Membrane potential refers to the charge across the plasma membrane of a cell.
Resting membrane potential
A chemical disequillibrium exists between the extacellular (ECF) and intracellular fluid (ICF) compartments of our body cells. This equillibrium is as follows:
- There is a higher concentration of K+ ions in the ICF as compared to the ECF
- There is higher concentration of Na+ ions in the ECF as compared to the ICF
This chemical disequillibrium exists in not just excitable cells, but all cells, and creates a slight negative charge on the intracellular region of the plasma membrane. This is known as the resting membrane potential
How is this achieved? The resting membrane potential is achieved and maintained by the work of 2 different transmembrane proteins. These are:
- The K+ leak channel
- The Na+/K+ ATPase pump
The higher concentrations of K+ in the intracellular region of the cell compared to the extracellular region creates a concentration gradient through which K+ ions move out of the cell. It does so through the K+ leak channel. This causes a slight accumulation of positive charge on the outer region of the plasma membrane, and the loss of this positively charged ion from the intracellular region cause there to be a slight negative charge on the inner region of the plasma membrane.
The higher concentrations of Na+ ions in the extracellular region when compared to the intracellular creates a concentration gradient for the Na+ ions move into the cell. Even though the plasma membrane is impermeable to Na+, some of its ions do manage to diffuse though the phospholipid bilayer following its concentration gradient. However, upon entering the cell, they are immediately actively transported out by the works of a Na+/K+ ATPase pump. The Na+/K+ ATPase is a antiport which actively transports out 3 Na+ ions for every 2 K+ ions it brings into the cell. The K+ ions that enter the cell in this way immediately diffuse back out of the cell again through the K+ leak channel.
An electrical gradient also exists between the intracellular and extracellular regions of the plasma membrane and is in the opposite direction to the concentration gradient. This electrical gradient exists because the K+ ions that have diffused out of the cell are attracted to the negative state of the cytosolic region of the plasma membrane. As a result they start to move in to the cell through the K+ leak channel and will continue to do so until the electrical gradient and the concentration gradient are equal but occuring in opposite directions. The Voltage at which this occurs for K+ ions is reffered to as the Equillibrium potential of Na+. (It is to be noted that the equillibrium potentials of the different ions are established at different voltages. For example the Equillibrium potential of K+ ions is -86 mV, where as for Na+ ions it is +60. If one is to sum up Equillibrium potentials of all of the ions that move across the membrane it would sum up to the Resting membrane potential of -70mV)
When a cell is stimulated, a rapid reversal of its resting membrane potential occurs, so that the outer region of the membrane harbours a slight negative charge where as the inner region has a slight positive charge. This depolarisation is called an action potential. An action potential can also be described as a rapid change in the permeability of the membrane from K+ to Na+ ions.
How is this achieved?
An action potential is generated by the work of another set of transmembrane proteins. These are:
- The voltage gated Na+ channel
- The voltage gated K+ channel
When a cell is stimulated, depolarisation starts to occure due to some of its votage gated Na+ channels being open. The Na+ ions hence move down their concentration gradient into the cell depolarising the cell membrane further. When the membrane potential reaches a voltage of +55 mV all the Na+ channels suddenly open because this is the threshold voltage required to open these Na+ channels. As expected, there is an Na+ influx and the membrane is continues to be rapidly depolarised until the the membrane reaches a voltage of +70mV as this is voltage at which the Na+ channels are inactivated. As a result the Na+ ions are no longer permeable across the cell membrane and stop moving into the cell.
This is when the voltage dependant K+ channels start to open causing the K+ ions to move down its concentration gradient out of the cell. This movement of the positively charged K+ ion out of the cell causes the charge on the intracellular region of the membrane starts to lower back to the resting membrane potential of -70 mV. This is called repolarisation. However due to the K+ ion channels being relatively slow at closing again there is a slight overshoot where the membrane is repolarised further than the resting membrane potential. This is called hyperpolarisation. Eventually, when they do turn off, the K+ leak channel and the Na+/K+ ATPase works to reset the membane potential in readiness for the next action potential.
The Refractory period
There are 2 types of refractory period. These are:
- The absolute refractory period
- The relative refractory period
The absolute refractory period is the period in which no action potentials can be generated no matter how big the stimulus. This is because the the voltage gated Na+ channels are inactivated during this time therefore can't open to depolarise the membrane again.
The relative refractory period is the period in which action potentials can still be generated but the stimulus has to be bigger than normal. This is because some of the voltage gated Na+ channels have recovered from inactivation but some of the voltage gated K+ channels are still open and hence the movement of K+ out of the cell and the Na+ into the cell counteract one another.
The action potential propagates forward via current loops. The refractory period ensures that the action potential does travel backwards.