Electrical Mechanisms Flashcards
What ion concentration sets up the RMP in cardiac cells?
K+
What are the intra and extracellular concentrations of K+ roughly in the body?
Intra-140mM
Extra-4mM
When are cardiac myocytes most permeable to K+?
At rest
What is Ek roughly?
-95mV
Why does the RMP not equal Ek exactly?
Because there is some very small permeability to other ion species at rest
During action potentials what happens to the cystolic Ca2+ conc?
It is increased
Why is calcium needed in cells during contraction?
Allows actin and myosin interactions
Roughly how long is the wave in a cardiac myocyte action potential?
300-400 ms
What is the ratio of systole to diastole in a ventricular cardiac cell action potential?
1:2
What ion channel is responsible for the upstroke in the ventricular cardiac action potential?
Voltage gated Na+ channels
What is the initial re-polarisation of the ventricular cardiac action potential caused by?
A transient outward K+ current
What ion channels are responsible for the plateau phase of the ventricular cardiac action potential
V-gated Ca2+ channels (L-type)
Why does the plateau phase of the ventricular cardiac action potential still continue to decrease slightly?
Because there are some K+ channels open also
When the membrane potential becomes increasingly positive, what happens to the Na+ channels?
The are inactivated
What is the large down stroke of the ventricular cardiac action potential graph caused by?
The inactivation of Ca2+ channels (L-type) and opening of voltage gates K+ channels
What is different about the cardiac myocytes that create the SA node? (2)
- They do not have much contractile ability
- no proper RMP
What is the minimum potential a cardiac myocyte in the SA node ever reaches as a membrane potential?
-60mV
When looking at a graph of a SA node action potential, what is the initial slope to threshold called?
The funny current
What channel causes the funny current of the SAN action potential?
HCN (hyperpolarisation-activated, cyclic nucleotide-gates channels )
What ion causes the funny current in a SAN action potential?
Na+ (influx into the cell- depolarises the cell)
At what potential is a HCN channel activated?
-50mV (more negative the potential the more likely it is that the HCN channels will open)
What causes the upstroke of the SAN action potential?
Opening of voltage gated Ca2+ channels (L-type)
What causes the re-polarisation of the membrane during a SA node action potential?
The opening of Voltage gated K+ channels
What is different about the RMP in normal cardiac cells and then the RMP of the cells in the SAN and AVN?
Those in the SAN and AVN have no proper/unstable resting potential
Why is SA node the thing to set the rhythm of the heart?
It is the fastest to depolarise
(There are other parts of the heart- AVN and purkinje fibre that have automaticity but they depolarise slower so are not the pacemakers)
What happens if the action potentials in the heart fire too slowly?
Bradycardia
What happens if the action potentials in the heart fail to fire?
Asystole
What happens if the action potentials in the heart fire to quickly?
Tachycardia
What happens if the electrical activity of the heart becomes random?
Fibrillation
What can fibrillation cause a loss of?
Cardiac output
What is the normal plasma K+ conc?
3.5-5.5mmol/L-1
What will happen to the interval between APs created by the pacemaker cells if the membrane depolarised more rapidly during diastole? How will this effect the heart rate?
The interval will decrease meaning there is a faster heart rate- tachycardia
What will happen to the heart rate if the pacemaker potential depolarises less rapidly?
Bradycardia- slower heart rate because the interval between action potentials increases
