Initiation of the heart beat Flashcards
How does the cardiac action potential differ to the neuronal action potential?
It is very long (200ms)
Which ionic movements underly the cardiac action potential?
Na+ enters
Ca2+ enters
K+ leaves
What does the cardiac action potential duration depend on?
Heart rate
- faster heart rate = shorter AP
What is the cardiac action potential roughly the same as?
The QT interval of the ECG
Why is the cardiac action potential so long?
- Prevents tetany
2. Protects against re-entrant arrythmias
What is the principal pacemaker of the heart (highest intrinsic rate)?
SAN
Which specialised conductive tissues conduct the signal from the AVN to the ventricles?
Bundle of His -> Purkinje fibres
Which cells show diastolic depolarisation?
SAN
AVN
Cells of cardiac conduction system
What is diastolic depolarisation?
No stable resting membrane potential
Cells depolarise between beats - ‘clock’ function
Can atrial and ventricular myocytes contract spontaneously?
NO - they have stable resting membrane potentials
Describe the cells of the SAN
Poorly differentiated
Empty bags - lots of membrane
No cytoplasm
Membranes have pseudopodia
What are the two theories about the ‘clock’ of the heart?
- Membrane clock - due to changes in ionic currents
2. Calcium clock - due to changes in calcium concentrations
What is the funny current?
Inward current (Na and K) that is activated when the membrane is hyperpolarised
Which hormones modify the funny current?
Adrenaline stimulates the funny current
Acetylcholine inhibits the funny current
Which drug blocks the funny current and is the only drug that changes HR directly?
Ivabradine
Which is more important in the clock theories of cardiac rhythm initiation/
Both are important and both modulate each other
What is the function of the ‘AV pause’?
- Allows time for ventricular filling
2. Prevents transmission of high rates from atria
What speed is conduction from SAN to AVN?
Slow
What speed is conduction through the ventricular conduction system?
Fast - allows apex to contract before base
What is the arrangement of the ventricular myocytes?
Synctium - ends are interdigitated where the cells meet their neighbours for mechanical strength and for conduction
What are the junctions between myocytes called?
Intercalated discs (contain gap junctions)
What is anisotropic conduction in the heart?
Unequal conduction
- electrical impulse travels easier along the fibre than across it
What is the cardiac dipole in the ECG?
A wave of positiveness followed by a wave of negativeness
From right shoulder to left hip
What does the P wave on an ECG represent?
Atrial depolarisation
What does the Q wave on an ECG represent?
Depolarisation of the septum
What does the R wave on an ECG represent?
Depolarisation of the ventricles (towards apex)
What does the S wave on an ECG represent?
Depolarisation of the ventricles (towards atria)
What does the T wave on an ECG represent?
Repolarisation of the ventricles
What does the P-Q interval represent?
Atrial conduction
example pathology: AV block
What does the QRS duration represent?
Ventricular conduction velocity
example pathology: Bundle branch block
What does the ST segment represent?
Heterogeneity of ventricular polarisation
example pathology: MI
What does the Q-T interval represent?
Ventricular action potential duration
example pathology: long QT syndrome
What is the mechanism of cardiac muscle contraction?
Calcium induced calcium release
Which type of calcium channel is found in T tubules and opens due to voltage changes in membrane?
L-type calcium channels
What is the mechanism of skeletal muscle contraction?
Voltage induced calcium release
What is the mechanism of smooth muscle contraction?
IP3 induced calcium release
Which mechanisms are responsible for calcium removal during relaxation of cardiac muscle?
SERCA
Na/Ca exchanger
What does a positive chronotrope do?
Increases heart rate
What does a positive inotrope do?
Increase the force of contraction
What does a positive lusitrope do?
Increase rate of relaxation
How do positive chronotropes (e.g. adrenaline) work?
Increase funny current
Faster rate of diastolic depolarisation
How do negative chronotropes (e.g. acetylcholine) work?
Inhibit funny current
Opens KAch channels
Slower diastolic depolarisation
How does B1 stimulation of the heart work?
B1 adrenoreceptor associated with G protein - works via AC/cAMP/PKA
PKA has many phosphorylation targets
