cardiac electrophysiology Flashcards
what is meant by myogenic?
The heart generates electrical activity and contraction without the need for nerves
How does SAN generate APs?
generates action potentials (APs) with rhythmical pacemaker activity in absence of any nerve stimulation
Hr and SAN
importance?
Firing rate of APs (resting about 1/s) equals heart rate (resting about 60-70bpm)
Therefore SAN controls HR
SAN cells not contractile?
generate electrical activity
why does SAN node keep generating AP?
unstable/non-equilibrium resting membrane potential (RMP)
what are If channels? (funny current)
what do they do?
Hyperpolarisation-activated Na+ channel (sometimes termed Ih)
Na+ influx produces depolarising slope (Phase 4)
explain phase 4 for SAN
what sets it off and what do they do?
If activated by hyperpolarisation
open If Na+ channels and the influx causes depolarisation till threshold hit
threshold hit leading to phase 0, explain (SAN)
Voltage-gated Ca2+ channels open leading to Ca2+ influx and rapid Depolarisation so AP generated
why is Ca2+ and Na+ depolarisation different? (3 things)
Na+ nerves
Ca2+ slow upstroke compared to Na+ due to opening slower
Ca2+ is open longer so wider AP
Phase 3 (SAN)
Voltage-gated K+ channels open leading to K+ efflux and rapid Repolarisation
conduction pathway from SAN through heart
conduction across both atrium to AV nodes to Bundle of His to purkinje fibres
why does Av node slow down conduction rate?
how does it slow it down?
has fatty insulated tissues to slow down conduction rate so it allows time for the ventricles to fill with blood from atrium
importance of purkinje fibres
allow both ventricles to be stimulated at the same time hence can contract and eject blood at the same time
what allows FAST electrical conduction in heart
Via low resistance pathways between myocytes called intercalated discs
how are APs generated in atrium/ventricles?
from electrical stimulation arising in S-A node
atrial/ventricular cells have a stable resting membrane potential
phase 0 in a/v ap
what opens, why and what does it do?
AP from SAN lead to Voltage-gated Na+ channels open and high Na+ influx leading to Depolarisation
phase 1 (a/v) - what opens?
k+ channels breifly open for repolarisation
phase 2 - plateau stage (a/v)
what opens and why plateau?
Voltage-gated Ca2+ channels (VGCCs) open and Sustained Ca2+ influx leading to Depolarisation
plateau cos k+ out and ca2+ in
importance of plateau stage
what does it prevent and what is it essential for?
Myocytes are unexcitable or REFRACTIVE – CANNOT fire more APs, no twitching
One AP only produce ONE contraction
ESSENTIAL for proper ejection of blood from heart
This is all due to Na+ channels being inactivated so can’t lead to another AP firing
phase 2 to 3 (a/v) - what close/open?
VGCCs switch off and K+ channels switch on
Phase 3 (a/v) - what open and effect?
Voltage-gated K+ channels open leading to K+ efflux and rapid Repolarisation
How does a/v APs produce a contraction?
Contraction occurs by an INCREASE in cytosolic Ca2+ levels
Higher increases in Ca2+ lead to increased force of contraction
Ca2+ rises from 0.1 µM to about 2-10 µM
difference between smooth and cardiac muscle contraction (2 things)
CICR (Ca2+-induced Ca2+ release) via RyR and contraction via troponin system are DIFFERENT to smooth muscle (IP3R + MLCK)
Link between Electrical Excitability and Contraction
1 heart beat = SAN AP to atrial contraction to electrical activity to AV node to ventricular contraction
Ecg and pqrst wave
what do the different waves correspond to?
P wave - atrial depolarisation
QRS wave - ventricle depolarisation
T wave - ventricle repolarisation
what effect does stimulation of the sympathetic nerves have on the heart?
2 effects?
Stimulation of these nerves as an effect on,
Heart rate : Positive chronotropic effect
Contractility : Positive inotropic effect
↑ If —-> ↑ Pacemaker frequency —–> ↑ Heart rate
↑ ICa + ↑ RyR —-> ↑ influx of Ca2+ levels + ↑ CICR ——> Contractility
what sympathetic nerves have an effect on the heart?
what nerves and what do they stimulate?
Sympathetic nerves from T1-T5 of spinal cord innervates both S/A node and atria/ventricles
How does the sympathetic nerves have an effect in the heart?
what is released and act on what receptors?
Releases noradrenaline which acts at B1 adrenoceptors on S-A node cells and cardiac myocytes
what effect does stimulation of the parasympathetic nerves have on the heart? ( 1 effect)
Heart rate : Negative chronotropic effect only
↓ If —-> ↓ Pacemaker frequency —–> ↓ Heart rate
what parasympathetic nerves have an effect on the heart?
Parasympathetic nerves from brainstem innervate S/A node
vagus nerve
How does the parasympathetic nerves have an effect in the heart?
what is released and what receptor?
Releases Ach which acts at muscarinic (M2) receptors
on S-A node cells
