Cardiac Action Potentials Flashcards
What is the major cause of depolarization of a neural action potential?
increased Na+ conductance
(voltage-gated na+ channels open rapidly –> close after minimal delay
What maintains resting membrane potential?
K+ permeability
For a normal AP, what is phase 4?
Phase 0?
Phase 3?
4 = resting membrane 0 = depolarization 3 = repolarization
What is the order of how the ventricles receive an action potential?
endocardium before epicardium
right ventricle epicardium before left ventricle epicardium
What connects cardiac myocytes?
Gap junctions - allow Na to flow from one cell to another to spread action potentials
What characterizes pacemaker cells?
modified myocytes that can’t contract anymore
do not have true resting membrane potential
have funny Na channels that allow them to create action potential spontaneously
What makes the fast depolarization of cardiac myocytes occur?
fast sodium channels open at threshold –> action potential
What happens in early repolarization of cardiac myocytes?
K channels are open, letting some K out of cell
Ca channels are still open too though, so ca can still go in little –> membrane potential decreases only slightly
What happens in the plateau phase of cardiac myocytes?
calcium influx in cell –> calcium-induced calcium release –> muscle contraction w/ sliding filaments about halfway through the plateau
(K+ channels are still open)
What are the major differences in action potential of cardiac muscle and skeletal muscle?
Have the plateau phase in cardiac –> much longer muscle contraction
have much longer refractory period in cardiac –> so muscles fully relax and you don’t get tetany
What channels are open in phase 0?
m type Na channels = fast
What channels are open in phase 1?
m type Na channels
a type K channel = very rapid partial repolarization
What channels are open in phase 2?
Ca channels (L type) (open slow, stay open until 3) b type K channels (slow to close, help initiate 3)
What channels are open in phase 3?
b type K
L type Ca
What channels are open in phase 4?
c type K channels = leak, maintain resting membrane potential
f type Na channels = specific to SA and Av nodes
What is the function of f type Na channels and where are they found?
specific to SA and AV nodes
causes slow depolarization during resting phase
What is the conduction velocity for atrial myocytes?
1 m/s
What is the conduction velocity of the AV node?
0.01-0.05 m/s
What is the conduction velocity of a His-purkinje fiber?
2-4 m/s
What is the conduction velocity of a ventricle myocyte?
1 m/s
What is the parasympathetic innervation of the heart?
vagus n –> SA and AV nodes –> uses AcH to muscarinic receptors
what is a chromotropic effect?
changes rate of depolarization of SA node –> chgs HR
+ = increased speed
- = decreased speed
what is a dromotropic effect?
changes the speed of conduction
What is an ionotropic effect?
changes the strength of muscular contraction
What is a lusitropic effect?
changes the rate of muscular relaxation
How does the parasympathetic NS create a negative chromotropic effect on the heart?
slows opening of funny Na channels in SA node
hyperpolarization by increasing K+ outward current through K+ - Ach channel
How does the parasympathetic NS create a negative dromotropic effect on the heart?
decreases inward ca current
increases outward K+ current via potassium-AcH channels
–> decreases speed of conduction
Where does the sympathetic NS innervate the heart, and through what NT and receptor?
SA node, AV node, and myocytes
through norepinephrine to muscarinic beta1 receptors
What are the 3 different sympathetic effects on the heart?
positive chronotropic
positive dromotropic
positive ionotropic
How does the sympathetic NS create a positive chronotropic effect on the heart?
increases funny Na channel opening in phase 4 of SA node
increases inward Ca current
How does the sympathetic NS create a positive dromotropic effect?
increased inward Ca current during phase 2
What is preload?
most accurate = amount of tension in the walls of the ventricles just before contraction is initiated
less accurate = pressure in ventricles before contraction
least accurate = volume in ventricles prior to contraction
What is afterload?
amount of pressure required to eject blood from the ventricles
(a little greater than, but essentially equal to that of the aorta or pulmonary artery)
What are inferior leads?
2, 3, AVF
What are septal leads?
V1 and V2
What are anterior leads?
V2, V3, and V4
What are lateral leads?
V4, V5, V6, I, and AVL
At what point in an EKG is the voltage difference btw ventricles the greatest?
T wave
What part of the action potential does the QRS represent?
phase 0
What is the diastolic pressure of the atria?
The ventricles?
4 mm Hg
closer to 0
What is diastolic pressure in the pulmonary A?
The aorta?
15 mm Hg
80 mm Hg
During atrial systole, why is there an increase in both atrial and ventricular pressure?
atria contract = increased pressure
blood flows into ventricles = increased pressure
Why does the atrial pressure increase during ventricular systole?
blood is still returning to atria –> can’t glow into ventricles, so collects in atria and P goes up
When does diastole start on the EKG?
at the end of the T wave
What happens as diastole comes to an end?
ventricles fill up w/ blood –> flow slows down –> causes slight increase in both atrial and ventricular pressures
