Nerves are just like wires aren’t they?

Electrical impulses move through neurones just like electricity flows through a wire, right?

No! Absolutely not. Nerves are commonly described as wires as an analogy to simplify their function, but it does not in any way describe how they actually work. The only similarity between wires and nerves is that there is a flow of charge. However, with a wire, there is a flow of electrons through the conducting metal. This is certainly not the case with a nerve.

figure 1. Representation of a nerve cell conducting impulses

So, how do nerves conduct electrical impulses then?

It is all to do with ions. The movement of ions through channels across a membrane to be more specific. But, before we can hope to understand how impulses propagate along an active nerve axon, we must first understand the cell at rest. This is known as the resting potential

What is a resting potential?

All cells have a resting potential. It is simply the charge of the inside of the cell with respect to the outside of the cells. The resting potential of a nerve cell is around -70mV. All that means is the inside of the cell has -70mV charge with respect to the outside of the cell.

How does the resting potential arise?

Ions and channels! I am going to explain this phenomenon using the example of the nerve cell.

The cell has a very high concentration of potassium ions (K⁺) inside the cell, and a very high concentration of sodium ions (Na⁺) outside the cell. Thus, with the K⁺ ions will tend to move outside the cell, while the Na⁺ ions will want to move into the cell. This is just common diffusion stuff down concentration gradients. Since both of the ions are positive, eventually they will not be able to continue to move into the place where there is a lower concentration of ions due to the repulsion of their positive charge, and an equilibrium will be reached.

Each ion has a specific value for the voltage at which this equilibrium is reached, depending on the concentration of ions inside and outside the cell. It is called a Nernst potential

The nerve cell is far more permeable to K⁺ than Na⁺, meaning more positive charges will leave the cell, than those entering the cell. This makes the inside of the cell more negative, giving us the resting potential value of -70mV. A visual representation of a resting potential is shown in figure 2.

figure 2. Resting potential

Now we are finally reading to tackle the nerve impulses and how they travel. This is down to depolarisation of the resting potential in response to a stimulus. This is known as an action potential. This is shown in figure 3 and the stages are described below.

figure 3. An action potential

Stage 1 = resting potential
Stage 2 = depolarisation in response to a stimulus. The stimulus causes voltage gated Na⁺ channels to open causing Na⁺ to move from outside to inside the cell (down its concentration gradient). Remember Na⁺ is in high concentration outside the cell. This makes the membrane become positive (around -30mV)
Stage 3 = Na⁺ channels close and K⁺ channels open in response to depolarisation
Stage 4 = repolarisation. Voltage gated K⁺ channels are now open and K⁺ move from inside the cell to outside the cell down its concentration gradient. This will bring the cell back to resting potential
Stage 5 = hyperpolarisation. The K⁺ channels are open too long and the voltage overshoots, woops!
Stage 6 = finally, resting potential again.

But how do the action potentials travel along the cell?

Thankfully, this is the simplest part. The channels mentioned above are voltage gated. This means if a stimulus is above the threshold for a specific nerve the voltage gated Na⁺ channels will be triggered to open. Once depolarisation in one place has a occurred, the disruption in membrane potentials triggers the next segment of the membrane to be depolarised and the next voltage gated Na⁺ channel to open. In this way the impulse travels along the nerve. Not by ions moving through the cell, but by a series of ions moving across the membrane and disrupting the voltages. This is seen in figure 4.

figure 4. Propagation of an action potential

If you are reading this, you made it! You have conquered nerve cells! The physiology of nerves and action potentials is extremely complicated, so I have tried to summarise it as simply as I can. I hope you have enjoyed reading it.

References:
https://en.wikipedia.org/wiki/Resting_potential
http://www.physiologyweb.com/lecture_notes/resting_membrane_potential/resting_membrane_potential_nernst_equilibrium_potential.html
https://www.khanacademy.org/test-prep/mcat/organ-systems/neuron-membrane-potentials/a/neuron-action-potentials-the-creation-of-a-brain-signal
http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/actpot.html