Action Potentials Flashcards
How does the Na/K ATPase pump work?
pumps 3 Na out & 2 K in creating a more negative membrane potential
Explain how the Na/Ca Exchanger will work under high & low concentrations of intracellular sodium.
Low IC Sodium: pumps 1 Ca out & 3 Na in creates a more positive membrane potential; High IC sodium: pumps out 3 Na & 1 Ca in creating a more negative potential
How many phases does a fast response action potential have?
Five
Describe what happens during phase 0 of a fast response AP.
Fast sodium channels open causing a sodium influx that will depolarize the membrane potential
What happens during phase 1 of a fast response AP?
major event is repolarization via transient outward potassium chain; these channels only stay open for a short period of time
What are the principle players of phase 2 for a fast response AP?
Major inward ion movement is calcium facilitated by L-type Ca channels; Major outward ion movement is potassium via delayed rectifier K+ channels; this phase is also referred to as the plateau phase b/c these channels open slowly which prolongs the phase
What happens during phase 3 of a fast response AP?
Ca channels are closed; Delayed rectifier K channels have reached maximum opening; stimulation of inward rectifying K channels to open
What happens during phase 4 of a fast response AP?
only IKs remain open; Na/K & Ca ATPases remove extra Na & Ca from cytoplasm and bring potassium back in
How does hyperkalemia affect the membrane potential?
makes it more positive which increases excitability and delays sodium channel recovery leading to cardiac arrest
How does hypokalemia affect membrane potential?
makes it more negative; this increase AP duration & delays repolarization thus increasing the risk for arrhythmia
What ion event occurs during a myocyte contraction?
influx of Ca from L-type Ca channels will trigger release of Ca from the SR; SERCA will facilitate Ca movement back into the SR after contraction is ended
How many phases does a slow response action potential have?
three
What happens during phase 0 of a slow response action potential?
AP generated by L-type Ca channels causing depolarization to happen slower
What happens during phase 3 of a slow response action potential?
K rectifier channels are open to repolarize the cells
What happens during phase 4 of a slow response action potential?
Spontaneous depolarization is initiated by pasmic chain/funny channels; Iks are deactivated & T-type Ca channels are activated to help facilitate spontaneous depolarization
after which phase can another AP be fired for fast response fibers?
beginning of phase 4
after which phase can another AP be fired for slow response fibers?
at the end of phase 4
What variable determines the duration of a cardiac AP?
ERP; increased ERP is associated w/ increased duration of an AP
Skeletal muscle fibers have prolonged contractions due to very short ERP and AP duration; the summation of APs for prolonged contraction is referred to as tentanization; longer cardiac ERP prevents tetanization; why is this a good thing?
it is essential for the heart to have a relaxation phase before firing another contraction AP; this gives the blood time to flow into the chambers b/t contractions
what ions can be used to activate a pacemaker current?
sodium & potassium
How do the membrane potentials differ with fast vs. slow response APs and what is the physiological significance of these differences?
Fast response APs have a more negative resting membrane potential compared to slow response APs; additionally, slow response APs have a much shorter AP peak; reduced distance b/t resting state & peak will make it easier to fire APs
What part of the heart initiated the relay of electrical propagation?
Right Atrium at the SA node
Via what pathway does electrical propagation from the RA reach the LA?
bachman’s bundle
describe the differences b/t longitudinal & transverse conduction.
pathways of longitudinal conduction have fewer cells w/ more gap junctions and therefore generate a much faster current; transverse conduction pathways have more cells w/ fewer gap junctions, slowing the rate of conduction
In which direction relative to the anatomical orientation of the heart, will the atrial currents flow?
they will flow downward into the ventricles
what is the main conductor that relays atrial electrical conductance to the ventricles?
the AV node
where is the AV node located?
lower interatrial septum b/t tricuspid valve & coronary sinus
What pathway relays electrical conductance to the ventricular myocytes?
His-Purkinje system; first through the His bundle; then to the purkinje fibers to be handed off to the ventricular myocytes
Myocytes in which ventricle and in what layer of the heart will be excited first?
endocardium of the left side via the interventricular septum
Which direction will the ventricular condance flow?
up towards the aorta
Why do epicardial myocytes have a shorter AP duration compared to endo & myocardial myocytes and why is this physiologically significant?
epicardial myocytes have a higher density of IK channels which will speed up repolarization; if the outer layer is contracted, the rest of the ventricle cannot naturally relax
What cardiac fibers have the fastest conduction velocity?
His bundle branches & purkinje fibers
which cardiac fibers have the slowest conduction velocity?
SA & AV nodes
which cardiac fibers have the largest cells?
purkinje fibers
which cardiac fibers have the smallest cells?
SA & AV nodal cells
How does the vagus nerve slow HR?
acetylcholine acts on M2 receptors to activate Gi to activate ligand gated potassium channels to hyperpolarize the cells
