Neurons, neural networks and the nervous system
Why do we need a brain? [All Ages]
What is the main reason that animals have brains?
Are there any animals that don’t have brains?
Show the audience the picture of a jellyfish from the image gallery below, ask them what is special about a jellyfish’s brain and solicit answers. This is a trick question – actually jellyfish don’t have brains. Explain that animals that do not decide where to go do not usually need brains. Jellyfish get swept around on the currents and simply trawl through the water with their tentacles to collect food as it goes by. Because they do not need to respond to their environment very much, they do not need brains. Starfish, sea urchins, jellyfish and other headless animals have nerves spread across their bodies but not a single bundle of nerves like the brain in other animals.
Q: What is the main reason that animals need a brain?
A: Most animals need a brain to navigate around the world, to make decisions and predictions about what will happen next and to react to external information and forces. Brains are for figuring out and predicting the external world we inhabit and for representing external information and telling our bodies how to respond.
Q: How do you think brains developed in the first place?
A: We currently think that very simple creatures, a bit like earthworms, began moving purposefully in a particular direction. This meant that one end of their body was always ahead of the other. As their senses evolved it would make sense that those animals that evolved senses clustered on this front-end would survive better than those that didn’t because they would be warned earlier about threats and opportunities (e.g. saltiness, light, food, water etc). It might then have been useful to have a bundle of nerves in the head as well to process all the sensory information that was coming in. We think that this evolved into the brain of every self-moving animal today.
Octopuses are very clever but they don’t have a single brain. About one-third of their brain is in their head but the other two-thirds is divided between their 8 tentacles. This means that each of the tentacles can think for itself and the only way the octopus knows what a tentacle is doing is by looking at it.
The Nervous System [All Ages]
How does information about the world get into the brain?
How do messages from the brain get to the parts of the body that need to be told what to do?
Show the audience the gallery image of the central and peripheral nervous system.
Explain that the brain sends and receives messages from the rest of the body via the ‘nervous system’. The most direct connection between the brain and the rest of the body is the ‘central nervous system’ which runs down your spine. The more distant parts of your body, like your arms and legs, are connected via the ‘peripheral nervous system’. The brain is the control centre that takes in information about the world from all the different parts of the body and then tells each of them how to react.
Q: Why are people sometimes unable to walk following damage to their spine?
A: Damage to the spine can either sever or disable the communication between the brain and the legs. This is because the part of the nervous system responsible for taking messages from the brain to the legs is no longer working properly because the pathway has been broken.
Both giraffes and humans have 7 vertebrae (bones) in their necks – obviously the giraffe vertebrae are MUCH bigger than human vertebrae.
Most mammals have only 7 neck bones but there are exceptions – the manatee and 2-toed sloth have 9 neck bones.
Neurons [Key Stages 3 & 4]
What are the building blocks of the nervous system?
How long are the cells that make up the nervous system?
Present the audience with the diagram of a synapse (see Image Gallery below).
Explain that the basic building blocks of the brain and nervous system are ‘neurons’. There are about 80 billion neurons in the human brain and most of these – the ones responsible for most of your thoughts and behaviour – are found in a very thin layer of your cortex.
Point out that there are three main parts to the neuron – the dendrites, the cell body and nucleus and the axon.
The dendrites receive signals to the cell body from other neurons. Each neuron has thousands of dendrites.
The cell body makes all the materials needed to keep the neuron alive and the nucleus controls all of the functions of the neurone.
The axon passes nerve signals from the cell body along to stimulate other neurons.
As you move around the world, neurons control your movements and receive information from the changing world. There are different types of neurons – for instance, Motor Neurons deliver signals to your muscles and Sensory Neurons pick up signals from your senses. All of this information is used to update and coordinate our actions. As you learn about your world and make decisions, you store information in the massive number of connections of neurons in your cortex. This is how we learn about the world around us.
Q: What are the building blocks of the nervous system?
A: The nervous system is made up of billions of cells called neurons that perform a variety of different functions including taking instructions from the brain to the body and taking sensory information from the body to the brain.
Q1: The myelin sheath (point this out) is a layer of fat around the axon. What is this for?
A1: The myelin sheath protects the long axon and speeds up electrical nerve signals. Some researchers think you can increase the thickness of your myelin sheath (and therefore how fast your brain works) by eating certain shellfish.
Q2: How are neuons similar to and different from other cells in the body?
A2: Neurons are similar to other cells in the body because they:
a) are surrounded by a cell membrane
b) have a nucleus that contains genes
c) contain cytoplasm, mitochondria and other organelles and
d) carry out basic cellular processes such as protein synthesis and energy production.
They are different to other cells in the body because they:
a) have specialized extensions called dendrites and axons that bring information to and take it away from the cell body (respectively).
b) communicate with eachother through electrochemical processes and
c) contain some specialised structures (e.g. synapses) and chemicals (e.g. neurotransmitters).
- If you laid all the neurons in your brain end to end they would be 162,000km long – enough to go around the equator of the earth 4 times!
- If you counted all the neurons in your brain at the rate of one a second and never lost count, it would take 645 years to count them all!
- The number of potential connections that can be made between neurons is greater than the number of known atoms in the universe!
- You could fit 30,000 brain neurons on the head of a pin!
- However other neurones can be several feet long. For instance, the length of a giraffe’s longest neurone (from it’s toe to its neck) is 15 feet!
How do neurons communicate with eachother? [Key Stages 3 & 4]
How are neurons connected to each other?
How do neurons communicate with each other?
Part 1: Show the audience this image and explain that neurons connect to each-other at junctions called ‘synapses’ (see diagram in the gallery).
Synapses are very small gaps between the tentacles of two neurones.
Part 2: Play the following video for the audience
The electrical signal can’t pass across the gap between synapses and so the synapse releases special chemicals, called ‘neurotransmitters’ which travel across the gap and trigger an electrical impulse in the next neuron.
Q: How do neurons communicate with each-other?
A: Neurons connect to each-other via their synapses. There is a gap between synapse endings so synapses have to transmit neurotransmitters across the gaps to trigger electrical impulses from one to the next.
Q: How do neurons transmit electrical signals?
A: When the neuron is resting, it pumps positively charged sodium atoms (from ordinary salt) to the outside of the cell where they build up like water behind a dam. When an electrical signal arrives, the floodgates open and the charged atoms rush back inside the cell, causing an electrical charge to shoot along the axon.
How fast do neurons communicate? [Key Stages 2-4]
How fast do neurons communicate?
Pen and paper to write down times
Part 1: Ask at least 10 members of the audience to come to the front, stand in a line facing the audience and place one arm on the shoulder of the person next to them. Explain that you will squeeze the shoulder of the first person who will then squeeze the shoulder of the second person and so on until it gets to the last person. Instruct the last person to say ‘Stop’ when his or her shoulder is squeezed.
[Diagram of people holding shoulders from Image Gallery below]
At the same time, squeeze the shoulder of the first person and start the stop watch. Stop the stop watch when the last person says ‘stop’. Note down how long it took the message to get from beginning to end.
Part 2: Repeat the experiment but this time ask the audience members to hold hands instead of shoulders. Again, start the stopwatch when you squeeze the first persons hand and stop it when the last person says ‘Stop’. Note down how long it took the message to get from beginning to end. This should take longer than Part 1.
[Diagram of people holding hands from Image Gallery below]
Explain that it took longer for the signal to transmit from beginning to end when the audience members held hands than when they had their hands on each-other’s shoulders. This is because the distance from hand to hand was twice as long (two arm lengths) as the distance from hand to shoulder (one arm’s length).
To calculate how long the average nerve signal took, divide the overall time to complete each task by the number of participants who took part.
Explain that although electricity in the world seems to travel instantaneously, electrical signals in the body can take more time and increase with the distance traveled. In fact, electricity in the body can travel up to 3 million times slower than electricity in the world. This is partly because electrical impulses have to be converted to neurotransmitters between each of the synapses along the way rather than travelling directly.
Watch the following video for an example of this demonstration in action:
Q: Why does the communication between the body and the brain feel so quick?
A: Electrical signals over short distances, like those within your brain, can travel as fast as 400km/hour. As the distances get longer the messages take more time but even over longer distances the perception is of something happening immediately. For instance, imagine eating grapes. Your hand picks a grape and sends the signal to your brain that it is squishy. Your brain decides on that basis that the grape is rotten and instructs the hand to throw it away rather than eating it. This process can seem almost instantaneous yet it does take a significant amount of time.
Q: From this demonstration, how would you work out how fast the message is passing?
A: By dividing the amount of time it took for the message to pass from one end of the line to the other by the number of people in the line.
How do neurons connect to form thoughts? [Key Stages 3 & 4]
How do neurones connect together to form pathways for thought?
Explain that neurons connect up together to create networks that allow us to think, remember and predict. It is not the number of neurons we have that makes us clever but the number of connections between neurones and the complexity of the patterns they form. This is what gives the brain its immense processing power. Each of the brain’s 80 billion neurons can have up to 10,000 connections. This means that the human brain has more than 500,000 times as many connections as even the most advanced computer chip. The human brain can also be running lots of these networks all at the same time, a problem that computer scientists have not yet been able to master.
Different experiences cause the neurons to fire messages to each-other and the more those pathways are used, the stronger they get. So, if I was to taste a grape for the first time, the network of neurons responsible for recognizing ‘green’, ‘round’ and ‘sweet’ would all fire together. Each time I ate a gape this network would get stronger and stronger. Cells that fire together, wire together. In the following video Bruce shows how a simple neural network can be created using members of the audience. Pause at 2:10.
Now, if I were to see an olive for the first time – I would notice that it was round and green like a grape so my network for grapes might start firing already. This would mean that my brain is expecting the olive to be sweet. If this is the case it will come as a great shock when the olive is actually salty! In the following video Bruce shows what happens to the neural network when it experiences something unexpected. Play the rest of the video now.
Q: How do neurons connect to form pathways for thought?
A: When clusters of neurons responsible for processing specific information are fired at the same time they form a stronger connection with each other. If this connection is reinforced through repeated use other weaker connections might die off. Strong connections between clusters of neurons lead to strong expectations about how the world will behave.
The world’s top supercomputer is only as powerful as half a mouse’s brain yet it takes up more than a million times as much space!
How do neural networks develop? [Key Stages 2-4]
Do you grow more neurons as you get older?
Is it true that we only use 10% of our brain?
Show the audience the video below. The models show the number of neurons and their connections in the brains of (from left to right) a newborn baby, a 3-month-old baby and a 3-year-old child.
When you are born you have almost all the neurons you will ever have in your cortex. Point out the black blobs in each picture and explain that these are the neuron bodies and all the thin lines between them are the connections. Note that the number of cell bodies (black blobs) doesn’t change from one age to the other but the number of connections do. When the baby is born the brain starts sending out connections to all the other neurons. At first these connections are very sparse and thin, as they are here in the picture of part of the brain of a newborn infant.
The human brain is made up of 80 billion neurons that, for the most part, remain the same throughout the person’s life. When babies are born they go through a period where their brain becomes massively connected as they experience everything for the first time. Over time and with experience and learning, some of these connections get strengthened through repeated use and some die away as a result of rarely being used – a rule called ‘Use it or lose it!”. The process of dieing away is called ‘pruning’ because it is like chopping off the branches of a tree to ensure the main part of the tree is as efficient as possible. Most pruning happens during childhood which is why children are often better at learning new skills than adults are. The networks that remain after this first surge of connection, pruning and strengthening form the basis for all thought, feeling and memory later in life.
Q: What does this understanding of neural-network development tell us about the statement that we only use 10% of our brains?
A: Clearly this statement must be a myth because any part of our brain that wasn’t being used would die off so that its resources could be used to strengthen the networks that are being used.
Q1: Why has the 10% myth persisted even though we now know it isn’t true?
A1: No-one is really sure where the 10% myth arose – its possibly a mis-quotation from the 1930s that the average human uses 10% of their brain at any one time. Even this much milder claim has been refuted – in fact we use nearly every part of our brain and most of the brain is active all of the time. The myth has been perpetuated in pop culture and is frequently used in advertisements. Part of its appeal may be the idea that we have a huge amount of potential that, if we only knew how, we could tap into to do incredible things beyond out current capabilities.
Q2: How might the “use it or lose it” idea help us understand why some people show a lot of mental decline in old age while others don’t?
A2: It is now well known that intellectual challenges help to slow down the decline of the brain in old age. People such as musicians and scientists who keep working well past retirement age often show very few signs of mental ageing. In fact, even doing regular crosswords and brainteasers may help to slow down mental decline in old age – although problem solving may seem like hard work, it probably is the exercise that keeps your brain in tip-top condition and ensures you don’t lose the parts of the brain that you need later on.