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59 Cards in this Set
- Front
- Back
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How much does Cl contribute to the resting Em of a vertebrate (frog) skeletal muscle fiber?
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NOT AT ALL
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So how does the lack of contribution of Cl to the resting Em in a frog muscle fiber affect the GHK equation?
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Cl is not included, just Na and K.
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Why doesn't Cl contribute to the resting Em of skeletal muscle fibers?
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Because there's no active pumping of Cl- into or out of the cells.
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How are muscle fibers different from nerve?
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Nerve fibers DO have Cl- pumping so it is a factor for them.
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Is the Ecl in squid axon fibers more or less negative than the resting Em?
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Less negative!
Ecl = -60 mV Em = -64 mV |
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What happens to the resting Em in a frog muscle fiber as extracellular K conc. increases?
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The Em approaches the Ek and becomes more negative - hyperpolarized - just like in nerve fibers.
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Because muscle fibers are not permeable to Chloride, what is the Ecl equal to?
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Ecl = Em for muscle fibers
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Because nerve fibers are permeable to chloride, what is the Ecl?
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Ecl = -60
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What is the Electrical Model of the plasma membrane?
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The representation of each ion's equilibrium potential and channel as a Battery + Conductor
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What is g?
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conductance - the ease with which an ion moves through a membrane via its channel.
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What regulates the g of an ion?
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The specific membrane channels through which that ion flows.
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What is Ion Conductance (g) directly proportional to?
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Ion permeability - the two are pretty much the same thing.
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Define ion permeability then:
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Ease of movement of a charged particle through its membrane channel under the influence of a concentration difference.
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Define Ion Conductance:
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Ease of movement of a charged particle through its membrane channel under the influence of a driving voltage difference.
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What is one factor that regulates BOTH ion permeability and conductance?
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The number of open ion channels per unit area of membrane.
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In the electrical model of the membrane what does a battery represent?
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The Equilibrium potential for a particular ion.
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In the electrical model of the membrane what does a Conductor (or resistor) represent?
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The pathway of the ion - the ease with which it passes thru the membrane - ppl to permeability.
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In what direction does current always flow?
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From neg to pos
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What is the phopholipid bilayer equivalent to in a circuit?
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A dielectric insulator
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What is the membrane equivalent to electrically?
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A parallel plate capacitor - it has the ability to store charge.
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What is the equation for Capacitance?
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Cm = Qm/Em - membrane capac. is the amt of charged stored per unit of membrane potent. diffnc.
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How can Capacitance be used to calculate the capacitative current (i) flowing through the membrane?
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i = C x (delta Em/time)
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What are the components of the membrane capacitor?
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Dielectric insulator = bilayer
Conductors = ECF and ICF |
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What is the value for the Cm of a cell membrane?
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1 uF / cm^2
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What is the cause of membrane capacitative current?
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Transient movement of ions across the membrane which charges or discharges the capacitor.
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So the electrical force driving each permeable ion across the membrane is equivalent to?
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The difference between Em and the Equilibrium potential for each ion.
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What is the equation for the current of an ion across the membrane?
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I(ion) = g(ion)(Em-Eion)
-current is the conductance times membrane potential minus ion potential. |
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What is the equation for the total ion current under steady state conditions at resting Em?
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Itotal = Ina + Ik + Icl = 0
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Why does the total current equal zero at resting steady state?
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Because if there were current moving the resting Em would always be changing! (charge would be getting stored/used)
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What is the purpose of the Goldman Chord equation?
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To determine the resting Em based on ion currents.
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What is the Goldman Chord equation?
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gk gna gcl
Em = --Ek + --Ena + --Ecl totalg totalg totalg |
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Most important thing to remember about the Goldman Chord eqn:
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The total ionic currents in the resting cell is ZERO otherwise the Em would be changing.
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Based on Goldman Chord eqn what 2 things is Em directly ppl to?
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-The equilibrium potentials of each permeable ion
-The fractional conductance of each permeable ion |
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How can you remember the current equation based on ohm's law?
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V = IR and R = 1/C
So I = VC = g(ion) x (Em - Eion) |
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IMPORTANT THING TO REMEMBER ABOUT Ecl:
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Ecl = more neg than Em for a mammalian axon!!!
Ecl = more pos than Em for a squid axon (-60 mV) |
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What is the state of the resting Em?
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Steady state - needs a constant supply of energy to be maintained.
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What is the energy source for maintaining the resting Em?
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ATP
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How is the resting Em maintained?
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Na/K ATPase
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What does Na/K ATPase do?
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Pumps 3 Na out per 2 K in
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What is the effect of Na/K ATPase?
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A net positive charge outside the cell and net negative inside.
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Why is the Na/K pump needed?
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Because Na and K leak across the membrane down their concentratio n gradients at a constant rate; if allowed to be unopposed, the gradients would dissipate and no potential difference would be available for establishing AP's and IPSPs/EPSPs!
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How much energy is used by the RBC, resting squid axon, and resting frog muscle fiber for Na/K Atpase?
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20%
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For the retina how much energy is used for Na/K active transport?
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Almost all of it
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How is Na/K ATPase regulated?
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By increasing intracellular Na or extracellular K
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What will cause loss of Na/K pump action?
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Ischemia or hypoxia
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Why do ischemia/hypoxia stop Na/K ATPase?
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Because they stop ATP formation and there's no more energy to pump the ions.
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Why is the Na/K pump termed electrogenic?
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Because it adds to the already negative cell interior by pumping out more positive charge than in.
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So what 2 things contribute to the resting cell's negative interior?
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-Ediffusion potential
-Eelectrogenic potential |
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What will cause an increase in the rate of the Na/K pump?
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Any condition causing a significant increase in intracellular Na or extracellular K
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Give 2 examples:
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-Increased activity in a nerve or muscle fiber (AP generation)
-Increased skeletal muscle activity during exercise |
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What happens to the cell membrane potential after tetanus?
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Post-tetanic hyperpolarization
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What causes post-tetanic hyperpolarization?
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An increase in Em negativity due to Na/K pump rate increase to get all the Na that influxed during tetani back out of cell.
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What happens to Na/K atpase in skeletal muscle during exercise?
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[K]o increases, Na/K pump activates to hyperpolarize cell, Vascular smooth muscle cells dilate, surrounding blood vessels reperfuse the muscle.
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In what type of cell is the Na/K electrogenic contribution most important?
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Smooth muscle
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2 possible effects of increasing extracellular potassium:
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1. Hyperpolarization due to electrogenic effect of Na/K pump
2. Depolarization due to effect on the diffusion potential |
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How do you know which effect of increased extracellular K will result?
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By looking at the concentration; -BELOW 20 mM = HYPERPOLARIZE
-ABOVE 20 mM = DEPOLARIZE |
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When is blocking Na/K ATPase done for clinical treatment?
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To increase heart contractility via Digitalis (cardiac glycosides)
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How does Digitalis increase the force of heart contraction?
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1. Decrease Na/K ATPase
2. Decrease Na-coupled secondary active transport of Ca2+ out of the cell. 3. Increased Ca in heart myocardium means increased force of contraction. |
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What is this effect called?
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Positive inotropism
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