RESTING MEMBRANE POTENTIALOverview • Ions flow along their electrochemical gradient (combination of concentration gradient and electric potential) • Neurons have open channels ("leak" channels) that allow potassium and sodium ions to travel across the membraneCREATION OF RESTING POTENTIAL BY POTASSIUM ONLY Stage 1 • We show a cell within an enclosed environment and specify the higher concentration of potassium within the cell. • The membrane potential is zero at the beginning. • Next, we introduce a potassium leak channel, which allows potassium to pass through based on its concentration gradient. Stage 2 • The efflux of potassium ions out of the cell makes the inside of the cell negatively charged. • There is a large concentration gradient driving potassium ions out of the cell. • A weak electric force attract potassium ions (which are positively charged) back inside the cell. • However, the force from the concentration gradient overpowers the electric force, so potassium ions continue to leave the cell though slower than before. Stage 3 • There is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell). • The concentration gradient still favors an efflux of potassium ions, but is weaker than before (because of the higher concentration of potassium ions in the extracellular space than before). • The electric force is stronger than before, because there is a greater negative charge within the cell. • So now both forces are equivalent, so the cell has reached its equilibrium potential and there is no net movement of potassium ions. • For potassium, the membrane equilibrium potential is about -90 mV.CREATION OF RESTING POTENTIAL WITH BOTH SODIUM AND POTASSIUM This scenario approximates what actually happens with neurons. Stage 1 • There is a high sodium concentration outside the cell and high potassium concentration inside the cell. • We show the sodium and potassium leak channels. • There is sodium influx into the cell and potassium efflux out of the cell (based on their concentration gradients). - Pay attention that our potassium arrow is larger than the sodium arrow because there are more potassium "leak" channels than sodium "leak" channels, not as a reflection of the volume of individual ions that pass through the channel types. Stage 2 • The concentration gradients still promote the influx of sodium into the cell and the efflux of potassium out of the cell. • There is still a slight negative charge within the cell and the slight positive charge outside the cell. • We show an electrical force that enhances the movement of sodium (a positively charged ion) into the cell and opposes the movement of potassium (also a positively charged ion) out of the cell. • This force slows down the flow of potassium ions and speeds up the flow of sodium ions. Stage 3 • Now we show that there is a greater negative charge (inside the cell) than before and a greater positive charge (outside the cell). • Ultimately the ion movement of sodium influx into the cell is balanced by the potassium efflux (make both arrows of equal size). - The resting state of the neuron refers to the steady state that occurs when there is no longer any net charge movement across the membrane (each positive charge that leaves the cell as a potassium ion is replaced by a positive charge entering the cell as a sodium ion). • The resting potential is measured at about -70mV. • The sodium/potassium pump we learned about elsewhere transports sodium out of the cell and potassium back into the cell to continually "recharge" their concentration gradients.