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Questions and Answers
What is the direction of blood flow pumped by the Right Ventricle?
What is the ability of cardiac cells to respond to a stimulus?
What occurs when cardiac cells lose their negativity?
What is the process by which cardiac cells return to their resting state?
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What is the phase of cardiac contraction equivalent to?
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What is the primary function of the epicardium?
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What occurs during early ventricular diastole?
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What is the term for ventricular contraction?
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What is the primary function of the endocardium?
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During which phase of the cardiac cycle does the atria begin to contract?
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What is the term for the thin protective coating of the heart?
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What is the primary purpose of an action potential in heart cells?
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What is the primary ion responsible for depolarization in pacemaker cells?
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What is the function of the plateau phase in myocardial cell action potential?
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Which type of channels are 'leaky' and maintain the resting potential in myocardial cells?
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Which phase of the myocardial cell action potential is characterized by rapid depolarization?
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What is the composite representation of action potentials from all cardiac cells?
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What is the cardiac action potential composed of?
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What is the function of myocardial cells?
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What characteristic allows pacemaker cells to depolarize spontaneously?
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Which type of cells is responsible for efficiently conducting electrical signals to distant regions of the heart?
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Where are the dominant pacemaker cells located?
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What influences the firing rate of the SA Node?
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What is the primary function of the Pacemaker Potential in pacemaker cells?
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What is the effect of Acetylcholine release on the heart rate?
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What is the native pacemaker of the heart?
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What is the sequence of contraction in the cardiac conduction system?
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What is the role of 'Backup' Pacemakers in the heart?
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What is theBundle of His?
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Study Notes
Cardiac Action Potential
- A complete cycle of depolarization and repolarization makes up the Cardiac Action Potential.
Cardiac Cells
- There are three types of cells in the heart: Pacemaker cells, Electrical conducting cells, and Myocardial cells.
- Pacemaker cells:
- Are small, about 5 to 10 μm long.
- Can depolarize spontaneously over and over again.
- Have no true resting potential due to “leaky K channels”.
- Are located in the upper Right Atrium (SA Node).
- Electrical conducting cells:
- Carry current rapidly and efficiently to distant regions of the heart.
- The AV Node is a slow conductor.
- Atrial conducting systems can vary anatomically, while the ventricular conducting system forms distinct pathways.
- Myocardial cells:
- Make up the largest part of heart tissue.
- Are about 50 to 100 μm long.
- Contain an abundance of actin and myosin (contractile proteins).
- Depolarization wave reaching a myocardial cell causes calcium release → cells contract (excitation–contraction coupling).
Pacemaker Cells
- The SA Node spontaneously fires 60 to 100 times per minute, and varies based on:
- Autonomic nervous system (Sympathetic vs. parasympathetic).
- Increased demand (exercise vs. rest).
- Autonomic regulation of heartbeat:
- Vagal Nerve slows HR → ↑Acetylcholine release = ↓ rate of rise of Phase 4 depolarization.
- Accelerant nerve speeds HR → ↑Norepinephrine release = ↑ rate of rise of the cardiac action potential during phase 0.
- ↑Epinephrine release = ↑ rate of rise and acetylcholine.
Cardiac Conduction System
- The cardiac conduction system consists of:
- Native/dominant pacemaker: Sinoatrial or Sinus node (60-100 BPM).
- Ectopic/latent pacemaker: Atrial Pacemaker (60-75 BPM), AV Node (Junctional) (40-60 BPM), Myocardial (Ventricular) cells (30-45 BPM), and Conducting (Purkinje) cells (30-40 BPM).
- The cardiac conduction system works as follows:
- SA node → Bachman’s Bundle (depolarization of LA) → SA node → AV node (short delay) → Bundle of His (splits) → L&R Bundle Branches (depolarization of ventricles) → Purkinje System (depolarization of rest of ventricles).
Cardiac Electrophysiology
- Physiological properties of myocardial cells:
- Automaticity: ability to initiate an impulse.
- Excitability: ability to respond to a stimulus.
- Conductivity: ability to transmit an impulse.
- Contractility: ability to respond with pumping action.
- Cardiac cells are electrically polarized (negatively charged) in the resting state.
- Depolarization: the loss of negativity in cardiac cells.
- Repolarization: return to resting polarity (negatively charged).
Action Potentials
- An action potential is a brief change in voltage (membrane potential) across the cell membrane of heart cells.
- Comprised of one complete cycle of depolarization and repolarization.
- Initiates a wave of depolarization that propagates throughout the heart.
- ECG is the composite representation of action potentials of all cardiac cells.
- Phases of action potential:
- Phase 0: Rapid depolarization.
- Phase 1: Slight repolarization.
- Phase 2: Plateau phase.
- Phase 3: Rapid Repolarization.
- Phase 4: Diastolic resting membrane potential.
Pacemaker Cell Action Potential
- Similar to depolarization in the myocardium, except:
- Depolarization results from the influx of Calcium (rather than Na).
- There is no plateau phase (no Phase 1 and 2).
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Description
Learn about the cardiac action potential, pacemaker cells, electrical conducting cells, and myocardial cells. Understand the structure and function of different heart cells and their roles in the heart's electrical and contractile functions.
