1.2 Membrane Potential: Its components and maintenance
1.2 Membrane Potential: Its components and maintenance
The plasma membrane surrounding every cell is selectively permeable. By itself, the lipid bilayer is not permeable to any ions. However, the introduction of channels and transporters allows for the exchange of some, but not all, ions. Channels are always selective for one to two ion types.
In a single ion system where the membrane is perfectly permeable to that ion, particles would exchange via channels until equilibrium is achieved. Given any concentration differential, an equilibrium potential that achieves equilibrium can be calculated. If the current charge differential across the membrane results in a membrane potential that deviates from the equilibrium potential, then ions would be exchanged in net until equilibrium is achieved.
In the brain, the concentration of ions surrounding a resting state neuron have already been characterized (Table 1). If a neuron was to be made perfectly permeable to only one ion, but perfectly impermeable to all other ions, then the Nernst equation could be used to calculate equilibrium potential. Ions would then be exchanged across the membrane to bring membrane potential closer to equilibrium. In this theoretical case, if one was to plug the numbers from Table 1 into the Nernst equation, they would calculate an equilibrium potential of +50mV for Na, -90mV for Potassium, +135mV for Calcium and -70mV for Chloride.
| Ion | Intracellular Concentration (mM) | Extracellular concentration (mM) |
| K+ | 140 | 4 |
| Na+ | 15 | 145 |
| Cl– | 4 | 110 |
| Ca2+ | 0.0001 | 5 |
In reality, neurons exist as multi-ion systems, with varying membrane permeability for different ions. The plasma membrane surrounding a neuron is characterized as being selectively permeable. By itself, the lipid bilayer which forms the membrane is not permeable to any ions. However, the introduction of channels and transporters allows for the exchange of some, but not all, ions. Channels are always selective for one to two ion types. Therefore, if no channels exist to facilitate the transfer of one ion type, then the membrane is impermeable to it. If a cell has a low concentration of sodium channels but a high concentration of potassium channels, then it is permeable to both, but relatively much more permeable to potassium. There are two major types of passive transport channels; constitutive/leak channels and gated channels. The former are always open and facilitate the transfer of ions towards equilibrium constitutively. The latter are only opened after an event has occurred, lifting the proverbial gates that otherwise keep the channel closed and unable to facilitate ion transfer. Gated channels exist in two major subtypes: ligand-gated and voltage-gated. The former is only open when chemically bound by a ligand, while the latter only opens at certain voltage thresholds.
In reality, neurons exist as multi-ion systems, with varying membrane permeability for different ions. As a consequence, the