Flashcards in Electrical Activity In The Heart Deck (19)
What is the intracellular and extra cellular concentrations of sodium ?
Intracellular = 15mM
Extracellular = 150mM
10 fold difference
What are the intracellular and extra cellular concentrations of potassium ?
Intracellular = 150mM
Extra cellular = 5mM
30 fold difference
What is the intracellular and extra cellular concentrations of calcium ?
Intracellular = 0.0001mM
Extra cellular= 2mM
20000 fold difference
What is the Nernst equation ?
It transduce the concentration gradient of an ion into a potential
How do we know that the resting membrane potential is mainly permeable to potassium ions ?
Because the resting membrane potential is negative and the only ion which produces a negative equilibrium potential is potassium so membrane potential is dominated by potassium concentrations
What are the properties of cardiac ions channels for sodium ?
They are fast voltage gated sodium channels
Open at negative voltages - -50mV
Activates rapidly but inactivates rapidly
Produces an inward current
What are the properties of cardiac ion channels for potassium ?
Produce an outward current
BACKGROUND POTASSIUM CHANNELS
- open at negative voltages and set the negative resting membrane potential of atrial and ventricular muscle
- they close at positive membrane potentials
DELAYED POTASSIUM CHANNELS
- they are closed at negative membrane potentials
- open at more positive membrane potentials because they are involved in repolarisation of the cell
What are the properties of cardiac ion channels for calcium ?
T-TYPE - transient opening / tiny conductance
- mainly in pacemaker and atrial tissue
- open at -55mV
- inactivate rapidly
L-TYPE - large conductance/long lasting opening
-found throughout the heart
- open at -40mV
- inactivate slowly
What are the properties of mixed conductance channels ?
1- BACKGROUND SODIUM CONDUCTANCE
- small conductance of sodium and potassium
- their reversal potential is -20mV
- makes inward currents at potentials negative to the -20mV
2- FUNNY CURRENT
- permeable to both sodium and potassium
- activates slowly but has small conductance
- their reversal potential is -20mV
- present in he SAN
- called the funny current because they are the only channels opened by hyper polarisation
Why is there no stable resting membrane potential in the SAN ?
Because there are no background potassium channels present so ten resting potential is continually changing
What are the 4 overlapping currents which make up the pacemaker potential ?
1- decay of delayed potassium channel permeability
2- funny current
3- background sodium conductance
4- T-type calcium channels
What is the pacemaker potential ?
It is the period between action potentials
What induces repolarisation in the SAN action potential ?
Induced by closure of L-type calcium channels and opening of delayed potassium channels
Why does ventricular action potential have a stable resting potential ?
Because there are background potassium channels present
What is responsible for the depolarisation in the ventricular action potential ?
Opening of voltage gated sodium channels allowing sodium to enter the cell
Which channels are responsible for the plateau phase of the ventricular action potential ?
L-type calcium channels which open at -40mV
The inward current is long lasting
Which channels are responsible for the repolarisation of the ventricular action potential ?
Opening of the delayed potassium channels and the closing of the the L-type calcium channels
What is the hierarchy of pacemakers ?
SAN, AVN and purkinje fibres