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Questions and Answers
What is the definition of excitability in cardiac muscle?
What is the definition of excitability in cardiac muscle?
What is the resting membrane potential of a single cardiac muscle fiber?
What is the resting membrane potential of a single cardiac muscle fiber?
Which phase of the cardiac action potential involves rapid depolarization?
Which phase of the cardiac action potential involves rapid depolarization?
What ion influx is primarily responsible for initial depolarization in phase 0?
What ion influx is primarily responsible for initial depolarization in phase 0?
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During which phase does the cardiac muscle experience a plateau?
During which phase does the cardiac muscle experience a plateau?
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How long does the action potential last in a cardiac muscle cell?
How long does the action potential last in a cardiac muscle cell?
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What characterizes phase 1 of the cardiac action potential?
What characterizes phase 1 of the cardiac action potential?
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What is the primary reason for the prolonged contraction time in cardiac muscle?
What is the primary reason for the prolonged contraction time in cardiac muscle?
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Which channels open during the plateau phase of the action potential?
Which channels open during the plateau phase of the action potential?
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What happens during phase 3 of the action potential in cardiac muscle?
What happens during phase 3 of the action potential in cardiac muscle?
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What initiates the slow depolarization in the SA node pacemaker fibers?
What initiates the slow depolarization in the SA node pacemaker fibers?
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Which ion mainly contributes to the depolarization phase in the SA node?
Which ion mainly contributes to the depolarization phase in the SA node?
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What is the threshold level at which the action potential starts in the SA node?
What is the threshold level at which the action potential starts in the SA node?
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During repolarization in the SA node, the negativity decreases due to the efflux of which ion?
During repolarization in the SA node, the negativity decreases due to the efflux of which ion?
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Vagal stimulation affects the SA node by releasing which neurotransmitter?
Vagal stimulation affects the SA node by releasing which neurotransmitter?
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What effect does sympathetic stimulation have on the SA node?
What effect does sympathetic stimulation have on the SA node?
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In the SA node, what leads to hyperpolarization during vagal stimulation?
In the SA node, what leads to hyperpolarization during vagal stimulation?
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What characterizes the pacemaker potential in the SA node?
What characterizes the pacemaker potential in the SA node?
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How does the efflux of potassium ions affect the membrane potential during repolarization?
How does the efflux of potassium ions affect the membrane potential during repolarization?
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What role does cAMP play in the sympathetic stimulation of the SA node?
What role does cAMP play in the sympathetic stimulation of the SA node?
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What primarily maintains positivity inside the muscle fiber during prolonged depolarization?
What primarily maintains positivity inside the muscle fiber during prolonged depolarization?
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During final repolarization, which ions are primarily exiting the cell?
During final repolarization, which ions are primarily exiting the cell?
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What role do gap junctions play in cardiac muscle?
What role do gap junctions play in cardiac muscle?
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Which structure in the heart is known as the primary pacemaker?
Which structure in the heart is known as the primary pacemaker?
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What is the resting membrane potential range of the sinoatrial node?
What is the resting membrane potential range of the sinoatrial node?
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Which phase occurs after the threshold level of –40 mV is reached in the pacemaker action potential?
Which phase occurs after the threshold level of –40 mV is reached in the pacemaker action potential?
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What is the production rate of impulses in the sinoatrial node?
What is the production rate of impulses in the sinoatrial node?
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Which node can also function as a pacemaker but has a lower rate than the SA node?
Which node can also function as a pacemaker but has a lower rate than the SA node?
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What primarily happens at the end of the final repolarization phase?
What primarily happens at the end of the final repolarization phase?
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Which ions are moved out of the cell by the sodium-potassium pump after repolarization?
Which ions are moved out of the cell by the sodium-potassium pump after repolarization?
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Study Notes
Cardiac Muscle Properties (Part 1)
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Excitability: The ability of living tissue to respond to stimuli, initiating electrical activity (action potential). This is a fundamental property found in all tissues.
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Electrical Potentials in Cardiac Muscle:
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Resting Membrane Potential:
- Single cardiac muscle fiber: ~90 mV
- Sinoatrial (SA) node: 55-60 mV
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Action Potential: Different from skeletal, smooth, and nervous tissue. Duration is 200-350 milliseconds (0.2 to 0.35 seconds).
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Phases of Cardiac Action Potential
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Phase 0 (Initial Depolarization): Rapid, lasting ~2 milliseconds (+20 mV amplitude).
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Phase 1 (Initial Repolarization): Brief period of rapid repolarization (~2 msec).
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Phase 2 (Plateau/Final Depolarization): Prolonged depolarized state before repolarization; crucial for contraction time. Plateau lasts 50-80 msec, significantly longer than in skeletal muscle.
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Phase 3 (Final Repolarization): Slow process following the plateau (~50-80 msec).
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Phase 4 (Resting Membrane Potential): Re-establishment of the resting potential after repolarization.
Ionic Basis of Action Potential
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Phase 0 (Initial Depolarization): Fast sodium channels open, rapid sodium influx.
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Phase 1 (Initial Repolarization): Transient potassium channels open, potassium efflux, and slow sodium channels open (modest).
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Phase 2 (Plateau): Slow calcium channel opening, maintaining depolarization. Calcium influx and slow sodium influx maintain positivity and prolonged depolarization.
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Phase 3 (Repolarization): Calcium channels close, potassium channels remain open; potassium efflux restores negativity.
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Phase 4 (Resting Membrane Potential): Sodium-potassium pump restores ionic balance, returning membrane to resting potential. Excess calcium is pumped out.
Spread of Action Potential
- Gap junctions between cardiac muscle fibers facilitate rapid action potential spread from one cell to another, crucial for coordinated heart contractions.
Ventricular Myocyte Action Potential
- A graphic representation of the membrane potential changes during a cardiac action potential in ventricular cells (myocytes).
2- Rhythmicty (Pacemaker)
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Pacemaker: The structure that initiates the heartbeats regularly.
- Sinoatrial (SA) Node: Key pacemaker, located in the right atrium near superior vena cava. Its cells have a faster rate of depolarization than other parts like the AV node.
- Rate of the SA node: ~60-100 beats per minute (BPM)
- Rate of the AV node: ~40-60 BPM
- Purkinje Fiber rate: ~15-40 BPM
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Sinoatrial (SA) Node Activity: The SA node is unstable and continually depolarizing (creating its own signals) until it reaches the threshold required to depolarize the rest of the heart.
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Resting Membrane Potential of the SA Node: -55mV to -60 mV (unstable).
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SA Node Ionic Basis of Depolarization: Involves slow sodium, calcium channel opening, and slow depolarization.
Automic Control of SA Node
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Vagus Nerve (Parasympathetic):
- Decrease heart rate.
- Releases acetylcholine (Ach), which acts on muscarinic receptors, leading to increased potassium outflow.
- This process hyperpolarizes the cell (inhibits depolarization) and slows the heart rate.
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Sympathetic Nervous System:
- Increase heart rate.
- Release of norepinephrine (NE), acting on beta-1 adrenergic receptors.
- Promotes increase in intracellular cyclic AMP (cAMP) and opens calcium ($Ca^{++}$) channels.
- Increased $Ca^{++}$ influx promotes rapid depolarization and speeds up the heart rate.
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Description
Explore the fundamental properties of cardiac muscle, including its excitability and electrical potentials. This quiz covers key aspects of the action potential phases that are essential for understanding cardiac function. Delve into details like resting membrane potential and the significance of each phase in muscle contraction.
