Membrane potential – Definition, Types, Equilibrium and Ions

The membrane potential is crucial for the functioning of cells, particularly in the nervous system. It helps in the transmission of signals between neurons. The membrane potential of neurons is important for the transmission of nerve impulses. Understanding the membrane potential graph and equation helps us in learning how nerve cells communicate.

In this article, we will cover resting membrane potential, membrane potential and action potential and more, in detail.

Membrane potential is the electrical difference across a cell’s membrane. It is created by the unequal distribution of ions, like sodium (Na+) and potassium (K+), inside and outside the cell. This difference in charge is essential for various cellular processes, especially in nerve and muscle cells, where it plays a key role in transmitting signals. The membrane potential can change rapidly, allowing cells to respond to stimuli and communicate with each other.

The resting membrane potential, typically around -70 millivolts (mV) in neurons, is maintained by ion pumps and channels, primarily the sodium-potassium pump, which actively transports Na+ out of the cell and K+ into the cell. Changes in membrane potential, called depolarization and hyperpolarization, allow cells to respond to stimuli and communicate through electrical signals. These changes are fundamental to the functioning of the nervous system and muscle contraction.

Membrane potentials are defined by various ionic attention configurations outside and in the membrane of a cellular. These potentials are:

• Resting membrane potential: the membrane ability at rest, steady-nation situations.
• Action potential: a non-graded ability, similar to binary code (on/off).
• Post-synaptic potentials: graded potentials, that may be summated/subtracted by using modulation from presynaptic neurons.

Membrane potential is the electrical potential difference across a neuron’s membrane, resulting from the unequal distribution of ions such as sodium (Na+), potassium (K+), and chloride (Cl-) inside and outside the neuron. This potential is crucial for neuron function and communication. The resting membrane potential, typically around -70 millivolts (mV), is maintained by the sodium-potassium pump, which transports three Na+ ions out of the neuron and two K+ ions into the neuron, creating a negative charge inside the cell.

When a neuron receives a signal, ion channels open, causing depolarization as Na+ ions flow in, making the inside less negative. If this change reaches a threshold, an action potential is triggered, sending an electrical signal along the neuron’s axon. Afterward, repolarization occurs as K+ ions flow out, restoring the resting potential. These rapid changes are essential for the nervous system’s function, enabling quick and efficient information transmission.

Resting membrane potential (RMP) is the difference in electric potential between the intracellular and extracellular matrices of a cell when it is not actively transmitting signals. While every cell in the body possesses its own membrane potential, only excitable cells like nerves and muscles are capable of altering and generating action potentials.

For excitable cells, the membrane potential during rest is termed the resting membrane potential, with variations associated with the generation of action potentials. RMP arises from the differing concentrations of ions, expressed in millimoles per liter (mmol/l), on either side of the cell membrane. Various tissues exhibit distinct RMP values:

• Skeletal muscle mobile = -90 millivolts (mV) 
• Smooth muscle cellular = -55mV
• Cardiac muscle mobile = -80mV
• Neuron = -65mV 

Negative values indicate that the cytoplasm is more electronegative than the extracellular space. The values of RMP are influenced by several factors:

• Concentration gradients of ions inside and outside the cell, primarily sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-).
• Activity of the sodium-potassium pump, which actively transports ions against their concentration gradients.
• Variable permeability of the cell membrane to ions.

Producing and Maintaining Resting Membrane Potential (RMP)

RMP is produced and maintained by:

• Donnan effect-described as large impermeable negatively charged intracellular molecules attracting positively charged ions (e. g.: Na+ and K+) and repelling negative ones (e. g.: Cl−)
• Membrane selectivity is the difference in permeabilities between different ions
• Active transport (Na+/K+ ATPase pump)-is the mediated process of moving particles across a biological membrane, against the concentration gradient.
  • Primary active transport – if it spends energy. This is how the Na+/K+ ATPase pump functions.
  • Secondary active transport – if it involves an electrochemical gradient. This is not involved in maintaining RMP.

Affection of Resting Membrane Potential by Ion Channels

Protein channels in the cell membrane allow ions to diffuse passively. These channels, including potassium-selective and sodium-selective ion channels, facilitate the movement of ions across the membrane, with potassium (K+) channels being more permeable due to their higher abundance.

The membrane potential equation, also known as the Nernst equation, calculates the equilibrium potential for a specific ion based on its concentration gradient across the cell membrane. The equation is:

[Tex]E ion​=RT/ZF​ ln ([ion inside] / [ion outside]​)[/Tex]

where:

• ?ion​ is the equilibrium potential for the ion.
• ?R is the universal gas constant (8.314 J/(mol·K)).
• ?T is the temperature in Kelvin.
• ?z is the valence (charge) of the ion.
• ?F is Faraday’s constant (96,485 C/mol).
• [ion outside][ion outside] is the ion concentration outside the cell.
• [ion inside][ion inside] is the ion concentration inside the cell.

For neurons, the Goldman-Hodgkin-Katz (GHK) equation is more accurate for calculating the membrane potential because it considers the permeability and concentrations of multiple ions (Na+, K+, and Cl-):

[Tex]E_{\text{ion}} = \frac{zF}{RT} \ln \left( \frac{[\text{ion}]_{\text{inside}}}{[\text{ion}]_{\text{outside}}} \right)'[/Tex]

Where:

• ??Vm​ is the membrane potential.
• ??+,???+,PK+​,PNa+​, and ???−PCl−​ are the permeabilities of potassium, sodium, and chloride ions, respectively.

These equations are fundamental in understanding how ions contribute to the electrical potential across the cell membrane, influencing how neurons transmit signals.

While some cells exhibit continually changing RMP, others have stable resting potentials, measurable by inserting electrodes into the cell or using dyes that alter their optical properties based on membrane potential. The values of resting membrane potential vary across cell types, influencing their physiological functions.

Resting membrane potential varies according to the types of cells
For example:

• Skeletal muscle cells: −95 mV
• Smooth muscle cells: −50 mV
• Astrocytes: −80/−90 mV
• Neurons: −70 mV
• Erythrocytes: −12 mV

The difference between membrane potential and action potential is given below:

Understanding membrane potential and action potential is essential for comprehending how cells, particularly neurons and muscle cells, transmit signals. Membrane potential represents a cell’s steady-state electrical charge, while action potential involves rapid changes that propagate signals. These processes rely on different ion channels and mechanisms, with membrane potential being static and action potential dynamic. Together, they play crucial roles in cellular communication and response to stimuli.

Also Read:

• Plasma Membrane – Definition, Structure, Components, Functions
• Diagram of Cell Membrane
• Diagram of Plasma Membrane
• Cell Membrane
• Transport Across Cell Membrane
• Nuclear Membrane – Function, Structure, and Diagram

What do you Mean by Membrane Potential?

Voltage difference across a cell membrane due to unequal ion distribution (mainly K+ inside, Na+ outside) is called membrane potential.

What is Membrane Potential in Biology?

It’s the electrical potential difference created by ion concentration gradients across a membrane.

What do you Mean by Transmembrane Potential?

Transmembrane potential is the electric potential difference across a cell’s membrane.

What is the Resting Membrane Potential of Neuron?

The resting membrane potential of a neuron typically ranges from -65 to -70 millivolts (mV).