Resting Membrane Potential

Resting Membrane Potential
The human organism is composed of multiple cells, all of them with different components and therefore with differents Resting Membrane Potentials.

Some of these cells are excitable (eg:. cardiac pacemaker cells; neurons; muscle fibers), generating an action potential when subjected to an external stimulus, causing its membrane depolarization.

The Resting Membrane Potential (RMP) is due to changes in membrane permeability for potassium, sodium, calcium, and chloride, which results from the movement of these ions across it.

Once the membrane is polarized, it acquires a voltage, which is the difference of potentials between intra and extracellular spaces.

What is a RMP?
Resting Membrane Potential is:


 * the unequal distribution of ions on the both sides of the cell membrane;


 * the voltage difference of quiescent cells;


 * the membrane potential that would be maintained if there weren’t any stimuli or conducting impulses across it;
 * determined by the concentrations of ions on both sides of the membrane


 * a negative value, which means that there is an excess of negative charge inside of the cell, compared to the outside.

How is the Resting Membrane Potential produced and maintained?
RMP is produced and maintained by:


 * Donnan Effect

described as large impermeable negatively charged intracellular molecules attracting positively charged ions (eg:. Na+ and K+) and repelling negative ones (eg:. Cl-)


 * Membrane Selectivity

is the difference of permeabilites 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

Secondary Active Transport – if it involves an electrochemical gradient

How do ions affect Resting Membrane Potential?
RMP is created by the distribution of ions and its diffusion across the membrane.

Potassium ions are important for RMP because of its active transport, which increase more its concentration inside the cell.

However, the potassium-selective ion channels are always open, producing an accumulation of negative charge inside the cell. Its outward movement is due to random molecular motion and continues until enough excess negative charge accumulates inside the cell to form a membrane potential.

How does the Na+/K+ - ATPase pump affect the RMP?
The Na+/K+ - ATPase pump creates a concentration gradient by moving 3 Na+ out of the cell and 2 K+ into the cell.

Na+ is being pumped out and K+ pumped in against their concentration gradients.

Because this pump is moving ions against their concentration gradients, it requires energy.

How do ion channels affect Resting Membrane Potential?
The cell membrane contains protein channels that allow ions to diffuse passively without direct expenditure of metabolic energy.

These channels allow Na + and K+ to move across the cell membrane from a higher concentration toward a lower.

As these channels have selectivity for certain ions, there are potassium- and sodium- selective ion channels.

All cell membranes are more permeable to K+ than to Na+ because they have more K+ channels than Na+.

The Nernst Equation
Is a mathematical equation applied in Physiology, to calculate equilibrium potentials for certain ions.

$$Ei = R.T/F.z ln [X]1/[X]2$$

Where:


 * R = Gas Constant
 * T = Absolute temperature (K)
 * E = The potential difference across the membrane
 * F = Faradays Constant (96500 coulombs/mole)
 * z = Valency of ion

The Goldman-Hodgkin-Katz Equation
Is a mathematical equation applied in Physiology, to determine the potential across a cell's membrane, taking in account all the ions that are permeable through it.

$$Em = 58 log (PNa*[Na]out + PK*[K] out / PNa*[Na]in + PK*[K]in)$$

Where:


 * E = The potential difference across the membrane
 * P = Permeability of the membrane to sodium or potassium
 * [ ] = Concentration of sodium or potassium inside or outside

Measuring resting potentials
In some cells, the RPM is always changing. For such, there is never any resting potential, which is only a theoretical concept.

Other cells with membrane transport functions that change potential with time, have a resting potential.

This can be measured by inserting an electrode into the cell. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential.

Resting Membrane Potential varies according to 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

Refferences
Internet:


 * http://www.wisegeek.com/topics/the-resting-membrane-potential.htm


 * http://en.wikiversity.org/wiki/The_Resting_Membrane_Potential


 * http://people.eku.edu/ritchisong/301notes2.htm


 * http://www.getbodysmart.com/ap/nervoussystem/neurophysiology/restingpotentials/measurement/tutorial.html


 * http://www2.yvcc.edu/Biology/109Modules/Modules/RMP/RMP.htm


 * http://en.wikipedia.org/wiki/Resting_potential


 * http://advan.physiology.org/cgi/content/full/28/4/139


 * http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-13/CB13.html


 * http://www.colorado.edu/intphys/Class/IPHY3730/03plasmamembrane.html


 * http://www.biologymad.com/NervousSystem/nerveimpulses.htm


 * http://www.ucl.ac.uk/~sjjgsca/NerveRestingPot.html


 * http://jimswan.com/237/channels/channel_graphics.htm

Books:


 * Hall, John E., and Arthur Clifton Guyton. Guyton and Hall Textbook of Medical Physiology. 12th ed. Philadelphia, PA: Saunders/Elsevier, 2011. Print.


 * Ward, Clarke, Linden. Physiology at a Glance. 3. Blackwell publishing


 * W. Gannon, Review of Medical Physiology. Lange Medical Books/McGraw-Hill, 21st edition


 * R. Berne, M. Levy, Physiology. Mosby, 4th edition


 * A. Despopoulos, S. Silbernagl, Color Atlas of Physiology. 5th edition