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Date Posted: 21:41:55 05/16/02 Thu
In reply to:
shrift
's message, "Smallville" on 07:48:14 05/13/02 Mon
The Limbic System
A low growling from the woods makes you freeze. You are alone on a small path holding a basket of the sun-kissed blackberries you’ve been picking all morning. The growling becomes louder, and the bushes shake and rustle violently. The putrid smell of dead flesh washes over you.
The low growling, the smell of decaying flesh, the rustling leaves: all of this information is is being passed to the amygadala, an almond shaped structure in your brain that acts as the gateway to the limbic system. Once past the gateway, the hippocamups, thalamus, septum, mammilary body and cingulate cortex receive this information and react. Files are pulled from the brain’s reservoir of memory as your limbic system strives to identify the threat. Once the proper file has been found, it is opened and associated emotions to that memory are recalled. At the same time, the limbic system is writing all of this action into a new file so that it can be stored in the form of memory as well.
Your heart begins to race. Your eyes widen. Wiping the sweat from your palms onto your jeans, you look around wildly. Inside your head, a voice is screaming, "Run! Run! Run!"
At this point, vital information has filtered through the limbic system and landed in the hypothalamus. This structure is the ‘brain’ of the limbic system. The hypothalamus has three major functions. First, it sends messages to the pituitary gland, also known as the ‘conductor’ of the endocrine system. Hormones released by the endocrine system affect almost every aspect of the body. Increased hormone levels lead to sweat, thus the sweat coating the palms of the hand. Second, the hypothalamus is connected to the autonomic nervous system, which in turn controls body functions such as blood pressure, respiration, sweat, body temperature, and tears. This is why your heart is racing. The third function of the hypothalamus is the control of actual behaviors, often known as the ‘four F’s”: fighting, fleeing, feeding and sexual behavior (Ramachandran, p. 177). The hypothalamus is the survival center of the brain.
By drawing on the past memory your limbic system found earlier, along with the associated emotions, and linking them to the present situation, your brain makes a decision to save your body. You run away. Thus, you survive another day to pick more blackberries in the July heat.
But what happens if the limbic system is damaged? In his book, Phantoms in the Brain, Dr. V.S. Ramachandran relates the case of Arthur, a patient whose limbic system had been damaged in a car accident. Arthur suffered under the delusion that his parents are actually imposters of his real parents. Through a series of tests, Dr. Ramachandran found that, although Arthur can recognize faces and experience emotions, he cannot link the two together. When Arthur sees his parents he feels no connection to them. Therefore, Dr. Ramachandran theorized, when Arthur sees his parents and feels no ‘warm glow’, he copes by assuming they are imposters.
In two other cases, Dr. Ramachadran tells of Willy and Ruth, two people who literally died laughing. Willy began laughing at his mother’s funeral and couldn’t stop. Since this was obviously inappropriate behavior, his friends took him to the hospital. Although Willy had stopped laughing by that time, he was admitted. Two days later, Willy died of a subarachnoid hemorrhage. The postmortem found that a large anuerysm in an artery at the base of his brain had compressed the hypothalmus, mammilary bodies and other parts of the limbic system (p. 200).
Ruth, a librarian, started laughing after a violent headache. She was unable to stop laughing, though she was able to follow her doctor’s instructions. She laughed until she became comatose and then died. The postmortem revealed a blood-filled cavity in the middle of her brain. This had compressed the thalamus and several nearby structures as well, causing the unstoppable laughter (p. 200).
A more classic example of limbic damage is rabies. In 1935, James Papez, an anatomist, noted that patients suffering from rabies often experienced extreme rage or terror. Upon dissecting the victim’s brains, he saw that the rabies virus had attacked the limbic system. He concluded that the limbic system dealt with emotions and that the destruction of it by the rabies virus caused these rages and frights (p. 177).
The next week, you are on the path again, carrying your bucket in the early morning sun and enjoying the solitude that comes with picking blackberries. As you pass the spot you ran from the previous week, you pause for a moment. The heavy scent of pine trees fill the air. A tiny buzz hums in your ear as a June bug flutters past.
Your limbic system assesses this information and relates it to the memory file it created of this spot last week. It finds no danger today, but the terror you felt last week urges you forward. Your heart beats a little faster. You comply with your body’s wish and move on.
You relax as the spot fades from sight, and as your heart rate returns to normal, you start to hum. Deep in your brain, though, your limbic system is always on alert. If it’s not prepared, it could be the death of you.
The Limbic System
Prepared for
Sylvia Taylor
Medical Terminology
Thursday, 6p.m.
Prepared by,
Shannon Swearingen
November 30, 1999
References
NOVA “Brain Geography; The Limbic System”. Online. Internet. 28 November
1999. Available at http://www.pbs.org/wgbh/nova/coma/geography/info1.html
Ramachandran, V.S. Phantoms in the Brain. New York: Quill William Morrow,
1998.
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