Excitation Contraction Coupling

Question 1

What is excitation contraction coupling?

Answer 1

The conversion of electro-chemical signal to a mechanical event
- the autorhythmic cells fire impulses/APs spontaneously which are then spread to adjacent cardiomyocytes through gap junctions

**Results in all cells working as a unit= cardiac syncytium

Question 2

Steps of Excitation contraction coupling

Answer 2

1.Current spreads through gap junctions to contractile cells
2. APs travel along the plasma membrane and T tubules
3. L-type Ca channels open in plasma membrane and sarcoplasmic reticulum (Ca enters)
4. Ca induces Ca release from sarcoplasmic reticulum through RyR (Ryanodine receptors) channels
5.Ca binds to troponin, moving tropomyosin off of binding site and exposing myosin-binding sites
6.Crossbridge cycle begins (muscle fibers contract)
7.Ca is actively transported back into sarcoplasmic reticulum and extracellular fluid
8. Tropomyosin blocks myosin-binding sites (muscle fiber relaxes)

Question 3

How does heart failure impact excitation contraction coupling?

Answer 3

-heart failure will impact Ca reuptake in cardiomyocyte

Question 4

Methods in which Ca is actively transported out of cell after contraction (STEP 8)

Answer 4

-Sarcoendoplasmic reticulum calcium ATPase (SERCA)- pumps calcium back into sarcoplasmic reticulum
-Calcium ATPase- pumps calcium out of cell
-Sodium calcium exchanger- pumps calcium out for exchange for sodium
-Na-K pump – helps restore membrane potential due to sodium calcium exchanger

Question 5

Excitation contraction coupling and sympathetic nervous system

Answer 5

1. Catecholamines bind to beta adrenergic receptor which will activate cAMP, Protein kinase A, and then phosphorylates L-type Calcium channel
2. L-type calcium channel activity is increased, and more Ca enter cell
3. RyR activity increases, and more calcium released from sarcoplasmic reticulum
4. Myofilament calcium sensitivity is decreased. More calcium for SERCA to reuptake, but also more left behind for Sodium calcium exchanger.
5. Sodium calcium exchanger exchanges Ca for Na. Double the amount of Na is brought in due to higher amounts of Ca.
6. More Na, means increased use of Na/K pump and therefore more ATP/O2 used = greater energetic cost!!

Question 6

Norepinephrines impact on excitation contraction coupling

Answer 6

-increase in Ca release, but also increase in Ca reuptake so that heart can fill and pump back out for next beat