# Electric impluses inside nerve cells

I'm not a biologist, nor physicist, but there is one thing I try to understand about the elecricity inside human body. I wasn't sure whether place this question here, or perhaps on biology portal, but I consider physics to be more descriptive of the processes occuring in nature (than e.g. biology).

I always was taught about electricity to be an ordered motion of electrons - and that they are transferred from one atom to another one when electricity "happens" (which means that one atom looses its electron, but acquires a new one from the other atom instead). But after reading some science-popularizating articles on the net on human body, I found out that they mention about whole ions being moved in the human nervous system in order to transfer an electric impulse ("message" inside nervous system).

Is it really the case that whole atoms, or even bigger particles, can be involved in the electricity? Even if so, it should have been probably much slower than the electricity with electrons, or even smaller particles, I guess? Whole atoms being "mediums" for electrical charges is something new to me.

The key to electricity is that it is moving charge. In our electrical wires we use electrons, which carry charge, to send signals. The current is the net flow of charge through a wire (or whatever is carrying the current) which then depends on the speed. The speeds at which electrons move involve a few distinctions. Firstly, the electrons move randomly due to energy from temperature (thermal energy). This speed is quite high, but does not involve any movement of current, since for each electron moving one way there will be another moving the opposite. Especially so when we have $10^{23}$ electrons in our wire.

When we apply a voltage across the wire, the electrons "drift" along at a very slow speed, giving us a current. Usually the speed is in the millimetres per second range, which is quite slow. But the effect of the current travels much faster, because the electrons exert an electrical force on their neighbours, which travels along the wire very quickly. This is why when you plug in your phone you don't need to wait for electrons to flow along the cable before it starts charging.

Now in the body, the electrical signals are carried by ions. An ion is an atom that is missing electrons, so it carries a net charge. This means we can use it to create a current. Since atoms are much much heavier than electrons, they will travel even slower (in the same applied voltage).

The nerves involve the movement of positive sodium ions and negative chlorine ions. When the ions are released into the ion channels, they move along and change the potential energy (voltage) which is the signal they are sending. This signals can move along between 1 m/s and 100 m/s (a brisk walking pace to faster than a race car), depending on the type of nerve.

So fundamentally, there is no difference between using electrons and ions for electricity, in terms of physics principles. What does differ is the way they interact with the medium they flow through. Electrons are good for wires since the conduction electrons aren't bound to their individual atoms so they are free to move. Ions are good for the body as electrons wouldn't be contained properly, and we can get sodium and chlorine fairly easily into the body.

• Also nerve impulses travel at a max 119 m/s. Nerve impulses such as pain signals travel slower at 0.61 m/s. Touch signals travel at speeds of 76.2 m/s. Where as in a wire the speed is around 200,000,000 m/s depending on the wire's dialectic structure. – user6972 Apr 5 '14 at 18:00
• Thank you for great, descriptive answer. Now I recall that really electrons are just a medium through which current flows, than being a current-particles themselves. Thanks for claryfying this and other things. It's interesting however that all you mention Cl ions, while on articles (including Wiki) I saw Na as negative ions (next to mentioned positive K ions). Also, I wonder why "electrons wouldn't be contained properly" - because I was really thinking of "having" something like a wire inside human body (in order to make signals being moved faster - and thus ability to react faster)? – forsberg Apr 6 '14 at 8:50
• Na and K are both positive ions, and are involved in nerve processes, as is negative Cl ions. The reason I say electrons wouldn't be contained is that to get electrons we need to ionise atoms. Ionise means to give enough energy to the electrons to escape their bond to the atom. In metals this happens easily and a "sea" of free electrons exists inside the metal. So if we had metal wires instead of nerves and axons, that would work. Na and Cl ions (for example) can be obtained by taking salt (NaCl) and splitting it up, because their bond is caused by already ionised Na+ + Cl- bonding. – Nuclear_Wizard Apr 6 '14 at 9:00

I found out that they mention about whole ions being moved in the human nervous system in order to transfer an electric impulse

They aren't really electric impulses, just signals. Take look at the Wikipedia page, with a picture that shows the propagation of the impulse. Action potentials are not currents themselves, nor ions moving from the head of the neuron to the tail. The signal is local change of membrane potential, which moves like wave. The electricity, in the form of ions, flows between the outside of the cell and the inside through ion channels, essentially perpendicular to the action potential. The flow of ions eventually brings the local voltage potential to a certain threshold. This causes changes in the surrounding molecules, opening more ion channels and spreading the signal forward.

When you think about a neuron, you have to keep in mind that it is not a piece of wire where you can apply a voltage to the ends to get a current. It's more like a rope where you can wave one end to transmit a signal to the other end. It's just that the wave happens to be made out of voltage.

Very interesting theme, but it's really a bio question. :)

It depends on what exactly you mean by electricity. When you're thinking about electricity moving this could be an electric current, or it could be an electric field. The obvious example of the first is current flowing in a wire, and the obvious example of the second is a radio wave travelling (at the speed of light) between the transmitter and your radio. The wave a nerve cell works is more like the second than the first.

Electric current is the flow of charge. It's certainly true that in most cases the charge is carried by electrons, and in most cases an electric current means a flow of electrons. However anything with charge can carry a current. For example if you pass a current through sodium chloride solution the current is carried by Na$^+$ and Cl$^-$ ions.

By contrast a propagating electric field, like a radio wave, does not carry any charge but instead it transports energy as an oscillating electric/magnetic field. I'm not sure it would be wise to try and explain in detail here as it would be long explanation.

Anyhow, a nerve cell does not work by a current flowing along the axon, like a current flowing in a wire. Instead what travels along the axon is an action potential, which is effectively a travelling electric field. Charge (Na$^+$ and K$^+$ ions) does flow, but it flows in and out through the axon walls not along the axon. What travels is the electrical potential difference between the inside and outside of the axon.

• This difference between carrying a charge and propagating electric field (being radio wave as well, if my memory is correct) is something surprising to me. I was always thinking so far about the electric waves as propagating very small charge-potential - so small that they're unable to affect anything in the way that "matter-based-electricity" does, like through wire (and alike, a charge moved through electrons also generates electromagnetic field)? What am I missing here? The field propagation can't be controlled, so how neurons manages this? This field could be accidentaly captured by other? – forsberg Apr 6 '14 at 9:10
• @forsberg: Electromagnetic waves do not carry any charge. – John Rennie Apr 6 '14 at 10:09
• But, isn't the electromagnetism (like Sun rays incoming to the Earth) the original source of power, which then can be used in a number of forms, and finally is responsible for any energy existing on the planet? E.g. electrons needs energy to be moved outside from atoms (and electricity as you mentioned is all about it) - so, this energy should have something in common with charge, I guess? – forsberg Apr 6 '14 at 12:45
• @forsberg: electromagnetic waves certainly carry energy, but not charge. – John Rennie Apr 6 '14 at 15:14