Chapter 51 – Resting Membrane Potential

Abstract

The cell membrane provides a selectively permeant electrical barrier between the intracellular and extracellular compartments. The membrane potential of a cell is the electrical voltage of its interior relative to its exterior. The electrical potential across the membrane will be zero when there are exactly equal numbers of positively and negatively charged ions on either side of the cell membrane. A non-zero membrane potential arises from inequalities in the distribution of charged ions across the cell membrane.

Chapter 51 Resting Membrane Potential

How is the membrane potential produced?

The distribution of ions across a cell membrane results from a combined effect of:

The different ionic compositions of the intracellular fluid (ICF) and extracellular fluid (ECF).
The selective permeability of the cell membrane to the different ions.
The presence of negatively charged intracellular proteins, whose large molecular weight and charge mean that they are unable to cross the cell membrane. These proteins tend to bind positively charged ions and repel negatively charged ions.

Thus:

A negative membrane potential is produced when there is a greater number of positively charged ions on the outside of the cell membrane relative to the inside.
A positive membrane potential is produced when there is a greater number of positively charged ions on the inside of the cell membrane relative to the outside.

A quiescent cell typically has a negative resting membrane potential (RMP). RMP is more negative in excitable cells (–70 mV in nerve cells, –90 mV in skeletal muscle cells) than non-excitable cells (around –30 mV). Cell excitation results in an action potential (see Chapter 52), a transient change in the membrane potential from the RMP to a positive value; the cell membrane has been briefly depolarised.

The RMP is particularly influenced by the concentrations and membrane permeability of three major ions:

K⁺: the intracellular K⁺ concentration is normally greater (150 mmol/L) than the extracellular K⁺ concentration (5 mmol/L). The phospholipid bilayer of the cell membrane itself is impermeable to K⁺ ions, as they are polar. However, the cell membrane contains open K⁺ leak channels¹ that permit K⁺ to pass down its concentration gradient from the ICF to the ECF.
Na⁺: the extracellular Na⁺ concentration is higher (140 mmol/L) than the intracellular Na⁺ concentration (20 mmol/L). Na⁺ ions are polar and therefore do not traverse the cell membrane, and Na⁺ channels present in the membrane are normally closed at RMP, leaving the resting cell membrane impermeable to Na⁺.²
Cl‾: membrane permeability varies with cell type:
1. – In neurons, the cell membrane is relatively impermeable to Cl‾: permeability to Cl‾ is about 1000 times less than that of K⁺, and therefore its contribution is often ignored.
2. – Muscle contains open membrane Cl‾ channels. Cl‾ therefore distributes itself across the cell membrane passively according to its electrochemical gradient. At RMP, Cl‾ is driven out of the cell by the negatively charged cell interior. However, membrane depolarisation results in a positively charged cell interior, producing a Cl‾ influx. Therefore, in most cells, Cl‾ movement does not influence the RMP; rather, the membrane potential passively influences Cl‾ movement. Passive Cl‾ movements do, however, act like a ‘ballast’, making changes to the RMP more difficult, thus increasing membrane stability.

What is the Nernst equation?

Consider a particular membrane-permeant ion, X:

X will distribute on either side of a cell membrane according to its chemical (i.e. concentration) and electrical gradients across the membrane.
The movement of X ceases when the net chemical and electrical gradients of X across the membrane are zero; that is, at electrochemical equilibrium.
The contribution that ion X makes to the RMP may be calculated using the Nernst equation from its valency, the concentration difference across the membrane and the temperature: