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Physiology > Cardiac Electrophysiology I > Flashcards

Cardiac Electrophysiology I Flashcards

1
Q

resting cardiac muscle cell

A
  • biophysicists look at the cell from the inside

- electrocardiologists look at the cell from the outside

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2
Q

types of biological electrical potentials

A
  • equilibrium potential
  • gibbs donnan equilibrium
  • diffusion potential
  • epithelial membrane potentials
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3
Q

equilibrium potential

A
  • voltage obtained for a given concentration gradient of a single ion at equilibrium across a semi permeable membrane
  • given single ion and represent by nernst equilibrium equation
  • Na is 60
  • K is -90
  • Ca is 120
  • if membrane is more permeable to one of the ions, the resting potential is closer to it
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4
Q

Gibbs-Donnan equilibrium

A
  • special
  • impermeable polyelectrolyte on one side of a membrane that is permeable to salts
  • capillary membranes if albumin and other charged plasma proteins are in the blood but not the interstitial fluid
  • pH different from isoelectric point of the polyelectrolyte, leads to unequal distribution of salts across the membrane and slight potential that has the same sign as the charge on the polyelectrolyte
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5
Q

diffusion potential

A
  • when two or more ions are permeable to a membrane, but the various ions have different permeabilities
  • calculated by goldman hodgkin katz equation using the theory of electrodiffusion
  • independent passive movements of ions across membranes under influence of their concentration gradients and electrical forces
  • cell resting potential and action potentials are diffusion potentials
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6
Q

epithelial membrane potentials

A

-differences in electric potential between two dilute solutions when the membrane itself it a layer of cells
kidney and GI

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7
Q

nernst equilibrium for K

A
  • raising external K decreases outward K gradient and makes Ek less negative, which is depolarizing
  • raising internal K increases outward flow of K and makes Ek more negative, hyperpolarizing
  • at low external K, membrane potential is more positive than predicted by nernst equation due to Na presence inside the cell that brings the charge difference back down (more positive)
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8
Q

raising internal K

A

increases the gradient out and makes Em more negative inside- hyperpolarizing

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9
Q

raising external K

A

decreases the gradient out and makes Em more positive inside- depolarizing

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10
Q

raising internal Na

A

-decreases gradient in and makes the membrane more negative- hyperpolarizing

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11
Q

raising external Na

A

-increases gradient in and makes the membrane more positive- depolarizing

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12
Q

GHK diffusion potential

A
  • equation includes permeabilities of whatever ions happen to be moving- a is Pna/Pk
  • chloride is in equilibrium and doesn’t have any pumps, which is why its not in the equation
  • accounts for biological diversity of cell membrane diffusion potentials
  • different cells have different permeability constants and therefore different membrane potentials (compared to nernst prediction of all the same since concentrations of K are the same)
  • nerve and muscle cells much more permeable to K, Em -80, alpa is 0.05
  • RBCs a is 1/4, so resting potential only -11mV
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13
Q

conductance

A
  • predicted by current voltage plot
  • outward currents of pos ions are pos, inward are neg
  • conductance is 1/R
  • depends on number of open channels
  • Ohm’s law predicts linear
  • biological channels not Ohmic-rectification, conductance differs for inward and outward currents
  • iK=Gk (Em-Ek)
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14
Q

outward rectification

A
  • conductance of outward currents is greater than for inward
  • current voltage slopes up non-linearly
  • closed at hyperpol, open at depol (when membrane potential is positive)
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15
Q

inward rectification

A
  • conductance of inward currents is greater than for outward currents
  • current voltage plots slope downward non-linearly
  • closed at depol, open at hyperpol (when membrane potential is negative)
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16
Q

K+ channel

A
  • iK1
  • resting potential
  • inward rectifier
  • cell is hyperpolarized at rest, conductance of the channel is high
  • upon depolarization, the conductance is less, allowing inward Na to depolarize membrane potential
  • upon repolarization, iK1 re acquires high conductance, maintaining inside neg resting potential
  • direction of current flow depends on electrical and ionic concentration gradients
  • inward rectifier iK1 mediates a positive efflux of K (brings K in which increases gradient and causes more to leak out)??
17
Q

molecular structure of mammalian potassium channel

A
  • 4 identical subunits
  • transmembrane domain forms a pore that allows the ions to cross the membrane
  • the selectivity filter filters out the ions other than potassium
  • ion selectivity depends on the strength of the electric filed at the binding site and on the size of the hydrated ion
18
Q

single file electro diffusion

A

-three K ions may simultaneously occupy a channel but do not pass each other in transit

19
Q

channel blockers

A
  • occlude calcium at extracellular side
  • can reduce heart rate and contractility
  • lower CO and BP
  • ACE inhibitors block conversion of angiotensin I and II relax smooth muscle decreasing TPR and BP
20
Q

membrane voltage and open channels

A
  • cell becomes more permeable to a certain ion
  • the Vm moves closer to the Ex of that ion
  • when a cardiac muscle cell at -60 becomes permeable to Na, INa increases and Na flows in and the value of Vm moves toward 60, causing the depolarizing upstroke of the AP
  • concentration doesn’t change much
21
Q

three stage model for Na and Ca channels

A
  • open, inactivated, closed
  • charged domain is voltage sensor that responds to membrane voltage and causes conformational change in channel that opens outwardly rectifying channels-conductance low at hyperpolarization
  • open at a threshold and cause more depol
  • even if voltage inside is pos, concentration gradient still drives movement
  • membrane switches from neg to pos, moves from Ek to Ena
  • sodium channels rapidly inactivate- h gates, occludes cytoplasmic side
  • inactivation in Ca much slower
  • both progress to closed with m gate down and h gate up
22
Q

refractory periods

A
  • comes from inactivation
  • absolute-no AP-inactivation gates mostly closed, inward current can’t reach another threshold
  • relative-normal stim not good enough, but raising stim does give AP; some but not all channels have returned to resting state
  • refractory period in cardiac is diastole- heart can refill with blood
  • prevents chambers from contracting before they have enough blood
23
Q

lower membrane potential

A
  • reduces rate of depol and amp of AP
  • initial resting potential more depolarized than usual, some channels inactivated, AP is smaller and slowing
  • ischemia will depolarize resting potential
24
Q

stimulating voltage

A
  • if stimulating voltage to threshold has slower upstroke (takes longer to reach threshold)
  • some channels will inactivate before they get the chance to do anything
  • AP smaller and slower upstroke
  • AV node- arrhythmias from small AP with slow uptakes
25
Q

voltage gated activation of outwardly rectifying K channels

A
  • charged transmembrane domains move when the transmembrane voltage becomes pos (depol)
  • lever causes a change in the conformation of the channel pore, thereby opening channel
  • conductance is higher at depolarized than at hyperpolarized voltages and currents are outward for K channel
  • works toward repolarizing
  • not leak channels
26
Q

evolution

A

gave us many different types of channels
calcium and sodium probably related
-potassium and camp related

27
Q

delayed rectifying K channels

A
  • activate with time delay and inactivates by ball and chain mechanism
  • repolarization by bringing K out
  • open at depol with delay
28
Q

cardiac action potentials in different regions

A
  • SA and AV from Ca out and K in
  • all others are variations with Na in, Ca in delayed and K out
  • a lot slower than muscle cell AP and there is no plateau in skeletal muscle
29
Q

purkinje fiber AP

A
  • resting potential is -90
  • overshoot is 30
  • amp up to 120 mv
  • low internal resistance
  • long duration with long refractory period-prevents ventricular muscle from reactivating the conducting system
  • membrane conductance is high early in the plateau, but falls at the end of the plateau
  • this is because Ca in and K out are almost balanced, both currents decline and then the delayed rectifier channels open and repolarize the membrane
  • see last slide- very important

Physiology (13 decks)

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