Oct 24, 2011
(Editor’s note: below you have part 2 of the 6-part The Neurobiology of Stress series. If you are joining the series now, you can read the previous part Here.)
The term gray matter usually evokes an image of the cortex, because that ’ s the part most visible in pictures of the brain. In fact, gray matter makes up not only the cerebral cortex but also the central portion of the spinal cord and areas called the cerebellar cortex and the hippocampal cortex. This dense tissue is packed full of neuronal cells, their dendrites (branching, root – like endings), axon terminals (the other end), and those sticky glial cells I mentioned earlier. The cortex is the area of the brain where the actual processing of information takes place. Because of its relative size and complexity, it ’ s easy to understand why it plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness.
A Division of Labor
A central groove, or fissure, runs from the front to back of the cortex, dividing it into right and left hemispheres. In general, the left hemisphere controls functions on the right side of the human body and the right hemisphere controls the left side, but there are significant exceptions and much sophisticated interaction between the two hemispheres. This communication between the left and right hemispheres is facilitated by the corpus callosum, a wide, fl at bundle of axons located in the center of the brain, beneath the cortex. Think of it as the Lincoln Tunnel, connecting Manhattan and Jersey City. (I ’ ll leave it to you to decide which one represents which hemisphere.)
The corpus callosum makes up the largest area of so – called white matter in the brain. White matter is made of bundles of axons each encased in a sheath of myelin. These nerve bundles lead into and out of the cortex and the cerebellum, and branch to the “ old brain, ” the hippocampus. About 40 percent of the human brain is made up of gray matter, and the other 60 percent is white matter. It’s the white matter that facilitates communication between different gray matter areas and between the gray matter and the rest of the body. White matter is the Internet of our brains. (Al Gore did not invent it.)
Evolution, tempered by experience, has employed gray matter to build what might be considered very well – developed “ cognitive condos ” that sit above the hippocampus. This arrangement is very important to a discussion of stress. Our old or primitive brain was primed for survival in our ancestors ’ environment. It’s interesting to note that the brains of lower vertebrates like fish and amphibians have their white matter on the outside of their brain. We are blessed (and cursed) with lots of gray matter that gives us the ability to think things through (especially if we are anxious). Frogs and salamanders and their pond – side friends don’t think about danger so much — they just get out of its way! (And while I can ’ t be sure, I don ’ t think that they have nightmares about giant human children armed with nets.)
How do you feel about that? In case you ever get this question on Jeopardy or in a game of Trivial Pursuit, the limbic system is made up of the amygdala, the hippocampus, the cingulate gyrus, fornicate gyrus, hypothalamus, mammillary body, epithalamus, nucleus accumbens, orbitofrontal cortex, parahippocampal gyrus, and thalamus. These structures work together to process emotions, motivation, the regulation of memories, the interface between emotional states and memory of events, the regulation of breathing and heart rate, the production of hormones, the “ fight or flight ” response, sexual arousal, circadian rhythms, and some decision – making systems. Pretty impressive job description, eh? The word limbic comes from the Latin word limbus, which translates to “ belt ” or “ border, ” because this system forms the inner border of the cortex. The limbic system is part of the old brain and developed first, followed by the new brain: the cortex, which is sometimes referred to as the neocortex. Put very simply, the limbic system feels and remembers; the cortex acts and reacts. And they communicate with each other. Why is this important? The limbic system figures prominently in what ’ s called the stress response, which is a central player in this book.
These days, both our old and new brains are activated when we ’ re under stress. The primitive part, the limbic system (notably the hippocampus), sniffs out danger well before the new brain (the neocortex) actually processes it. The old brain responds first, acting as a sort of fi re alarm system. It is the neocortex, and in particular, the frontal lobe (the pre-frontal cortex), that helps us make sense of the alarms.
The cortex is made up of four major sections, arranged from the front to the back. These are called the frontal, parietal, occipital, and temporal lobes. Each of the four lobes is found in both hemispheres, and each is responsible for different, specialized cognitive functions. For example, the occipital lobe contains the primary visual cortex, and the temporal lobe (located by the temples, and close to the ears) contains the primary auditory cortex.
The frontal lobes are positioned at the front most region of the cerebral cortex and are involved in movement, decision making, problem solving, and planning. There are three main divisions of the frontal lobes. They are the prefrontal cortex, the premotor area, and the motor area. The frontal lobe of the human brain contains areas devoted to abilities that are enhanced in or unique to humans. The prefrontal cortex is responsible for planning complex cognitive behaviors, the expression of personality, decision making, and social behavior, as well as the orchestration of thoughts and actions necessary for a person to carry out goals. A specialized area known as the ventrolateral pre-frontal cortex has primary responsibility for the processing of complex language. It is more commonly called Broca ’ s area, named for a nineteenth – century French physician who determined its role.
In humans and other primates, an area located at the forward part of the prefrontal cortex is called the orbitofrontal cortex. It gets its name from its position immediately above the orbits, the sockets in which the eyes are located. The orbitofrontal cortex is very involved in interpreting rewards, decision making, and processing social and emotional information. For this reason, some consider it to be a part of the limbic system.
The amygdala, a part of the limbic system, is a brain structure that is responsible for decoding emotions, especially those the brain perceives as threats. As we evolved as a species, many of our alarm circuits have been grouped together in the amygdala. Not surprisingly, many regions of the brain send neurons into the amygdala. As a result, lots of sensory messages travel instantaneously to the amygdala to inform it of potential dangers lurking in our neighborhood. The amygdala is our guard dog.
The amygdala is directly wired to the hippocampus, also a part of the limbic system. Since the hippocampus is involved in storing and retrieving explicit memories, it feeds the amygdala with strong emotions triggered by these recollections. Why is this important? If a child has a negative experience in school, like being terribly embarrassed when asked to read in front of the class, the hippocampus just won’ t let go of this memory, and it shouts it out to the amygdala. Since the amygdala has signed a no confidentiality agreement, it sends a warning to the rest of the brain to go into protection mode. A rather amazing arrangement, don’t you think?
What’s really interesting about this is that the hippocampus specializes in processing the context of a situation. As a result, the child under stress generalizes the entire situation and uses it as justifi cation for anxiety or stress: “ Hey, they’re telling me to go to social studies class. ”Even though not everything about social studies may be a threat — perhaps just the fact that they read out loud in there — the hippocampus sends out a general alert. So the student responds by protesting the whole enchilada: “No way I’m going there.”
The amygdala is also wired to the medial prefrontal cortex. Want to know why this is important? This is the area of the brain that seems to be involved in planning a specific response to a threat to safety. Here ’ s how it works: the child is hit with the gigantic Titanic news (which may be just “social studies coming up next” to the rest of the group, but it’s “Submerged iceberg ahead!” to the kid worried about perceived horrors there). This two – way communication between the prefrontal cortex and the limbic system (particularly the amygdala) enables us to exercise conscious control over our anxiety. The emotion – cognition connection allows us to feel that we can do something about the danger that lies ahead. The child is then faced with the necessity of choosing a course of action that looks best for getting out of danger. This seems very protective but tends to be counterproductive, because the very mechanism that allows us to create an escape plan can actually create anxiety. “ Oh crud — now we have to do something! ” The brain not only allows us to imagine a negative outcome, which can help us avoid danger, it makes it possible for us to imagine dangers that do not actually exist. This is a problem for children who have ADHD, and a huge problem for students who have both anxiety disorders and ADHD. If you do a brain scan of a person with ADHD while putting on pressure to perform in a certain way, you see that this “to do” order results in a decrease in activity in the prefrontal cortex (instead of increasing it, as it does in most people). This helps explain why kids with ADHD don ’ t respond well to lists. These are read as “thou shalt” messages. What helps some of us stay organized sends some kids with ADHD up the wall.
The Thalamus Bone’s Connected to the . . .
Of course, it’s not really a bone; it’s a plum – shaped mass of gray
matter that’s multilayered and multifaceted. The thalamus, another part of the limbic system, sits on top of the hypothalamus which, in turn, sits on top of the brain stem, which is in the center of the base of the brain. This is a great location for the thalamus because it acts as a relay system that sends nerve fi bers upstairs to all parts of the cerebral cortex as well as many sub-cortical (underneath the cortex) parts of the brain. The thalamus receives information from every sensory organ and its associated neurons except the olfactory (smell) system. The hypothalamus gets information from the eyes, the ears, the skin, and the tongue, and it forwards these messages to the corresponding areas of the cortex where they are processed. In terms of stress, this relay system is how the brain knows that it’s in a dangerous environment.
There’s a bulb – shaped brain structure (called, as you might guess, the olfactory bulb ) that has the specialized task of making sense of scents. Think about this: you can’t see when you are sleeping, but you can smell. This is awfully helpful at night, especially when there’s a fire. And when you get hold of a bad piece of fish. That’s probably why the nose gets its own special receptor. It’s another example of how sensitive the brain is to changes in the environment, and how it’s always on alert!
To Be Continued…
- October 31st: The Little Brain Down Under
- November 7th:Stress Response Explained
- November 14th: The Human Brain Likes Balance
- November 21st: To Fight, Flee or Freeze –That is the Question
– Jerome J. Schultz, Ph.D., the Author of Nowhere to Hide: Why Kids with ADHD and LD Hate School and What We Can Do About It (Jossey-Bass; August 2011), is a clinical neuropsychologist and is on the faculty of Harvard Medical School in the Department of Psychiatry. He served until recently as the Co-Director of the Center for Child and Adolescent Development, CCAD, a multi-disciplinary diagnostic and treatment clinic which is a service of the Cambridge Health Alliance, a Harvard Teaching Hospital.