Excitation-Contraction Coupling in the Myocardium

Multiple Choice Questions (MCQs)

1. What triggers the release of calcium from the terminal cisternae of L-tubules during myocardial contraction?
   a) Sodium influx
   b) Action potential in T-tubules
   c) Calcium-induced calcium release
   d) Potassium efflux
   Answer: c) Calcium-induced calcium release
2. What is the primary role of calcium during myocardial contraction?
   a) Activate sodium channels
   b) Bind to troponin to initiate the sliding filament mechanism
   c) Depolarize the membrane potential
   d) Close voltage-gated calcium channels
   Answer: b) Bind to troponin to initiate the sliding filament mechanism
3. Which ion transport mechanism ensures myocardial relaxation?
   a) Sodium-potassium pump
   b) Calcium pumps actively transporting calcium into the terminal cisternae
   c) Calcium channels allowing calcium influx
   d) Potassium efflux through delayed rectifiers
   Answer: b) Calcium pumps actively transporting calcium into the terminal cisternae
4. What is the normal heart rate of a horse?
   a) 28–40 beats/min
   b) 48–84 beats/min
   c) 70–80 beats/min
   d) 120–140 beats/min
   Answer: a) 28–40 beats/min
5. Which structure is primarily responsible for spreading the action potential into the myocardial cell?
   a) Intercalated discs
   b) T-tubules
   c) Sarcoplasmic reticulum
   d) Gap junctions
   Answer: b) T-tubules
6. What mechanism allows calcium to enter the myocardial cell during the plateau phase of the action potential?
   a) Voltage-gated calcium channels on T-tubules
   b) Ryanodine receptors in the sarcoplasmic reticulum
   c) Sodium influx through fast sodium channels
   d) Potassium efflux through rectifiers
   Answer: a) Voltage-gated calcium channels on T-tubules
7. Which species has the highest normal heart rate among the given animals?
   a) Cow
   b) Cat
   c) Goat
   d) Pig
   Answer: b) Cat
8. What happens to calcium levels in the cytosol during myocardial relaxation?
   a) They increase to sustain contraction.
   b) They decrease as calcium is actively transported back into the terminal cisternae.
   c) They remain constant due to steady calcium influx.
   d) They increase due to the closure of calcium pumps.
   Answer: b) They decrease as calcium is actively transported back into the terminal cisternae.
9. What structure activates the release of calcium from the sarcoplasmic reticulum?
   a) Gap junctions
   b) Ryanodine receptors
   c) Sodium-potassium pump
   d) T-tubules
   Answer: b) Ryanodine receptors
10. What is the normal heart rate range for sheep?
    a) 48–84 beats/min
    b) 70–80 beats/min
    c) 28–40 beats/min
    d) 120–140 beats/min
    Answer: b) 70–80 beats/min

Fill in the Blanks

1. During myocardial contraction, calcium binds to ________, initiating the sliding filament mechanism.
   Answer: troponin
2. Calcium pumps transport calcium back into the ________ to facilitate myocardial relaxation.
   Answer: terminal cisternae
3. The ________ phase of the action potential involves the opening of voltage-gated calcium channels.
   Answer: plateau
4. The normal heart rate of a cow is ________ beats per minute.
   Answer: 48–84
5. The release of calcium from the L-tubules is regulated by ________ receptors.
   Answer: ryanodine

True or False

1. Ryanodine receptors are responsible for calcium influx into the myocardial cell.
   Answer: False
2. T-tubules are responsible for spreading the action potential into myocardial cells.
   Answer: True
3. Calcium-induced calcium release is crucial for myocardial contraction.
   Answer: True
4. The normal heart rate of a goat is the same as that of a sheep.
   Answer: True
5. Potassium efflux is the primary driver of myocardial contraction.
   Answer: False

Subjective Questions

1. Explain the role of calcium in excitation-contraction coupling in the myocardium.
   Answer: During excitation-contraction coupling in the myocardium, the action potential spreads through T-tubules, leading to the opening of voltage-gated calcium channels. Extracellular calcium enters the cell and activates ryanodine receptors on the sarcoplasmic reticulum, causing a large release of stored calcium into the cytosol. This calcium binds to troponin, triggering the sliding filament mechanism and myocardial contraction. After contraction, calcium pumps actively transport calcium back into the terminal cisternae, reducing cytosolic calcium levels and allowing relaxation.
2. Discuss the significance of the plateau phase in myocardial action potential.
   Answer: The plateau phase of the myocardial action potential is critical for maintaining prolonged depolarization, which ensures sustained contraction of the heart muscle. This phase, characterized by a balance between calcium influx and potassium efflux, prevents tetanization and allows the heart to contract and relax rhythmically. The plateau phase also aligns with the mechanical twitch duration, ensuring effective blood ejection and proper cardiac output.