Cardiac electrical activity Flashcards
What is the resting membrane potential for a cardiac muscle cell and why
-90mV, near the Nernst or equilibrium potential for Potassium. At rest the predominant channels that are open are K+ channels meaning the RMP is closest to K+ EP.
What is the duration of a cardiac muscle cell action potential compared with nerve AP
200-300 msec, nerve and skeletal muscle AP 1 msec.
What is threshold potential for cardiac muscle cells?
-70mV. The point at which voltage gated Na+ channels open.
Describe phase 0 of the fast action potential
Rapid influx of Na+ ions as voltage gated Na+ channels open. Na+ is moving inwards downs its concentration (Na+ much higher out of the cell than in the cell) and electrical (RMP is negative Na+ is positive) gradients.
Why do the Na+ channels close almost as quickly as they open
Because they are voltage and time sensitive
Describe phase 1 of the fast action potential
Early repolarisation. Whereby there is a transient outward K+ current. Membrane potential close to 0mV
Describe phase 2 of the fast action potential
Plateau phase Na+ channels shut and are refractory, inward going current Ca2+ and outward K+ current. Na+/Ca2+ exchangers 3 Na+ in for 2 Ca2+ out. Also Ca2+ dependent Ca2+ release from SER. Net current 0.
Describe phase 3 of the fast action potential
Voltage and time dependant K+ channels (iK) open and repolarisation occurs,
What happens if there is early or delayed repolarisation?
Potential for cardiac arrhythmia
Describe phase 4 of the fast action potential
Resting phase. K+ open and membrane potential at -90mV
Where does the fast action potential occur
In the atria, ventricles and fast parts of the specialised conduction system
Where does the slow action potential occur?
In the cells of sinoatrial node, and atrioventricular node. Maybe pacemaker cells.
What is the difference in upstroke in the the slow AP compared with the fast?
Slower upstroke on depolarisation as the currents are not as intense.
What causes the upstroke in the slow AP
Upstroke due to opening of Ca2+ channels and inward going calcium. Also ‘funny’ channels (If) where Na+ channels are opened in response to hyper polarised membrane
Why is refractory period essential
To avoid tetanic contraction. Contract to eject blood, needs relaxation to fill.
Describe relative refractory period (RRP)
Can generate an AP with higher than normal stimulus but weaker AP than normal.
Describe super normal period (SNP)
When you can stimulate AP with smaller than normal stimulus but slower conduction of AP occurs.
Describe absolute refractory period (ARP)
When there is no possibility of generating another AP
Describe the properties of cardiac muscle for spreading AP
Cardiac muscle fibres are conductive. Activation spreads through the muscle cell to cell via gap junctions in the syncytium
Pacemaker cells in the heart
Sinoatrial node, Atrioventricular node, purkinje fibres
How does the PNS work to slow HR
ACh, the PNS NT, increases the permeability of the cell membrane to K+. Cell membrane becomes hyperpolarized. strong vagal stimulation can also stop the SAN or block the AVN.
How does the SNS increase the HR
Noradrenaline at the SAN increases the slope of the pacemaker depolarisation through phosphorylating Ca2+ channels which generates longer opening of the channels, more inward Ca2+ faster depolarisation, faster upstroke.
What is the role of the AV node
The AV nodes role is to slow the conduction of AP to the ventricles. This is so the atria contracts slightly before ventricles this assists in filling the ventricles before they contract.
What is the speed of conduction of the bundle branches and purkinje fibres?
2-4/m/s fast to activate ventricular endocardium synchronously
What is the speed of conduction of the SA node
Very slow
What is the speed of conduction of the AV node
Slow 0.02- 0.05m/s to slow conduction allowing ventricular filling
Conduction velocity depends on
- Radius of cells
- Rate of depolarisation and amplitude of action potential (driven by different currents, different channels)
What would occur with blockage of the right coronary artery?
The right artery supplies the SA and AV nodes so rhythm disturbances may occur
What may occur with blockage of the left coronary artery?
Supplies the circumflex and LAD arteries. Blockage results in pump failure.
What is the name of the band shortening transmission in the right ventricle.
The septomarginal trabecular. Allows the right ventricle to contract uniformly with the left.
