The document outlines the roles of the nervous and hormonal systems in responding to environmental changes, highlighting the functions of receptors, neurones, and reflex actions. It explains the components of the human eye and their functions, as well as hormonal control in processes like the menstrual cycle and fertility treatments. Additionally, it discusses plant responses to stimuli, emphasizing tropisms and the influence of plant hormones.

1. Biology B1.2
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2. Nerves & Hormones
The nervous system allows the body to respond to changes in the environment. This is a process
usually co-ordinated by the brain. Reflex actions are extra-rapid responses to stimuli, and this process
also involves the nervous system, but bypasses the brain.
Receptors:
Receptors are groups of specialised cells. They can detect changes in the environment, which are
called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs,
such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.
Receptors & Effectors
Biology - B1.2
Sense Organs: Stimulus:
Skin Touch, Pressure, Pain, Temperature
Tongue Chemicals In Food
Nose Chemicals In The Air
Eyes Light
Ears Sound & Position Of The Head
The central nervous system –
CNS- in humans consists of
the brain and spinal cord.
When a receptor is
stimulated, it sends a signal
along the nerve cells –
neurons – to the brain. The
brain the co-ordinates the
response.
Effectors:
An effector is any part of the body that produces the response. Here are some examples of effectors:
- A Muscle Contracting To Move The Arm.
- A Muscle Squeezing Saliva From The Salivary Gland.
- A Gland Releasing A Hormone Into The Blood.

3. Neurones
Neurons are nerve cells. They carry information as tiny electrical signals. There are three different
types of neurones, each with a slightly different function.
1. Sensory Neurones - Carry Signals From Receptors To The Spinal Cord And Brain.
2. Relay Neurones - Carry Messages From One Part Of The CNS To Another.
3. Motor Neurones - Carry Signals From The CNS To Effectors.
The diagram below shows a typical neurone – in this case, a motor neurone. It has tiny branches at
each end and long fibre carries the signals.
A Motor Neurone
Synapses:
Where two neurones meet, there is a tiny gap called a Synapses, Signals cross this gap using
chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and
makes the next neurone transmit and electrical signal.
Biology - B1.2

4. Reflex Actions
When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-
ordinates the response. But sometimes a very quick response is needed, One that does not need the
involvement of the brain. This is a reflex action.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away
from a hot flame without thinking about it. The animation below allows you to step through each stage of
the reflex arc.
c c
c
c c c
c
c
c c
cc
c
1. Stimulus
(Heat)
2. Pain Receptor
Stimulated
c c
3. Signal Sent Along
Sensory Neuron
5. Signal Sent
Along Motor Neuron
c 6. Effector Muscle
Contracts (Response)
cc
4. Signal Passed
Along Relay Neuron
(Message Sent To
Brain)
c
Biology - B1.2

5. This is what happens:
1. Receptor Detects A Stimulus – A Change In Environment.
2. Sensory Neurone Sends Impulses To Relay Neurone.
3. Motor Neurone Sends Impulses To Effector.
4. Effector Produces A Response. – Muscle Contracts To Move Hand Away.
The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex
arc happens if we accidentally touch something hot.
A reflex action is a way for the body to automatically and rapidly respond to a stimulus to minimise any
further damage to the body. It follows this general sequence and does not involve the brain:
Stimulus Receptor
Sensory
Neurone
Relay
Neurone
Motor
Neurone
Effector Response

6. The Eye
The eye is a sense organ that responds to light.
Structure: Function:
Comea Refracts Light – Bend It As It Enters The Eye
Iris Controls How Much Light Enters The Pupil
Lens Focuses Light Onto The Retina
Retina Contains The Light Receptors
Optic Nerve Carries Impulses Between The Eye And The Brain
Biology - B1.2

7. The Retina
Light passes through the eyeball to the retina.
There are two main types of light receptors – rods
and cones. Rods are more sensitive to light than
cones. There are three different types of cone
cells which produce colour vision.
A photograph of the human retina seen through
the eye.
Rods: Cones:
Number In The Retina 12 Million 7 Million
Where Concentrated Outer Edges Of The Retina The Fovea
Works Best In Dim Light Bright Light
Accommodation:
The ability of the lens to change it shape to focus near and distant objects is called ‘Accommodation’.
Position Of
Object
Ciliary Muscles Suspensory
Ligaments
Muscle Tension
On Lens
Lens Shape
Near Contract Slackened Low Fat
Distant Relax Stretched High Thin
Biology - B1.2

8. The Pupil Reflex
The amount of light entering the eye is controlled by a reflex action. The size of the pupil changes in
response to bright or dim light. This is controlled by the muscles of the iris.
The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex
action:
In Dim Light:
- Radial Muscles Of The Iris Contract.
- Circular Muscles Of The Iris Relax.
- More Light Enters The Eye Through The
Dilated Pupil.
In Bright Light:
- Radial Muscles Of The Iris Relax.
- Circular Muscles Of The Iris Contract.
- Less Light Enters The Eye Through The
Contracted Pupil.
Biology - B1.2

9. Antagonistic Pairs
Muscles work in antagonistic pairs. This ensure that when apart of the body is moved, it can move back
to its original position. Examples are:
- The Biceps And Triceps In The Arm.
- The Quadriceps And Hamstring In The Leg.
The slideshow explains how the lower arm is moved upwards (flexed) when the biceps muscle
contracts and the triceps muscle is relaxed. It is moved downwards (extended) when the triceps is
contracted and the biceps is relaxed.
The Synapse:
Where two neurones meet there is a tiny gap called a ‘Synapse’. Information crosses this gap using
neurotransmitters, rather than using electrical impulses. One neurone releases neurotransmitters into
the synapse. These diffuse across the gap and make the other neurone transmit electrical impulses.
Biology - B1.2

10. Control In The Human Body And In Plants
A hormone is chemical substance, produced by a gland and carried by the blood, which alters the
activity of more specific target organs (and is then destroyed by the liver). Like the nervous system,
hormones can control the body.
There are important differences between the two systems as described in the table.
What It Is
Biology - B1.2
Nervous: Hormonal:
Type Of Signal: Electrical (Chemical At
Synapses)
Chemical
Transmission Of Signal: By Nerve Cells By Nerve Cells By The
Bloodstream
Effectors: Muscles Or Glands Target Cells In Particular
Tissue
Type Of Response: Muscle Contraction Or
Secretion
Chemical Change
Speed Of Response: Very Rapid Slower
Duration Of Response: Short (Until Nerve Impulses
Stop)
Long (Until Hormone Breaks
Down)

11. Control In The Human Body
Hormones are chemical substances that regulate processes in the body. Hormones are secreted by
glands and travel to their target organs in the blood stream. Several hormones are involved in the
female menstrual cycle. Hormones can be used to control human fertility and have advantages and
disadvantages.
Control Of Internal Conditions:
The internal environment of the body is controlled by the nervous system and hormones. The
maintenance of a constant internal environment is called homeostasis. Below are some of the internal
conditions that need to be controlled.
Water Content In The Body:
Water in the body has to be controlled to protect cells from either too much water entering them or too
much water leaving them. Water content is controlled by water loss from:
- The Lungs = When We Exhale
- The Skin = Through Sweating
- Passing Urine = Produced By The Kidneys.
Ion (Salt) Content In The
Body:
Ion levels are controlled to
protect cells from too much
water entering or leaving them.
Ion content is controlled by the
loss of ions from:
- The Skin = Through
Sweating.
- Passing Urine = Produces
By The Kidneys.
Temperature Of The Body;
This is controlled to maintain
the temperature at which
enzymes work best. Body
temperature is controlled by
sweating, shivering and
controlling blood flow to the
skin.
Blood Sugar Levels:
This is controlled to provide
cells with a constant supply of
energy. Blood sugar level is
controlled by the release and
storage of glucose controlled by
insulin.
Biology - B1.2

12. Water Content In The Body:
Too Much Water Too Little Water
Biology - B1.2

13. Hormones In The Human Body
Hormones are chemical secreted by glands in the body. Different hormones affect different target
organs. The bloodstream usually transports hormones from the glands to their target organ. Hormones
regulate the functions of many cells and organs.
Biology - B1.2

14. Hormones In The Menstrual Cycle
The menstrual cycle in women is a recurring monthly process in which the lining of the uterus (womb) is
prepared for pregnancy. If pregnancy does not happen, the lining is shed at menstruation.
Several hormones control this cycle, which includes controlling the release of an egg each month from
an ovary, and changing the thickness of the uterus lining. These hormones are secreted by the ovaries
and pituitary gland.
Follicle Stimulating Hormone (FHS):
The hormone FSH is secreted by the pituitary gland. FHS makes two things happen:
- It causes an egg to mature in an ovary.
- It stimulates the ovaries to release the hormone oestrogen.
Oestrogen:
The hormone oestrogen is secreted by the
ovaries. Oestrogen makes two thing happen:
- It stop FHS being produced = So that only
one egg matures in a cycle.
- It stimulates the pituitary gland to release
luteinizing hormone (LH), which triggers
ovulation (the release of the mature egg
from the ovary).
How the level of oestrogen changes during the
menstrual cycle.
Luteinising Hormone (LH):
This photo shows how the levels of FSH and
LH change during the menstrual.
Biology - B1.2

15. Oral Contraceptives
Human fertility is controlled by hormones. This means that knowledge of hormones can be used to
decide to increase, or reduce, the chances of fertilisation and pregnancy.
Contraceptives:
The oral contraceptive, commonly known as the
pill, greatly reduces the chances of mature eggs
being produced. It contains oestrogen and
progesterone (another hormone). These
hormones inhibit the production of FSH, which in
turn stops eggs maturing in the ovaries.
Benefits And Risks:
Oral contraceptives allow couples to chose the
time they start a family, and choose the time they
stop having children.
The first birth-control pills contained higher
amount of oestrogen than the pills taken today.
This caused women to have significant side
effects, such as changes in weight, mood and
blood pressure. Modern birth-control pills contain
much less oestrogen. Some only contain
progesterone, which also leads o fewer side
effects.
Biology - B1.2

16. Fertility Treatments
Some women have difficulty becoming pregnant because they don’t produce enough FSH to allow their
eggs to mature. Fertility drugs contain FSH and LH, which stimulate eggs to mature in the ovary.
Fertility treatments increase a woman’s chance of becoming pregnant, although the treatment may not
always work. On the other hand, because the treatment boosts the production of mature eggs, multiple
conceptions sometimes occur, with twins or triplets being expected. This increases the risk of
complications in pregnancy and childbirth, and may lead to premature or underweight babies.
In Vitro Fertilisation (IVF):
If a couple are having difficulty
conceiving a child because the
quantity or quality of the mans
sperm is poor then IVF can be
used. This is where the egg Is
fertilised outside the women’s
body and then implanted back into
her uterus. As FSH can also be
used to encourage the production
of several mature eggs at once, it is
used as part of IVF to increase the
number od egg available for
fertilisation.
Some people worry about the ethical implications of IVF. They are
concerned that couples may want ‘designer babies’ with ‘desirable’
qualities, so may only want certain fertilised eggs. For example, they
may want a girl if they have lots of boys in the family, or they may
wish to avoid producing a baby with an inherited defect,
Biology - B1.2

17. Control In Plants
Plants produce hormones and respond to external stimuli, growing towards sources of water and light,
which they need to survive.
A tropism is a growth in response to a stimulus and an auxin is a plant hormone produces in the stem
tips and roots, which controls the direction of growth. Plant hormones are used in weed killers, rooting
powder and to control fruit ripening.
Sensitivity In Plants:
Plants need light and water for photosynthesis. Plant responses – called tropisms – help make sure that
any growth is towards sources of light and water.
There are two main types of tropism:
- Positive Tropism = The plant grows towards the stimulus.
- Negative Tropism = The plant grows away from the stimulus.
Phototropism is a tropism where light is the stimulus. A gravitropism (also called a ‘geotropism’) is a
tropism where gravity is the stimulus. The roots and shoots of a plant respond differently to the same
stimulus.
Part Of Plant: Light Stimulus: Gravity Stimulus:
Shoot Positive Phototropism (Grows
Towards Light)
Negative Gravitropism
(Grows Against The
Force Of Gravity)
Root Negative Phototropism (Grows
Away From Light)
Positive Gravitropism
(Grows In The
Direction Of The Force
Of Gravity)
Different Types Of Tropism:
The tropism of shoots
mean that the shoots are
likely to grow into the air,
where there is light for
photosynthesis. The
tropisms of roots mean
that the roots are likely to
grow into the soil, where
there is moisture.
Biology - B1.2