Cardiac Cycle: Electrical Events of Cardiac Cycle Flashcards
What is the ‘pacemaker ‘ part of the heart?
Atrioventricular node
What is the ‘cardiac cycle’?
the relationship between electrical, mechanical, and valvular events during one complete heartbeat
The sum of cardiac potentials at any given time is recorded by what machine?
Electrocardiogram (ECG)
is it easier to create an action potential in a pacemaker cell or a normal myocardial cell?
a pacemaker cell (sinoatrial/atrialventricular cell) - has a higher resting membrane potential in pacemaker cells compared to ventricular cells
- they have a tendency to depolarize
what ion channel is missing in pacemaker cells?
voltage gated sodium - therefore there is no sodium influence on the nodal cells - depends on slow depolarisation activated Ca2+ channels to produce action potential
What are the pacemaker cells called?
atrioventricular cells
sinoatrial cells
describe the action potential process in a non- pacemaker cell?
pretty standard electrophysiology - normal muscle cell process
where are your highest concentrations of sodium, potassium and calcium?
sodium = highest out of the cell
potassium = highest inside of the cell
calciu = highest outside of the cell
name a drug that inhibits fast sodium channels
tetrodotoxin from a pufferfish
Describe thet Fast Response Action Potential
- phase 0 = Na+ enters through fast channels
- phase 1 = efflux of K+ through channels (partially repolarize)
- phase 2= influx of Ca2+ through channels is balanced by efflux of K+ through K+ channels
- phase 3= forces favor efflux of K+ through channels over influx of K+ through same channels
- phase 4= restoration of ionic concentraion through 3 enzymes
what are the three enzymes that restore ionic concentration in fast response action potential?
- Na/K ATPase
- 3Na+/1Ca++ antiporter
- ATP driven Ca++ pump
what is the main ion which contributes to the upstroke of an action potential in non-pacemaker cells?
Sodium
Which cells in the heart do you see slow response action potentials?
pacemaker cells of the heart
How does depolarization in the nodal cells occur?
it is achieved by influx of Ca2+ throuh the Ca2+ channels (instead of by influx of Na+ through fast Na+ channels)
What is the “IF” in the nodal cells?
they are channels for sodium channels that differ from the fast sodium channels present in the non-nodal cells of the heart - it’s called the ‘funny Na current’ because no one expected to find them in these celsl
what is automaticity?
the spontaneous generation of action potentials due to unstable RMP
nodal cells depend on what for depolarization\?
they depend on slow Calcium channels -
are the sodium channels on the nodal cells similar to the sodium channels on the non-pacemaker cels?
no they vary significantly
for automaticity to occur, what occurs to the ions of the pacemaker cells?
- calcium - moves inwards to depolarize threshold to -40mv
- sodium and potassium - initial depolarisation to -55mv by inward Na+ current (IF channel) and outward K+ current
which of the following ions is not involved in the automaticity of the nodal cells
- calcium
- sodium
- potassium
- magnesium
-magnesium
do parasympathetic/sympathetic nerves influence whether or not a heart beat occurs?
no, they only effect the rate aat which it occurs
does noradrenaline/sympathetics shift action potential to be more likely or less likely to occur?
it makes the aciton potential more likely to occur
how does acetylcholine influence the action potential of the nodal cells?
it slows down the heartbeat - so it makes a longer gap inbetween action potentials -
there are acetylcholine dependent potassium channels
What is the refactory period?
frame during which stimulation cannot generate action potentials
= Effective refractory period
or = relative refractory period
what is the effective refractory period?
some of the sodium channels are closed, some are open
but you cannot have any sodium conductance
what is the relative refractory period?
many but not all of sodium channels re-open between -50mv and complete repolarizaiton?
*sodium channels are starting to open, so they are closed but they can be opened*
Why does tetany not happen in cardiac cells?
the purkinje fibres have the longest refractory period - therefore they prolong the absolute refractory period in the cardiac muscle so tetany cannot occur
how does the action potential spread from cell to cell in cardiac muscle?
travels from muscle cell to muscle cell through gap junctions in intercalated discs separatig cells - its syncronus
describe normal cardiac conduction through the heart
- action potential originates in the sinoatrial nodes - moves across muscle cells through gap junctions and intercalated discs seperating cells
- proceeds sequentially to the atrioventricular node
- moves into ventricles via bundle of his through ventricles (functional sycytium)
What are the purkinje cells?
fast conducting muscle cells carrying current from AV node through ventricles
Where is the atrioventricular node located?
loctaed on the right side of interatrial septum - imposes a delay of about 0.1 s to the passage of AP
what is the rate of conduction at the atrioventricular node, compared to the other conduction fibers of the heart?
What occurs in Wolff-parkinson White syndrome?
there is a ‘new ‘ pathway in the heart for the electrical signal called the bundle of Kent or the bypass tract
- so the Sino atrial node flows through the bundle of kent instead of through the AV node, then it goes back into the atrium after passing around the ventricle (this phenomenon is called Global AV reentry) - this leads to supraventricular tachyarrhythmias (above the ventricle fast heartbeat)
*there can also be local reentry sites where the action potentials go around and around in circles - these cause the heart to beat incorrectly *
what causes Wolff-Parkinson White Syndrome?
associated with mutations in 5’AMP activated prtoein kinase subunit gamma (PRKAG2 gene)
how do you treat Wolff-parkinson White syndrome?
“radiofrequency catheter ablation”- destroy the abnormal heartbeat through a catheter to use heat or cold to bypass the ‘bundle of kent’
or
“electro cardioversion” = electrical charge is produced that slows down the heart beat
a denervated heart has a faster or slower heart rate than a noramlly innervated heart?
a dennervated heart would have a faster heart rate than an innervated heart b/c the ANS slows down the heart
When you examine a patient, you may describe the normal pulse as being in sinus rhythm. Explain?
*sample short note question*
- Describe location of SA node, then it’s ability to produce action potential
- describe how potential spreads through heart
- describe what occurs when any bit of this process goes wrong (wolf syndrome)
what does the term functional syncytium mean?
it is a functional syncytium not a true syncytium where all the cells are fused together
a functional syncytium = electrical impulses propagate freely between communicating cells via gap junctions, so tha thte myocardium functions as a single contractile unit - this allows rapid, synchronized depolarization of the mycardium
the AV node delays the signal by what amount of time?
by 0.1 s
What are the characteristics of the AV node?
found in the right side of the interatrial septum
- narrow muscle fibre diameter
- low density of gap juncitons
- lack of fast Na+ channels
- low amplitude action potentials
- effect on ANS
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