Cardiac AP - Slow Response/Timing Cells Flashcards
what maintains phase 4 in slow response cells?
funny current (Na/K)
what does funny current do?
provokes spontaneous depolarization
what controls heart rate?
rate (slope) of spontaneous depolarization
what dominates upstroke in slow response cells?
Ca channels
what do muscarinic receptors do to channels?
decrease iF (funny channel) to hyperpolarize, and increases iK (leading to decreased cAMP)
what do adrenergic receptors do to channels?
increase iF and iCa to depolarize faster and decreases iK to be less polarized (more frequent AP)
is there a phase 1?
no
is there a plateau?
no
what causes phase 3? repolarization
increased gK through iK and iK1 channels
is there iK1 channel during phase 4?
no
difference between SA and AV node
SA depolarizes more, faster, triggered first, and reaches threshold faster
what if SA node is knocked out?
slower AP from AV node, reduced force
what occurs if iF turns on during repolarization?
causes spontaneous depolarization
how does gCa increase?
responds to spontaneous depolarization from iF (and initiates AP)
action during phase 4
iK and iK1 turn off during phase 4, allowing funny current to depolarize
when does funny current turn on?
once cell repolarized and causes spontaneous depolarization
what channel repolarizes cells?
iK
what is threshold of slow response cell?
-40mV
threshold of fast response cell
-65mV
what is resting potential of slow response cell
less negative than myocardial cells
which cell is more likely to be blocked ?
slow response cells
what is conductance speed of slow response?
slower than fast
what is post-repolarization refractoriness?
that the relative refractory period extends into phase 4 resting potential, which decreases conduction velocity of impulses that arrive early
how do slow respond cells keep rhythm
relative refractory period lasts until phase 4 resting potential and decreases conduction velocity of the impulses that arrive early
what occurs if interval between AP is short?
next AP amplitude is short, or may be blocked.
benefit of post-repolarization refractoriness
allows ventricles to fill
calcium current in SA node
little calcium current, but it is important for membrane potential
why is resting potential less than ventricular myocytes?
- fewer iK1 so there is no efflux of K
2. ratio of gK and gNa is low so cell deviates more from the E of K
What influences AP frequency of SA node?
- slope of phase 4 spontaneous depolarization
- maximal negativity during phase 4 (starting more hyperpolarized vs more depolarized)
- threshold potential
how do you change threshold potential?
close some Ca channels to make threshold less negative, therefore need to depolarize more to reach threshold (takes longer)
how do you change slope of phase 4?
decrease iF
how do you change negativity of phase 4?
hyperpolarize cell so starts from lower starting point)
what is overdrive suppression?
when HR is excessively high, Na builds up in the cell, which drives Na/K ATP pump at a higher rate, which makes cell more hyperpolarized. therefore, after period of high HR (once stimulus is turned off) may be period of unusually low rate
what do muscarinic receptors do to slow response cells?
M2 increases gK (which hyperpolarizes the cell) by decreasing cAMP
how does acetylcholine affect M2?
- regulates K channels (suppresses them)
- depresses iF Na current and Ca current (less depolarization)
- M2 increases gK by decreasing cAMP
effect of muscarinic activation on HR?
- lowers slope of phase 4
- raises threshold by closing ca channels
- slows HR!
what occurs during hypocalcemia?
depresses AP amplitude and rate of spontaneous depolarization
how does norepi affect slow response cells?
- increases Na permeability during phase 4 (increase iF slope)
- increases gCa via increasing cAMP (opens Ca channels) (increases amplitude)
what do CCB (calcium channel antagonists) do
reduce phase 4 and phase 0 slopes in pacemaker cells, reduces plateau in myocytes
what finding by EMG most strongly and specifically supports dx of neurogenic myopathy?
decreased frequency of muscle depolarization with motor nerve stimulation
what would a decrease in gCl do in a muscle cell?
spontaneous depolarization, causing spontaneous muscle contractions
electrical stimulation has normal amplitude and conduction velocity but AP decreases in amplitude with each stimulus. resting membrane potentials of neurons and myocytes is normal. what’s wrong.
myasthenia gravis, something wrong at motor end plate
what causes irreversible neural disorders?
deficiency in NO synthase NOS, poly-ADP-ribose polymerase PARP, or superoxide dismutase (SOD)
delayed inactivation of voltage gated Na channels in skeletal muscle most likely to cause what?
spontaneous AP and efflux of K causing hyperkalemia
what causes neurogenic myopathy?
destruction of alpha motor neurons
what would demyelination cause
decreased sensory transmission and motor disorders
what does hypercalcemia do?
inacctivates na channels
