Health

The human brain weighs only about three pounds, but it is one of the most complex systems known to science. It has an array of interconnecting nerve cells that chatter incessantly in languages both chemical and electrical.

A micro view of the brain

The brain consists of about 100 billion neurons, specialized nerve cells that communicate with one another using chemical and electrical signals. Each neuron has a body that contains a nucleus, a long fiber called an axon, and many shorter branching fibers called dendrites. Neurons talk with one another across microscopic gaps called synapses. When a cell body or dendrite receives a message from a neighboring neuron, an electrical impulse is generated. This signal travels the length of the neuron to the end of the axon, where it prompts the cell to release a chemical messenger called a neurotransmitter into the synapse. Certain neurotransmitters pass on messages by creating an electrical impulse in the receiving cell, and the process of electrical-to-chemical signaling is repeated. Other neurotransmitters suppress the transmission of signals to neighboring neurons (see Figure 2).

The connections between neurons are where the brain's work is done. This is where thinking and feeling take place and all the basic processes of human life, including movement and breathing, are controlled. In the early stages of development, the brain is highly flexible: Damage to a specific area can often be repaired because existing neurons can form new connections with other nerve cells. By the end of childhood, however, the brain loses much of this reparative power. While the adult brain can rewire itself to some degree, most of the neurons that die cannot be replaced. Unlike the heart, which can still support a marathon runner after losing 10% of its tissue, a 10% loss in the adult brain can result in devastating disability.

A macro view of the brain

The human brain is the product of millions of years of evolution. It has three interconnected levels: the brainstem and cerebellum, the limbic system, and the cerebral cortex. The brainstem, an extension of the spinal cord, is the oldest part of the brain in evolutionary terms. It connects the rest of the brain to the spinal cord and regulates blood pressure, breathing, chewing, swallowing, and eye movements. At the top of the brainstem is the thalamus, which relays sensory information to other parts of the brain. At the back, near the brainstem, lies the cerebellum, which is responsible for maintaining balance and posture and coordinating movement.

The cerebellum's role is crucial. For example, when you are learning a new, complex skill such as playing the violin, the larger, domed part of the brain known as the cerebral cortex helps you understand the process of playing the violin. But with practice, it is the cerebellum that helps you accomplish real mastery and makes the activity second nature.

The limbic system, a step up the evolutionary ladder from the brainstem, is a group of related structures that help regulate emotion, memory, and certain aspects of movement. One of these structures, the hippocampus, is vital to the storage of recently acquired information — one of the brain's most important functions. Damage to the hippocampus can destroy the ability to learn new information or cause the loss of recent knowledge. Emotions such as fear, anger, and pleasure are stored nearby in the amygdala; damage to this structure can erase emotion-charged memories, such as details of your wedding day or a family reunion. Deep in the brain, clusters of neurons called basal ganglia play an important role in movement.

At the top of the evolutionary ladder sits the cerebral cortex, which is involved in all forms of conscious experience. It is the large, dome-shaped mass of gray matter that most people imagine when they picture the human brain. The cerebral cortex covers the top and the outermost sides of the brain and is divided into left and right hemispheres. The two halves communicate through a vital superhighway of neurons called the corpus callosum.

Each hemisphere of the cerebral cortex consists of four lobes. Although there is some overlap among these sections, each has distinct functions. The front portion is the frontal lobe, which controls motor function, planning, organization skills, and the expression of language. Behind it lies the parietal lobe; the left side of it enables you to understand language and do mathematical calculations and the right side controls tasks that involve visual and spatial orientation. Farther back is the occipital lobe, which perceives and interprets vision. Wrapped around the sides is the temporal lobe, which is involved in hearing, long-term memory, and behavior (see Figure 3).

The visual cortex, where vision is processed, is in the occipital lobe, in the back of the cerebral cortex. The right visual cortex controls the left visual field for both eyes, and the left visual cortex controls the right visual field. That's why defective vision or blindness in half the visual field of one or both eyes — a condition called hemianopia — is a common consequence of stroke.

The left side of the brain usually controls activity on the right side of the body, and the right side of the brain controls the left side of the body. Damage to the left hemisphere can produce paralysis or loss of sensation on the right side of the body, and vice versa. Such impairment on one side of the body is one of the hallmarks of stroke. Weakness that is limited to one side is called hemiparesis; paralysis on one side of the body is known as hemiplegia.

Speech centers are on the left side of the brain. Motor speech (the physical movements of the mouth, tongue, and lips) is formulated in Broca's area of the frontal lobe, while understanding written and spoken words occurs in Wernicke's area. These two regions are in constant communication through a dense bundle of neurons.

When stroke strikes

The symptoms of a TIA or stroke provide clues about its type and its location in the brain. A stroke on the left side of the brain, for example, may result in confusion, abnormally slow movements, abnormal speech, an inability to speak or understand speech, or an inability to see objects in the right half of the visual field. People with this type of stroke may have difficulty reading, writing, or calculating. They may also become slow, cautious, and disorganized when faced with an unfamiliar situation.

People with right-sided strokes may ignore or fail to recognize objects or sensations on their left side or even neglect parts of the left side of their body. Deficits on the right side of the brain may also interfere with spatial perception, leading to difficulties with common tasks such as eating, drinking, dressing, operating a wheelchair, or reading. People with this type of stroke may engage in impulsive or dangerous behavior.

If you notice any telltale symptoms in yourself or in a loved one, dial 911 right away. A stroke is a medical emergency, and the sooner you seek treatment, the better.

The brain is especially vulnerable to injury, which is one reason it has a strong skull for protection. Although the brain accounts for only 2% of a person's total body weight, it uses about one-quarter of the body's oxygen and expends more than two-thirds of the body's chief source of energy, glucose. Despite these intense needs, the brain cannot store oxygen or glucose and therefore must be supplied by a constant flow of blood. Cutting off the brain's blood supply for only 30 seconds can cause unconsciousness, and after only 4 minutes permanent damage begins to set in. That's why it's important to get medical help quickly when the signs and symptoms of stroke first appear.

Blood delivers fuel and nourishment through a complex system of blood vessels that reaches every neighborhood of the brain (see Figure 1). The carotid arteries, which run up the left and right sides of the neck, are especially important, supplying blood to the front of the brain and the cerebral cortex (see Figure 4). The vertebral arteries, which run up the back of the neck and join at the base of the skull to form the basilar artery, supply the cerebellum, the brainstem, and the back parts of the brain. These major arteries join at the base of the brain to form the Circle of Willis, from which other vessels branch off. Problems in any of these blood vessels may lead to stroke.

Source: from Harvard Health Publications, Copyright © 2008 Harvard University. All rights reserved. Harvard Medical School does not endorse products.
Used with permission of StayWell.
Terms of Use
Medical Disclaimer