Brain Health Article

Brain

Part of the central nervous system located in the skull. Controls mental and physical actions of the organism.

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 weight 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 threelayer membrane called the meninges. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. The four principal sections of the human brain are the brain stem, the diencephalon, the cerebrum, and the cerebellum.

The brain stem

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, which also contains visual and audio reflex centers involving 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

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

The cerebrum constitutes nearly 90% of the brain's weight. Specific areas of the cerebrum interpret sensory impulses. For example, spoken and written language 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 functions mainly in 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.