Excitation Contraction Coupling in Cardiac and Skeletal Muscle

Question 1

What is excitation contraction coupling?

Answer 1

The process by which an electrical signal (excitation) is translated into a mechanical response (contraction) in muscle fibers

Question 2

What is the latent period?

Answer 2

The latent period is the duration between the peak potential and the onset of tension ( start of muscle contraction). This is the the time it takes for the signal to propagate and top be transmitted through the muscle fibre

Question 3

Explain the role of the transverse tubules and the terminal cisternae in contraction

Answer 3

• T tubules brings action potentials into interior of muscle fibre. It is sandwiched between the SR in a triad:
  
  • Terminal cisternae - calcium store (x2 on each side of the t tubule)
  • T tubules
• Allows SR to sense changes in the membrane as the AP is delivered.
• Depolarisation of the T tubule membrane is ‘signalled’ to the membrane of the terminal cisternae

Question 4

Explain the role of “junctional foot proteins”

Answer 4

• Junctional foot proteins
  
  • DHPR- L type voltage gated calcium channel in the T tubule membrane
  • RYR - Calcium release channel in SR

Question 5

Explain the role of ryanodine and dihydropydine receptor binding proteins

Answer 5

• T tubule system contains voltage gated calcium channels called dihydropyradine receptors (DHP receptor)
• DHP receptor sense change in membrane potential which activates a conformational change in the DHP receptor
• DHP receptor is mechanically coupled with type one ryanodine receptors in the SR
• Ryanodine receptor is activated which causes a large influx of Ca2+ from the SR
• Ca2+ binds to troponin C

Question 6

Detail the changes in the cytoplasmic calcium concentration during activation and relaxation

Answer 6

• Calcium is recycled between 2 intracellular compartments
  
  • Cytoplasm
  • Terminal cisternae (SR)
• The open calcium channels allow calcium to diffuse down a steep concentration gradient into the cytoplasm
• Cytoplasmic calcium concentration increases form <10^-7 M to > 10^-5
• This increase ultimately leads to force generation through the interaction of actin and myosin filaments in the SR membrane
• The increase in calcium concentration activates SERCA

Question 7

Explain of Role of Ca2+ ATPase (SERCA)

Answer 7

• The increase in intracellular calcium concentration activates a Ca2+ ATPase (calcium pump) in the SR membrane
• Active transport of calcium from the cytoplasm into SR
• For 1 molecule of ATP 2 Ca2+ ions

Question 8

Explain of role calequestrin

Answer 8

• Stores calcium at high concentrations in the terminal cisternae to establish a concentration gradient from the SR to the cytollasm
  
  • Calcium binding protein
  • Binds 43 Ca2+ per molecule

Question 9

Differences in excitation coupling in cardiac muscle

Answer 9

• The key difference is there is no mechical coupling bewteen the DHPR and the RYR
• In cardiac muscle, 25% of the required Ca2+ enters through the voltage gated L-type Ca2+ channels (DHP receptor protein) in the transverse tubular membrane
• The calcium acts as a ligand for ligand gated RYR channels, resulting in the RYR channels on the SR surface opening, and remaining 75% of calcium required for contraction entering into the cell
• This process is known as Ca2+ induced Ca2+ release (CICR) in cardiac muscle