Electrical events of the cardiac cycle Flashcards
What cells are the myocardial pacemaker cells
Cells in SAN
Cells in AVN
Cells in Purkinje fibres
Cells in Bundle of His
What are 99% of the cardiac cells
What are 1% of the cardiac cells
Contractile
Autorhythmic
What is the natural pacemaker and why
SAN because it has the fastest firing cells (depolarise at a faster rate)
In a pacemaker cell (SAN),
what happens to the cells at -60mV
The voltage gated channels are activated
What channels open at -60mV and what moves where?
Na+ voltage gated channels open and sodium moves outside to inside
What happens when the potential becomes more positive because of the influx of Na+
Calcium channels open because of the threshold potential and Ca2+ moves inside the cell as conc is higher outside
What channels also open
-when and what does this do
K+
-When a high mV is reached and the K+ moves out of the cell
What slows the excitation and why
AV node so the excitation can spread across the atria
What is the rapid spread of excitation through the heart facilitated by
Intercalated discs between fibres which are found in the gap junctions
What do the gap junctions provide and what does the cardiac muscle function as
Low resistance pathways
-Cardiac muscle functions as a syncytium (single unit)
What does the wave of excitation cause in the other 99% of cells (the contractile cells)
Increase in mV because of the opening of sodium channels so there is a sodium influx
Why is there a dip in membrane potential after the increase in membrane potential
Because as the potential becomes more positive, the potassium channels open and there is a potassium eflux so mV becomes more negative
Why is there a plateau after the K+ efflux
Because of the influx of calcium
Why is the plateau important
It is a refractory period which protects the heart from tetanus
What happens after the plateau
-why
There is a fall in the membrane potential
-Calcium channels close and then potassium efflux overtakes and mV becomes more negative and brings it back to -90mV
Where does contraction occur from and to
From inside to outside (endocardium to epicardium) and bottom to top (apex to base)
What happens during systole
- Influx of Ca2+ during action potential
- Triggers release of further Ca2+ from sarcoplasmic reticulum
- Free Ca2+ activates contraction of myocardial fibres
What determines the force of heart contraction
Amount of Ca2+
What happens in diastole
- Uptake of Ca2+ by sarcoplasmic reticulum and extrusion of Ca2+ by Na+/Ca2+ exchange and outward Ca2+ pump
- Lowers free Ca2+ allowing relaxation
What nerves increases heart rate
Sympathetic nervous system
How does the sympathetic nervous system increase heart rate
Adrenaline and noradrenaline activate Beta1 adrenoceptors in SA node to increase heart rate (increase frequency of action potentials)
How does the sympathetic nervous system increase the slope of pacemaker
BY increasing the funny current and the influx of Ca2+
What nerves decrease heart rate
Parasympathetic via the vagus nerves
How does the parasympathetic nervous system decrease heart rate
AcH moves membrane potential to -80mV (hyper polarise) so takes longer to reach threshold to fire off action potential
