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GCSE OCR A Biology Notes - B3: Organism Level Systems
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Conscise notes with diagrams and key words highlighted, sticking to the OCR A GCSE combined science spec. Notes got me an A*A - 98
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Biology B3 – Organism level systems3.1.1 The Nervous System
Key Terms
Human nervous system – central nervous system (CNS) and peripheral nervous system
(PNS).
Enables humans to react to their surroundings and coordinate their behaviour.
Stimulus: A change in the internal / external environment (e.g. decrease in temperature
outside)
Receptor: A cell or organ which detects a stimulus
Neurone: A nerve cell, specially adapted to carry electrical charges, called nerve impulses
from one part of the body to another
Sensory neurone: A neurone which carries nerve impulses from receptors to the central
nervous system
Motor neurone: A neurone which carries nerve impulses from the central nervous system
to effectors
Relay neurone: A neurone that acts as a coordinator, transmitting impulses from the
sensory to the motor neurone in the spinal cord
Effector: A cell, tissue, organ, or organ system that responds to a stimulus
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,Biology B3 – Organism level systems
Voluntary response: A nerve pathway which produces a conscious response to a stimulus
Reflex response: A nerve pathway which produces an automatic response to a stimulus
Electrical impulse: A signal which is passed through the nervous system as electrical
charge to instigate a response
Central nervous system (CNS) – The part of the nervous system which includes brain and
spinal cord
Peripheral nervous system (PNS) – All of the nerves in the body which extend from the
CNS
Generating a nervous response
Nervous Co-ordination
Neurone – a nerve cell. Lots of neurones make up a large part of the nervous system.
Or axon terminal
/ fatty sheath
Direction of electrical impulse
Synapse: gap between the axon terminal of one neurone, to the dendrites of the next.
Chemicals (neurotransmitters) diffuse across the gap to cause an electrical impulse in the
next neurone.
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,Biology B3 – Organism level systems
Dendrites
- Highly branched
- Large surface area – to be able to receive electrical impulses / signals from
neighbouring neurones
Axon
- Long, thin extensions of cytoplasm
- Where electrical impulses travel
Myelin / fatty sheath
- Electrical insulator that surrounds the axon
- Speeds up transmissions of electrical impulses and prevents cross-communication
with neighbouring neurones
Axon terminal
- End of axon
- Highly branched to pass electrical impulses to a neighbouring neuron in a
communication pathway
Components of the Nervous System
1) Receptor
Group of specialised cells – detects changes in the environment. Converts the stimuli into
electrical impulses. Often located in the ear, eye (receptor for light), and skin (receptors
for temperature and touch).
2) Sensory neurone
Carries electrical impulses from the receptor to the CNS.
3) Relay neurone
In the CNS. Passes electrical impulses from the sensory neurone to the motor neurone.
4) Motor neurone
Carries electrical impulses to the effector.
5) Effector
Causes a response (e.g. shivering in the cold to warm up).
[Spinal cord
is part of the
CNS.]
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, Biology B3 – Organism level systems
Summary of neurones [just read]
Neurones carry electrical impulses from one place to another, around many parts of the
nervous system. They connect receptors to the CNS and also connect one part of the
nervous system to another, for example, in the brain and the spinal cord. They also carry
impulses from the nervous system to the effector organs, such as muscles and glands.
When neurones are stimulated, they transmit an electrical impulse.
A motor neurone has a nucleus surrounded by a cytoplasm. The cytoplasm forms a long
fibre that is surrounded by a cell membrane. This is called an axon. The axon carries the
electrical impulse and is protected by a fatty sheath. The fatty sheath increases the speed
at which the nerve impulse is transmitted. The nerve ending is branched to make good
contact with other neurones or the effector organ.
Two neurones do not make direct contact. Where they meet, there is a very small gap called
a synapse. The impulse needs to cross this gap to continue its journey to, or from the CNS.
This is done by means of chemicals which diffuse across the gap between two neurones.
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