4. Electrical Control Flashcards
Is the conc K+ higher or lower inside cardiac myocytes than extracellularly?
Higher inside
Is the concentration of Na+ higher or lower inside cardiac myocytes than extracellularly?
Lower inside
What sets up the resting membrane potential in cardiac myocytes?
Permeable to K+, so moved out of cell, making inside negative. There is very small permeability to other ion species, making the RMP not quite equal to Ek.
What is the resting membrane potential of cardiac myocytes?
-90 to -85 mV
What does an action potential in cardiac myocytes trigger an increase in for contraction?
Cytosolic Ca2+ conc
What causes the upstroke in the ventricular action potential?
Opening of voltage gated Na+ channels causing an influx of Na+
What causes the initial repolarisation in the cardiac action potential?
Opening of voltage-gated K+ channels and so an transient outward movement of K+
What causes the plateau in the cardiac action potential around 0mV?
Opening of voltage-gated Ca2+ channels and so influx of Ca2+ which balances with the K+ efflux
What causes repolarisation in the cardiac action potential?
Ca2+ channels inactivate and voltage-gated K+ channels remain open, so K+ efflux, making inside cell more negative again
Do the cells in the sinoatrial node have a resting membrane potential?
No, as they spontaneously depolarise
In the sinoatrial node, what is the name of the initial slope to threshold? What causes it?
Funny current
HCN channels - Na+ channels, influx of Na+ depolarises the cell
What causes the upstroke in the sinoatrial node action potential?
Opening of voltage-gated Ca2+ channels and Ca2+ influx
What causes sinoatrial node repolarisation?
Closing of Ca2+ channels and opening of voltage-gated K+ channels and K+ efflux
What type of voltage-gated Ca2+ channels are involved in cardiac myocytes and SA action potentials?
L-type
Then CICR in the SR
What happens if the action potentials in the heart fire too slowly?
Bradycardia
What happens if the action potentials in the heart fail?
Asystole (heart ceases to beat)
What happens if the action potentials in the heart fire too quickly?
Tachycardia
What happens if the electrical activity in the heart becomes random?
Fibrillation
Why are cardiac myocytes so sensitive to changes in K+ conc?
RMP is so close to Ek and K+ permeability dominates the resting membrane potential
What effect does hyperkalaemia (high blood plasma K+ levels) have on cardiac myocytes?
Depolarises the myocytes as Ek gets less negative
Slows upstroke of action potential by inactivating some voltage-gated Na+ channels
What is the main risk with hyperkalaemia?
Asystole
What is the treatment for hyperkalaemia?
Calcium gluconate - makes heart less excitable
Insulin + glucose - promotes K+ movement into cells
NB dont work if heart has already stopped beating
What affect does hypokalaemia (low plasma K+ conc) have on the action potential of cardiac myocytes?
Lengths the action potential
Delays repolarisation
Longer action potentials lead to early after depolarisations, and oscillations in membrane potential
What is the main risk with hypokalaemia?
VF and tachycardia
How does Ca2+ cause cardiac myocytes contraction?
Ca2+ binds to troponin C
Conformational change moves tropomyosin, to reveal myosin binding site on actin filament. Myosin head then binds to myosin binding site on actin
How does Ca2+ conc return to resting levels in repolarisation?
SERCA - pumps back into SR
Sarcolemmal Ca2+ ATPase and Na+/Ca2+ exchanger - Ca2+ exits across cell membrane
Where are the vascular smooth muscle cells located in blood vessels?
Tunica media
What are the two ways of increasing Ca2+ conc in excitation contraction coupling in vascular smooth muscle?
Depolarisation opening voltage gated calcium channels
Noradrenaline activating the alpha1 receptors
What does calcium bind to in excitation contraction coupling in vascular smooth muscle? What does it activate?
Calmodulin
MLCK
What needs phosphorylating in excitation contraction coupling in vascular smooth muscle to permit action with actin?
Myosin LIGHT chain
What inhibits the action of MLCK in excitation contraction coupling in vascular smooth muscle?
Phosphorylation of MLCK by PKA
