Cerebrum

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Introduction

    The cerebrum is the icing on the brain!  It is the outermost covering of the telencephalon, and is very large.  The gray matter of the cerebrum is all located on the most peripheral edges of it, with the white matter underneath.   Because the gray matter is peripheral, there's only as many neurons as their is surface area of the cerebrum.  So the cerebrum is highly folded to have more room for cerebral neurons.  Each fold, or dip toward the lower brain areas, is called a sulcus (plural:  sulci), and each surface ridge is called a gyrus (plural:  gyri).  The gray matter of the cerebrum is referred to as the cortex of the cerebrum, which just means the outer layer.

    I took this picture from Mosby's "Body Spectrum" program, and then labeled it with my own labels.  cerebrum.gif (21634 bytes)It shows the cerebrum only.  In this figure we are looking at the right half of the brain, since anterior is to the right (the eyes would be looking at this text).

    The entire cerebrum is divided into two halves, called cerebral hemispheres.  The right hemisphere controls the left side of the body, while the left hemisphere controls the right side of the body.

    One can divide the cerebrum into four lobes quite readily:  Frontal Lobe, Parietal Lobe, Occipital Lobe, and Temporal Lobe.   These four lobes are indicated somewhat by the four general-color regions in this figure.  All blue regions are parts of the frontal lobe, all reddish regions are part of the parietal lobe, the yellow region is the occipital lobe, and the purple region is the temporal lobe.  Some other, more specific regions are identified on this image, and they are explained in a bit more detail below.  You will learn how to distinguish these four regions in lab on Monday.

Mapping within the Brain

    Our cerebral hemispheres have to carry out so many functions that they need some more detailed type of intrinsic organization.  This organization is by function and by body area.  As an example of organization by function, all of the neurons involved in creating our personality are in the same region, and that is a different region from the one where the neurons that help us understand what we see are found.  The cerebrum organizes many of these areas even more by body area.  In this way, the part of your brain that controls your finger movements is right near the part that controls your wrist movements, and that is right near the part that controls your elbow movements.  This type of organization can also be found in the region of the brain that helps us understand what we see-- those neurons that are trying to understand the visual world directly ahead of us are found near one another, and far from those neurons that are getting information from our peripheral vision.  An organization according to such a spatial plan (body regions, visual world location, etc.) is called mapping.  The specific map of our body on our brains is called a somatotopic map, while a map of our visual world in our brains is called a visuotopic map.

Type of Cortex

    As you keep reading, you'll see the word "primary" and the word "association" to describe regions of cortex.  I'd like to try to explain that to you here.  A primary cortical region is one that is the first to receive its input or the simplest and most direct one for output.  For example, the primary visual cortex receives the visual information from the thalamus (that came from the eyes).  This primary cortical area is where visual information processing begins.   But any sensory information processing is quite complex, and needs more advanced cortical regions, the association regions, to be completed.  For example, are you seeing a person or an animal?  Do you recognize the face on the person?  Is that person your mother?  These types of questions need to be answered by relating what you see to what you know.  That relating of information happens in the association cortex.

Frontal Lobe

    The main functions of the frontal lobe are:  motor commands, motor planning, thought & personality, and olfaction.

Motor Control:

    The motor functions are carried out by the Primary Motor Cortex, as shown above in the figure, and by "motor association areas."   The motor association areas lie immediately anterior to the primary motor cortex.   If you want to perform a relatively-simple, voluntary motor task, like waving, your brain signals your spinal cord to carry out this task from the primary motor cortex.   If, however, you want to do something more complex, like reach around something in order to wave or avoid hitting something while waving, you will have to plan your movement a bit more.  The planning part is carried out more anteriorly, in the motor association areas.

    There is a special region within the motor areas of the frontal lobe, called Broca's area, named for the scientist who described it.   This region only really exists on the left side of the brain, that is why I only put the initials "B.A." on the drawing above of the right side of the brain.   I'll explain Broca's area more below in the section on speech.

    The neurons of the primary motor cortex send their axons down to run through the corticospinal tract that you learned about.  I hope that makes sense.

Thought and Personality:

    Exactly how thought and personality are stored in the frontal lobe remains a mystery.  But we certainly know that they exist in the frontal lobe.   Like with many other things, we often learn about brain regions from people with damaged regions.  Have you ever heard of a "frontal lobotomy?"  That is a surgical procedure where portions of the most anterior frontal lobe are removed.   The words "surgical procedure" are rather nice ways of describing this, since frontal lobotomies were often done against individual's wishes.  In fact, a great uncle of mine was given one, because it was thought that he could have been a danger to himself.  Anyway, after a frontal lobotomy, people are different; they have diminished intellectual capacity and lose some of their personality.  This deficit after anterior frontal lobe tissue removal indicates that this brain region really does hold our thoughts and personality.

    The anterior portion of the frontal lobe is called the "prefrontal" cortex.  This is where our personality and intellect reside.

Olfaction:

    Our olfactory information enters the frontal lobe at its anterior and inferior region (by the back of the nose), through the cribiform plate of the ethmoid bone in the skull.  We'll discuss this in lab.

Occipital Lobe

    This lobe is basically responsible for our visual processing.   Lying at the very tip of the occipital lobe is our primary visual cortex.   The rest of the occipital lobe contains more and more advanced visual association areas.  There is even a specific area just for facial recognition.  Visual information arrives in the primary visual cortex from the thalamus; it gets to the thalamus from the eyes via the optic nerve.

Parietal Lobe

    The parietal lobe houses our somatosensation (sensation of the body from touch, pain, and proprioceptive senses) and relates this information to the rest of our sensory information.  So, our primary somatosensory cortex lies in the parietal lobe.  Immediately posterior to it are the somatosensory association areas.  As we continue to go even more posteriorly within the parietal lobe, we approach the occipital lobe with its visual information.  So, the more posterior regions of the parietal lobe are where somatosensory and visual information come together.   For example, when I look forward toward this screen, I see two hands in front of me.  Because I also can sense my arm positions and hand positions, I can integrate my sensation of my body with my visual information and come to the understanding that those two hands are my hands.  If we couldn't do that, seeing those two hands would be eerie!

    A bit more inferior in the parietal lobe is where we integrate our visual and somatosensory information with other senses, hearing and taste.

Temporal Lobe

    The temporal lobe houses our primary auditory cortex and our auditory association areas.  Auditory information comes to the primary auditory cortex from the thalamus (it gets to the thalamus from the ears through a cranial nerve, the vestibulocochlear nerve). 

Insular Lobe

    The insular lobe is not shown in the picture at the top.  Why not?  Because with all the folding of the cortex, some of it got folded inside!   There are folds that run deeply, right beneath the temporal lobe and under the parietal lobe.  These deep folds are referred to as the insular lobe.  The insular lobe houses our sense of taste and is also considered to be part of our limbic system for emotions.

Understanding and Production of Speech

    Communication is a very complicated task.  We have to listen to what someone else is saying (or read it on a page), interpret the words they say (or that we read), decide how we want to respond, put our desired response into words, and then speak (or write) those words.  Do you see how this is a multi-step task?   Because of the many steps involved, we need many brain areas to accomplish this task.  Let's consider these one-by-one.  (Keep in mind that these areas are only on the left side of the brain, even though labeled above on the right side).

    Understanding words:  In order to figure out what words mean, either spoken or written words, we use the area of the brain that is called Wernicke's area, but your book refers to it as the "general interpretative area."   You see, we have to take either auditory (spoken) or visual (read) words or even words we feel (Braille reading) and turn those into meaningful ideas in our brain.   Wernicke's area (take a look at where it is, labelled as "W.A." above) lies right at the border of the temporal, occipital, and parietal lobes.  Because of that, it is in a position to interpret words or even body language that we perceive through any sense.  It is also the same area that we use to pick words for ourselves to use in speech; so if I see an apple, I can send the visual information into Wernicke's area to come up with the label, "apple," for that seen item.

    Producing words:  In order to either speak or write words, we have to turn the concept of the word into a motor action.  Motor systems are found in the frontal lobe, and this is no exception.  Broca's area is located right by the primary motor cortex and association areas for movements of the mouth.  This is where we turn our words into motor commands for either the mouth (for speaking) or the hand (for writing).

    Let's put the whole thing together.  I say, "hello."   You hear this with your auditory cortex in the temporal lobe.  You send this auditory information to Wernicke's area where you gain an understanding of what I just said.  Then, you decide to say "hi" back to me.  You come up with the word "hi" in your Wernicke's area, and send that information along to Broca's area.  Broca's area comes up with the motor plan for your mouth and tongue and larynx and respiratory muscles, and it sends this plan down to the motor neurons controlling those muscles.  That causes you to actually say the "hi" back to me.

    Both Wernicke's area and Broca's area are specifically found on the left side of the brain.  The right side of the brain is not really involved in our spoken/written language.  The left side of the brain is considered to be more logical, whereas the right side is considered more artistic.  Well, although words are dealt with on the left side, similar regions exist on the right side.  The right brain homologues to Wernicke's area and Broca's area are thought to be involved with understanding and producing, respectively, body language and intonation.

Brain trauma can cause odd effects because of the segregation of function in the brain

    Brain trauma can be due to a stroke, tumor, or head trauma due to a head impact like when someone falls off a motorcycle when they aren't wearing a helmet or to something more focal like a bullet.

    A stroke occurs when a blood vessel in the brain gets clogged.   Neurons are highly demanding in their need for oxygen with which to make their ATP.   Without ATP, neurons die quickly.  So, when a stroke occurs, the blood stops arriving at particular brain regions and the neurons within those brain regions begin to die.  Lack of oxygen to the brain is called ischemia, and neurons can only withstand about 4 minutes without oxygen before they begin to die.   Sometimes, the clogged blood vessel is in the brainstem and the neurons that die are in vital centers, like the respiratory center.  However, other times the clogged blood vessel is in the cortex, and only specific cortical regions are damaged.

    Tumors are when cells divide out of control, as I described in the cerebellum page.  This growth puts pressure on the brain areas around it because of the limited amount of space within the skull.

    When injuries occur to specific regions of the cerebrum, loss of very particular functions can occur.  A stroke can destroy Broca's area in a person-- in that case, the person could understand what is being said to them and can want to respond, but will not be able to respond.  A stroke can also destroy Wernicke's area, so that a person cannot understand anything that is being said, and can only babble meaninglessly.  These are horrible conditions due to localized brain trauma.

    There are cases where brain injury has caused people to be unable to recognize other people's faces-- in those cases the injured person can see eyes and a mouth and a nose, but they can't put that together to mean that those things are part of another person.

    When brain trauma occurs and someone suddenly has problems moving one side of their body or sensing touch on one side of their body, it is obviously the opposite side of the brain that has the trauma.  A stroke can cause limpness and lack of sensation on one side of the body due to either affecting cortical areas or to affecting the axon pathways as they run up and down the brainstem, from and toward the spinal cord.

    This section of the web page on brain trauma is just meant to introduce you to the idea of what can happen.  Certainly, there are many things we cannot begin to discuss in our limited amount of time we have to spend on the brain.   But you should be able to see that very specific deficits can occur when very limited regions of the brain are injured

A little fun...

    If you like thinking about right versus left brain, you can download this program that is a fun test to see if you are right or left-brained and to see if you are a visual or an auditory learner.  It will take a while to download, since it is over 1 MB in size.  But I didn't want to put it up there in a condensed (zipped) format, just in case you couldn't figure out how to unzip it.  When you run the program it randomly picks 20 questions to ask you, and most of the questions seem weird.  There are no correct answers, so it's not really a test.   You can take it more than once, because there are so many questions that you'll get new ones each time (with only some repeats).  I think it's fun.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.