The brain has three main divisions: (1) the cerebrum, (2) the cerebellum, and (3) the brain stem. Each part consists chiefly of nerve cells called neurons, and supporting cells called glia. The cerebrum makes up about 85 percent of the weight of the human brain. A large groove called the longitudinal fissure divides the cerebrum into halves called the left cerebral hemisphere and the right cerebral hemisphere. The hemispheres are connected by bundles of nerve fibres, the largest of which is the corpus callosum. Each hemisphere, in turn, is divided into four lobes (regions). Each lobe has the same name as the bone of the skull lies above it. The lobes are: (1) the frontal lobe, at the front; (2) the temporal lobe, at the lower side; (3) the parietal lobe, in the middle; and (4) the occipital lobe, at the rear. Fissures in the cerebral cortex form the boundaries between the lobes. The two major fissures are the central fissure and the lateral fissure. The cerebellum is the part of the brain most responsible for balance, posture, and the coordination of movement. It lies below the back part of the cerebrum. The cerebellum consists of a large mass of closely packed folia. The cerebellum has a right hemisphere and a left hemisphere, with a finger-shaped structure called the vermis in the middle.
The brain stem is a stalk like structure that connects the cerebrum with the spinal cord. The bottom part of the brain stem is called the medulla oblongata or medulla. The medulla has nerve centers that control breathing, heartbeat, and many other body processes essential to life. At the upper end of the brain stem are the hypothalamus and the thalamus. There are actually two thalami, one on the left side of the brain stem and one on the right side. Each thalamus receives nerve impulses from various parts of the body and routes them to the appropriate areas of the cerebral cortex. The thalami also relay impulses from one part of the brain to another. The hypothalamus regulates body temperature, hunger, and other internal conditions. It also controls the activity of the nearby pituitary gland, the master gland of the body.
The endocrine system links the brain to the organs that control body metabolism, growth and development, and reproduction is a control system of ductless endocrine glands that secrete chemical messengers called hormones that circulate within the body via the bloodstream. The limbic system operates by influencing the endocrine system and the autonomic nervous system. The limbic system is highly interconnected with a structure known as the nucleus accumbens, commonly called the brain’s pleasure center. The nucleus accumbens plays a role in sexual arousal and the “high” derived from certain recreational drugs. These responses are heavily modulated by dopaminergic projections from the limbic system. Rats with metal electrodes implanted into their nucleus accumbens will repeatedly press a lever which activates this region, and will do so in preference over food and water, eventually dying from exhaustion.
Injuries, diseases, and inherited disorders can damage the brain. However, the seriousness of brain damage depends chiefly on the area of the brain involved rather than on the cause of the damage. Stroke is the most common serious disorder of the brain. A stroke occurs when the blood supply to part of the brain is cut off. Nerve cells in the affected areas die, and the victim may lose the ability to carry out functions controlled by those areas. Tumors are abnormal growths that can cause severe brain damage. The effects of a tumor depend on its size and location. A tumor may destroy brain cells in the area surrounding it. As the tumor grows, it also creates pressure, which may damage the other areas of the brain or at least interfere with their normal function. A number of diseases caused by bacteria or viruses can damage the brain. The most common of these infectious diseases are encephalitis and meningitis, either of which may be caused by bacteria or viruses. Encephalitis is an inflammation of the brain. Meningitis is an inflammation of the meninges, the membranes that cover the brain and spinal cord. Chorea is a disease of the brain that mainly affects children from 7 to 15 years old. Most cases of chorea occur with rheumatic fever and may be caused by the same bacteria which cause that disease. A virus disease called poliomyelitis attacks the brain and spinal cord.
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