The brain is the part of the central nervous system located in the skull. It controls the mental processes and physical actions of a human being.
The brain, along 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 digestion.
The human brain is a soft, shiny, grayish-white, mushroom-shaped structure. The brain of an average adult weighs about 3 lb (1.4 kg). At birth, the average infant's brain weighs 13.7 oz (390 g); by age 15, it has nearly reached full adult size. The brain is protected by the skull and a three-layered membrane called the meninges. The brain's surface is covered with many bright red arteries and bluish veins that penetrate inward. Glucose, oxygen, and certain ions pass easily across the blood-brain barrier into the brain, although other substances, such as antibiotics, do not.
The four principal sections of the human brain are: the brain stem, the diencephalon, the cerebrum (divided into two large paired cerebral hemispheres), and the cerebellum.
The brain stem connects the brain with the spinal cord. Every message transmitted between the brain and spinal cord passes through the medulla oblongata—a part of the brain stem—via nerve 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 regulates the heartbeat, breathing rate, and blood-vessel diameters; it also helps coordinate swallowing, vomiting, hiccuping, coughing, and sneezing.
Another brain stem component, the pons (meaning "bridge"), conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses between the cerebral cortex, pons, and spinal cord, and 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 lies above the brain stem and embodies the thalamus and hypothalamus. The thalamus is an important relay station for sensory information, interpreting sound, smell, taste, pain, pressure, temperature, and touch; it also regulates some emotions and memory. The hypothalamus controls a number of body functions, such as heartbeat and digestion, and helps regulate the endocrine system (hormonal system) and normal body temperature. The hypothalamus signals hunger and thirst, and also helps regulate sleep, anger, and aggression.
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