If we look at a piece of a nerve fibre,
we see that the cell produces large negatively charged ions called A-
meaning large anions which are trapped inside the cell and can't get out.
It also actively excludes sodium ions, the Na+.
And that leads the inside of the cell to be strongly negative.
To some extent this is compensated by passive migration of chloride ions.
Negative. Out. Cl.
And potassium ions. K+. Inwards.
Which are then not in equal concentrations inside and out.
When part of the cell fibre becomes active however,
what happens is that there is a leakage of sodium.
Sodium is allowed to leak into the cell.
Thus carrying positive charge inside and making the inside go positive.
As soon as that happens, about a 1000th of a second later
the cell becomes leaky to potassium and potassium moves out
taking the positive charge outside the cell again and restoring the status quo.
If we have an active region in the middle, which becomes positive,
then electric currents are sent out from the active region to the surrounding regions.
The currents go from positive to negative.
And they cause the resting region to become leaky to sodium.
So that region in turn becomes active
and the action potential moves down the membrane.
And it can move at quite a high speed.
At the end of the fibre there is a slightly swollen terminal
which contains a number of little capsules called vesicles.
And inside those are chemicals that are made by the cell.
The action potential comes along to the end there
and it causes a few of those vesicles
to come to the outside of the cell, burst and release their contents
and they diffuse across to the next cell
which is just a short distance away across the gap.
In some nerve terminals those chemicals cause a leakage of sodium
so that they tend to excite activity in the following cell.
In other terminals different chemicals cause the leakage of potassium
which makes the next cell become less active.
Now in the brain that next cell may have ten thousand terminals coming onto it.
Some exciting it, some inhibiting it.
And where its own action is determined on the sum total of all that happens,
all these ten thousand inputs.
So it's rather like a computer that is receiving a whole lot of information
through different channels and having its own activity coordinated.
When it fires its own action potential that may go out through branches of the fibre
to as many as several thousand other cells.
And this is a real piece of tissue.
It shows a motor fibre branching to terminate on muscle fibres.
And each receives a terminal from the motor fibre
and that whole collection forms a motor unit.
When we make a muscle contract an action potential goes down
and each one makes a number of muscle fibres contract
and the whole muscle is controlled in that sort of a way.
What I'd like to show you now - an actual action potential.
We're going to show it in earthworms.
Earthworms have, in common with other invertebrate animals,
their nervous systems just inside the skin on the ventral or underneath side.
Here's some worms living in a wormarium.
There's one on top here.
You know that if you poke a worm it will contract quickly along its length.
See if this one will do it now.
I pulled on its tail quickly.
This is part of an escape reaction.
If a worm is unwise enough to put its head out of its burrow on the lawn
during the day and a bird comes along and pecks at it
it'll pull back into its burrow very, very quickly.
It's important to do it quickly.
But a worm can't afford to run its nervous system at high speed.
Because the speed at which an action potential goes
depends for electrical reasons on the size of the fibre.
Big fibres conduct faster than small ones.
So for its escape reaction the earthworm has a couple of giant fibres
that run along the body and tell it to contract
to get back into its burrow as quickly as possible.
We have a demonstration here. I'm going to be helped by David,
a graduate student from Queen Mary College.
Hi David.
What we've done here is to hang an earthworm over some wire electrodes
from which we can record the action potential of its giant fibre system.
And stimulate action potentials by passing a small shock
through the body of the worm at the other pair of electrodes.
It would of course be terrible for this experiment if the earthworm were to wriggle
so this one has been sedated by being dropped for a few minutes into 10% alcohol.
Roughly equivalent to the strength of table wine.
This doesn't seem to do them any harm at all.
They recover afterwards and they don't even seem to get a hangover.
So I will now give it a switch on the stimulator.
And what we see here on the screen is a spike representing the initial shock
that goes down the length of the worm at the speed of light.
If I go to the next scale I can turn it up a lot more and I get a slower potential there
from a second smaller and therefore slower fibre system.
Let's just measure the time interval. Can we record that please, David, on the screen?
The speed of this now then is about nine milliseconds.
And the distance traveled is near enough nine centimetres.
So the speed then of the action potential in that faster giant fibre
is just about ten metres per second.
Well I said that this giant fibre is used for escape reactions.
One of the most spectacular of giant fibre systems is that contained in marine fan worms.
It's a different group of worms that live on the bottom of the sea
on a muddy environment and construct for themselves tubes of mud in which they live.
And you see some here in the tank.
They push their front ends out of the tube as a big fan that collects food
by filtering the seawater and collecting microorganisms.
But of course that's a very tempting morsel for any fish that's going by.
And they're very vulnerable.
The only escape they can make is to whip back into their tubes at high speed.
Now I'm not going too close to the tank because as I walk up
they often react just to my footfalls on the floor.
I creep up on them now.
You see these very beautiful crowns of tentacles here.
If I tap the tank [tap] they'll shoot in.
They nearly all shoot in.
And that is brought about by a single giant fibre that runs down the body of the worm
and is truly enormous. Fan worms have some of the largest fibres known.
A section through a fan worm.
At the bottom there, just above the skin is the nervous system
containing lots of small nerve cells.
And above that, the pale area arrowed there, is the single giant fibre.
Which is much bigger than the whole of the nerve cord of the rest of the nerve cord.
It's up to 1.8mm in diameter. Which is huge.
It was so big that for a long time people thought
that this was a strengthening, skeletal element
and didn't recognise it as a single nerve fibre.
Nevertheless, that fibre can only conduct at 15-20 metres per second.