CVS Cardiac Action Potentials and Conduction Pathway Flashcards
How does the heart generate electrical activity?
The heart generates electrical activity and contraction
without external stimulation, e.g., from nerves or hormones
Termed : Myogenic
How does skeletal muscle generate electrical activity?
Skeletal muscle: needs motor nerve input
How do smooth muscle generate electrical activity?
Smooth muscle: needs ANS input
What does the sinoatrial node(SAN) generate?
SAN generates action potentials called pacemaker potentials
in absence of any external stimulation, e.g. nerves or hormones
What does the firing rate of the action potentials generated from the SAN equate to?
Firing rate of these action potentials equates to heart rate
Rhythm of these action potentials equates to heart rhythm
How does the SAN produce pacemaker potentials?
- Starts at resting membrane potential of -60mv
- Phase 4: If channels are activated. These cause hyperpolarization by activate Na+ channels. Na+ influx produces depolarising slope
- Phase 3: Voltage gated K+ channels open which cause a K+ efflux. This results in repolarisation
4 . Phase 0: Voltage gated Ca2+ channels open, causing depolarization by a Ca2+ influx - K+ efflux continues until cardiac resting membrane potential threshold is met
What do intercalated discs allow in conduction of electrical activity in heart?
Low resistance pathways
between atrial and ventricular cells called
intracalated discs
- Allows fast conduction -
What are the steps involved in conduction of electrical activity from SAN through heart?
(1) Conduction across both atria to atrial-ventricle node (AVN)
(2)AVN is non-contractile tissue which slows electrical conduction
between atria and ventricles
- This allows atrial ejection to correctly fill ventricles before electrical activity
is conducted to ventricles and contraction occurs -
(3)Bundle of His to purkinje fibres is a fast conduction pathway which
allows both ventricles to be stimulated and contracted together
What does the P wave in an ECG represent in the conduction pathway?
atrial depolarisation
What does the QRS complex in an ECG represent in the conduction pathway?
ventricle depolarisation
What does the ST interval in an ECG represent in the conduction pathway?
Interval between ventricle depolarisation and repolarisation
What does the T wave in an ECG represent in the conduction pathway?
Ventricle repolarization
Electrical-contraction coupling through the heart
(1)Electrical activity generated in SA
node (non-contractile tissue)
spreads out into (2) atria to produce
contraction
Electrical activity then enters AVN
which delays conduction before
stimulating ventricles
- allows ventricles to fill properly
during atrial contraction -
(3) Excitation conducts rapidly through
bundle of His into purkinje fibres
(4) Excitation through purkinje fibres is
conducted throughout the ventricular
contractile tissue producing contraction
Where does ventricular contraction begin and spread?
Ventricular excitation and contraction begins at the apex then spreads to base of heart
What are action potentials in atrial/ventricular cells generated by?
Action potentials in atrial/ventricular cells are ultimately generated from
electrical activity arising in SAN
atrial/ventricular cells have a stable resting membrane potential
What are the phases involved in atrial/ventricular action potential?
- Phase 4 is resting memrbane potential which starts at -90mv
- Phase 0 is where voltage gated Na+ channels open causing depolarisation by a Na+ influx
- Phase 1 is where voltage gated calcium channels open resulting in sustained Ca2+ influx causing depolarisation
- phase 2 is the plateau phase lasting 200-400 ms
- When going form phase 2 into phase 3, VGCCs switch off and K+ channels switch on
- Phase 3 is where voltage gated K+ channels open causing repolarization by a K+ efflux
What is the function of plateau phase?
Tissue is unexcitable or REFRACTIVE – CANNOT fire more action potentials
Voltage-gated Na+ channels are inactivated
No twitching - One action potential produces ONE contraction
ESSENTIAL for proper ejection of blood from heart
What is contraction of atria and ventricles caused by?
Contraction caused by an INCREASE in cytosolic Ca2+ levels
How much does Ca2+ rise in health resting individuals?
In a healthy resting individual Ca2+ rises from about 0.1 μM to about 1 μM
How do atrial and ventricular action potentials couple to contraction?
- Voltage gated Ca2+ channel open which causes a Ca2+ influx
- Some Ca2+ will bind to ryanodine receptors (RyR) on sarcoplasmic reticulum which results in calcium induced calcium release(CICR)
- This increases intracellular Ca2_ which interacts with contractile proteins(Troponin system)
- This leads to contraction
