A Stimulus is a change in your
environment. It is something that you have
to react to.
Your sense organs
are:
Eyes, ears, nose, tongue
and skin.
All of these sense organs contain different
receptors. Receptors are groups of cells which
are sensitive to a stimulus. A stimulus could be;
light, sound, touch, pressure, pain, chemical or
a change in position or temperature.
The sense organs and the
receptors they contain:
Eyes: Light Receptors
Ears: Sound and "balance"
receptors.
Nose: Smell receptors-
sensitive to chemical stimuli.
Tongue: Taste receptors-
chemical stimuli- sensitive to
bitter, salt, sweet and sour.
Skin: Sensitive to touch, pressure
and temperature changes.
Your central nervous
system (CNS) is where all
the information from the
sense organs is sent, and
where your actions and
reflexes are coordinated.
The central nervous
system consists of your
brain and spinal cord only.
Neurones (nerve cells) transmit the
information- as electrical impulses- very
quickly to and from the CNS.
"Instructions" from
the CNS are then
sent to the effectors
(muscles and glands
which respond
accordingly.
Sensory Neurones- These are nerve
cells that carry signals as electrical
impulses from the receptors in the
sense organs to the CNS.
Motor Neurones- These are the nerve
cells that carry signals to the effector
muscles or glands.
Effectors- Muscles and glands
are known as effectors- these will
respond in different ways.
Muscles contract in response to a
nervous impulse, whereas your
glands secrete hormones.
Reflexes help you to prevent injury.
Reflexes are automatic responses to a certain stimuli. E.g. if
someone shines a bright light in your eyes, your pupils automatically
become smaller so that less light goes into your eye- which in turn
stops them becoming damaged.
The passage of information in a reflex (from your receptor to
effector) is called a reflex arc.
The reflex arc passes through your CNS. The neurones in reflex
arcs go through the spinal cord or through an unconscious part of
the brain.
When a stimulus is detected by
receptors an impulse is sent along
a sensory neurone to the CNS.
When in the CNS the sensory
neurone is passes on the
message to another type of
neurone- a relay neurone.
A relay neurone relays the impulse to a motor
neurone.
The impulse then travels along the motor neurone to
the effector. The muscle then contracts and moves
hyour hand away from the problem (e.g.a bee).
Synapses connect neurones- The connection between two
neurones is called a synapses.
The nerve signal is transferred by chemicals which diffuse
(move) across the gap.
These chemicals then set off a new electrical signal in the next neurone.
Hormones are chemical messengers sent in the blood.
Hormones are carried in the blood plasma to
other parts of the body, but they only affect
particular cells;(these are called target cells).
Hormones control things in organs and cells
that need constant adjustment.
Hormones are produced
in various glands. They
travel trough your body at
'the speed of blood'.
Hormones have long-
lasting effects.
The Pituitary Gland.
This produces many important
hormones- including LH, FSH
and ADH. ADH controls water
content.
The Ovaries- (females only)
Produces oestrogen- this
controls the menstrual cycle and
promotes all female secondary
sexual characteristics during
puberty.
Pancreas
Produces insulin for the control of
blood sugar.
Testes- (male only)
Produces testosterone which
promotes all male secondary
sexual characteristics at puberty-
e.g. changes in body proportions.
Hormones are chemical
messengers which travel in the
blood to activate target cells.
Hormones and nerves do similar jobs however
there are differences between the two.
Nerves: very FAST
messages, act for a very
SHORT TIME, act on a very
PRECISE AREA.
Hormones: SLOWER message,
act for a LONG TIME, act in a
more GENERAL way.
The Menstrual Cycle
Stage 1: When the
bleeding starts. - the
uterus lining breaks down.
Stage 2: The lining of the womb builds
up again.- Ready to receive a fertilized
egg.
Stage 3: An egg is developed and
then released from the ovary at
around day 14.
Stage 4 The wall in the
maintained for about 14 days,
until day 28.
Hormones control the different stages.
FSH (Follicle- Stimulating Hormone)-
This is produced by the pituitary
gland. It causes an egg to mature in
one of the ovaries and stimulates the
ovaries to produce oestrogen.
Oestrogen: produced in the ovaries. This causes the
pituitary gland to produce LH (Lutenising Hormone).
It inhibits the further release of FSH.
LH (Luteinising Hormone): This is produced by the
pituitary gland. It stimulates the release of an egg
at around the middle of the menstrual cycle.
Controlling Fertility.
Hormones can be used to increase or reduce fertility.
Oestrogen can be used to prevent the release of an
egg. The pill is an oral contraceptive that contains
oestrogen.
If oestrogen is taken every day to keep the
level of it permanently high- it will inhibit the
production of FSH. This means that after a
while egg development and production stops
and will stay stopped.
Advantages:
The pill is over 99% effective at preventing pregnancy.
It reduces the risk of getting some types of cancer.
Disadvantages:
It isn't 100% effective- there's
still a slight chance of getting
pregnant.
It can cause side effects like
headaches, nausea, fluid
retention and irregular
menstrual cycle.
It doesn't protect against sexually
transmitted diseases (STD's).
Some women have levels of FSH
that are too low to cause their eggs to
mature. This means that no eggs are
released and the women can't get
pregnant.
The hormone FSH can be taken by
these women to stimulate egg
production in their ovaries.
Advantages:
It helps a lot of women to get pregnant.
Disadvantages:
It doesn't always work- some
women may have to do it many
times, which can be expensive.
Too many eggs could be
stimulated, resulting in
unexpected multiple
pregnancies (twins, triplets
etc.)
IVF (In-vitro fertilization)
involves collecting eggs from
the woman's ovaries and
fertilizing them in a lab using the
man's perm. These are then
grown into embryos, which are
transferred to the woman's
uterus.
Hormones are given before egg collection to
stimulate egg production.
Oestrogen and progesterone are often
given to make implementation of the
embryo into the uterus more likely to
succeed.
But the use of hormones in IVF can cause problems for some women:
Some women have a very
strong reaction to the
hormones- including
abdominal pain, vomiting
and dehydration.
There have been some
reports of an increased
risk of cancer due to the
hormonal treatment.