In order for you to wiggle your toes, a message must travel from your brain down to your feet. That might sound simple
but it in fact requires some remarkable events to have happened.
This is a story about you when you were very young or more accurately a very very tiny part of you. Specifically,
It's about a cell in your brain which we're going to call Betzy.
Betzy is a brand-new neuron, only a few hours old, and she's got a job to do. She has received
chemical instructions which have activated the string of biological processes inside her. Kind of like a robot receiving orders.
Betzy is to be part of a team of cells that will carry messages from your brain all the way down your spine and deliver
them to neurons that can then control the muscles in your arms and your legs.
But in order for Betzy to be able to send messages all that way she has to first grow a very long cable which can
carry electrical signals, and this cable is called an axon.
Amazingly, she manages this while a billion other neurons around her are growing their own axons to other places in your body.
Understanding just how Betzy grows her axon so precisely over such a long distance is hugely important, so one day
we might help people who have suffered from nerve damage, which is currently very difficult to repair.
First Betzy creates a growth cone, from which extend small sticky fingers called filopodía.
When these sticky fingers stick to the outsides of nearby cells, tiny muscle-like filaments within them
contract, which pulls the growth cone forwards through your body trailing the growing axon behind it. At the same time, Betzy is sending new
material down to her growth cone along tiny microtubule tracks that run along the inside of her axon. These
microtubules tracks continuously extend further into the growth cone,
and so push it forward from within. This is how Betzy's growth cone moves through your body,
and she grows her axon. The filopodía stick to nearby cells and pull the growth cone forward, whilst
microtubules push the growth cone forwards from within.
But with so many possible paths to choose from, how does Betzy's growth cone determine in which direction to go?
The material that Betzy sends down her axon to her growth cone includes tiny chemical sensors,
similar to the ones found in your nose and your tongue. However, these sensors are not for food
But to detect special signalling proteins that either attract or repel the advancing growth cone. These signaling molecules do this by either
stabilizing the microtubule tracts within the growth cone or collapsing them, and in so doing change the direction in which the
microtubules push the cone from within, and so guide its growth.
These signaling proteins can be found on the outer membranes of many cells and so influence the growth cone when it makes
contact, and are also released into the spaces between cells, and these affect the growth cone's direction from a distance.
We're doing well now. Whilst others have gone off course,
Betzy's growth cone has reached your brain stem at the base of your skull and is now approaching the point where it has to cross
over the midline of your body.
Betzy lives in the right side of your brain and therefore will eventually control the toes on your left foot.
Nobody quite knows why neurons on the right side of your brain need control muscles on the left side of your body and vice versa,
but it evolved ages ago, long before even
lizards were around. So Betzy's growth cone crosses in your brain stem from right to left and then continues on down your spinal cord.
Time is running out now.
Betzy's growth cone has to reach its target
soon as we're now approaching a special stage in your development where any neuron whose axon has not reached its designated target will be
instructed to begin apoptosis and will die.
This is an important step so that your nervous system ends up wired correctly
because a poorly wired nervous system simply would not work very well.
Betzy, however, is on course, and is getting closer. Her growth cone detects the specific mix of signalling proteins being excreted by her
designated target and begins to home in.
The target is a motor neuron in your spine which has, in this same time, grown its own axon from your spinal cord down to
your tippy toes and connect it to a muscle there.
When Betzy finally makes contact she receives an elixir from the motor neuron which, when the clock hits zero, keeps her alive
When all the neurons that did not reach their targets died.
Right now, whilst you are watching this video, if you wiggle your fingers or toes
it is these
successful Betzy cells in your brain's motor cortex that carry messages from your brain all the way down to the motor neurons in your spine,
which then carry the signal to muscles in your hands and your feet making them
contract and your digits wiggle -- all in just a fraction of a second. And the instructions to build this
remarkable system and much much more can all be found in your DNA. Which I think is awesome
We already know an awful
lot about how the brain works
But there is still so much more to learn. If you would like to know more about how the brain works today
You can visit brainfacts.org and also subscribe to this channel for more fun videos
Thanks for watching