PSL301: Cardio 2 Flashcards
Cardiac action potential
2 types of cardiac action potentials
- Non-pacemaker
2. Pacemaker
What are non-pacemaker action potentials?
“fast response” AP: rapid depolarization
Where can non-pacemaker AP be found?
Everywhere in the heart except where pacemakers are
What are pacemaker AP?
“slow response” AP: slower rate of depolarization
Pacemakers generate spontaneous AP
Where can pacemaker AP be found?
SA and AV node
Function of pacemaker
Generate resting heart rate
Respond to changes -> increase / decrease HR
Difference b/t AP in the heart and everywhere else
- Duration: 200 - 400 ms
- Resting membrane potential is -90mV instead of -70 mV
What is the purpose of the increased duration for cardiac AP?
Refractory period will end when muscle is almost done relaxing -> prevent tetanus
What causes depolarization in a cardiac contractile cells?
Na+ influx
What causes the plateau in cardiac contractile cells?
Ca++ influx
What causes the final repolarization in cardiac contractile cells?
Slow K+ channels
What is resting membrane potential for contractile myocardium?
-90 mV
How is cardiac AP prolonged?
Influx of Ca++
Why does cardiac AP have no hyperpolarization?
resting potential is -90 mV, the equilibrium potential for K+
Why do pacemakers have spontaneous AP?
Unstable membrane potentials: -60 mV
If channels are found on pacemaker cells -> cause depolarization when Na+ influx > K+ exflux
If channels
A HCN channel that is permeable to K+ and Na+.
Open at -60 mV
HCN
Hyperpolarization-activated cyclic nucleotide gated channel
Describe the AP for pacemaker cells
- If channels allow Na+ and K+ to move
- Na+ influx > K+ exflux cause slight depolarization
- Slight depolarization causes If channel to close, and some Ca++ channels to open
- Ca++ brings membrane potential to threshold
- T & L type Ca++ channels open and cause depolarization to +20 mV
- Ca++ channels close, slow K+ channels open
- K+ repolarizes
Only ___ cells use Ca++ to depolarize
Pacemaker
Describe AP for contractile myocardial cells
- AP arrives through gap junctions
- Voltage gated Na+ channels open: Na+ depolarizes cell to +20mV then gates close
- Fast K+ channels open: K+ repolarizes cell
- Ca++ channels open; fast K+ channels close
- Ca++ causes membrane potential to plateau
- Slow K+ channels finally open: completey repolarize cell
Tetanus
Sustained muscle contraction
Why is tetanus dangerous in myocardium?
Muscles must relax for ventricles to fill with blood.
If ventricles cannot fill with blood, body gets no fresh blood.
Tissues will die.
___ prevents electric signals from passing from atrium to ventricle anywhere else except at AV node
fiberous skeleton
Steps of electrical conduction in the heart
- SA node depolarizes
- Internodal pathway (atrial contraction)
- AV node
- AV bundle (Purkinje fibres)
- Bundle branches (Purkinje fibres)
- Apex of heart (smaller Purkinje fibres)
- Signals to contractile cells
- Spiral arrangement of muscles pull apex & base together to squeeze blood upwards (ventricular contraction)
Purpose of AV node
- Routes direction of electrical signals
2. Delays transmission of AP (ventricle doesn’t contract when atria isn’t done yet)
SA node sets HR at…
70 bpm
AV node sets HR at…
50 bpm
Purkinje fibres set HR at…
25-40 bpm
If the SA node is isolated from the body, at what rate does it beat?
90 bpm
What slows the SA node down to 70 bpm?
Parasympathetic system constantly acting on SA node
What nerve connects the PNS to SA and AV node?
Vagus nerve
Atropine
Muscarinic receptor antagonist (bind to ACh receptors) -> increase HR by 20-40 bpm
What drug can be used to increase HR?
Atropine
Does vagal tone increase or decrease HR?
Decrease
What causes increased heart rate?
- Decreased vagal tone
- Innervation of SA node by SNS
- Catecholamines acting on B-adrenoceptors on SA node
What caused decreased heart rate?
- Increased vagal tone
HCN is ____ gated.
Give an example
cyclic nucleotide
cAMP
Explain how catecholamine can increase heart rate?
- E bind to B1-adrenergic receptor (G protein)
- Subunit activates adenylyl cyclase
- Production of cAMP
- cAMP affects If channels and Ca2+ channels
How does cAMP affect If channels?
Increase funny current: Na+ enter cell faster
How does cAMP affect calcium channels?
cAMP activates PKA -> {P} Ca++ channels
Keeps Ca channels open longer, so more Ca can get into cell.
What kind of receptors do pacemaker cells have?
Muscarinic M2 Gi protein coupled receptors
ACh acts on which subunit of the ___ receptor?
B-gamma
Muscarinic M2 Gi protein receptor
What affect does ACh have on pacemaker cells?
- Opens K+ channels, making cell harder to depolarize
2. Decrease cAMP concentration (If channels & Ca channels less active)
Which hormone slows heart rate?
ACh
Which hormone speeds up HR?
Epinephrine
The PNS / SNS can affect force of contraction in addition to rate of contraction? Why?
SNS
Sympathetic fibres also innervate atria & ventricles
