The cardiac action potential and ECG Flashcards
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
what is the refectory period
the period in which the cell membrane is recovering from the action potential (repolarizing)
how long the refractory period in cardiac muscle?
250ms
what does the prolonged phase in the AP of cardiac muscle prevent
-summation of contractions and tetanus
- this is a protective mechanism because pumping mechanism requires alternating phases of contraction and relaxation
