The brain’s functions are both mysterious and remarkable. All thoughts, beliefs, memories, behaviors, and moods arise within the brain. The brain is the site of thought and intelligence, and the control center for the entire body. The brain coordinates the abilities to move, touch, smell, taste, hear, and see. It enables people to form words, speak, and communicate, understand and manipulate numbers, compose and appreciate music, recognize and understand geometric shapes, plan ahead, and even to imagine and fantasize.
The brain reviews all stimuli—from the internal organs, surface of the body, eyes, ears, nose, and mouth. It then reacts to these stimuli by correcting the position of the body, the movement of limbs, and the rate at which the internal organs function. The brain can also determine mood and levels of consciousness and alertness.
No computer has yet come close to matching the capabilities of the human brain. However, this sophistication comes with a price. The brain needs constant nourishment. It demands an extremely large amount and continuous flow of blood and oxygen—about 20% of the blood flow from the heart. A loss of blood flow to the brain for more than about 10 seconds can cause a loss of consciousness. Lack of oxygen or abnormally low sugar (glucose) levels in the blood can result in less energy for the brain and can seriously injure the brain within minutes. However, the brain is defended by several mechanisms that can work to prevent these problems. For example, if blood flow to the brain decreases, the brain immediately signals the heart to beat faster and more forcefully, and thus to pump more blood. If the sugar level in the blood becomes too low, the brain signals the adrenal glands to release epinephrine (adrenaline), which stimulates the liver to release stored sugar.
The blood-brain barrier also protects the brain. This thin barrier prevents some toxic substances in the blood from reaching the brain. It exists because in the brain, unlike in most of the body, the cells that form the capillary walls are tightly sealed. (Capillaries, the smallest of the body’s blood vessels, are where the exchange of nutrients and oxygen between the blood and tissues occurs.) The blood-brain barrier limits the types of substances that can pass into the brain. For example, penicillin, many chemotherapy drugs, and most proteins cannot pass into the brain. On the other hand, substances such as alcohol, caffeine, and nicotine can pass into the brain. Certain drugs, such as antidepressants, are designed so that they can pass through the barrier. Some substances needed by the brain, such as sugar and amino acids, do not readily pass through the barrier. However, the blood-brain barrier has transport systems that move substances the brain needs across the barrier to brain tissue. When the brain is inflamed, as may occur when people have certain infections or tumors, the blood-brain barrier becomes leaky (permeable). When the blood-brain barrier is permeable, some substances (such as certain antibiotics) that normally are unable to pass into the brain are able to do so.
The activity of the brain results from electrical impulses generated by nerve cells (neurons), which process and store information. The impulses pass along the nerve fibers within the brain. How much and what type of brain activity occurs and where in the brain it is initiated depend on a person’s level of consciousness and on the specific activity that the person is doing.
The brain has three main parts:
Cerebrum
Brain stem
Cerebellum
Each has a number of smaller areas, each with specific functions.
