The brain is guarded by several highly developed
protective mechanisms. The bony cranium, the surrounding meninges, and the
cerebrospinal fluid all contribute to the mechanical protection of the
brain. In addition, a filtration system called the blood-brain barrier
protects the brain from exposure to potentially harmful substances carried
in the bloodstream.
Brain disorders have a wide range of causes, including head injury, stroke,
bacterial diseases, complex chemical imbalances, and changes associated with
aging.
Head injury can initiate a cascade of damaging
events.
After a blow to the head, a person may be stunned or may become unconscious
for a moment. This injury, called a concussion, usually leaves no permanent
damage. If the blow is more severe and hemorrhage (excessive bleeding) and
swelling occur, however, severe headache, dizziness, paralysis, a
convulsion, or temporary blindness may result, depending on the area of the
brain affected. Damage to the cerebrum can also result in profound
personality changes.
Damage to Broca's area in the frontal lobe causes difficulty in speaking and
writing, a problem known as Broca's aphasia. Injury to Wernicke's area in
the left temporal lobe results in an inability to comprehend spoken
language, called Wernicke's aphasia.
An injury or disturbance to a part of the hypothalamus may cause a variety
of different symptoms, such as loss of appetite with an extreme drop in body
weight; increase in appetite leading to obesity; extraordinary thirst with
excessive urination (diabetes insipidus); failure in body-temperature
control, resulting in either low temperature (hypothermia) or high
temperature (fever); excessive emotionality; and uncontrolled anger or
aggression. If the relationship between the hypothalamus and the pituitary
gland is damaged (see Endocrine System), other vital bodily functions may be
disturbed, such as sexual function, metabolism, and cardiovascular activity.
Injury to the brain stem is even more serious because it houses the nerve
centers that control breathing and heart action. Damage to the medulla
oblongata usually results in immediate death.
A stroke is damage to the brain due to an
interruption in blood flow. The interruption may be caused by a blood clot
(see Embolism; Thrombosis), constriction of a blood vessel, or rupture of a
vessel accompanied by bleeding. A pouchlike expansion of the wall of a blood
vessel, called an aneurysm (see Artery), may weaken and burst, for example,
because of high blood pressure.
Sufficient quantities of glucose and oxygen, transported through the
bloodstream, are needed to keep nerve cells alive. When the blood supply to
a small part of the brain is interrupted, the cells in that area die and the
function of the area is lost. A massive stroke can cause a one-sided
paralysis (hemiplegia) and sensory loss on the side of the body opposite the
hemisphere damaged by the stroke. For detailed information regarding stroke
go here.
Epilepsy is a broad term for a variety of brain
disorders characterized by seizures, or convulsions. Epilepsy can result
from a direct injury to the brain at birth or from a metabolic disturbance
in the brain at any time later in life.
Some brain diseases, such as multiple sclerosis and Parkinson disease, are
progressive, becoming worse over time. Multiple sclerosis damages the myelin
sheath around axons in the brain and spinal cord. As a result, the affected
axons cannot transmit nerve impulses properly. Parkinson disease destroys
the cells of the substantia nigra in the midbrain, resulting in a deficiency
in the neurotransmitter dopamine that affects motor functions. For detailed
information regarding epilepsy go
here.
Cerebral palsy is a broad term for brain damage
sustained close to birth that permanently affects motor function. The damage
may take place either in the developing fetus, during birth, or just after
birth and is the result of the faulty development or breaking down of motor
pathways. Cerebral palsy is nonprogressive—that is, it does not worsen with
time.
A bacterial infection in the cerebrum, encephalitis. or in the coverings of
the brain, meningitis, swelling of the brain, edema, or an abnormal growth
of healthy brain tissue, tumor, can all cause an increase in intracranial
pressure and result in serious damage to the brain.
Scientists are finding that certain brain chemical imbalances are associated
with mental disorders such as schizophrenia and depression. Such findings
have changed scientific understanding of mental health and have resulted in
new treatments that chemically correct these imbalances. It is important to
note however, that drugs are not the solution in balancing these chemical
imbalances. For more information regarding mental illness go
here.
During childhood development, the brain is particularly susceptible to
damage because of the rapid growth and reorganization of nerve connections.
Problems that originate in the immature brain can appear as epilepsy or
other brain- function problems in adulthood.
Several neurological problems are common in aging. Alzheimer's disease
damages many areas of the brain, including the frontal, temporal, and
parietal lobes. The brain tissue of people with Alzheimer's disease shows
characteristic patterns of damaged neurons, known as plaques and tangles.
Alzheimer's disease produces a progressive dementia (see Senile Dementia),
characterized by symptoms such as failing attention and memory, loss of
mathematical ability, irritability, and poor orientation in space and time.
For detailed information regarding Alzheimer's disease go
{alzheimer} here.
Several commonly used diagnostic methods give images
of the brain without invading the skull. Some portray anatomy—that is, the
structure of the brain—whereas others measure brain function. Two or more
methods may be used to complement each other, together providing a more
complete picture than would be possible by one method alone.
Magnetic resonance imaging (MRI), introduced in the early 1980s, beams
high-frequency radio waves into the brain in a highly magnetized field that
causes the protons that form the nuclei of hydrogen atoms in the brain to
reemit the radio waves. The reemitted radio waves are analyzed by computer
to create thin cross-sectional images of the brain. MRI provides the most
detailed images of the brain and is safer than imaging methods that use X
rays. However, MRI is a lengthy process and also cannot be used with people
who have pacemakers or metal implants, both of which are adversely affected
by the magnetic field. For detailed information regarding MRIs go
here.
Computed tomography (CT), also known as CT scans, developed in the early
1970s. This imaging method X-rays the brain from many different angles,
feeding the information into a computer that produces a series of
cross-sectional images. CT is particularly useful for diagnosing blood clots
and brain tumors. It is a much quicker process than magnetic resonance
imaging and is therefore advantageous in certain situations—for example,
with people who are extremely ill. For detailed information regarding CT
scans go here.
Changes in brain function due to brain disorders can be visualized in
several ways. Magnetic resonance spectroscopy measures the concentration of
specific chemical compounds in the brain that may change during specific
behaviors. Functional magnetic resonance imaging (fMRI) maps changes in
oxygen concentration that correspond to nerve cell activity.
Positron emission tomography (PET), developed in the mid-1970s, uses
computed tomography to visualize radioactive tracers (see Isotopic Tracer),
radioactive substances introduced into the brain intravenously or by
inhalation. PET can measure such brain functions as cerebral metabolism,
blood flow and volume, oxygen use, and the formation of neurotransmitters.
Single photon emission computed tomography (SPECT), developed in the 1950s
and 1960s, uses radioactive tracers to visualize the circulation and volume
of blood in the brain.
Brain-imaging studies have provided new insights into sensory, motor,
language, and memory processes, as well as brain disorders such as epilepsy;
cerebrovascular disease; Alzheimer's, Parkinson, and Huntington's diseases;
and various mental disorders, such as
{schizo} schizophrenia.
In lower vertebrates, such as fish and reptiles, the
brain is often tubular and bears a striking resemblance to the early
embryonic stages of the brains of more highly evolved animals. In all
vertebrates, the brain is divided into three regions: the forebrain
(prosencephalon), the midbrain (mesencephalon), and the hindbrain
(rhombencephalon). These three regions further subdivide into different
structures, systems, nuclei, and layers.
The more highly evolved the animal, the more complex is the brain structure.
Human beings have the most complex brains of all animals. Evolutionary
forces have also resulted in a progressive increase in the size of the
brain. In vertebrates lower than mammals, the brain is small. In meat-eating
animals, particularly primates, the brain increases dramatically in size.
The cerebrum and cerebellum of higher mammals are highly convoluted in order
to fit the most gray matter surface within the confines of the cranium. Such
highly convoluted brains are called gyrencephalic. Many lower mammals have a
smooth, or lissencephalic ("smooth head"), cortical surface.
There is also evidence of evolutionary adaption of the brain. For example,
many birds depend on an advanced visual system to identify food at great
distances while in flight. Consequently, their optic lobes and cerebellum
are well developed, giving them keen sight and outstanding motor
coordination in flight. Rodents, on the other hand, as nocturnal animals, do
not have a well-developed visual system. Instead, they rely more heavily on
other sensory systems, such as a highly developed sense of smell and facial
whiskers.
Recent research in brain function suggests that
there may be sexual differences in both brain anatomy and brain function.
One study indicated that men and women may use their brains differently
while thinking. Researchers used functional magnetic resonance imaging to
observe which parts of the brain were activated as groups of men and women
tried to determine whether sets of nonsense words rhymed. Men used only
Broca's area in this task, whereas women used Broca's area plus an area on
the right side of the brain.
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