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 thinking 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 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 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:
Each has a number of smaller areas, each with specific functions.
The cerebrum, the largest part of the brain, contains the following:
The cerebrum is divided into two halves—the left and right cerebral hemispheres. The hemispheres are connected by nerve fibers that form a bridge (called the corpus callosum) through the middle of the brain. Each hemisphere is further divided into lobes:
Each lobe has specific functions, but for most activities, several areas of different lobes in both hemispheres must work together.
The frontal lobes have the following functions:
Particular areas of the frontal lobes control specific movements, typically of the opposite side of the body. In most people, the left frontal lobe controls most of the functions involved in using language.
The parietal lobes have the following functions:
The occipital lobes have the following functions:
The temporal lobes have the following functions:
Subcortical structures include large collections of nerve cells:
The limbic system, another subcortical structure, consists of structures and nerve fibers located deep within the cerebrum. This system connects the hypothalamus with other areas of the frontal and temporal lobes, including the hippocampus and amygdala. The limbic system controls the experience and expression of emotions, as well as some automatic functions of the body. By producing emotions (such as fear, anger, pleasure, and sadness), the limbic system enables people to behave in ways that help them communicate and survive physical and psychologic upsets. The hippocampus is also involved in the formation and retrieval of memories, and its connections through the limbic system help connect those memories to the emotions experienced when the memories form. Through the limbic system, memories that are emotionally charged are often easier to recall than those that are not.
The brain stem connects the cerebrum with the spinal cord. It contains a system of nerve cells and fibers (called the reticular activating system) located deep within the upper part of the brain stem. This system controls levels of consciousness and alertness.
The brain stem also automatically regulates critical body functions, such as breathing, swallowing, blood pressure, and heartbeat, and it helps adjust posture. If the entire brain stem becomes severely damaged, consciousness is lost, and these automatic body functions cease. Death soon follows. However, if the brain stem remains intact, the body may remain alive, even when severe damage to the cerebrum makes movement and thought impossible).
The cerebellum, which lies below the cerebrum just above the brain stem, coordinates the body's movements. With information it receives from the cerebral cortex and the basal ganglia about the position of the limbs, the cerebellum helps the limbs move smoothly and accurately. It does so by constantly adjusting muscle tone and posture.
The cerebellum interacts with areas in the brain stem called vestibular nuclei, which are connected with the organs of balance (semicircular canals) in the inner ear. Together, these structures provide a sense of balance, making walking upright possible.
The cerebellum also stores memories of practiced movements, enabling highly coordinated movements, such as a ballet dancer's pirouette, to be done with speed and balance.
Both the brain and spinal cord are covered by three layers of tissue (meninges) that protect them:
The space between the arachnoid mater and the pia mater (the subarachnoid space) is a channel for cerebrospinal fluid, which helps protect the brain and spinal cord. Cerebrospinal fluid flows over the surface of the brain between the meninges, fills internal spaces within the brain (the four cerebral ventricles), and cushions the brain against sudden jarring and minor injury.
The brain and its meninges are contained in a tough, bony protective structure, the skull.
Last full review/revision December 2014 by Steven A. Goldman, MD, PhD