The cell membrane is a partially permeable membrane that allows only hydrophobic or lipid soluble molecules to move in and out freely. Ions, being polar molecules, are unable to pass freely and therefore channel proteins are responsible for carrying them across the cell membrane. These channel proteins are gated and thus open and close in response to a certain stimulus.

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Measurement of membrane potential

Image: “Measurement of membrane potential” by OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface. License: CC BY 4.0


Membrane Potential

basis of membrane potential

Image: “Figure 1.0: Diagram showing the ionic basis of the resting potential” by Synaptidude. License: CC BY 3.0

Membrane potential (Vm) is the difference in the electrical potential between the interior and exterior of a biological cell. A typical value of the membrane potential ranges from -40 mV to -70 mV.

Figure 1.0 shows that the inside of the cell is more negative as compared to the extracellular environment. This is because of the permeability of the cell membrane to certain ions.

The concentration of certain ions in the intracellular and extracellular environment is listed in the table below:

Sr. No

Ion

Extracellular concentration (mM/L)

Intracellular concentration (mM/L)

1

Sodium (Na+)

145

12

2

Potassium (K+)

4

120

3

Chloride (Cl-)

110

15

4

Calcium (Ca+2)

2.5

0.0001

 Table 1.0: Concentration of ions across the cell membrane.

Measurement of membrane potential

Image: “Figure 2.0: Measurement of membrane potential” by OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface. License: CC BY 4.0

It is evident from Table 1.0 and Figure 1.0 that the inside of the cell is more negative than the outside environment. This is due to the presence of more negative ions inside the cell. This state is also known as resting membrane potential since there is no active stimulus.

The membrane potential is measured with the help of microelectrode as shown in figure 2.0.

Equilibrium Potential

Equilibrium potential is the membrane potential where the net flow of specific ions through an open channel across the cell membrane is zero. So, the equilibrium potential across the cell membrane is 0 mV.

Resting Membrane Potential

In the absence of a stimulus, the membrane potential of a cell is called resting membrane potential.

action potential

Image: “Figure 3.0: Schematic of an action potential” by Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom – Own work. License: CC BY-SA 3.0

During resting membrane potential, the sodium potassium pump uses ATP to move three sodium ions out of the cell and two potassium ions into the cell. This creates negativity inside the cell. In addition, there are leaky potassium channels that allow K+ to diffuse out of the cell. The cell membrane is said to be polarized as shown in Figure 3.0.

  • The resting membrane potential is -70 mV.

Generation of Action Potential

Action potential is generated in the presence of a stimulus. The nerve endings are stimulated by a change in the environment. This results in opening of sodium channels. Sodium ions move into the cell and the membrane is said to be depolarized.

The membrane potential changes from -70 mV to +40 mV.

It is important to note that not every stimulus is able to generate an action potential. The minimum amount of potential change required to generate an action potential is known as threshold potential.

A stimulus opens some sodium ion channels, which slightly decrease the negativity inside the cell. If threshold potential is reached, all sodium channels open and let more sodium ions move into the cell.

Once generated, action potential moves along the entire length of the membrane, until the whole membrane is depolarized.

Example: Action Potential

action-potentialThese point where these voltage gated channels open is called the threshold.

Increasing Vm past the threshold can cause an action potential in certain tissues like nerves.

Repolarisation

Once an action potential is generated and the cell membrane is depolarized, the sodium channels close and the potassium channels open. This allows the potassium ions to move out of the cell.

The movement of potassium ions out of the cell recreates negativity inside the cell. The cell membrane is said to be repolarized.

If the membrane potential becomes more negative than the resting membrane potential, it is termed as hyperpolarization.

During this state, a new action potential cannot be generated. It is therefore called refractory period or resting state as shown in Figure 3.0.

The cell membrane retains its polarized state by the sodium-potassium pump, which realigns the ions. Sodium ions which moved into the cell during action potential are expelled against the concentration gradient. Similarly, potassium ions are moved into the cell. The cell membrane regains the potential of -70 mV.

If another stimulus arrives, the membrane will be depolarized again and action potential will be generated.

Effect of Ion Manipulation on Membrane Potential

Under normal physiological conditions, the membrane potential fairly remains in a constant range. However, if there is an electrolyte imbalance, the equilibrium potential as well as the membrane potential change.

In case of hypokalemia, more potassium ions leak out of the cell during resting state, which changes the membrane potential to a more negative value of -90 mV.

Similarly, if hyperkalemia occurs, less potassium ions will move out of the cell through leaky potassium channels, and the resting membrane potential will become less negative.

ion

Effect of Current on Membrane Potential

The direction of the current decides the state of membrane potential. In case of efflux of positive ions, the cell membrane will become hyperpolarized.

If there is an influx of positive ions, the cell membrane may become depolarized and generate an action potential.

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