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59 Cards in this Set

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How much does Cl contribute to the resting Em of a vertebrate (frog) skeletal muscle fiber?
NOT AT ALL
So how does the lack of contribution of Cl to the resting Em in a frog muscle fiber affect the GHK equation?
Cl is not included, just Na and K.
Why doesn't Cl contribute to the resting Em of skeletal muscle fibers?
Because there's no active pumping of Cl- into or out of the cells.
How are muscle fibers different from nerve?
Nerve fibers DO have Cl- pumping so it is a factor for them.
Is the Ecl in squid axon fibers more or less negative than the resting Em?
Less negative!
Ecl = -60 mV
Em = -64 mV
What happens to the resting Em in a frog muscle fiber as extracellular K conc. increases?
The Em approaches the Ek and becomes more negative - hyperpolarized - just like in nerve fibers.
Because muscle fibers are not permeable to Chloride, what is the Ecl equal to?
Ecl = Em for muscle fibers
Because nerve fibers are permeable to chloride, what is the Ecl?
Ecl = -60
What is the Electrical Model of the plasma membrane?
The representation of each ion's equilibrium potential and channel as a Battery + Conductor
What is g?
conductance - the ease with which an ion moves through a membrane via its channel.
What regulates the g of an ion?
The specific membrane channels through which that ion flows.
What is Ion Conductance (g) directly proportional to?
Ion permeability - the two are pretty much the same thing.
Define ion permeability then:
Ease of movement of a charged particle through its membrane channel under the influence of a concentration difference.
Define Ion Conductance:
Ease of movement of a charged particle through its membrane channel under the influence of a driving voltage difference.
What is one factor that regulates BOTH ion permeability and conductance?
The number of open ion channels per unit area of membrane.
In the electrical model of the membrane what does a battery represent?
The Equilibrium potential for a particular ion.
In the electrical model of the membrane what does a Conductor (or resistor) represent?
The pathway of the ion - the ease with which it passes thru the membrane - ppl to permeability.
In what direction does current always flow?
From neg to pos
What is the phopholipid bilayer equivalent to in a circuit?
A dielectric insulator
What is the membrane equivalent to electrically?
A parallel plate capacitor - it has the ability to store charge.
What is the equation for Capacitance?
Cm = Qm/Em - membrane capac. is the amt of charged stored per unit of membrane potent. diffnc.
How can Capacitance be used to calculate the capacitative current (i) flowing through the membrane?
i = C x (delta Em/time)
What are the components of the membrane capacitor?
Dielectric insulator = bilayer
Conductors = ECF and ICF
What is the value for the Cm of a cell membrane?
1 uF / cm^2
What is the cause of membrane capacitative current?
Transient movement of ions across the membrane which charges or discharges the capacitor.
So the electrical force driving each permeable ion across the membrane is equivalent to?
The difference between Em and the Equilibrium potential for each ion.
What is the equation for the current of an ion across the membrane?
I(ion) = g(ion)(Em-Eion)
-current is the conductance times membrane potential minus ion potential.
What is the equation for the total ion current under steady state conditions at resting Em?
Itotal = Ina + Ik + Icl = 0
Why does the total current equal zero at resting steady state?
Because if there were current moving the resting Em would always be changing! (charge would be getting stored/used)
What is the purpose of the Goldman Chord equation?
To determine the resting Em based on ion currents.
What is the Goldman Chord equation?
gk gna gcl
Em = --Ek + --Ena + --Ecl
totalg totalg totalg
Most important thing to remember about the Goldman Chord eqn:
The total ionic currents in the resting cell is ZERO otherwise the Em would be changing.
Based on Goldman Chord eqn what 2 things is Em directly ppl to?
-The equilibrium potentials of each permeable ion
-The fractional conductance of each permeable ion
How can you remember the current equation based on ohm's law?
V = IR and R = 1/C

So I = VC = g(ion) x (Em - Eion)
IMPORTANT THING TO REMEMBER ABOUT Ecl:
Ecl = more neg than Em for a mammalian axon!!!
Ecl = more pos than Em for a squid axon (-60 mV)
What is the state of the resting Em?
Steady state - needs a constant supply of energy to be maintained.
What is the energy source for maintaining the resting Em?
ATP
How is the resting Em maintained?
Na/K ATPase
What does Na/K ATPase do?
Pumps 3 Na out per 2 K in
What is the effect of Na/K ATPase?
A net positive charge outside the cell and net negative inside.
Why is the Na/K pump needed?
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!
How much energy is used by the RBC, resting squid axon, and resting frog muscle fiber for Na/K Atpase?
20%
For the retina how much energy is used for Na/K active transport?
Almost all of it
How is Na/K ATPase regulated?
By increasing intracellular Na or extracellular K
What will cause loss of Na/K pump action?
Ischemia or hypoxia
Why do ischemia/hypoxia stop Na/K ATPase?
Because they stop ATP formation and there's no more energy to pump the ions.
Why is the Na/K pump termed electrogenic?
Because it adds to the already negative cell interior by pumping out more positive charge than in.
So what 2 things contribute to the resting cell's negative interior?
-Ediffusion potential
-Eelectrogenic potential
What will cause an increase in the rate of the Na/K pump?
Any condition causing a significant increase in intracellular Na or extracellular K
Give 2 examples:
-Increased activity in a nerve or muscle fiber (AP generation)
-Increased skeletal muscle activity during exercise
What happens to the cell membrane potential after tetanus?
Post-tetanic hyperpolarization
What causes post-tetanic hyperpolarization?
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.
What happens to Na/K atpase in skeletal muscle during exercise?
[K]o increases, Na/K pump activates to hyperpolarize cell, Vascular smooth muscle cells dilate, surrounding blood vessels reperfuse the muscle.
In what type of cell is the Na/K electrogenic contribution most important?
Smooth muscle
2 possible effects of increasing extracellular potassium:
1. Hyperpolarization due to electrogenic effect of Na/K pump
2. Depolarization due to effect on the diffusion potential
How do you know which effect of increased extracellular K will result?
By looking at the concentration; -BELOW 20 mM = HYPERPOLARIZE
-ABOVE 20 mM = DEPOLARIZE
When is blocking Na/K ATPase done for clinical treatment?
To increase heart contractility via Digitalis (cardiac glycosides)
How does Digitalis increase the force of heart contraction?
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.
What is this effect called?
Positive inotropism