Brain

The brain, with the spinal cord and network of nerves, controls information flow throughout the body; voluntary actions, such as walking, reading, and talking; and involuntary reactions, such as breathing and heartbeat. The human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, the brain of an average adult weighs about 3 lb. (1.4 kg). At birth, the average human infant's brain weighs 13.7 oz. (390 g); by age 15, the brain has nearly reached full adult size. The brain is protected by the skull and by a three-layer membrane called the meninges. Many bright red arteries and bluish veins on the surface of the brain make their way inward. The veins carry oxygen and other substances to the brain, but some drugs and substances do not pass into the brain. The four main sections of the human brain are the brain stem, the diencephalon, the cerebrum, and the cerebellum.

The brain stem connects the brain with the spinal cord. All the messages that are transmitted between the brain and spinal cord pass through the medulla—a part of the brain stem—via fibers. The fibers on the right side of the medulla cross to the left and those on the left cross to the right. As a result, each side of the brain controls the opposite side of the body. The medulla also controls the heartbeat, the rate of breathing, and the diameter of the blood vessels and helps to coordinate swallowing, vomiting, hiccupping, coughing, and sneezing. Another component of the brain stem is the pons (meaning bridge). It conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. Conveying impulses between the cerebral cortex, the pons, and the spinal cord is a section of the brain stem known as the midbrain. The midbrain also contains reflex centers involved in the movement of the eyeballs and head.

Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Among these cranial nerves are the olfactory nerves that bring messages about smell, and the optic nerves that conduct visual information.

The diencephalon lies above the brain stem and embodies the thalamus and hypothalamus. The thalamus is an important relay station for sensory information, interpreting sensations of sound, smell, taste, pain, pressure, temperature, and touch; the thalamus also regulates some emotions and memory. The hypothalamus controls a number of body functions, such as heartbeat rate and digestion, and helps regulate the endocrine system and normal body temperature. The hypothalamus interprets hunger and thirst, and it helps regulate sleep, anger, and aggression.

The cerebrum makes up nearly 90% of the brain's weight. Specific areas of the cerebrum interpret sensory impulses. For example, spoken and written languages are transmitted to a part of the cerebrum called Wernicke's area, where meaning is extracted. Motor areas of the cerebrum control muscle movements. Broca's area translates thoughts into speech, and coordinates the muscles needed for speaking. Impulses from other motor areas direct hand muscles for writing and eye muscles for physical movement necessary for reading. The cerebrum is divided into two hemispheres— left and right. In general, the left half of the brain controls the right side of the body, and vice versa. For most right-handed people (and many left-handed people as well), the left half of the brain is dominant. By studying patients whose corpus callosum had been destroyed, scientists realized that differences existed between the left and right sides of the cerebral cortex. The left side of the brain mainly controls functions such as speech, logic, writing, and arithmetic. The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.

The cerebrum's outer layer, the cerebral cortex, is composed of gray matter made up of nerve cell bodies. The cerebral cortex is about 0.08 in. (2 mm) thick and its surface area is about 5 sq ft. (0.5 sq m)—around half the size of an office desk. White matter, composed of nerve fibers covered with myelin sheaths, lies beneath the gray matter. During embryonic development, the gray matter grows faster than the white matter and folds on itself, giving the brain its characteristic wrinkly appearance. The folds are called convolutions or gyri, and the grooves between them are known as sulci.

A deep fissure separates the cerebrum into a left and right hemisphere, with the corpus callosum, a large bundle of fibers, connecting the two.

Researchers have discovered that neurons carry information through the nervous system in the form of brief electrical impulses called action potentials. When an impulse reaches the end of an axon, neurotransmitters are released at junctions called synapses. The neurotransmitters are chemicals that bind to receptors on the receiving neurons, triggering the impulse to continue. More than 100 different neurotransmitters have been discovered since the first one was identified in 1920. By studying the chemical effects of neurotransmitters in the brain, scientists are developing treatments for mental disorders and are learning more about how drugs affect the brain.

Scientists once believed that brain cells do not regenerate, thereby making brain injuries and brain diseases untreatable. Since the late 1990s, researchers have been testing treatment for such patients with neuron transplants, introducing nerve tissue into the brain. For example, scientists at Harvard University were able to transplant human-derived stem cells into the brains of mice in 2014 to help control seizures in the mice.

Technology provides useful tools for researching the brain and helping patients with brain disorders. An electroencephalogram (EEG) is a record of brain waves, electrical activity generated in the brain. An EEG is obtained by positioning electrodes on the head and amplifying the waves with an electroencephalograph; it is valuable in diagnosing brain diseases such as epilepsy and tumors.

Scientists use other techniques to study and understand the brain and diagnose disorders. Magnetic resonance imaging (MRI) uses a high-powered magnet and radio frequency waves to display the living brain on a computer screen in high detail. Adding a contrast agent can also show blood flow and some brain function. Positron emission tomography (PET) results in color images of the brain displayed on the screen of a monitor. During this test, a technologist injects a small amount of a substance, such as glucose, that is marked with a radioactive tag. The marked substance shows where glucose gathers in the brain. PET is used to study the chemistry and activity of the normal brain and to diagnose abnormalities such as tumors. Magnetoencephalography (MEG) measures the electromagnetic fields created between neurons as electrochemical information is passed along.

Using tools such as functional MRI and MEG, physicians and scientists can look into the brain without using surgery. Research on brain activity might be used to help people with physical problems related to the brain and central nervous system, such as spinal cord injuries, and to help people with learning and emotion. Cognitive psychologists study how people gain, interpret, process, and store information. They use the information to improve education and help people who have problems such as speech disorders or Alzheimer's disease.