Lecture 3 - Excitation Flashcards
What causes RMP?
An uneven distribution of ions across the cell membrane
What is the ion permeability through the cell membrane?
High K+ permeability
Low Na+ permeability (10%)
Very low Ca+ permeability
What is the structure of the conduction system in the heart?
Sinoatrial node (SA)
Origin of action potential (AP)
Atrioventricular node (AV)
Bundel of His
Purkinje fibers
Cardiomyocytes
Where is the SA node located?
Right atrium
Why does the AV node go through atrial muscle?
Because AP cannot go through valves
What is the sinoatrial (SA) node?
The origin point of action potentials in the heart.
The SA node sends out an AP which is then conducted through the atria.
What is the the AV node?
Once the atria are fully depolarised, the signal reaches the AV node.
The AV node is the only location in which an AP can travel from the atria to the ventricles.
What allows for the delay between depolarisation and contraction of the atria and the ventricles?
Slow conduction through AV node
What is the Bundel of His?
The Bundel of His has branches through the ventricular septum. These branches rapidly conduct the AP down to the apex of the heart.
Once AP is at the apex of the heart, where does it go?
The AP is sent through the Purkinje fibres and consequent cardiomyocytes from the bottom of the ventricle to the top
Why is depolarisation in the ventricles down up?
This down-up depolarisation means that the ventricles will contract first at its base, aiding them to be able to push all the blood out of the heart.
What is the structure of cardiomyocytes?
Interwoven
Branch at either end
Intercalated disks
Gap-junctions
How does Cardiomyocyte Structure support Conduction of AP?
Cell-to-cell conduction
(charge movement)
Electrical coupling
Functional syncytium
all-or-none contraction
What are conduction of AP rates in the atria?
SA node generates AP’s at ~100 min-1
Conducted through atrium at ~0.5 m s-1
Conducted slowly through AV node at ~0.05 m s-1
Delay permits full depolarisation and contraction of the atria before depolarisation and contraction of the ventricles
What are conduction of AP rates in the ventricles?
Rapidly ~5.0 m s-1 through
Bundle of His
Bundle branches
Purkinje fibres
Ventricular myocardium spreads at ~0.5 m/s
Allows synchronous depolarisation and contraction of
all ventricular regions
The speed of conduction of the action potential spreads slowest in the:
A. SA Node
B. Atria
C. AV Node
D. Bundle of His
C. AV Node
What is the difference in AP in pacemaker cells vs ventricular cells?
Origin of AP in sinoatrial node (SA)
(‘Leaders’)
AP in ventricular cell (cardiomyocyte)
(‘Followers’) - Sitting at RMP until signal comes
What are the 3 phases of AP at the SA node (pacemaker cells)?
Pacemaker or pre potential (Phase 4)
Upstroke (Phase 0)
Repolarisation (Phase 3)
Why is membrane potential not completely flat in pacemaker cells?
Due to funny sodium channels which are active at RMP, letting slow influx of Na+ - this is reduction.
What happens during Pacemaker or pre potential (Phase 4)?
“Rest” membrane potential is -60/-70 mV
Unstable due to funny Na+ channels: slow influx of Na+
Late phase T-type Ca2+ channels (TTCC): influx of Ca2+
Threshold reached at -50/-40 mV
What happens during Upstroke (Phase 0) in pacemaker cells?
L-type or Voltage operated Ca2+ channels (LTCC): Ca2+ influx
The threshold for these calcium channels to open is around -50/-40mV. The influx of calcium then rapidly depolarises the cell
What happens during Repolarisation (Phase 3) in pacemaker cells?
slow K+ efflux
What are the phases of AP propagation in ventricular cells?
Stable rest membrane potential: -90 mV (Phase 4)
Upstroke (Phase 0)
Early repolarisation (Phase 1)
Cardiac Ca2+ Plateau (Phase 2)
Late repolarisation (Phase 3)
What happens during phase 4 in ventricular cells?
At phase 4 we have a stable rest membrane potential at -90mV and we have the flux of potassium
