The nervous system is so important- it makes us notice danger, makes us react 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, as a nerve is a BUNDLE of nerve cells packed together.
Now, there are 3 types of neurones: Motor, Sensory and Relay Neurones, and while they are unique, they also share some common functional adaptions too:
|Motor Neurones||Sensory Neurones||Relay Neurones/ Interneurons|
Motor neurones carry signals from the CNS to the target organs and muscles to perform an action
They detect changes in our environment, or stimuli, from loud noises to temperature changes.
They carry this sensory information from stimuli in the peripheries to the brain and spinal cord of the CNS.
A relay neurone can be in the spinal cord and the brain (anywhere in the CNS) and they help pass information between the sensory neurones and the motor neurones.
Neurones have a cell body or soma 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. This can be very long e.g. one axon can stretch from your hip to your toe!
To speed up the electrical signals, the axon is wrapped in myelin sheaths, and it’s the myelin that insulates the axon. This area doesn’t conduct electricity, meaning the signal jumps down the fibre through uninsulated gaps called the nodes of Ranvier, ending in the axon terminals of the neurone.
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, as you can imagine, if our finger is on fire, we need to register this and react fast, so the faster the signal can travel the more control our bodies have!
The relay neurone can also be called an interneuron.
(think that it runs interference 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 AND 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.
When the neurones want to do their functions, they need synapses to communicate and pass along the electrical signals.
Synapses are the connection between the dendrites and axon terminals of two cells.
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.
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 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.