The cell membrane is a lipid bi-layer that separates the intra- and extra-cellular fluids. Sodium ions are actively pumped out of the cell in exchange for potassium ions , a process that depends on the sodium-potassium pump which uses energy in the form of adenosine triphosphate (ATP); this pump creates differences in the ionic composition of these fluid compartments.
Neurones have a transmembrane potential, with the inside of the neurone being negative with respect to the outside. This potential is dependent on the presence of potassium channels in the neuronal cell membrane
The neuronal cell membrane consists of a bi-lipid layer with a high electrical resistance, and containing protein insertions such as passive channels and active pumps that allow the movement of ions.
This membrane separates the intra- and extra-cellular fluids, which have different ionic compositions.
The presence of channels that allow only selected ions to move across the membrane allow the generation of a transmembrane potential; this Resting Potential is largely due to the permeability of the membrane to potassium ions.
Key Words: Active Transport; Sodium-Potassium Pump; Ion channels; Electrochemical Equilibria; Nernst Equation; Membrane permeability to ions.
Composition of Intracellular and Extracellular Fluid
The cell membrane is a lipid bi-layer that separates the intracellular and extracellular fluids, which have every different ionic compositions as a result of the action of the sodium-potassium pump.
high [Cl -]
high [K+] (150mM)
Sodium ions are in high concentration outside the membrane, because sodium is pumped out of the cell, and cannot re-enter the cell because the resting membrane is impermeable to sodium.
Potassium ions are in high concentration inside the cell; as a result potassium ions tend to move out of the cell down their concentration gradient through open potassium channels in the membrane. (The membrane is selectively permeable to potassium ions).
The movement of positively charged potassium ions through ion-selective pores in the membrane generates a transmembrane potential, which in turn attracts potassium ions back into the cytoplasm.
Movements of potassium in and out of the cell are balanced, i.e. form an equilibrium, because an electrical potential is generated by movement of the positively charged ion out of the cell. This equilibrium is known as an Electrochemical Equilibrium, and a physical equation, the Nernst Equation, describes the mathematical relationship between the chemical gradient and the electrical gradient that form the equilibrium.
The Nernst Equation for potassium states that:
E (mv)= -58 log10[K+ inside] / [K+ outside]
The Equilibrium Potential (or Electrochemical Potential) of Sodium, Potassium and Chloride, using their normal concentration gradients, are as follows:
Sodium : +60 mV
Potassium : -90mV
Chloride : -70mV
These are approximate figures because the concentrations of ions vary slightly.
These ions enter into electrochemical equilibria and move down their concentration and electrical gradients, because open ion-selective potassium channels in the membrane allow movement of potassium ions in either direction.
Outward movement of K+ down its concentration gradient is balanced by an equal inward movement down the electrical gradient.
How do neurones generate electrical currents?
Image source: www.anselm.edu
The Resting Potential is measured by inserting a microelectrode into a neurone.
When a microelectrode is inserted into an axon, a transmembrane potential can be measured: it is about -70mv to -90mv, meaning that the inside of the membrane is negative to the outside by 70 to 90mv.
This Potential Difference across the axonal membrane depends mainly on an electrochemical equilibrium for potassium which can move freely through open potassium channels in the cell membrane.
The Resting Potential is due mainly to the fact that open potassium channels allow [K+] to move freely and eqully across in either direction.
So the resting potential is determined mainly by an equilibrium of potassium ions:
the concentration difference of potassium between inside and outside the axon (causing OUTward movement).
the potential difference (resting potential) across the membrane (causing INward movement)
For the resting nerve membrane, the transmembrane potential is about -70mv (70mv negative inside with respect to the outside of the cell).
For the known concentration gradients of each ion, the Nernst Equation predicts an equilibrium potential of -90 mV and -70 mV for potassium and chloride, and + 60 mV for Na+
So the resting membrane is negaative and close to the equilibrium potential of K+ and Cl- but quite unlike the equilibrium potential of Na+.
The conclusion is that the resting membrane is permeable to K+ and,to ome extent, also Cl- , but impermeable to Na+.
More details on Ionic Equilibria
Some cells have a resting potential of -90mv - in which case a potassium equilibrium exists; for other cells with lower membrane potentials, chloride channels are also open to some extent.
In natural conditions, several ions may contribute to the membrane potential, and chloride is an important example in some cells.
The Goldmann Equation is a more complex physical consideration of ionic equilibria where several ions may be involved; it can be used to predict the transmembrane voltage predicted by ions that are in equilibrium across the membrane (in this case, Na+, K+ and Cl-):
Note that each ion has a factor 'p' with the Goldmann equation, which refers to the permeability of each ion. Sodium is included in the equation, but its permeability is negligible in resting neurones.
The Nernst and Goldmann Equations predict that an elevated external potassium concentration will affect the membrane potential. As expected from the Nernst Equation, raised external potassium levels cause the cell to depolarise (which is why cardiac arrhythmias are common when plasma K+ is raised).
Similarly, low potassium concentrations in the extracellular fluid are associated with hyperpolarisation of cells, which become less excitable.
Chloride channels are present in some neuronal membranes, such as the post-synaptic membrane of synapses, and these can be opened by the neurotransmitter GABA, which moves the potential towards ~-70mv by allowing chloride to move towards its equilibrium potential.
Membrane Resistance, Conductivity and Current
Ohm's law: describes the relationship between voltage, current and resistance: V=IR
V(millivolts)=I (milliamps) x Resistance (Ohms)
Ohms' law is a very basic equation relating a potential gradient and current flow through a Resistor. The current flow depends on the voltage gradient and the resistance (I=V/R).
For biological membranes the voltage gradient is the transmembrane potential. The resistance depends on the number if open channels and their nature; and current flow occurs when ions flow through those channels as they open or close.
In a later section there will be mention of the sodium currents at the start of the action potential. This is a measure of the amount of sodium ions moving across the membrane, due to the opening of sodium channels in response to an electrical stimulus (and flowing through voltage-gated sodium channels).