As covered in the previous lesson, At rest there is an uneven distribution of ions on either side of the membrane. The inside of the neuron is more negatively charged than the outside. For a typical neuron at rest, sodium, chloride, and calcium are concentrated outside the cell, whereas potassium is concentrated inside. This ion distribution leads to a negative membrane potential. How the ions are distributed across the membrane plays an important role in the generation of the resting membrane potential.
When the cell is at rest, some non-gated or leak ion channels are actually open. Significantly more potassium channels are open than sodium channels, and this makes the membrane at rest more permeable to potassium than sodium. Since the membrane is permeable to potassium at rest due to the open non-gated channels, potassium will be able to flow across the membrane.
The electrochemical gradients at work will cause potassium to flow out of the cell in order to move the cell's membrane potential towards potassium's equilibrium potential of negative 80 millivolts. You might ask though, if the cell has these open, non-gated ion channels, and ions are moving at rest, won't the cell eventually reach potassium's equilibrium potential? If the non-gated potassium channel is open, channels were the only open channels, the membrane potential would eventually reach potassium's equilibrium potential.
However, the membrane has other open, non-gated ion channels as well. There are fewer of these channels compared to the potassium channels, though. The permeability of chloride is about half of that of potassium, and the permeability of sodium is about 25 to 40 times less than that of potassium.
potassium. This leads to enough chloride and sodium ion movement to keep the neuron at a resting membrane potential that is slightly more positive than potassium's equilibrium potential. As ions move across the membrane, both at rest and when the neuron is active, the concentrations of ions inside and outside of the cell would change. This would lead to changes in the electrochemical gradient.
that are driving ion movement. What then maintains the concentration and electrical gradients critical for the ion flow that allows the neuron to function properly? The Zodium Potassium Potassium is a pump is the key.
The sodium-potassium pump is embedded in the cell membrane and uses ATP to move sodium out of the cell and potassium into the cell, maintaining the electrochemical gradients necessary for proper neuron functioning. Three intracellular sodium ions enter the pump. ATP is converted to ADP, which leads to a conformational change. change of the protein, closing the intracellular side and opening the extracellular side. The sodium ions leave the pump while two extracellular potassium ions enter.
The attached phosphate molecule then leaves, causing the pump to again open toward the inside of the neuron. The potassium ions leave and the cycle begins again. It is possible to calculate the membrane potential of a cell if the concentrations and relative permeabilities of the ions are known. Recall from the last lesson, the Nernst equation is used to calculate one ion's equilibrium potential.
Knowing the equilibrium potential can help you predict which way one ion will move, and it also calculates the membrane potential value that the cell would reach if the membrane were only permeable to that one ion. However, at rest, the membrane is permeable to potassium, chloride, and sodium. To calculate the membrane potential, the Goldman equation is needed.
Like the Nernst equation, The constant 61 is calculated using values such as the universal gas constant and temperature of mammalian cells. The p variables are the relative permeabilities of each ion, with potassium being 1, sodium being 0.04, and chloride being 0.4. Then the intracellular and extracellular concentration values of each ion are needed.
Note that since chloride is a negative ion, the inside concentration is in the numerator and the outside concentration is in the denominator, opposite of potassium and sodium. With the concentration and relative permeability values that we have, the calculated resting membrane potential would be negative 65 millivolts.