4-Cardiac Action Potentials Flashcards
What establishes the resting membrane potential of cardiac myocytes?
The permeability to K+ ions
Does resting membrane potential = Ek?
No, RMP is reached at Ek but are not equal
Describe the stages of a ventricular myocyte action potential
1-upstroke, as opening of voltage gated Na+ channels allows an influx of Na+
2-Small downstroke, as outwards K+ channels repolarise membrane
3-plateau, as l type voltage gated calcium channels open, influx of ca balanced by K efflux
4-downstroke, repolarisation due to efflux of k through voltage gated k channels
What part of the heart depolarises the fastest?
Sinoatrial node
Describe the stages of a SAN action potential
1-slight increase over long period, funny current activated, HCN channels allow,influx of Na
2-upstroke, opening of voltage gate l type calcium channels
3-downstroke, voltage gated k channels allow efflux of k, causing repolarisaton
How many action potentials are responsible a single heartbeat
1
What happens if action potentials are fired too slowly?
Bradycardia
What happens if action potentials fail?
Asystole
What happens if action potentials are fired to quickly?
Tachycardia
What happens if electrical activity becomes random?
Fibrillation
Define hyperkalaemia
Plasma potassium concentration above 5.5mmol/l
Define hypokalaemia
Plasma potassium concentrations below 3.5mmol/l
Why do variations in potassium concentration have such a large effect on the heart?
Cardiac myocytes resting membrane potential is very close to Ek and potassium permeability sets the resting membrane potential
What effect does hyperkalaemia have on cardiac myocyte action potentials?
Makes Ek less negative, membrane depolarises a little, inactivating voltage gated sodium channels, slowing the upstroke
What are the risks of hyperkalaemia?
Initially, increase in excitation
Asystole
How is hyperkalaemia treated?
Insulin and glucose
Calcium gluconate
What effect does hypokalaemia have on cardiac myocyte action potentials?
Lengthens action potential
Delays repolarisation
What are the risks of hypokalaemia?
Longer action potential - early after depolarisation - oscillations in membrane potential - ventricular fibrillation - no cardiac output
Describe how calcium is made available for contraction in cardiac myocytes
Depolarisation opens L type calcium channels in T tubule system
Localised calcium entry opens calcium induced calcium release (CICR) channels in sarcplasmic reticulum
How are calcium levels controlled following a contraction in cardiac myocytes?
Most calcium pumped back into SR by SERCA, the pumps stimulated by the high intracellular calcium levels
Some is pumped out of the cell by Na/Ca exchanger or SERCA
What carrier protein does calcium bind to in the cell?
Calmodulin
What must be phosphorylated to enable actin myosin interactions?
Myosin light chain kinase
How is MLCK inhibited?
Phosphorylated by PKC
