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The nervous system is so important - it enables us to notice and react to danger, and protects us by using our senses!
Let’s begin with the most fundamental part of the nervous system: nerve cells!
Individual nerve cells are called neurones, and their main function is to transport electrical signals around the body to control our internal and external processes.
But a nerve is not the same as a nerve cell - a nerve is a bundle of nerve cells packed together.
There are three types of neurones: motor, sensory and relay, and while each of them is unique, they also share some common functional adaptations.
| Motor Neurones | Sensory Neurones | Relay Neurones/ Interneurons |
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FUNCTION: Motor neurones carry signals from the CNS to the target organs and muscles to perform an action.
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FUNCTION: They detect changes in our environment, or stimuli, from loud noises to temperature changes, and carry this information from around the body to the brain or spinal cord. |
FUNCTION: They help pass information between the sensory neurones and the motor neurones. |
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STRUCTURE:
Motor neurones have a cell body with branches called dendrites which communicate with other cells. The cell body leads to the axon, a long fibre that guides the signal to its destination.
To speed up the electrical signals, the axon is wrapped in myelin sheaths. It’s the myelin that insulates the axon, which ends in the axon terminals of the neurone. |
STRUCTURE:
Sensory neurones have specialised cells called receptors in the periphery of our bodies that detect stimuli, and so sensory cells have no dendrites.
Their axons are short but also insulated with myelin. |
STRUCTURE:
The relay neurone can also be called an interneuron.
(The preface 'inter' means between and they go between the sensory and motor neurones!)
Their structure is very similar to motor neurones, although their dendrites are short and their axons can be long or short! |
Important Note: Dendrites are not axon terminals!!
Dendrites are on the cell body and axon terminals are on the other end of the neurone after the axon.
Synapses
Neurones need synapses to communicate and pass along the electrical signals, allowing the axon terminals of one nerve cell to pass on information to the dendrites of the next cell.
Electrical signals can't pass through the air, so in the axon terminal, it's converted into specific chemicals called neurotransmitters.
These pass across a gap called the synaptic cleft to the receiving receptors on the dendrites of the other cell, where they trigger another electrical signal to be passed down the axon.
Important Note: Dendrites are not axon terminals!!
Dendrites are on the cell body and axon terminals are on the other end of the neurone after the axon.
Synapses
For neurones to function, they need synapses to communicate and pass along the electrical signals.
Synapses are the connections between the dendrites and axon terminals of two cells.
Electrical signals can't pass through the air, so in the axon terminal, they're converted into specific chemicals called neurotransmitters.
These pass across a gap called the synaptic cleft to the receiving receptors on the dendrites of the other cell, where they trigger another electrical signal to be passed down the axon.
So let's put everything together - what happens when you touch something sharp?
1. Sensory receptors detect pain in the skin cells of your finger.
2. An electrical signal is generated at the cell body and travels down the sensory axon to the axon terminals.
3. Here, the sensory axon terminals synapse with a relay neuron and communicate with specific neurotransmitters.
4. The relay neurone carries the signal to its terminals, which then synapse with the motor neurone.
5. A motor neurone then takes the electrical signal to the muscles of the finger, and makes them contract to make you move away from the stimulus.
There's a lot to remember in this activity, so let's have a go at the questions now to see how much you can recall!
