Membrane Potentials.pdf

Physiology Aida Lecture 4 Membrane Potentials What is the electrical state of Most living cell membranes are electrically p...

Physiology Aida Lecture 4 Membrane Potentials What is the electrical state of Most living cell membranes are electrically polarized, meaning there's a most living cell membranes? difference in electrical charge between the inside and outside of the cell. What is the transmembrane +30 to -100mV voltage range? Why is membrane potential Membrane potential is necessary for signaling, especially in important in cells? neuromuscular systems. It allows cells to communicate and respond to stimuli. Which transporters create Na-K ATPase ionic gradients across cell Ca2+ ATPase membranes, and what is their These are active transport proteins that create large concentration function? differences of sodium and potassium across the membrane, allowing for depolarization or repolarization of the cell. How does the body maintain a When we add potassium to the ECF, it will move inside cell constant ratio of extracellular When we reduce potassium in the ECF, it will move out the cell to intracellular potassium? Why is the inside relatively Because K+ tends to move out of the cell more negative inside the membrane? What is the equilibration for -94 mV (no net movement of the ion), [K+]i = 155 mM, [K+]o = 2.5-4.5 potassium? mM What about sodium? Sodium is very high outside and low inside so it makes the outside more positive. This makes it want to enter the cell What is the equilibration for +61mV (no net movement of the ion), [Na+]i = 12 mM, [Na+]o = 145 mM sodium? Nernst equation, what does it E = equilibrium potential (potassium), RT and zD are constants describe? E = potassium outside/potassium inside Describes the relationship of voltage to ion concentration differences The greater the concentration, the greater the tendency of an ion to move in one direction What is most of the Potassium, so the movement of a positive ion moving out of cell causes conductance? cell membrane to become negative How is the sign of the Negative - positive ion leaves cell like potassium or negative ion like membrane potential decided? chloride enters cell Positive - position ion enters cell like sodium or negative ion like chloride leaves cell When is the Goldman equation When membrane is permeable to multiple ions used? Takes into account? Ionic conductance and permeability What is the equilibrium Polarity (direction), concentration diff for each ion (gradient), potential dependent upon? permeability of membrane to each ion (net movement) What is the Goldman-Hodgkin It is the same as equilibration potential but it is the voltage at which Katz equation? direction of current reverses What is the ionic current? Driving force - how much the ions want to move (expressed in voltage) x conductance for that ion How do you measure One electrode is filled with an electrolyte solution which is impaled into membrane potential? the interior of fiber while the other is in the ECF (indiff electrode). This measures the voltage difference on either side of membrane What if the charges align on The electrical potential will change either side of the membrane? What are the two important Potassium diffusion potential and sodium diffusion through nerve factors for the origin of normal membrane resting potential? What is the resting membrane -86 mV b/c the conductance of potassium is 100 times greater than potential for sodium? Why? sodium What are the minor factors for Na-K ATPase pump because it makes the interior of cell -4mV more the origin of normal resting negative potential? Chloride Action potential terminology Depolarization - process of making membrane less negative (Vm), it can overshoot and become positive Repolarization/Hyperpolarization - make membrane more negative (Vm) can undershoot Inward current - flow of positive charge into cell, depolarizing Outward current - flow of positive charge out of cell, repolarizing Threshold potential - membrane potential at which AP commences, depolarization becoming self-sustaining, inward current greater than outward Overshoot - membrane potential is positive Under shoot - membrane potential is more negative than at rest What causes action potential? A wave of depolarization which spreads down motor neurons and excitable tissues triggered at -55 mV which is called threshold. What is the difference between Much larger amplitude, AP is propagated down length of tissue without subthreshold response and downplaying strength, All or NONE action potential? For a gated channel, when is it Moves through central pore (Sodium). Activation gates open during open? Where does something depolarizations and inactivated during repolarization move through? Closed, but available - depolarization will open activation gate and Sodium channels and AP inactivation gate open gates Open - both gates open, beginning of AP Inactivated - inactivation gates close at peak of AP, begin repolarization Absolute - No AP is possible (sodium inactivation gates are closed) What are refractory periods? Relative - Begins at end of absolute RP because of high potassium conductance The spread of local potentials from activated to inactivated regions What is the propagation of Depolarization of local segment, spread of local current to adjacent action potential? How does regions, becomes self sustaining that happen? Neuron diameter (large diameter have lower internal resistance to How do you increase electrical movement) conduction velocity? Myelination (acts as insulation, prevents loss of signal, allows signal to jump node to node - node of ranvier) Increases conduction velocity. If it’s not myelinated, the AP has to be What does myelination do for spread along the entire length of the neuron. Myelin reduces the conduction velocity? bleeding of current out of neurons Resting stage - before AP, membrane is polarized bc potassium AP Stages conductance greater than sodium Depolarization stage - membrane becomes permeable to sodium ions and cell now depolarizes so na+ conductance increases (must reach threshold) Repolarization stage - sodium channels begin to close and potassium channels open, inactivation so K+ conductance increases and Na+ pumped out to return to rest 1. Closed but available, when AP arrives the attendant voltage change Sodium activation gate steps opens sodium channel 2. open, sodium conductance increases 3. Inactivated, refractory period - absolute or relative (needs more stimulus to fire an AP but enough sodium channels are in resting state so excitement travels in one direction) B/c the membrane potential is positive and it needs to go back to its Why can’t the membrane normal state potential be stimulated when It can get into relative refractory period when membrane potential sodium channel inactivates? becomes more negative Voltage = Resistance * Current (or V = R * I). We can see that the higher Ohm’s law the membrane resistance, the lower the current required to maintain a given membrane voltage.