Question

Explain what an action potential is, and describe how it moves down the axon and across the synapse?

Answer 1

The action potential (electrical impulse) is pulled along the cell by positive ions entering and attracting it before leaving again.

Explanation:

An action potential is the electrical signal that travels down the neuron cell.

The electrical signal is negatively charged, because it is, obviously, electrical. It is drawn along the neuron by a series of positive ions appearing in front of it and pulling it forward. Imagine if you tied a string to a ten pound note and pulled it along the street with a cartoon character chasing after it - that's how I like to think of it.

The inside of the neuron cell is normally negatively charged relative to the outside. Ion channels open when the electrical signal enters the cell and pump `Na^+` ions inside, which attracts the electrical signal along the cell.

Since the cell is normally negative, or polar, the influx of positive ions is known as a depolarisation, because it turns the cell more positive in that area. However, once the cell has reached a certain level of depolarisation, the `Na^+` supply cuts off and the cell begins pumping `K^+` ions out of the cell instead. This turns the cell back to negative, causing a repolarisation.

While this is happening, some of the `Na^+` spreads out and changes the charge slightly further along the cell, which activates more sodium gates and lets sodium flow in further along, which depolarises that segment of the axon and pulls the electrical signal even further along.

The series of depolarisation and repolarisation along the cell makes the action potential move down the axon.

When an electrical impulse reaches the synapse, it opens calcium channels. The calcium enters and causes vesicles (bubbles) of neurotransmitters to bind to the membrane and release the chemicals. The neurotransmitter molecules drift across the synapse and bond to receptors on the next neuron, which initiates the electrical signal and the process of de- and repolarisations repeats.