Equilibrium Potential vs. Reversal Potential

Attribute	Equilibrium Potential	Reversal Potential
Definition	The membrane potential at which the net flow of a particular ion is zero	The membrane potential at which the ion current is zero
Ion	Specific to each ion (e.g. Na+, K+)	Specific to each ion (e.g. Na+, K+)
Calculation	Determined by the Nernst equation	Determined by the Goldman equation
Role	Helps maintain resting membrane potential	Plays a role in determining action potential threshold

Introduction

Equilibrium potential and reversal potential are two important concepts in the field of physiology, particularly in the study of cell membranes and ion channels. While they may sound similar, they have distinct attributes that are crucial to understand in order to grasp the underlying mechanisms of cellular function. In this article, we will delve into the differences between equilibrium potential and reversal potential, exploring their definitions, calculations, and implications in biological systems.

Equilibrium Potential

Equilibrium potential, also known as Nernst potential, is the membrane potential at which the electrical gradient driving an ion across the membrane is exactly balanced by the chemical gradient. In other words, it is the voltage at which there is no net movement of a specific ion across the membrane. The equilibrium potential for a given ion can be calculated using the Nernst equation, which takes into account the concentration gradient of the ion across the membrane.

Equilibrium potential plays a crucial role in determining the resting membrane potential of a cell, as well as the direction and magnitude of ion fluxes during action potentials. For example, the equilibrium potential for potassium (EK) is typically around -70 mV in most cells, which means that the membrane potential of a cell at rest is close to this value due to the permeability of the membrane to potassium ions.

Furthermore, changes in the concentration gradient of ions can alter the equilibrium potential for that ion, leading to shifts in the resting membrane potential and affecting the excitability of the cell. For instance, an increase in extracellular potassium concentration can depolarize the cell membrane by shifting the potassium equilibrium potential towards a less negative value.

Reversal Potential

Reversal potential, also known as the Goldman equation, is the membrane potential at which the net flow of an ion across the membrane is zero. Unlike equilibrium potential, which refers to a specific ion, reversal potential takes into account the combined effects of multiple ions with different permeabilities. The Goldman equation considers the permeability of each ion and their respective equilibrium potentials to calculate the overall reversal potential.

Reversal potential is particularly important in neurons and other excitable cells, where the opening and closing of ion channels lead to changes in membrane potential. For example, during an action potential, the opening of sodium channels causes depolarization of the membrane potential towards the sodium reversal potential, which is typically around +60 mV.

Moreover, the concept of reversal potential is essential for understanding inhibitory and excitatory synaptic potentials in neurons. Inhibitory synapses typically involve the influx of chloride ions, which have a reversal potential close to the resting membrane potential. On the other hand, excitatory synapses involve the influx of sodium ions, which have a reversal potential significantly higher than the resting membrane potential.

Comparison

While equilibrium potential and reversal potential both involve the balance of electrical and chemical gradients across the membrane, they differ in several key aspects. Equilibrium potential is specific to a single ion and is calculated based on the concentration gradient of that ion, whereas reversal potential takes into account multiple ions and their permeabilities.

• Equilibrium potential is determined by the Nernst equation, which considers the concentration gradient of a specific ion, while reversal potential is calculated using the Goldman equation, which factors in the permeabilities of multiple ions.
• Equilibrium potential is crucial for establishing the resting membrane potential of a cell and determining the direction of ion fluxes, whereas reversal potential is more relevant in situations where multiple ions are involved in generating membrane potential changes.
• Changes in the concentration gradient of ions can shift the equilibrium potential for that ion, leading to alterations in the resting membrane potential, while changes in ion channel permeability can affect the reversal potential by altering the relative contributions of different ions.

Conclusion

In conclusion, equilibrium potential and reversal potential are fundamental concepts in the study of membrane physiology and cellular excitability. While equilibrium potential pertains to the voltage at which the net flow of a single ion is zero, reversal potential considers the combined effects of multiple ions with different permeabilities. Understanding the distinctions between these two concepts is essential for unraveling the complex mechanisms underlying the generation and propagation of electrical signals in biological systems.