Action Potential

Welcome to today’s lesson on nerve impulse and action potential. In this lesson today, you will be learning specifically about:

1. Nerve Impulses
2. Action Potential Steps

1. Nerve Impulses & Action Potentials

An action potential is basically just another way of saying a nerve impulse. An action potential occurs when a stimulus causes the voltage difference across a cell membrane to shift. This voltage difference is caused by concentrations of potassium and sodium ions. So potassium and sodium are very important in setting the stage for an action potential and the threshold is the minimum shift needed for an action potential to occur.

• Action Potential
• A nerve impulse.
• Threshold
• The minimum voltage shift across a membrane required for an action potential to take place.

2. Action Potential Steps

In order for this action potential to occur, the voltage difference across a cell membrane has to shift by a certain amount. Once it's shifted by that amount, an action potential can happen. Here are the steps that set the stage for an action potential; they're the steps that occur in an action potential.

Step 1: Electrical Disturbance

First, an electrical disturbance has to occur. Signals will reach the input zone of a neuron and those signals will change what occurs in the cell membrane of that neuron.

Step 2: Open Sodium Gates

This electrical disturbance will cause sodium gates in the membrane of the cell to open which allows sodium to rush into the cell. Now normally in a resting membrane, the outside of the cell is positive relative to the inside of the cell. As these sodium gates open, sodium will rush in, causing more gates to open until that threshold we discussed is reached causing the voltage difference across the cell membrane is reversed.

Step 3: Restore Resting Membrane Potential

Now in order for another action potential to occur, you have to restore the resting membrane potential which is done so by sodium/potassium pumps. To understand this, it is important to review the structure of a neuron here. A neuron is a nerve cell, and it's made up of dendrites, which are part of the input zone. Information will move through the input zone, through the dendrites, to the cell body. From there, the signal will travel along this long narrow part of the neuron called the axon, and then down to the axon endings. So information goes through the input zone, through the cell body, along the axon, to the axon endings. And then from there, that signal will be sent either to another neuron or to a muscle or a gland cell.

• Resting Potential
• The steady voltage difference that occurs across a neuron’s membrane when it is not being stimulated.
• Sodium/Potassium Pump
• A protein that pumps sodium potassium against their concentration gradients to restore the resting membrane potential following an action potential.
• Input Zone
• The region of a neuron where signals enter.
• Trigger Zone
• The part of the neuron located at the base of the cell body that initiates action potentials.
• Conducting Zone
• The part of the neuron that consists of the axon where action potentials propagate away from the trigger zone toward the axon endings.
• Output Zone
• The part of the neuron that consists of the axon endings where signals are sent on to another neuron or to a gland or muscle cell.

To better understand what happens in action potential, take a look at this diagram below.

As mentioned, the outside of the neuron is generally positive relative to the inside. You have your cell membrane, and then embedded within that, you have something called a sodium potassium pump. Then you have your gated sodium channels which normally, these gated sodium channels are closed, making the cell membrane more or less impermeable to sodium. Sodium is not allowed to just flow through freely.

When a disturbance happens and it causes an action potential, sodium gates will open allowing sodium to flow into the cell. As that happens, more and more of these gates will open allowing more and more sodium to flow in. This will continue to happen until the threshold is reached and the voltage difference is reversed.

Now this isn't all happening throughout the whole membrane at the same time; it occurs in patches of the membrane. As sodium moves in to the cell in one patch, the previous patch of that membrane will allow potassium to leak out. This is happening reversing the voltage difference across the cell membrane and will cause the impulse to propagate along that cell membrane in patches.

Eventually, resting membrane potential has to be restored and that's where our sodium potassium pumps in pink here come into play. So your sodium potassium pumps will restore that resting membrane potential. In order perform this it has to use the cell's ATP which is a form of active transport because it's using ATP in order to restore this membrane potential. What the sodium potassium pump will do is it will put sodium back out and potassium back in. As that action potential was happening, you had your sodium diffusing in and potassium diffusing out, which was reversing our voltage difference across the membrane. However, in order to restore that resting membrane potential you have to get the sodium back out and the potassium back in and then our resting membrane potential can be restored.

This lesson has been an overview on nerve impulses and action potentials. You also got to learn about the specific steps involved in action potential.

Keep up the learning and have a great day!