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Reading Exercise 3:
Maintaining Control
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Functions such as breathing, thinking,
reflexes and memory are all controlled
by the central nervous system. It is
comprised of highly specialized cells
called neurones or nerves. These cells
make electrical impulses and transmit
them in a similar way to wire carrying
electricity. The central nervous system
comprises four parts. The brain and
spinal cord are the main pathways by
which an organism's cells communicate. The receptor cells detect a
stimulus, and make up the five
senses—sight, smell, touch, taste and
hearing. The sensory nerves carry
messages from the receptor cells to the
spine and brain. The motor nerves (/)
carry messages to the effectors; these
are glands or muscles that respond to a
stimulus. A reflex, such as flinching at a
pain, is a response to a danger.
Humans have a large brain compared
to other mammals. In an adult the brain
has about 100 billion neurones. Brain
cells cannot be replaced once they are
damaged; this is why brain damage is
generally permanent. The cerebrum is
the movement center of the brain. The
area is highly folded, which increases
the surface area of its cells and therefore the number of messages that it can
process simultaneously. The neurones
(/) do not touch each other and the gap
between them is called the synapse.
Impulses cross the gap via neurotransmitters, which are subsequently
removed by biological catalysts called
enzymes. Some toxins produced by
snakes and spiders act by disrupting
the enzymes. Over fifty neurotransmitters have been identified. Dopamine is
one of them and is responsible for emotional behavior. The brain also produces pain killing neurotransmitters,
such as encephalin and endorphins.
Heroin and morphine are artificial forms
of these neurotransmitters.
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Some drugs act in ways similar to neurotransmitters and are called psychoactive for this reason. Some are
legal, such as tobacco, alcohol and
prescription drugs. Others, such as
heroin and marijuana, are illegal. A
person can come to believe that they
need a particular drug to function and
become psychologically dependent on
a drug. The body itself can become
physically dependent on a drug. This
can occur when the neurotransmitters
in the brain become conditioned to the
presence of the drug and can’t function
correctly if the drug is withdrawn. On
the other hand, the brain may build up
a tolerance to the drug and the user
finds that a larger amount of it is
required to get the same response.
Another of the body's message networks is the endocrine network. The
endocrine glands include the pituitary
glands thyroid gland, pancreas, adrenal
glands, and the gonads. These glands
produce hormones that lock onto the
outside of a target cell and induce a
change in that cell. While the pituitary
gland produces some hormones, its
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main role is to direct the other endocrine
glands. The thyroid gland produces two
hormones involved in the homeostasis of
calcium in the blood. Homeostasis is a
constant level of a substance in the system; calcitonin removes soluble calcium
and stores it in the insoluble form in
bones, while thyroxin removes it from the
bones and returns it to the blood.
Pheromones are the chemical messengers used by organisms of the same
species to attract a mate. While insects
and other mammals are known to use
these, no human equivalent has been
identified.
The regulation of cellular respiration is
controlled by the supply of the simple
sugar glucose. After eating, glucose
levels in the blood rise. The pancreas
excretes insulin to turn the excess glucose into glycogen, which is then
stored by the liver. If glucose is consumed during exercise, the pancreas
releases glucagons to turn glycogen
into glucose again. In this way the pancreas maintains homeostasis of glucose levels in the blood.
The regulation of cellular respiration is
controlled by the supply of the simple
sugar glucose. After eating, glucose
levels in the blood rise. The pancreas
excretes insulin to turn the excess glucose into glycogen, which is then
stored by the liver. If glucose is consumed during exercise, the pancreas
releases glucagons to turn glycogen
into glucose again. In this way the pancreas maintains homeostasis of glucose levels in the blood.
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In general, diabetes is caused by the
destruction of the cells in the pancreas
that produce insulin. Insulin-dependent
diabetics must inject themselves daily
with the hormone and follow a strict diet
and exercise plan. This helps them
avoid hypoglycemia, where the blood
sugar level drops dramatically. The
treatment used to avoid confusion,
shaking or coma is to give the sufferer
some sugar. Prior to 1980, the insulin
used to treat diabetics was extracted
from the pancreases of cows and pigs.
Today, the insulin is produced in bulk
from genetically engineered bacteria.
Other treatments include the implantation of donor pancreases, or the pancreas of a genetically altered pig. The
other option is to inject the cells
responsible for insulin production. If
another type of cell can be induced to
produce insulin, they can be introduced
then into the pancreas to replace the
destroyed cells.
The kidneys are responsible for
removing most of the waste produced
by the body. Urea is the broken down
amino acids produced by the liver. The
kidney treats this to conserve water,
salts, glucose, and complete amino
acids, while removing toxic waste. Kidneys are involved in maintaining the
homeostasis of water and salts in the
body and can produce more concentrate or dilute urine to do so. Illness,
injury, or overuse of some drugs can
cause kidney failure. While a human
can survive with the use of one fully
functioning kidney, anything less would
require the person to undergo dialysis
two to four times a week. Dialysis is
when a machine is used to perform the
same function as the kidneys. It takes
approximately four hours, and most
people on dialysis live in hope of a kidney transplant.
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Reptiles and insects are ectodermic—
the external environment regulates their
body temperature. Mammals, birds and
fish are endotherms, maintaining a stable body temperature through internal
metabolism. The human body needs to
stay around 37C, and hypothermia will
result if the temperature drops dramatically. On the other hand, an elevated
body temperature causes hyperthermia. The body cools itself by sweating,
or through vasodilatation. The blood
vessels close to the skin expand to
increase the volume of blood flowing
through and so heat is lost to the environment. Heat is generated through
shivering, vasorestriction or through
goose bumps. Goose bumps raise the
hair on the skin, trapping warm air
between fur and skin. On relatively
hairless humans, this contributes little
to heat generation.
Plants also respond to stimuli, but in a
simpler fashion. These responses are
called tropisms. Phototropism is the
response to light—this helps the plant
to measure the length of the day, and
hence flower or die at the correct time
of year. Gravitropism is the response to
gravity—when a seed germinates it
needs to point its roots down and its
shoots up. The hormone responsible
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for this is called auxin. For example, it
causes cells on the dark side of the
plant to elongate, so bending the plant
to the light source. A plant will remove
all food from a leaf, and transfer waste
there before releasing a growthinhibiting hormone to cause the leaf to
drop off. The gas ethylene is involved in
the ripening of fruit, so fruit can be
picked before it ripens and is caused to
do so at a more desirable time.
(bbc.co.uk)
Total number of words in 1 minute: _______