Membrane Potentials

1. What is a Membrane Potential?
2. The Resting Membrane Potential
3. Changing the Membrane Potential
4. Synaptic Transmission
5. Quiz

• The membrane potential is the difference in electrical potential (the voltage) across the plasma membrane.
• Generating a membrane potential requires both ionic concentration gradients (different ion concentrations inside and outside the cell) and selective permeability (the membrane lets some ions cross more easily than others).
• An ion’s equilibrium potential is the membrane potential when the membrane is selectively permeable to that ion only; it can be calculated using the Nernst equation.
• The membrane potential can be altered by opening and closing different ion channels.
• A synapse is a connection between two excitable cells; synaptic transmission can be fast or slow, and synapses can be excitatory or inhibitory.

What is a Membrane Potential?

A membrane potential is the difference in electrical potential (the voltage) across a plasma membrane, expressed as the potential inside the cell relative to the extracellular solution (the fluid outside the cell). It is measured in millivolts (mV).

All cells have a membrane potential, although the resting membrane potential and the ability to change it (excitability: how readily a cell can produce electrical signals called action potentials) vary between different cell types. Membrane potentials are the basis of electrical signalling in excitable cells such as neurones and muscle cells, and in other cells they help to control processes such as cell volume and the cell cycle (the cycle of cell growth and division).

Table - The resting membrane potentials of different cell types

SimpleMed original by Dr. Joshua Bray

The establishment of a membrane potential requires two key criteria:

1. Ionic concentration gradients (resulting in an asymmetrical distribution of ions)
2. Selective permeability to particular ions (the membrane lets some ions cross more easily than others, via ion channels) - mainly potassium (K⁺), sodium (Na⁺) and chloride (Cl^-)

The movement of ions results in a separation of charge across the membrane: this is the membrane potential.

The Na⁺/K⁺ pump (a protein that uses energy to move sodium out of the cell and potassium in) is slightly electrogenic: it moves three Na⁺ out for every two K⁺ in, which adds a small negative component to the resting membrane potential. However, its main role is indirect: it sets up and maintains the ionic gradients required to generate the membrane potential, and without it the membrane potential would gradually run down.

Diagram - The ionic concentration gradients responsible for the membrane potential

Creative commons source by smonsays [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The Resting Membrane Potential

There are two gradients which determine the net movement of ions:

1. Chemical gradient - Equivalent to the concentration gradient.
2. Electrical gradient - The tendency of positive ions to move to areas of negative charge and negative ions to move to areas of positive charge.

Assume that the membrane is selectively permeable to one ion only - for example, K⁺. If the concentration of K⁺ is greater inside the cell compared to outside the cell, then K⁺ will begin to move out of the cell. However, as K⁺ leaves the cell, the cell will become less positive and the extracellular solution will become more positive - this causes the opposing electrical gradient to increase. When the chemical and electrical gradients become equal and opposite, they cancel each other out so that there is no net movement of K⁺. However, a negative charge remains across the membrane. In this simplified case, that potential is the equilibrium potential for K⁺ (defined below). In a real cell, the resting membrane potential is strongly influenced by K⁺ but other ions also cross the membrane, so it is less negative than the equilibrium potential for K⁺ (in a neurone, about -70 mV compared with -95 mV).

The membrane potential when the membrane is selectively permeable to only one type of ion is known as the equilibrium potential (E) for that ion. We can calculate an ion’s equilibrium potential using the Nernst equation. The Nernst equation for K⁺ is as follows: E_K = (RT/zF) ln([K⁺]_out/[K⁺]_in), where:

• R is the gas constant (8.314 J mol^-1 K^-1)
• T is temperature in kelvin (K)
• z is the charge on the ion (+1 for K⁺, -1 for Cl^-)
• F is the Faraday constant, the charge carried by one mole of singly charged ions (96500 C mol^-1)
• [K⁺]_out and [K⁺]_in are the concentrations of K⁺ outside and inside the cell respectively.

To make calculations easier, we can work out the value of RT/F (T = 37°C, which is 310 K) and convert the natural log (ln) to log₁₀ (the base-10 logarithm, as on a calculator). This gives a constant of about 61 mV, so: E_K = (61 mV / z) × log₁₀([K⁺]_out / [K⁺]_in).

The Nernst equation can be used for any ion (both positive and negative ions).

If a membrane is selectively permeable to K⁺ alone, its membrane potential will be at E_K. However, in reality a membrane is never permeable to K⁺ alone, as real cells are permeable to other ions too.

The equilibrium potentials of K⁺, Na⁺ and Cl^- at physiological (normal body) temperature and concentrations are as follows:

• E_K = -95 mV
• E_Na = +60 mV
• E_Cl = -70 mV to -90 mV

Preview the Membrane Potentials quiz