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All cells have a resting membrane potential, which is the voltage difference between the fluids inside and outside the cell. This is important for transmitting electrical signals in nerve and muscle cells. The voltage difference is maintained by channels that control the flow of ions, such as potassium and sodium. When a cell receives a stimulus, it can trigger an action potential, causing a change in the voltage. After this, the cell enters a refractory period, during which it cannot be stimulated again until it returns to its resting membrane potential.
The resting membrane potential is the voltage difference of the fluids inside a cell and outside a cell, which is usually between -70 and -80 millivolts (mV). All cells have this difference, but it is especially important in relation to nerve and muscle cells, since any stimulus that changes the voltage and makes it different from the resting membrane potential is what allows cells to transmit electrical signals. If the cells had no voltage difference, they would be neutral and would not transmit any information.
background
All cells have a membrane that acts as a barrier between the external and internal fluid and to control what types of particles can enter and leave the cell. Some particles, such as oxygen, can cross the membrane on their own, but larger ones need special channels to get through them. Some of these channels only let one type of particle in and out and don’t actively push or pull particles in any direction, while others can take in multiple types of particles and can actively push them in or out of the cell. Both types can be opened or closed at specific times by the cell to control the flow of particles.
Resting potential
When a cell is at rest, the fluid inside it is slightly more negative than the fluid outside it, which generally has a charge of 0 mV. This is due to electrically charged particles called ions. The ions that cause the voltage difference are the type of particles that need channels to pass through the membrane and include things like potassium (K+) and sodium (Na+). When a cell is at rest, it contains a concentration of large negative ions within it, as well as some K+ and some Na+. The outside of the cell is surrounded by Na+ and some K+, among other things.
Since fluids ideally want to have different types of particles dispersed evenly throughout them, the K+ within the cell wants to go out of it and the Na+ wants to go in, so that the ions are evenly distributed. However, they cannot do this, because the channels that allow Na+ to pass through the membrane are closed when the cell is at rest, and those for K+ are only slightly open, which lets only a little K+ out. Additionally, there is a third type of channel that actively pushes any extra Na+ out of the cell and takes the leaking K+ back into the cell. This means that the slightly negative voltage is maintained within the cell, creating the resting membrane potential.
Action potentials
An action potential is how cells transmit electrical information and it occurs in response to a stimulus. If a cell that is at rest receives a sufficient stimulus to bring the charge of the fluid inside it down to -55 mV, then the Na+ channels open, causing a lot of Na+ to flow into the cell. This further increases the charge of the fluid inside, up to about +30mV. Once the fluid reaches this charge, the Na+ channels close and the K+ channels fully open, allowing K+ to flow out of the cell. However, these channels take longer to open than Na+ channels, so the cellular fluid remains positively charged for a while.
Refractory period
Once the K+ channels are fully open, a lot of K+ flows out of the cell, bringing its internal voltage to about -90 mV. This is called hyperpolarisation and prevents the stimulus from going back and affecting the same cell again, as the fluid is now at a much lower voltage, meaning that a much greater stimulus would be required to bring it back down to -55mV. After this happens, the channels that take in K+ and excrete Na+ start functioning and eventually return the cell to the resting membrane potential of -70mV.
