Initiation of the Heartbeat Flashcards
Definition of absolute refractory period
No action potentials possible here whilst depolarization and depolarization is happening
Definition of relative refractory period
Action potentials possible here but stimulus must be greater
Definition of funny current
Inward current activated when membrane potential hyperpolarises
Definition of anisotropic
Property of being directionally dependent which implies different properties in different directions
Definition of isotropy
Uniformity in all orientations, same properties in all directions
Definition of isoelectric line
Straight horizontal line on ECG, no +ve or -ve changes of electricity to create deflections
Definition of electric dipole
Wave of +veness followed by a wave of -veness
Definition of chronotropy
Anything that affects heart rate
Definition of inotropy
Anything that affects strength of contraction
Definition of lusitropy
Anything that affects rate of relaxation
How long is the neuronal action potential duration
AP duration = 500us, v short
Absolute refractory period is v short
How long is the cardiac action potential duration and how does this compare to the neuronal AP
AP duration = 200-400ms, v long
Longer than neurons but can vary with HR
Describe the ionic changes that occur in a cardiac action potential in cardiac contractile cells
1, Na influx
2, Ca influx and K efflux
3, More K efflux
4, NaKATPase, NaCa exchange to maintain resting potential
Have a stable resting membrane potential so do not contract spontaneously
How does the heart rate affect the action potential duration
What is the average AP duration at rest
As HR increases, AP duration decreases
As HR decreases, AP duration increases
APD roughly equal to QT interval on ECG
350-380ms
Why is the cardiac action potential long
Prevents tetany unlike skeletal muscle
Protects against reentrant arrhythmias
Long AP = long ARP, needed to allow for contraction and relaxation for cardiac output
Describe the shape of the electrical impulse in the cardiac conducting cells
Why is this important
All have diastolic depolarisation, don’t have stable resting potential
Results in pacemaker function
Diastolic depolarisation sets tempo for HR
What are the relative intrinsic rates of conduction
What happens if the SAN is out of control
SAN (fastest intrinsic rate)
AVN
His Bundle
Purkinje Fibres (slowest intrinsic rate)
If SAN goes wrong, other structures can take over pacemaker function but at a slower rate
Describe the cells in the SAN and their structure
What is the function of the SAN
Embryologically derived from muscle
- Poorly differentiated
- Empty membrane bags with no cytoplasm
- Numerous cavaeolae
- Lots of membrane for AP generation
- Many pseudopodia, coupling between adjacent cells
Designed for AP generation, not contraction
What are the 2 clocks that generate pulse
Membrane clock
Calcium clock
Describe the membrane clock
Cyclic changes in ionic currents driven by NaK help the membrane potential exceed the threshold
When the membrane potential hyperpolarises, funny current activated
Stimulated by NA/A
Inhibited by Ach, both alter funny current
Describe the calcium clock
Cyclical Ca release from intracellular stores drives membrane potential to threshold
Describe the steps in cardiac conduction
SLOW conduction from SAN=>AVN via atrial muscles
SLOW conduction through AV (AV pause)
- Ventricles given time to fill
- Prevents high transmission rates from atria
FAST conduction through Bundle of His to apex
Conduction spreads through ventricle via muscle cells from apex => base
How are electrical impulses spread throughout cardiac contractile cells
100um long
Intercalated discs with connexons allows diffusion of ions, small molecules from cell to cell
Describe anisotropic conduction and why its is found in the ventricular myocytes
Electrical impulses travel faster along fibres connected via more connexons.
Many connexons at the ends of myocytes than the sides
Main direction of conductance is along the fibre
How does fibre orientation affect conduction
Fibre orientation dictates impulse direction
Fibre orientation depends on whether its epicardium, myocardium or endocardium
Where are the electrodes placed for an ECG
Reference electrode on right shoulder
Recording electrode on left leg
Describe the causes of each section of the ECG wave
P, atrial depolarization
Q, Septum depolarization towards atria
R, ventricular depolarization towards apex (spread by PF)
S, ventricular depolarization towards atria (endocardium => epicardium)
T, depolarization of ventricles (epicardium => endocardium)
What do the upward lines on the ECG mean
Net depolarisation (increased positivity) towards measuring electrode
What do the downwards lines on the ECG mean
Net repolarisation (increased negativity) towards measuring electrode
What is happening during the PQ section
What could be occurring if this section cannot be seen?
Atrial conduction
AVN delay
AV block
What is happening during the QRS section
What could be occurring if this section cannot be seen
Ventricular conduction velocity
Bundle branch block
What is happening during the ST plateau section
What could be occurring if this section cannot be seen
All of ventricle depolarised
MI
What is happening during the QT section
What could be occurring if this section cannot be seen
Ventricular AP duration
Long QT syndrome
How much does the intracellular [Ca] increase by during excitation contraction coupling
200nM -> 1uM
How does cardiac muscle contract
Calcium induced calcium release
Depolarization of T tubule causes L type channels to open, Ca enters
Increase in intracellular Ca triggers ryanodine receptors
Ryr open and release more Ca which diffuses to myofilaments
How does cardiac muscle relax
SERCA uses ATP to reabsorb Ca back into SE
SERCA activity regulated by PLB
Some Ca removed via NaCa exchanger
What is chronotropy and causes positive and negative chronotropic events
Affects HR and SAN
Sympathetics (NA/A)
- increase funny current
- increase rate of diastolic depolarisation
- increase HR
Parasympathetics (Ach)
- decrease funny current
- Opens Kach channels
- Decreases rate of diastolic depolarization
- decreases HR
What is inotropy and lusitropy and how are they stimulated?
B1 receptor stimulation (PKA and Gs) (NA/A)
Results in accelerated rate of contraction in inotropy
Higher rate of Ca removal in lusitropy
What happens when L type Ca channels are phosphorylated by PKA
Increased channel opening
+ve chronotropy
+ve ionotropy
What happens when ryanodine receptors are phosphorylated by PKA
Increased SR ca release
+ve ionotropy
What happens when the myofilaments are phosphorylated by PKA
Troponin I and myosin binding protein C
Increases rate of cross bridge cycling
+ve ionotropy
+ve lusitropy
What happens when the pacemakers are phosphorylated by PKA and cAMP
Activates membrane and Ca clocks
+ve chronotropy
What happens when ATPases are phosphorylated
PLB (for SERCA), PLM (for NaKATPase) phosphoryated to increase Ca uptake
+ve lusitropy
