Initiation of the Heartbeat Flashcards
(43 cards)
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
