Cardiac Electromechanical Coupling

Question 1

What are the steps of excitation contraction coupling of cardiac muscle cells? Generally speaking

Answer 1

1. Cardiac AP initiated
2. Ca2+ enters the cell via L type Ca2+ channels
3. Ca2+ binds troponin c
4. Tension
5. Relaxation

Question 2

What happens after a cardiac AP is initiated?

Answer 2

Depolarization spreads to the interior of the cell via T Tubules

Question 3

Through which type of channels is Ca2+ able to enter the cell? At which phase of the cardiac AP is this occurring?

Answer 3

L type Ca2+ channels, phase 2

Question 4

What is calcium induced calcium release?

Answer 4

Increase in intracellular concentration of Ca2+ due to the opening of L-type Ca2+ channels triggers Ca2+ to be released into the cell from the Ca2+ stores of the SR lumen

Question 5

What two factors control how much Ca2+ is released from the SR during cardiac muscle excitation contraction coupling?

Answer 5

• Amount of Ca2+ stored in SR
• Size of inward Ca2+ current during phase 2 of AP

Question 6

What happens after Ca2+ is released from SR?

Answer 6

• Binds troponin c
• Moves tropomyosin out of the way of myosin binding sites on actin
• Cross bridge formation between actin and myosin
• Contraction

Question 7

How long does cross bridge cycling occur?

Answer 7

Until there is no longer enough Ca2+ available to bind troponin c

Question 8

What is a key feature of tension in cardiac myocytes?

Answer 8

Magnitude of the tension is proportional to intracellular Ca2+ concentration

Question 9

What is happening during the relaxation phase of excitation contraction coupling?

Answer 9

• Ca2+/ATPase pumps Ca2+ back into SR lumen
• Na+/Ca2+ exchanger in sarcolemma membrane pumps Ca2+ out of cell
• Both of which help bring the cell back towards RMP

Question 10

What are some key features of skeletal muscle cells?

Answer 10

• Striated
• Under voluntary control
• Single, long multinucleate cells
• Obvious striations
• Have no cell-cell junctions

Question 11

What are some of the key features of cardiac cells?

Answer 11

• Striated
• Involuntart
• Short length, branching (y shaped) chains of cells
• Uni or binucleate
• Striations
• Intercalated discs/transverse junctions that allow passage of electrical current
• Purkinje cells

Question 12

Where are intercalated discs/transverse junctions located?

Answer 12

With Z line

Question 13

How are cardiac muscle and skeletal muscle similar?

Answer 13

Both have an optimal length-tension relationship (though cardiac optimum length is 2.25, compared to 2.1-2.2 in skeletal muscle)

Question 14

When is Lmax obtained in cardiac muscle?

Answer 14

As the ventricules fill with blood

Question 15

Cardiac muscle converts _ tension to _contraction. This enables blood to be pumped out of the cell when the ventricles contract

Answer 15

Isometric, isotonic

Question 16

What are cardiac glycosides used to treat?

Answer 16

Congestive Heart Failure

Question 17

What type of agent is a cardiac glycoside?

Answer 17

Positive inotropic (increase in contractility)

Question 18

How do cardiac glycosides work?

Answer 18

• Inhibit Na+/K+ ATPase at extracellular K+ binding site
• Less Na+ pumped out of the cell
• Decreased function of Na+/Ca2+ exchanger
• Increase in intracellular Ca2+ concentration
• Increase in cardiac muscle tension
• Positive inotropic effect