Membrane Potential -2 10/5/10

Cells have a membrane potential, a slightly excess of negative charges lined up along the inside of the membrane and a slight excess of positive charges on the outside of the membrane.

The specialization of nerve and muscle cells depends on the ability of these cells to alter their potential on appropriate stimulation. The plasma membrane maintains the difference in the ionic concentration between the cell interior and exterior.

These ionic differences are important in the electrical activity of the cells. Also the plasma membrane plays an important role in the ability of the cell to respond to changes or signals, in the enviroment.

Diffusion of K+ and Na+ ions can take place through the channels present in the cell membrane which are called K+ and Na+ ion leak channels. Na and K-ion inaddition, to the active carrier mechanism can passively cross the memb. through theses protein channels that are specific for them.

These sodium channels accommodate the passage of only Na-ions whereas only K-ions can pass through the K- channels. This selectivity is due to specific arrangements of various chemical groups on the interior surfaces of the proteins that form the channel walls.

The channel can open or close to its specific ion as a result of changes in the channel shape and in response to the controlling mechanisms.

Various ions tend to diffuse from one side of the membrane to the other depending upon their Electrochemical gradients. The permeability of cell membrane to these ions which varies greatly.

K-ion can cross more easily as the memb. has more channels open for it. Also at resting potential in a nerve cell membrane is 50-70% more permeable to K-ion than Na-ion.

There are still other processes by which these ions can move across the cell membrane these are: The voltage-gated Na+ and K+ channels. The electrogenic Na+ K+ pump.

Effect Of Na-K Pump: About 20 % of the membrane potential is directly, generated by the Na-K pump. This active transport mechanism pumps three Na-ions out for every two K-ions transported in.

Na and K ions are both positive ions, this unequal transport generates a membrane potential with outside becoming relatively positive and than inside as more positive ions are transported out than in. The remaining 80% is caused by the passive diffusion of K and Na-ions down the conc. gradients.

Effect Of Movement Of Potassium Alone On The Membrane Potential

Initially, the concentration gradient is stronger than the electrical gradient so, the net diffusion of K –ion out of the membrane would continue and the membrane potential would increase.

As more and more K-ion move out the electrical gradient also increases as the outside becomes increasingly positive and inside becomes more negative.

Effect Of Movement Of Sodium Alone On The Membrane Potential

Resting Membrane Potential of Nerves

The resting membrane potential of large nerve fibers when not transmitting nerve signals is about -90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber.

Sodium-Potassium (Na+-K+) Pump. All cell membranes of the body have a powerful Na+ K+-pump. It is an electrogenic pump that pumps more positive charges outside than to the inside.

It continuously pumps 3 Na-ions to the outside of the cell and 2 K-ions to the inside of the cell. Leaving a net deficit of positive ions on the inside this causes a negative potential inside the cell membrane.

The Na-K- pump cause large concentration gradients for sodium and potassium across the resting nerve membrane: Na+ (outside):142 mEq/L Na+ (inside):14 mEq/L K+ (outside):4 mEq/L K+ (inside):140 mEq/L

Potassium-Sodium Leak Channel. These are protein channels in the nerve membrane through which potassium and sodium ions can leak. These channels are 100 times more permeable to potassium than to sodium.

Origin of the Normal Resting Membrane Potential Contribution of the K-ion diffusion potential. Contribution of the Na-ion diffusion through the nerve membrane. Contribution of the Na-K pump.

Na-K-Pump

Diffusion of potassium and sodium alone would give a membrane potential of about -86 millivolts (mainly due to potassium diffusion). An additional - 4 millivolts is contributed to the membrane potential by the electrogenic Na+-K+ pump, giving a net membrane potential of -90 millivolts.

Nerve Action Potential Nerve signals are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane by action potential.

Action potential begins Sudden change of normal resting negative membrane potential To positive potential Ends by rapid change back to the negative potential. A.P moves along the whole nerve fiber.

Stages Of Action Potential

Resting Stage. It is the resting membrane potential before the action potential begins. The membrane is "polarized" with -90 millivolts negative membrane potential. Depolarization Stage. The membrane suddenly becomes very permeable to sodium ions.

It allows tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon. Inflow of positive Na-ion immediately neutralizes -90 millivolts memb. Potential. Leading to rapidly rising potential in the positive direction.

Large Nerve Fibers: Great excess of positive sodium ions moving to the inside. Causing the membrane potential to actually "overshoot" That is it crosses beyond the zero level and to become somewhat positive.

Whereas, in some: Some Smaller fibers Many Central nervous system neurons. The potential merely approaches the zero level and does not over­shoot to the positive state.

Repolarization Stage Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions. The sodium channels begin to close The potassium channels open more than normal.

Lead to rapid diffusion of potassium ions to the exterior Re-establishes the normal negative resting membrane potential. Which is called repolarization of the membrane.

Transport Channels In The Nerve Membrane

Special characteristics of two other types of transport channels through the nerve membrane area: Voltage-gated sodium channels. Voltage-gated Potassium channels.

Depolarization of the nerve membrane during the action potential is mainly by the voltage-gated sodium channel. Voltage-gated potassium channel play an important role in increasing the rapidity of repolarization of the membrane.

Voltage-Gated Sodium Channel It has two gates: Activation Gate: Near the outside of the channel. Inactivation Gate. Near the inside of the channel.

when the membrane potential is -90 millivolts. The activation gate of Na-ion is closed. Which prevent any entry of sodium ions to the interior of the fiber through these sodium channels.

Activation of the Sodium Channel.

Membrane potential becomes less negative during the resting state, rising from -90 millivolts toward zero, Due to a sudden conformational change in the activation gate, flipping it all the way to the open position.

It is called the activated state During this state, sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to fold.

The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. Conformational change causes the inactivation gate to close which is a slower process.

Sodium ions no longer can pour to the inside of the membrane. K- channels open just about at the same time when the sodium channels are beginning to close because of inactivation.

At this point, the membrane potential begins to recover back toward the resting membrane state, which is the repolarization process. Inactivation gate will not reopen until the membrane potential returns to or reaches near the original resting membrane potential level.

Voltage-Gated Potassium Channel Can occur in two states: During the resting state Toward the end of the action potential.

During the resting state, the gate of the potassium channel is closed, preventing efflux of K-ion. When the membrane potential rises from -90 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium efflux through the channel.

The decrease in sodium entry to the cell and the simultaneous increase in potassium exit. Both processes speed the repolarization process. Leading to full recovery of the resting membrane potential within another few 10,000ths of a second.

Summary Opening of the potassium channels. These open slowly and reach their full open state only after the sodium channels have almost completely closed.

Once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value.

End Of Todays Lecture