A Macro View Of The Brain - How The Brain Works: Alzheimers

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A macro view of the brain

Neuroscientist Paul MacLean coined the term "triune brain" to describe in evolutionary terms what he viewed as the three separate but interconnected levels of the human brain: the brainstem (and cerebellum), the limbic system, and the cerebral cortex (see Figure 1). An extensive two-way network of nerves connects these three levels of the brain. Ongoing communication between the cerebral cortex and the limbic system inextricably links thinking and emotions (see Figure 2). Each influences the other, and both direct all voluntary action. This interplay of memory and emotion, thought and action is the foundation of each individual's unique personality.

Figure 1: The three levels of the brain The most primitive level of the brain is made up of the brainstem and cerebellum (1). The brainstem regulates the kinds of body functions you rarely stop to think about, such as your heartbeat, breathing rate, and blood pressure. The cerebellum coordinates posture, muscle tone, and skilled movements. The next level, the limbic system (2), provides the link between animal drives and rational behavior. The third level, the cerebral cortex (3), is the wrinkled "gray matter" that covers the brain. Less than a quarter-inch thick, the cerebral cortex is responsible for higher-level thought, memory, and language.

The brainstem and cerebellum

Operating at the first level, these two primitive structures control basic survival. The brainstem oversees vital functions such as heartbeat and body temperature, and the cerebellum orchestrates movement.

The limbic system

Nestled deep inside the brain is the limbic system, the second level of MacLean's "triune brain." This wishbone-shaped complex of nerve centers is found in all mammals. The limbic system links emotions and behavior, as numerous scientific experiments and observations of people with brain damage have proved. Stimulating one area of the limbic system produces feelings of anger and aggression, while stimulating another area prompts feelings of pleasure and relaxation. The limbic system is the interface between our animal drives and the constraints of civilization, between irrational impulses and practical decisions, between raw emotions and rational behavior.

The limbic system has another major function: It is central to memory and learning. Although memories are not stored in a single location, discrete structures within the limbic system orchestrate memory formation. Furthermore, these structures process different kinds of memory. The hippocampus, for example, is active in converting information into long-term memory and in memory recall. Repeated use of specialized nerve networks in the hippocampus enhances memory storage, so this structure is involved in learning from both commonplace experiences and deliberate study.

Damage to the hippocampus or its nerve connections can cause amnesia (inability to learn and then recall new information). People with amnesia are unable to form new long-term memories, and they forget information soon after they hear or see it. For example, researchers have found that patients with amnesia can continue doing things like playing checkers as well as they used to (because it was a skill that was acquired over years through practice), but they can't remember whom they're playing against.

But not all experiences in a person's life are indelibly etched in memory, nor is it necessary to retain every bit of information one encounters. This is where emotions enter the memory process. Some neuroscientists believe the hippocampus helps select which memories are stored, perhaps by attaching an "emotion marker" to some events or other information so they are more likely to be recalled.

The amygdala, which sits next to the hippocampus, is concerned with a different magnitude of emotional memory: It comes into play in situations that arouse feelings such as fear, anger, pity, or outrage. Researchers have discovered that memories that have an emotional component are more likely to be retained. But damage to the amygdala can abolish an emotion-charged memory. For example, an animal might no longer fear its predators.

The cerebral cortex

The third level of the brain is the cerebral cortex, commonly called the "gray matter." The cerebral hemispheres contain two specialized regions, one dedicated to voluntary movement and one to processing sensory information. But most of the gray matter is the association cortex, which becomes progressively larger as animals move up the evolutionary ladder. The association cortex is the region of conscious thought: It is where you store memory and language skills, process information, and carry out creative thinking.

Figure 2: Inside the brain In Alzheimer's disease, brain cells die and neuronal connections wither in all parts of the brain, but especially in the hippocampus and the amygdala — important parts of the limbic system that coordinate memory storage and recall — and the cerebral cortex, the seat of higher-level thinking, memory, and language.

Last updated:	January 23, 2007