Whew! We just covered a tremendous amount of information. Learning about how the human brain is organized is interesting to most people for a simple reason: we all have one. Knowing where in your brain the things you do during the day are directed from adds interest to some of the most mundane things we do.

Let's stay in touch on the Message Board. How will knowing more about the human brain help you?



 


Thanks, Brainstem!

It is the brainstem that controls reflexes. When you instantly recoil from momentary contact with the heat of a whistling teapot, you have your brainstem to thank for your quick reaction. If you had to take the time to consciously think about how to react to the pain of heat, your burn would be much worse!


   
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