The cardiac action potential and ECG Flashcards
(27 cards)
autorythmicity
The heart contracts rhythmically as a result of APs it generates itself
contractile cells
- 99% of cardiac cells
-do not not initiate action potentials
Autorhythmic cells
-do not contract
-They initiate or conduct action potentials
pacemaker potential
autoryythmic cells membrane’s slow drift to threshold
what is the main differences between autoryythmic cardiac cells and skeletal muscle cells
-autoryythmic cells do not have a resting membrane potential
- they display pacemaker activity
- they cyclically initiate APs without any nervous stimulation
where are autoryythmic cells located ?
1) Sinoatrial node
2) atrioventricular node
3) bundle of His
4) purkinje fibres
what is the normal pacemaker of the heart ?
sinoatrial node
what alters the discharge frequency of the Sino-arterial node
parasympathetic and sympathetic stimulation
what is the normal rate of AP discharge from the SA node ?
70-80 per min
what is the normal rate of AP discharge from the AV node ?
40-60 per min
what is the normal rate of AP discharge from the bundle of his and purkinje fibres ?
20-40 per min
functions of the AV node
- forms the only conducting pathway between the atrial muscle and bundle of his
-introduces a delay to spread of excitation ( by about 100ms) to allow blood to flow from atria to ventricles
what takes over peacemaking if SA node fails ?
AV node
what are the 5 distinct phases in the cycle of voltage change across cardiac myocytes ?
1) Depolarization
2) Early repolarization
3) plateau phase
4) late repolarization
5) resting potential
what is the threshold membrane potential for autoryythmic cells ?
approximately 40mv
describe the ionic activity during the pacemaker potential
1) funny channels open allowing Na+ to flow into the cell making the membrane potential more +
2) Funny channels close and transient Ca2+ channels open
3) Ca2+ flows into the cell bringing potential to threshold
what is a transient channel
Channel that’s only open for a short period of time
Describe the ionic activity when the membrane potential of autoryythmic cells reach threshold
1) L type Ca2+ open
2) allows entry of Ca2+ further depolarizing membrane giving rise to the upstroke of the Action Potential
How does the membrane of an autoryythmic cell repolarize after an AP ?
Lat the peak of the action potential, L channels close and K+ channels open allowing K+ to flow out of the cell, causing repolarization
what triggers Funny Channels to open ?
The membrane potential becoming more negative
describe the depolarization phase of the action potential in cardiac contractile cells
Voltage gated Na+ channels open and Na+ flows into the cell
describe the early repolarization phase of the action potential in cardiac contractile cells
Na+ channels close
transient K+ channels open and K+ flows out of the cell causing a slight repolarization
describe the plateau phase of the action potential in cardiac contractile cells
Slow L - type channels and Ca2+ slowly flows in
K+ channels close
describe the late repolarization phase of the action potential in cardiac contractile cells
Ca2+ channels are inactivated
delayed activation of ordinary K+ channels
K+ leaves the cell and the membrane potential returns to rest
