Welcome to this lesson today entitled membrane potential. In this lesson, you will be learning about how the differences in concentration of ions across the plasma membrane can set the stage for a nerve impulse. Specifically, you will look at:
Sodium and potassium are the two ions that are very important for setting the stage for a nerve impulse. Sodium and potassium ions, or the differences in the concentration of these ions across the membrane, produce an electrical charge and the difference in the electrical charges across the membrane of a neuron are what set the stage for a nerve impulse or an action potential to actually occur.
Take a look at the diagram below to help show how the stage is set for a nerve impulse.
The diagram is just illustrating a small part of the plasma membrane of a neuron. If you take a look, you have a couple different things going in here. You have these little proteins which are our sodium channels. You also have something called a sodium potassium pump. If you take a look on the left side of the membrane, you’re going to pretend it's the outside of the neuron and you’re going to pretend the right side of the membrane is the inside of the neuron.
Now you'll notice that the inside of the neuron is more negative relative to the outside of the neuron. That has to do with the concentrations of sodium and potassium ions across the membrane. On the outside of the membrane, you have a higher concentration of sodium ions whereas inside, you have a higher concentration of potassium ions. What this does, this difference in concentration of ions across the membrane, sets up a voltage difference.
In a resting membrane potential, that voltage difference is about negative 70 millivolts. The inside is more negative relative to the outside. Now normally these gated sodium channels are closed in a resting membrane. When you’re speaking of a resting membrane, you’re talking about a neuron that is not stimulated or not experiencing any activity. Sodium channels are generally closed in a resting neuron which means that sodium is not allowed to pass through this membrane easily. Since sodium channels are generally closed, therefore the membrane is usually more permeable to potassium and you’ll have a little bit of potassium leaking out. Sometimes you’ll have a little bit of sodium leaking in other channels that are sometimes open, but generally you have a higher concentration of sodium outside and a higher concentration of potassium inside. You now have a concentration gradient that has developed for these ions.
When a voltage or when some sort of disturbance occurs, what happens is it causes the sodium channels to open. Since you have this concentration gradient of sodium outside, once those channels open, sodium is going to flow through those channels. It's going to flow from an area of high concentration to an area of low concentration and then potassium will leak out as well. Those concentration gradients build up setting the stage for a nerve impulse, so when there's a disturbance and those gates open, they're going to flow with their concentration gradient. Sodium will flow in and potassium will flow out reversing the voltage across the membrane, therefore allowing an action potential to occur. The difference in charge across the cell membrane is called the resting membrane potential and it has potential for the launch of a nerve impulse.
This lesson has been an overview on membrane potential and how sodium and potassium ions set the stage for a nerve impulse.
Keep up the learning and have a great day!
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND
The difference in the charge across a cell membrane which has the potential for an action potential to occur.