1. Cardiac Action Potentials Flashcards
What is the general path for the conduction system of the heart?
- Sinoatrial Node (pacemaker)
- Atrioventricular Node
- Bundle of His (common bundle)
- Right bundle branch & left bundle branch
- Purkinje fibers/subendocardial
What is important to remember about the purkinje fibers?
They are larger in diameter than myocytes surrounding them, mean they have a much faster AP than the AV/SA
Unlike nerves/skeletal M and smooth muscle, the conduction occurs directly between _________ in the cardiac muscle.
cardiac myocytes (gap junctions)
What is phase 0?
depolarization, inwards Na+ current
What is phase 1?
Initial repolarization, inward Na+ current ceases but outward K+ is present
What is phase 2?
Plateau, then inwards Ca+ current and slowed outward K+ current. K+ becomes less than at rest during phase 2 due to a special class of voltage dependent channels that close during depolarization and open a predetermined time follow closure. These opening plus Ca2+ closing causes phase 2 to end.
What is phase 3 during the action potential of V/A and purkinje system?
repolarization, restoration of outward K+ current and cessation of inwards Ca2+ current
What is phase 4?
resting membrane potential
Regarding the conduction system of the heart, what areas of the heart receive action potentials before others?
Endocardium of ventricles receive AP before the epicardium, and right ventricle epicardium before left ventricle epicardium
What is the order of conduction velocity, from highest to lowest of the heart?
Purkinje fibers (atrial pathway)> atrial and ventricular muscle > Av node (slow!!! will see pause)
What does the delay or pause in the AV allow?
Allows the atria to empty into ventricles before they contract, allowing maximal volume in the heart
What phase of the sinoatrial node is the determinant of the heart rate?
The rate of phase 4 depolarization of the SA node!
Na+(m) is activated when membrane potential crosses threshold, opening channels and contributing to phase 0 and 1, doing what?
very fast generation of depolarization then automatically closes allowing partial repolarization
K+ (a) or IKTO is activated when the cell depolarizes, and will open channels and close quick allowing?
very rapid partial repolarization
Ca2+ (L type) is activated when the cell depolarizes, and will open channels during phase 2.. doing?
slow to open and remains open for a determined amount of time and closes causing phase 3
K+ (b - Kinward/rectifier) is activated when the cell depolarizes and close channels during phase 2, allowing?
–slow to close and remains closed for a determined amount of time then opens and helps with phase 3 (balances phase 2)
K+ (c) remains open all the time, helping with phase 4, meaning it is?
a leak channel which maintains resting potential
Na+ (f/funny) is activated when SA or AV node repolarizes, opening channels during phase 4 and?
causes slow depolarization during the resting phase
In the SA node, RMP gradually depolarizes spontaneously until it reaches threshold and then will fire, although slower than other regions. This feature is why the SA node is considered?
The intrinsic, spontaneous depolarization makes the SA node the PACEMAKER due to its automaticity
RHYTHMICITY
What allows phase 4 to occur in the pacemaker of the heart?
Phase 4 is due to opening of the funny voltage gated Na+ channels that open when membrane is repolarized
What is important to remember about phase 4 in the AV node?
Its depolariation is SLOWER than SA node, it doesnt reach threshold until SA node triggers it
What causes Phase 0 in the SA/AV nodes?
(action potential) Opening of the slow Ca2+ channels and closing of special K+(b) channels… balancing act between Ca2+ in and modulating K+ out
K+ b channels help modulate the heart rhythm. What are they sensistive to and what does that mean?
Sensitive to acetylcholine meaning there is a parasympathetic effect
What causes phaes 3 in the SA/AV nodes? (**there are no phase 1/2 d/t scarcity of Na+(m) channels)
Repolarization: Closing of Ca2+ gates and opening of special gates K+(b)
the bundle of his and purkinje fibers remain polarized. If not stimulated for a period of time what will they begin to do?
spontaneously depolarize during phase 4.
**rate of bundle/purkinje fibers is slower than AV node
The equilibrium potential is lower for fast tissues/fibers (-90). Phase 4 is the resting potential and is sustained by what channels?
high K+ (c) conductance
What occurs in phase 0 of fast tissues/fibers?
Rapid upstroke (depolarization) caused by crossing threshold and voltage gated Na+ channels (m) opening (AP is generated)
What occurs in phase 1 of fast tissues/fibers?
Small repolarization cause by inactivation gates in Na+ (m) channels closing and K+(a) channels opening
What occurs in phase 2/plateau in tissues/fibers and functionally what does this do?
Plateau: Slow opening of Ca2+ channels and closing of K+(b) channels
Functionally prolongs contraction and is fundamentally different than observed in skeletal muscle
What occurs during phase 3 in tissues/fibers?
Complete repolarization caused by Ca2+ channels closing and K+(b) channels opening
Conduction velocity usually relates to phase 0, how and what is the velocity related to?
The more rapid phase 0 occurs, (faster conduction velocity) the steeper the phase is.
It is related to the differences in the speed of either Na+ or Ca2+ channels
AV node damage delays conduction of the heart called AV block. What can happen if the purkinje fibers are damaged?
Disrupts the natural conduction and contraction of ventricles (arrhythmias)
Refractory period is when electrolyte gates have not ‘reset’ sufficiently to allow a second AP to be generate and is important to help prevent arrythmias, and are longer in cardiac cells than neurons. What occurs during absolute and relative refractory periods (ARP RRP)?
ARP: no depolarization
RRP: AP can be generated but will have an abnormal conduction
What is a supranormal period? (SNP)
cell is more exictable than normal
plays roles in arrythmias
What effect changes the rate of depolarization of SA node and therefore the heart rate HR?
What does a positive or negative effect cause?
Chronotropic effect
If there is a positive chronotropic effect, there will be a faster heart rate, if negative then slower heart rate
What effect changes the speed of conduction (velocity) in the heart?
Dromotropic (increase in dromotropic = increase in conduction velocity)
What is the difference between inotropic and lusitropic effects on action potentials in the heart?
Inotropic effect changes the strength of muscular contraction
Lusitropic effect changes the rate of muscular relaxation
A parasympathetic stimulus is carried by the vagus nerve to the SA and AV node but not really to ventricular myocytes. What neurotrasmitter and receptor is used for parasympathetic signaling?
Acetylcholine and muscarinic M2/M3
Reduces kinase activity to slow down HR
What do negative chronotropic effects cause? (2)
Decrease HR by Slowed opening of Na+(f) channels during phase 4
Hyperpolarization by increasing outward K+ current via K+-Ach channel
What do negative dromotropic effects cause? (2)
decrease conduction velocity by :
Reduced Ca2+ inward current
Increased outward K+ current via K+-Ach
A sympathetic stimulus goes to SA/AV nodes AND into myocytes via which neurotransmitter and receptor?
Via norepinephrine and muscarinic-B1-adrenergic receptors
What do positive chronotropic effects casue? (2)
Increase in heart rate by increased opening of Na+ (f) channels during phase 4 and increased inward Ca2+ current
What do positive dromotropic effects cause? (1)
increase conduction velocity (and HR) by increased Ca2+ inward current during phase 2
What are the differences between atropine and propranolol?
Atropine is a muscarinic receptor antagonist and increases HR
Propranolol is a B adrenergic antagonist and decreases HR
