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Biology 202
2001 Third Web Report
On Serendip

How Flexible is the Brain's Circuitry?

Dena Gu

The brain is a complicated organ, containing an estimated 100 billion neurons and around 1,000 to 10,000 synapses for each of those neurons (1). This organ has the great responsibility of not only controlling and regulating the functions of the body but also sensing and perceiving the world around it. In humans, it is what we believe makes us the highly adaptive and intelligent organisms that we are, as well as give us our individuality. But with so many parts and connections to it, what happens when the brain's delicate circuitry is disrupted? We've all heard of brain damage, and its horrible results, whether is a news report on TV or science books. It seems that with trauma, disruption of blood supply, and disease; neurons and their connections could be destroyed and the organism's behavior exceedingly affected. Yet I've read about how people have overcome tremendous damage to their brains and gone on to function with very minimal handicaps.

In elementary biology, we are all taught that cells in our body go through systems that replaces old, worn out cells with new cells. Most cell types go through programmed cell death, or PCD, but there was always an exception in the neuron; very early in mammalian development, neurons stop growing (4). PCD would be disastrous, as the depleted neurons would never be replaced. Since we need all our neurons and their connections to function, how do individuals with damage to both these neurons and connections survive, much less functioning within any definition of normality? After all, remove a few chips from a computer's motherboard and you won't have functioning computer. Yet there are children living their lives with only half their brains intact

One of the most memorable case studies I read about in high school psychology was the procedure of removing large portions, sometime half the brain, to treat young children with epileptic seizures. This procedure, called hemi-spherectomy, was developed in the 1920's but rarely performed due to complications (8). With the advances in medicine today, it has become a more common practice in treatment severe epilepsy. At first, though the procedure was expected to stop the seizures, doctors did not expect these children to ever function normally. After all, with so much of the brain missing, it is hard to expect much of the mental functions of these children. Surprisingly, these children often retained much of their personality, memories and sense humor (8), awing their doctors with the flexibility of the brains to adjust after such invasive surgery. Particularly amazing to me is that the child's memory is rarely lost in any great amount (8). That must mean memory is not stored in any particular region in the brain, as removing half of it doesn't erase half the memories.

There is even a case where the surgery actually improved the mental functions of a child. A British boy named Alex was born in 1980 with the left side of his brain choked in a tangle of abnormal blood vessels, leaving him mute, half-blind, half-paralyzed and epileptic (8). At the age of 8, doctors decided that the only way to control the seizures would be to remove that diseased left side of his brain (8). They didn't expect any other improvements to his condition; after all, it is a common belief that a lot of skills learned at a very early age (2-3 yrs) become impossible to learn later on. Much to everyone's surprise, Alex started to talk around ten months later (8). At age 11, he had some trouble pronunciation only certain words and by 16 he was speaking quite fluently. Previously, language was believed to be under the control of the left side of the brain, but obviously, the right side can fill in for it without great difficulty.

Alex's progress is amazing mainly because without the removal of half his brain, his quality of life and ability to function wouldn't come close to what it is now. I think you can see quite clear how the brain affected Alex's behavior. When the brain's abilities were disrupted and confused by the diseased left side, Alex could not function normally at all; his behavior was drastically altered by his brain. When the problem was alleviated by way of surgery, a lot of the behaviors caused by the removed portions stopped. The healthy portion of his brain then was able to affect his behavior without the disruptions by the left side. It also probably changed his perceptions on the world around him. Alex was now able to communicate in a spoke language, something he had never done in all eight years of his life. Was this because the left side dominates management of language until it is no longer present? Or that the left side was overwhelming the right side's potential to control language? Regardless, Alex was now able to perceive and apply something not though possible for a individual with his condition.

So how does the brain adapt and change to deal with it's experiences and even damages its sustained? Why is it still able to function with half of the necessary neurons? It is unlikely that only half the neurons are needed to function, because then why would human consistently have around 100 billion neurons instead of a varied range. The brain's ability to adapt and rearrange pathways is called plasticity. Plasticity refers to how circuits in the brain change and rearrange in response to experience, or sensory stimulation (6). Periods of plasticity occur in the brain under four main conditions (6). First, developmental plasticity, when the immature brain first begins to process sensory information (6). Second, activity-dependent plasticity, when changes in the body alter the balance of sensory activity received by the brain (6). An example of this would a blind individual learning to read by touch or a child learning to ride a bike and swing a baseball bat (5). Third, plasticity of learning and memory, this is where behavior is altered based on new sensory information (6). And fourth, injury-induced plasticity; this usually follows damage to the brain (6) and is what happens after hemi-spherectomies. It is believed that the same brain mechanisms drive all four types of plasticity: adjustments in the strength of the connections, or synapses, between brain cells (7).

Plasticity is a fair commonplace occurrence and comes into play as we learn new tasks and change our behaviors to accommodate new environmental stimuli. Perhaps plasticity is an important part of survival in an organism and 'chosen' for evolutionarily. After all, the ability to adapt your brain physically to change behaviors must be a trait that gives the organism an added advantage. But plasticity isn't something that stays consistently by our side for our whole lives. In developmental plasticity it is working overtime to get all the connection in the brain down as the organism is experiencing everything for the first time. When we are young, plasticity comes easily to our developing brain, but as we age and development slows in most senses, do does plasticity. Yet, plasticity does not go away with age as it was once though. Twenty something years ago, people thought that the structure of the brain develops only during childhood and once that order in the brain has been developed that there is very little room for changes (3). Today we know that the adult brain is not so set in its ways as previously considered, but still noticeably less able to rearrange essential pathways when they are destroyed or disrupted. Activity-dependent plasticity is still quite active, and can be seen in adults learning to play musical instruments (3). Brain scans show that regions controlling finger movement and dexterity become more active as the individual practices and builds more connections to control these movement (3).

Sadly, injury-induced plasticity diminishes with age. The hemi-spherectomies that show positive results for children under eight are not as effective for adults. There is more mental damage and facilities lost as the brain is physically manipulated in mature humans. The order of connections are not permanently set in the adult brain, but much less able to reorder around extensive damage. Maybe this is because overall plasticity lessens with age, and something as drastic as rearranging previously set pathways is just too much for the neurons to handle. There will have to be physical synaptic changes, as well as differences in neurochemicals produced (7). With older neurons, perhaps the changes are much slower, or not even an option to be considered.

Then there are things plasticity can't do anything about. After all, plasticity can't rearrange around components in the brain that developed to perform a certain function. In 1953, a young man named Henry M. went into surgery in the hopes of curing his severe epileptic seizures (2). The temporal lobes of both sides of his brain were removed, along with the hippocampus, the amygdala, the entohinal, and the perirhinal cortices (2). Though the seizures where effectively cured, Henry M. had another problem. He could no longer retain new memories. What wasn't known at the time was that the hippocampus was responsible for short-term memories to reach long-term memory storage (2). Any amount of rewiring in the brain isn't going to change this, and Henry M. today is no different than the one that woke up from surgery 48 years ago. The brain proves to be a complex organ that can make drastic changes to try to function as well as it can under trying situations. Henry M. can't retain new information, but he can communicate, experience emotions and joke about his condition-though he won't remember doing so (2).

WWW Sources

1) Brain Facts and Figures , Numerical figures on the brain of humans as well as a sample of animals.

2) Brain Connection, Numerous articles on a wide variety of topics on the brain.

3) Radio National ,

4) University of Nebraska Agricultural Research Division,

5) Annual Review of Psychology , Articles on Psychology

6) John F. Kennedy Center of Research on Human development,

7) Mechanisms Regulating Synaptic Plasticity in Brain,

8) NY Times, NY Times Article

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