The biology of the Brain
Brain is the master control
centre of the body. The brain constantly receives information from the
senses about conditions both inside the body and outside it. The brain
rapidly analyses this information and then sends out messages that
control body functions and actions. The brain also stores information
from past experience, which makes learning and remembering possible. In
addition, the brain is the source of thoughts, moods, and emotions.
In such simple animals as worms and
insects, the brain consists of small groups of nerve cells. All animals with
a backbone have a complicated brain made up of many parts. Animals that have
an exceptionally well-developed brain include apes, dolphins, and whales.
Human beings have the most highly developed brain of all. It consists of
billions of interconnected cells and enables people to use language, solve
difficult problems, and create works of art.
The human brain is a greyish-pink, jellylike ball with many ridges and
grooves on its surface. A newborn baby's brain weighs less than 0.5
kilogram. By the time a person is 6 years old, the brain has reached its
full weight of about 1.4 kilograms. Most of the brain's nerve cells are
present at birth. The increase in weight comes from growth of nerve cells,
development and growth of supporting cells, and development of connections
among cells. During this six-year period, a person learns and acquires new
behaviour patterns at the fastest rate in life.
A network of blood vessels supplies the brain with the vast quantities of
oxygen and food that it requires. The human brain makes up only about 2 per
cent of the total body weight, but it uses about 20 per cent of the oxygen
used by the entire body when at rest. The brain can go without oxygen for
only three to five minutes before serious damage results.
The brain is at the upper end of the spinal cord. The spinal cord is a cable
of nerve cells that extends from the neck about two-thirds of the way down
the backbone. The spinal cord carries messages between the brain and other
parts of the body. In addition, 12 pairs of nerves connect the brain
directly with certain parts of the body. For more information about the
nervous system and the brain's place in it..
The brain works somewhat like both a computer and a chemical factory. Brain
cells produce electrical signals and send them from cell to cell along
pathways called circuits. As in a computer, these circuits receive, process,
store, and retrieve information. Unlike a computer, however, the brain
creates its electrical signals by chemical means. The proper functioning of
the brain depends on many complicated chemical substances produced by brain
Scientists in various fields work together to study the structure, function,
and chemical composition of the brain. This field of study, called
neuroscience or neurobiology, is rapidly increasing our understanding of the
brain. However, much still remains to be learned. Scientists do not yet know
how the physical and chemical processes in the brain produce much of the
This article deals chiefly with the human brain. The last section of the
article discusses the brain in various kinds of animals.
The parts of the brain
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
per cent 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
Each hemisphere, in turn, is
divided into four lobes (regions). Each lobe has the same name as the bone
of the skull that 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.
A thin layer of nerve cell bodies
called the cerebral cortex or cortex forms the outermost part of the
cerebrum. Most of the cerebrum beneath the cortex consists of nerve cell
fibres. Some of these fibres connect parts of the cortex. Others link the
cortex with the cerebellum, brain stem, and spinal cord.
The cerebral cortex is folded into a surface with many ridges and grooves.
This folding greatly increases the surface area of the cortex and the number
of nerve cells it contains within the limited space of the skull. Some areas
of the cortex, called the sensory cortex, receive messages from the sense
organs as well as messages of touch and temperature from throughout the
body. Areas in the frontal lobes called the motor cortex send out nerve
impulses that control the voluntary movements of all the skeletal muscles.
The largest portion of the cortex is the association cortex. Every lobe of
the brain has areas of association cortex that analyse, process, and store
information. These association areas make possible all of our higher mental
abilities, such as thinking, speaking, and remembering.
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 (leaflike bundles of
nerve cells). The cerebellum has a right hemisphere and a left
hemisphere, with a finger-shaped structure called the vermis in the
Nerve pathways connect the right
half of the cerebellum with the left cerebral hemisphere and the right side
of the body. Pathways from the left half connect with the right cerebral
hemisphere and the left side of the body.
The brain stem is a stalklike 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 centres that control breathing,
heartbeat, and many other body processes essential to life.
Just above the medulla is the pons, which connects the hemispheres of the
cerebellum. The pons also contains nerve fibres that link the cerebellum and
the cerebrum. Above the pons lies the lic> Nerve centres in the midbrain
help control movements of the eyes and the size of the pupils.
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.
A network of nerve fibres called the reticular formation lies deep within
the brain stem. The reticular formation helps regulate and maintain the
brain's level of awareness. Sensory messages that pass through the brain
stem stimulate the reticular formation, which in turn stimulates alertness
and activity throughout the cerebral cortex.
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