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The Nervous System & Neurones
The Nervous System Sends Information as Nerve Impulses
1. The nervous system is made of a complex network of cells called neurones. There are 3 main
types of neurone:
Sensory neurones transmit nerve impulses from receptors to the central nervous system
(CNS) the brain & spinal cord.
Motor neurones transmit nerve impulses from the CNS to effectors.
Relay neurones transmit nerve impulses between sensory neurones & motor neurones.
2. A stimulus is detected by receptor cells & a nerve impulse is sent along a sensory neurone.
3. When a nerve impulse reaches the end of a neurone, chemicals called neurotransmitters take the
information across to the next neurone, which then sends a nerve impulse.
4. The CNS processes the information, decides what to do about it and sends impulses along motor
neurones to an effector.
Sensory Receptors Convert Stimulus Energy into Nerve Impulses
1. Different stimuli have different forms of energy e.g. light energy or chemical energy.
2. But your nervous system only sends information in the form of nerve impulses (electrical impulses).
3. Sensory receptors convert the energy of a stimulus into electrical energy.
4. Sensory receptors act as transducers they convert one form of energy into another.
How Receptor Cells that Communicate Via the Nervous System Work
1. When a nervous system receptor is in its resting state (not being stimulated), there's a difference in
voltage (charge) between the inside and the outside of the cell this is generated by ion pumps and
ion channels. The difference in voltage across the membrane is called the potential difference.
2. The potential difference when a cell is at rest is called its resting potential. When a stimulus is
detected, the cell membrane is excited and becomes more permeable, allowing more ions to move
in and out of the cell altering the potential difference. The change in potential difference due to a
stimulus is called the generator potential.
3. A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger
change in potential difference so a bigger generator potential is produced.
4. If the generator potential is big enough it'll trigger an action potential (nerve impulse) along a
neurone. An action potential is only triggered if the generator potential reaches a certain level called
the threshold level.
5. If the stimulus is too weak the generator potential won't reach the threshold, so there's no action
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The Structure of Sensory Neurones
1. All neurones have a cell body with a nucleus (plus
cytoplasm and all other organelles you usually get in a
2. The cell body has extensions that connect to other
neurones dendrites carry nerve impulses towards the
cell body & axons carry nerve impulses away from the cell
Here's a taster:
The sodiumpotassium pumps move sodium ions out of the neurones, but the membrane isn't
permeable to sodium ions, so they can't diffuse back in. This creates a sodium ion electrochemical
gradient (a concentration gradient of ions) because there are more positive sodium ions outside
the cell than inside.
The sodiumpotassium pumps also move potassium ions in to the neurone, but the membrane is
permeable to potassium ions so they diffuse back out through potassium ion channels.…read more