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:
Each has a number of smaller areas, each with specific functions.
The cerebrum, the largest part of the brain, contains the following:
The cerebral cortex: This convoluted layer of tissue forms the outer surface of the cerebrum. It consists of a thin layer gray matter about one eighth of an inch (2 to 4 millimeters) thick. In adults, the cerebral cortex contains most of the nerve cells in the nervous system.
White matter: White matter consists mainly of nerve fibers that connect the nerve cells in the cortex with one another, as well as with other parts of the brain and spinal cord. It also contains the support cells (glial cells) for the nerve cells. The white matter is located under the cortex.
Subcortical structures: These structures are also located under the cortex—hence, their name. They include the basal ganglia, thalamus, hypothalamus, hippocampus, and the limbic system, which includes the amygdala, olfactory connections (structures that help transmit smell signals), and related structures.
The cerebrum is divided into two halves—the left and right cerebral hemispheres. The hemispheres are connected by nerve fibers that form a bridge of white matter (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:
Initiating many voluntary actions, ranging from looking toward an object of interest, to crossing a street, to relaxing the bladder to urinate
Controlling learned motor skills, such as writing, playing musical instruments, and tying shoelaces
Controlling complex intellectual processes, such as speech, thought, concentration, problem-solving, and planning for the future
Controlling facial expressions and hand and arm gestures
Coordinating expressions and gestures with mood and feelings
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:
Interpreting sensory information from the rest of the body
Controlling body and limb position
Combining impressions of form, texture, and weight into general perceptions
Influencing mathematical skills and language comprehension, as do adjacent areas of the temporal lobes
Storing spatial memories that enable people to orient themselves in space (know where they are) and to maintain a sense of direction (know where they are going)
Processing information that helps people know the position of their body parts
The occipital lobes have the following functions:
The temporal lobes have the following functions:
Subcortical structures include large collections of nerve cells:
The basal ganglia, which coordinate and smooth out movements
The thalamus, which generally organizes sensory messages to and from the highest levels of the brain (cerebral cortex), providing an awareness of such sensations as pain, touch, and temperature
The hypothalamus, which coordinates some of the more automatic functions of the body, such as control of sleep and wakefulness, maintenance of body temperature, regulation of appetite and thirst, and control of hormonal activity of the adjacent pituitary gland
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. It also contains many of the nerve center collections that control movement of the eye,, face, jaw, and tongue, including chewing and swallowing.
The brain stem also automatically regulates critical body functions, such as breathing, blood pressure, and heartbeat, and it helps adjust posture and balance. 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 awareness, thought, and movement 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 helps to cushion the brain against sudden jarring and minor injury and also to remove waste products from the brain. The cerebrospinal fluid enters the brain along the outside of blood vessels and flows over the surface of the brain between the meninges. The fluid is taken up by support cells (glial cells) and distributed throughout the brain, filling internal spaces within the brain (the four cerebral ventricles). Eventually, the fluid leaves the brain to enter the body's blood vessels. As the cerebrospinal fluid flows through the brain, it removes discarded proteins and other waste products from the brain tissue. This removal process occurs mainly when people are sleeping, which highlights the importance of sleep.
The brain and its meninges are contained in a tough, bony protective structure, the skull. The spinal cord connects to the brain at the base of the brain stem.