M7 L3 : Cardiac muscle Flashcards
Describe the structural organisation of the heart and atrial cells
Heart has 2 atria and 2 ventricles. Atrial cells have central nucleus with no t-tubules and contract quite weakly. They have gap junctions to allow electric activity to spread.
Describe ventrical cardiac cells
Ventricular cells are larger and are branched intercalated discs that prevent cells from separating during contraction. Contains gap junctions that allow coordinated contraction: APs carried from one cell to the next.
They have lots of mitchondria. 1-3 nuclei. t-tubules at the z discs and SR less developed
Where are action potentials initiated in the heart? Are they controlled by nerves
AP initated in cells in the right atria (sino atrial node). Cardiac muscle is myogenic: initates contraction without nervous input
Describe the stages of Ventricular myocyte AP
The action potential is long lasting >100 ms. depolarisation is due to Na+ channels opening and then it turns off.
- Plateau phase where there is a large sustained calcium current through voltage gated slow channels. Ca2+ binds to RyRa triggers the release of calcium into cytosol from the SR known as calcium induced calcium release.
- repolarisation when Ca2+ channels close and K+ channels activate bringing it back to RMP
How is the the membrane potential depolarised throughout most of the heart beat - tentani highly unlikely
Ca2+ entering from inside which triggers SR to release Ca2+ into cell as well balanced by Ca/Na+ exchanger which also brings in 3Na+ for every 1 Ca2+ so it makes the membrane more positive anyway
How is heart beating regulated
Since membrane is highly depolarised through most of the twitch it makes the refractory period longer so the refractory period is for most of the twitch. it cannot be re excited and summation of responses is impossible.
Heart beats 1 per second. What is the division of contraction and relaxation during that time
It contracts for 1/3 (peak of depolarisation) and relaxes for 2/3 (fills with blood)
compared to skeletal muscle what is the ratio of contracting and APs
skeletal: full Ap back down to RMP then full contraction
cardiac: AP long slow, contraction pretty near finished before AP starts repolarising
If you have a premature contraction triggered in the relative refractory period
its small and it won’t pump out the blood.
On a T tubule what provides the structural basis for ECCoupling
LTCC: I type voltage gated calcium channel (Ical) on T-tubule and then on the SR going around it are RyR= ryanodine receptors that are Ca2+ triggered calcium channels in the SR
What are the 4 main ways to get relaxation to occur
Ca2+ must decline allowing Ca2+ to dissociate from troponin. This requires active transport of Ca2+ out of cytosol by SR Ca2+ ATpase, Sarcolemma Na+/Ca2+ exchanger, Sarcolemma Ca2+ ATPase and mitochondrial CA2+ uniport
What are the main differences in trigger for Ca2+ release, contraction type for skeletal vs cardiac
Cardiac is electrically coupled, with Ca2+ release triggered by Ca2+ influx. It has graded contraction as troponin usually not saturated but has all fibres or none whereas skeletal is electrically isolated, Ca2+ release triggered by Na+ and all or none contraction as troponin is always saturated with ability to recruit more for graded
How is Cardiac output ( L blood that comes out of the heart per minute)
CO= Stroke Volume * Heart Rate
What sets the heart rate
The heart rate is set by pacemaker cells in the sinoatrial node. The rate can then be modified, especially via autonomic nerves releasing neurotransmitters
In pacemaker cells what are is different about their RMP
It is unstable, it has a slow spontaneous depolarisation due to leaky “If” channels letting Na+ in.