Resting Potential

Resting Potential

Resting Potential At rest the cell membrane is polarized maintaining a negative interior charge of -70mv. This is called the electrical gradient. There is about ten times more sodium (Na+) on the outside and twenty times more potassium (K+) on the inside. This is called the concentration gradient. So how is this uneven distribution of electrically charged ions established? In part by properties of the membrane, and in part by the sodium/potassium pump. Remember that the membrane is semipermeable. Oxygen, carbon dioxide, urea, and water cross freely, but larger or electrically charged ions and molecules usually canÕt cross. Certain ions, potassium (K+), chloride (Cl-), and sodium (Na+) cross the membrane at specialized gates. The gates control the rates at which these ions enter. The Na+ gates are closed until the membrane is depolarized, so Na+ is held out. Therefore Na+ builds up in higher concentrations on the outside of the membrane. The Na+ ions are strongly attracted however to the negative interior of the neuron. The K+ ions, which are in greater abundance on the inside of the membrane, can cross the membrane at a controlled rate. Potassium tends to passively diffuse out of the cell because it is drawn to an area of lesser concentration, but it is partially held in the cell by the negative interior. Also the sodium/potassium pump returns K+ to the interior after it has diffused out. Chloride, which also crosses at a controlled rate, as well as large negatively charged proteins, also help to establish the negative interior. So Na+ is held out, K+ diffuses out, but is brought back in, and this imbalance of positive and negative charges (the electrical gradient) stops the movement of K+ ions. As a result, an equilibrium is reached at which point there is no net movement of K+ ions across the membrane (as many K+ are brought back in as diffuse out). This uneven distribution causes the excess of positive charge outside with respect to the -70mv interior. This is the resting potential and the neuron is ready to fire.

Glossary

Neuron Membrane

A) The membrane of a neuron is about 8 nm thick and is made up of two layers of fat molecules with larger protein molecules embedded in the fats.

B) The membrane is said to be semipermeable, which means that it allows some substances to pass but holds other substances out.

C) In its polarized state the membrane is not permeable to NA+ but it is permeable to K+ (potassium) so there is a tendency for K+ ions to leak out of the cell. The large negative anions can not diffuse across the cell membrane thus making the inside of the cell electrically negative (-70mv). This tends to attract the positive sodium (Na+) ions from the cell's exterior to reestablish electrical neutrality. The attraction of Na+ ions is counteracted by the cell's lack of permeability to sodium as well as its active transport of Na+ ions out of the cell against their own concentration gradient by the sodium-potassium pump.

Glossary

Sodium Potassium Pump

The pump is energy-dependent, but the work expended amounts to only a small fraction of the energy made available by the cell's own metabolism. The pump moves three sodium (Na+) ions out of the cell for every two potassium (K+) ions that it brings in, and because of the outward leakage of potassium and the presence of negatively charged ions within, the cell maintains a separation of charges; the inside of the neuron is made negative relative to the outside and is thus polarized. (-70Mv). This is called the membrane's resting potential.

Glossary

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