CVS 7: Electrical properties of the heart Flashcards
If you had 2 chamber separated with an impermeable membrane containing different concentrations of a solution what would be the p.d.?
No potential difference between the chambers.
Because of the impermeable barrier even though you have a concentration gradient.
If you had two chambers containing different concs of K+ separated by a permeable membrane what would happen at first?
- The membrane is more permeable to K+ than any other ions
- K+ ions diffuse down their Conc gradient carrying their +ve charge
- +ve charges build up on one side and en electrical gradient builds up
- The electrical gradient opposes the movement of K+
What happens at equilibrium according the potassium hypothesis?
- Electrical gradient = K+ gradient
- Ions move back and forth randomly through the channel but there’s no net movement of ions
Which equation helps you to predict the resting membrane potential? What is the resting membrane potential of a cardiomyocyte?
The Nernst equation
-80mV
What causes the membrane potential to change?
It changes based on the relative permeabilities to different ions
Which equation is used to give us a better understanding of membrane potential? and why is it better?
Goldman- Hodgkin- Katz equation
Takes into account of relative permeabilities of the membrane to different ions
How long do nerve and cardiac APs tend to last? Why is there this difference?
Nerve: 2ms
Cardiac: 200-400ms
(cardiac is much longer because long, slow contraction is needed to produce an effective pump)
Draw and annotate a diagram of a cardiac action potential, showing the different refractory periods as well
x= (ms) 0-300 y= membrane potential (mv) -100- =50 - flat line until 0 which is RP at -80mV - Straight line up at 0 to +30mV - a quick notch afterwards - plateau - quick drop back to -80mv by 280ms - ARP up to 180ms - RRP from 180-220ms
What is the absolute refractory period?
sodium channels are shut and cannot be opened for a long time.
What is characteristic of the ARP in cardiac tissue? What is the consequence of this?
- Cardiac cells have a long absolute refractory period
- so that you can’t restimulate the muscle for a long time and cardiac muscle will not tetanize
What is the relative refractory period?
Period after ARP where an AP can be elicited but only by a STRONG stimulus.
What are the different phases of the cardiac action potential?
Phase 0= upstroke Phase 1= early repolarisation Phase 2= plateau Phase 3= repolarisation Phase 4= resting membrane potential (diastole)
Phase 0/4: What determines the resting membrane potential?
K+ flowing out of the cells
Phase0: What determines he upstroke?
opening of Na+ and an increase in membrane permeability to sodium
Phase 1: What happens in this phase?
Early depolarisation caused by shutting of sodium channels
- transient outward K+ current starts
Phase 2: Why does the membrane potential plateau?
There is an increase in permeability to Ca2+ .
The influx of Ca2+ balances the efflux of K+
Phase 2: What sort of channel is used for the influx of Ca2+
L- type Calcium Channels (l= long- lasting)
Phase 2: Why is there an influx of Ca2+? What is it’s purpose?
- Ca2+ needed to trigger Ca2+ release from intracellular stores (CICR)
- Ca2+ needed for contraction
Phase 2: How is this phase targeted in anti-hypertensive therapy? Give examples of drugs used as well
Calcium Channel Antagonists are used which inhibit L-type calcium channels:
- Nifedipine
- Nitrendipine
- Nisoldipine
Phase 3: What happens during this phase? Which current is activated?
REPOLARISATION
- K+ currents are activated and K+ moves out > inward flow of Ca2+
- IK1 current is activated which fully repolarises the cell
- Ik1 is large and flows during diastole
Phase 3: What does IK1 reduce the risk of and how?
IK1 reduces the risk of arrhythmia because it requires a large stimulus to excite the cells
Why do different parts of the heart have different action potential shapes?
- They have different ionic currents flowing
- This is because they have different expression of ion channels
Why are the electric properties of the heart describe to be INTRINSIC?
- has its own independent generation and propagation system
- Myogenic= can beat independently even after being separated from its nerve supply
What modulates cardiac activity?
Sympathetic and Parasympathetic activity from the Autonomic Nervous system which controls the intrinsic beating of the heart
What are the differences between SAN and ventricular cells?
- No IK1 channels in the SAN
- Ca2+ influx not Na+ influx creates the upstroke
- more unstable membrane potential
- T- type Ca2+ channels not L-type Ca2+ channels because they activates at more -ve potentials
- presence of pacemaker current (the upward slope)
What is the consequence of SAN cells not having IK1 channels
IK1 is involved in stabilising the membrane potential
- no IK1= unstable membrane potential
What happens when you have sympathetic stimulation of the SAN?
(By ADRENALINE)
- pacemaker potential is steeper
- Threshold potential is attained more quickly
- heart rate increases
What happens when you have parasympathetic stimulation of the SAN?
(By ACETYLCHOLINE)
- pacemaker potential is less steep
- takes longer to attain threshold potential
- heart rate decreases
Where is the SAN located?
Below the epicardial surface a the boundary between the right atrium and superior vena cava
What are the four stages of the conduction system?
- SAN
- Inter-nodal fibre bundle (stimulates the atria)
- AVN
4) Ventricular bundles (left and right branches and Purkinje fibres)
What helps propagate the impulse?
neighbouring cells excite easily because of LOW MEMBRANE RESISTANCE between cells (gap junctions and intercalated discs)
What is the function of the inter-nodal fibre bundles?
Conduct the AP to the AVN faster than through atrial muscle
What is the function of the AVN?
- To connect the conduction systems between atrial and ventricular chambers
- To produce a short delay
What is the function of the Purkinje fibres?
Conducts AP to the base so that excitation of the ventricles proceed from the apex to the base
On an ECG what does and upward and downwards deflection mean?
Upwards: wave of DEpolarisation towards the +ve electrode
Downwards: Wave of DEpolarisation away from the +ve electrode
What effect does a wave of REpolarisation have on the eCG?
It is opposite to the effects of a wave of Depolarisation
Describe what PQRSTU on an ECG are?
P= atrial depolarisation QRS= ventricular depolarisation T= Ventricular repolarisation
