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Human Brain

What is the Brain?

The brain is the portion of the central nervous system in vertebrates (animals with bones) that lies within the skull. In humans, the brain weighs about 3 pounds. Differences in weight and size do not correlate with differences in mental ability.

The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survive.

The brain is a pinkish-gray mass that is composed of about 10 billion nerve cells. The nerve cells are linked to each other and together are responsible for the control of all mental functions. Nerve fibers in the brain are covered in a near-white substance called myelin and form the white matter of the brain. Nerve cell bodies, which are not covered by myelin sheaths, form the gray matter.

The entire brain is enveloped in three protective sheets known as the meninges, continuations of the membranes that wrap the spinal cord. The two inner sheets enclose a shock-absorbing cushion of cerebrospinal fluid. Nerve fibers in the brain are covered in a near-white substance called myelin and form the white matter of the brain. Nerve cell bodies, which are not covered by myelin sheaths, form the gray matter.

The brain is divided into three major parts, the hindbrain (including the cerebellum and the brain stem), the midbrain, and the forebrain (including the diencephalon and the cerebrum).

The brain is a delicate organ that must be protected. When participating in activities such as skiing or snowboarding, wear a helmet to help reduce brain injuries. Also, try to avoid heat stroke because the brain can only function in a very narrow temperature range.

What is the brain's major function?

Each area of the brain has an associated function, although many functions may involve a number of different areas.

The cerebellum is the hind part of the brain. It is made up of gray, unmyelinated cells on the exterior and white, myelinated cells in the interior. The cerebellum coordinates muscular movements and, along with the midbrain, monitors posture. It is essential to the control of movement of the human body in space. The brain stem, which incorporates the medulla and the pons, monitors involuntary activities such as breathing and vomiting.

The thalamus, which forms the major part of the diencephalon, receives incoming sensory impulses and routes them to the appropriate higher centers. The hypothalamus, occupying the rest of the diencephalon, regulates heartbeat, body temperature, and fluid balance. Above the thalamus extends the corpus callosum, a neuron-rich membrane connecting the two hemispheres of the cerebrum.

The cerebrum occupies the topmost portion of the skull. It is by far the largest part of the brain. It makes up about 85% of the brain's weight. The cerebrum is split vertically into left and right hemispheres, it appears deeply fissured and grooved. Its upper surface, the cerebral cortex, contains most of the master controls of the body. In the cerebral cortex ultimate analysis of sensory data occurs, and motor impulses originate that initiate, reinforce, or inhibit the entire spectrum of muscle and gland activity. The left half of the cerebrum controls the right side of the body; the right half controls the left side. (This explains why if a stroke occurs in the left half of the brain, the right side of the body is affected)

Other important parts of the brain are the pituitary gland, the basal ganglia, and the reticular activating system (RAS). The pituitary participates in growth regulation. The basal ganglia, located just above the diencephalon in each cerebral hemisphere, handle coordination and habitual but acquired skills like chewing and playing the piano. The RAS forms a special system of nerve cells linking the medulla, pons, midbrain, and cerebral cortex. The RAS functions as a sentry. In a noisy crowd, for example, the RAS alerts a person when a friend speaks and enables that person to ignore other sounds.

Diseases and Conditions of the Brain

Stroke, meningitis, multiple sclerosis, coma, paralytic polio, Parkinson's disease, Leu Gehrig's Disease, Cerebral Palsy, and migraine headaches are all diseases and conditions that affect the brain.

A stroke is damage to the brain due either to blockage in blood flow or to loss of blood from blood vessels in the brain.

Meningitis is an inflammation of the meninges (membranes covering the brain).

Multiple sclerosis affects transmission of electrical signals to nerve cells.

Coma is an extended period of unconsciousness from which a person cannot be aroused even with the most painful stimuli.

Paralytic polio, the polio virus invades the central nervous system -- the spinal cord and the brain.

Parkinson's Disease is a disorder of the brain characterized by shaking and difficulty with walking, movement, and coordination.

Cerebral palsy is a group of disorders are caused by injuries to the brain that occur during fetal development or near the time of birth.

A migraine headache is a recurrent, throbbing headache usually felt on one side of the head.

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