15. Resting Membrane Potential WEAK
Which channels are responsible for resting membrane potential?
*Na and K channels* 1. There are many different kinds of Na+ and K+ channels. 2. The Na+ and K+ channels open at the resting potential are probably *not the voltage-dependent Na+ and K+* channels that are responsible for the action potential. 3. For a typical cell at rest, the *K+ conductance is about 9 times greater than the Na+ conductance*, so the transference for K+ is 0.9 and the transference for Na+ is 0.1. 4. Typical values for equilibrium potentials: EK = -90 mV ENa = +60 mV Em=TKEK + TNaENa = 0.9 X (-90) + 0.1 X 70 = -75mV *5. Some cells have other ion conductances that must be considered (Ca2+ in cardiac muscle cells; Cl- in neurons).* REMEMBER THAT.
*Explain the difference between a steady state and equilibrium.* *Resting membrane potential is a steady state but NOT an equilibrium state*
*Steady state requires energy (pump), equilibrium does not* -- needs energy *Build up sodium in the cell, and losing potassium* 1. Net K+ current (flux) flowing out of the cell. 2. Net Na+ current (flux) flowing into the cell. 3. The outward K+ current equals the inward Na+ current, therefore the *membrane potential does not change (steady state)*. 4. These ion movements tend to dissipate the normal cellular ionic concentration gradients: They tend to lower the intracellular K+ concentration and increase the intracellular Na+ concentration. 5. Fortunately, cells have a Na+, K+ pump that "puts the ions back where they belong". Ionic concentration gradients remain normal. 6. The cell can remain at this resting potential only because the Na+-K+ pump is running, and *this requires energy. Therefore it is in a steady state, not an equilibrium.* 7. *A typical resting membrane potential (-70 mV) is not equal to either EK or ENa.* The proteins inside the cell cannot cross the membrane--they are negatively charged = *they are what really builds up negative charge inside the cell!*
The importance of resting potentials
Affects whether solutes enter or leave the cell. Creates driving force for Na+ entry, which is coupled to the transport of other molecules in and out of the cell Enables production and transmission of action potentials
Important
Alway movement across the membrane, but there is no net flux
Questions: Why is there a great K difference inside and outside the cell if the K gates are open most of the time? Solute concentration and charge table doesn't make sense to me
Because the sodium-potassium pump pumps K inside The proteins are making the negative charge
*Be able to calculate a nerve's resting membrane potential, Em, given the IONIC CONDUCTANCES and equilibrium potentials of the permeable ions.*
Each ion has its own specific conductance, gion, for example, gNa or gK. What this says: 1. The current of a specific ion = the specific ion conductance, g_ion times the effective voltage difference, Em - Eion. iNa = gNa (Em - ENa) iK = gK (Em - EK). 2. When Em = E_ion: --no effective voltage difference for that ion across the membrane and there is no flow of that ion though the channels for that ion are open. --there is a voltage difference across the membrane. In nerve and muscle cells, EK is about -90 mV. When Em is -90 mV --> *no flow of K+* through the K+ channels, *even though they are open*, and *even though there is a -90 mV voltage difference across the membrane*. In nerve and muscle cells, ENa is about +60 mV. When Em is +60 mV --> *no flow of Na+* through the Na+ channels, *even though they are open*, and even though there is a +60 mV voltage difference across the membrane.
*Know what the flux of an ion is when the membrane potential equals to or is more negative or more positive than the Nernst potential for that ion.*
Flux is proportional to concentration, and the difference will result in the difference in electrical potential.
*Nernst equation*
Follows from the equality of the electrochemical potential of an ion on both sides of a membrane. Used to calculate membrane potential where electrochemical potential of an ion will be the same on both sides of a membrane. In other words, the voltage where the ion is at equilibrium. *The common form of the Nernst equation assumes body temperature of 37 C (310 K).*
Nernst Potential of Ion
In a biological membrane the Nernst potential of an ion is the membrane potential at which there is no net (overall) flow of that particular ion from one side of the membrane to the other.
*Explain what is meant by the electrochemical potential of a solute in a solution.*
In the nerst equation. The difference in charge across the membrane
*Explain, for a nerve at its resting potential, what ionic currents are flowing across the membrane and in which direction.*
K+ is flowing out Na+ is trapped 3:2 sodium-potassium pump (sodium out, potassium in)
Solute concentrations in a real cell THIS GOES AGAINST EVERYTHING WE JUST LEARNED, AND HE DIDN'T REALLY EXPLAIN WHY *"Memorize that"*
Na concentration is much high than concentrations outside the cell K concentration is much higher inside the cell *Focus on sodium and potassium* Also calcium Chlorine inside the cell is low, outside the cell is high
*Explain the difference between a Nernst equilibrium potential and a resting membrane potential.*
Nernst is for one ion Resting membrane involves multiple ions
Definition of resting membrane potential
Resting membrane potential: a difference in electrical potential (a voltage difference) between the inside and the outside of the cell. The inside of the cell is negative in relation to the outside. Varies according to cell type but is usually between -50 and -90 mV.
*Know what would happen to a cell's resting potential if the sodium pump stopped working.*
Steady state could not be maintained
Ionic currents and ohm's law
The cell, a biological system, follows the physical law for electricity: Ohm's law: V = IR Any ion movement can be measured as current The specific ion conductance is proportional to the number of specific channels of that type that are open. The voltage difference we use is (Em - Eion).
How membrane potential is created *ENDING*: equilibrium has been reached ??????? (in OUR cellular environment)
The electrical force caused by the negative charge in the cell exactly counterbalances the effect of the chemical force (caused by K+ being in higher concentration inside the cell than outside the cell) 1. The membrane potential (Em) at which the sum of the electrical and chemical forces on an ion is zero is called the equilibrium potential of that ion (Eion) 2. In this example, since the only permeable ion was K+, the membrane potential at which the chemical and electrical forces were equal to each other was the potassium equilibrium potential (Ek)
Eion values in a real cell
The number of particles may not line up exactly, but because the absolute value of chloride's charge is great, it may make up for the odd numbers... BUT IDK Maybe -- The proteins inside the cell cannot cross the membrane--they are negatively charged = *they are what really builds up negative charge inside the cell!*
Chord equation can change to transference equation
The transference equation says that the membrane potential --is a weighted average of the equilibrium potentials of the permeable ions --the transferences are the weighting factors.
*Be able to calculate a nerve's resting membrane potential, Em, given the TRANSFERENCES and equilibrium potentials of the permeable ions.*
The transference equation says that the membrane potential: -- is a weighted average of the equilibrium potentials of the permeable ions --the transferences are the weighting factors.
*Know how the value of electrochemical potential for an ion inside and outside the cell compare, when the membrane potential is at the Nernst potential for that ion.* ????????
They should be equal?????? No net flow
Chord conductance and resting membrane potential (Calculating Em) Remember this equation!
When calculating, we assume resting membrane potential Assume only 2 types of channels -- sodium and potassium
How membrane potential is created *BEGINNING*: equilibrium has not yet been reached (in OUR cellular environment)
semipermeable cell membrane; permeable to K+, NOT to Cl- (in picture arrow = K+) K+ concentration difference, ∆C = 0.1 - 0.01 = 0.09 The membrane to is permeable to K+ --> K+ starts to diffuse out of the cell no Cl- leaves the cell, membrane is impermeable to Cl-. There are extra negative charges (Cl-) inside the cell and extra positive charges outside the cell (K+) --> A potential difference (voltage) forms because there is *transiently* a *net* efflux of positive charges from the cell. . The interior of the cell = negative