Cardiac muscle works like normal muscle however there are some changes. Principally it needs to contract and relax quickly, (up to maybe like 4 times a second? 3 times a second?), and it needs to be able to conduct and pass electrical impulses to the next cells along.
Quick excitation-contraction coupling
The heart muscle is like any other. actin and myosin interlace and the myosin engage in cross cycle bridging, but need calcium to trigger the troponin complex to bare the attachment points.
we need to get calcium in and out very quickly then.
calcium in (excitation contraction coupling)
We get calcium into cardiac cells through 1. voltage activated calcium channels (which does 25% of calcium) and 2. release from the sarcoplasmic reticulum (75% of calcium). The calcium is released from SR through CICR, calcium incuded calcium release. The action potential travelling though the T tubules trigger voltage gated calcium channels to open , and the calcium flowing in through these then trigger the RyR receptors to open leading to more calcium release.
This all outputs a lot of calcium for a quick strong contraction. but we also have to release and restitute
Calcium out
We quickly get calcium out through 3 ways.
- SERCA which is an atp dependant pump which pumps Ca2+ from the cytoplasm back into the SR. This is modulated by phospholamban (or phospholamdan) which inhibits serca while unphosphorylated.
- Na in C out exchanger pump. It lets in 3 na+ and 1 ca out. this is doen both chemical gradients.
- Ca out atp pump The order mention is the order in which they pump calcium out (serca does majority etc)
The best pump is a pump that always pumps enough
We need a heart that pumps to the amount we need. more and you have high bp and energy waste low we have hypoperfusion.
We can increase the contraction force of the heart with sympathetic stimulation. This works via the following pathway:
- Noradrenaline binds to ß1 adrenoreceptors on the cardiac muscle cell increased concentrationof cAMP
- This activates PKA which then phosphorylates the L-type calcium channel and RyR channel it increases opening.
- This means more calcium enters the cell per contraction.
- This means more contraction force per contraction we also need the heart to clear this calcium faster or we would have longer heartbeats or accumulation of Calcium.
Turns out adrenaline also increases lusitrophy (relaxation rate) 2. The activated PKA also phosphorylates the phospholamban which reduced serca inhibiton 3. This means calcium is cleared more rapidly from the cytoplasm
So in summary adrenaline makes the heart contract harder and relax faster
Additionally we with exercise we increase venous return meaning higher inotropy (starling law)
Additionally in heart damage (from MI or smth)