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Questions and Answers
What initiates the depolarization in contractile cells?
What initiates the depolarization in contractile cells?
Which protein does calcium bind to during myocardial contraction?
Which protein does calcium bind to during myocardial contraction?
What role does the sodium-potassium ATPase pump play in myocardial relaxation?
What role does the sodium-potassium ATPase pump play in myocardial relaxation?
What characterizes the membrane potential of autorhythmic cells?
What characterizes the membrane potential of autorhythmic cells?
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What is the average cardiac output in a healthy adult?
What is the average cardiac output in a healthy adult?
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How is stroke volume calculated?
How is stroke volume calculated?
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What prevents tetanus in myocardial contractile cells?
What prevents tetanus in myocardial contractile cells?
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What mechanism contributes to the long contraction duration in contractile cells?
What mechanism contributes to the long contraction duration in contractile cells?
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In smooth muscle, what mechanism is similar to the calcium process in cardiac muscle?
In smooth muscle, what mechanism is similar to the calcium process in cardiac muscle?
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Which of the following describes the primary function of the NCX antiporter in myocardial relaxation?
Which of the following describes the primary function of the NCX antiporter in myocardial relaxation?
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What is the formula for calculating ejection fraction?
What is the formula for calculating ejection fraction?
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Which neurotransmitter is primarily involved in increasing heart rate through sympathetic stimulation?
Which neurotransmitter is primarily involved in increasing heart rate through sympathetic stimulation?
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How does parasympathetic stimulation affect heart rate?
How does parasympathetic stimulation affect heart rate?
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What effect does increased afterload have on stroke volume?
What effect does increased afterload have on stroke volume?
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Which factor determines venous return?
Which factor determines venous return?
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What term describes the load placed on a ventricle during contraction?
What term describes the load placed on a ventricle during contraction?
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Which of the following is a positive inotropic agent?
Which of the following is a positive inotropic agent?
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What relationship does length-tension have with stroke volume?
What relationship does length-tension have with stroke volume?
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What is true about catecholamines like norepinephrine regarding contractility?
What is true about catecholamines like norepinephrine regarding contractility?
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What is primarily responsible for maintaining tonic control of heart rate?
What is primarily responsible for maintaining tonic control of heart rate?
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Study Notes
Myocardial Excitation-Contraction Coupling and Relaxation
- Myocardial contraction begins with an action potential in a pacemaker cell that propagates to adjacent cells.
- L-type calcium channels open, allowing calcium to enter from the extracellular fluid (ECF).
- Calcium triggers the release of more calcium from the sarcoplasmic reticulum (SR), a process called calcium-induced calcium release.
- This combined calcium influx creates a calcium spark, and the summation of these sparks forms a calcium signal.
- Calcium binds to troponin, initiating the cross-bridge cycle, similar to skeletal muscle contraction.
- Relaxation occurs when calcium is pumped back into the SR by ATPase pumps.
- The sodium-calcium exchanger (NCX) antiporter moves calcium out of the cell and sodium into the cell.
- The sodium is then pumped out by the sodium-potassium ATPase pump, maintaining the sodium gradient.
Autorhythmic vs. Contractile Cells
- Autorhythmic cells (pacemakers) are responsible for initiating the heartbeat, have unstable membrane potentials (pacemaker potentials), and are small in number.
- Pacemaker potentials arise from the inward "funny" current (If) through If channels.
- This allows the cells to continuously depolarize and set the heart beat.
- Contractile cells have stable membrane potentials
- Depolarization in contractile cells is initiated via gap junctions from adjacent cells.
- The prolonged plateau phase of the action potential in contractile cells is due to calcium entry, resulting in a longer contraction.
- The long refractory period in contractile cells prevents tetany.
Cardiac Output, Stroke Volume, and Heart Rate
- Cardiac output (CO) is the amount of blood pumped by the heart per minute
- CO = Heart Rate (HR) × Stroke Volume (SV)
- Stroke volume (SV) is the amount of blood ejected per ventricular contraction.
- SV = End-diastolic volume (EDV) - End-systolic volume (ESV)
- EDV is affected by venous return and preload (the degree of stretch of the heart).
- Venous return is affected by skeletal muscle pump, respiratory pump, and sympathetic innervation of the veins.
- SV is affected by contractility (the force of ventricular contraction).
Autonomic Nervous System and Heart Rate
- Sympathetic neurons release norepinephrine and epinephrine, binding to β1 receptors to increase heart rate by increasing sodium and calcium permeability.
- Norepinephrine and epinephrine also increase contractility.
- Parasympathetic neurons release acetylcholine, binding to muscarinic receptors to decrease heart rate by increasing potassium permeability and hyperpolarizing the cells.
Factors Influencing Stroke Volume
- Venous return: dictates end-diastolic volume (EDV), which affects stroke volume.
- Preload: degree of stretch on the heart, influenced by EDV.
- Contractility: the force of ventricular contraction, affected by catecholamines and inotropic agents.
- Afterload: the pressure the ventricles must overcome to eject blood.
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Description
This quiz covers the fundamental concepts of myocardial excitation-contraction coupling and relaxation. It explores the role of calcium in heart muscle contraction, including the mechanisms of pacemaker cells and the processes that lead to relaxation. Test your understanding of the electrical and chemical signals that regulate heart function.
