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Questions and Answers
What is responsible for the depolarization of cells in the SA node?
Parasympathetic fibers increase the heart rate by reducing the time to reach threshold.
False
What term describes the heart's ability to function as a single unit due to gap junctions?
functional syncytium
The action potentials in contractile cells have a duration of _______ milliseconds.
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Match the following components of cardiac muscle physiology with their descriptions:
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What primarily causes the rapid depolarization during Phase 2 of the cardiac action potential?
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The heart muscle can enter a tetanic state due to its longer refractory period.
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What phase follows the plateau phase in the cardiac action potential?
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During excitation-contraction coupling, the entry of ______ induces a significant release of Ca++ from the sarcoplasmic reticulum.
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Match the following phases of the cardiac action potential to their characteristics:
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Which type of cell in the myocardium is responsible for spontaneous action potential generation?
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The AV node has a faster pacing ability than the SA node.
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What is the primary function of gap junctions in cardiac muscle cells?
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The _____ node is known as the command center for heart contractions.
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Match the following cardiac structures with their functions:
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What is the significance of the electrical conduction system in the heart?
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The contraction of the atria occurs before the contraction of the ventricles.
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What happens at the AV node during the sequence of excitation?
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Which neurotransmitters mediate the actions of the sympathetic nervous system on the heart?
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The parasympathetic nervous system increases the heart rate by reducing the firing of the SA node.
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What is the primary function of the QRS wave in an ECG?
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The __________ wave in an ECG indicates repolarization of the ventricles.
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Match the following heart conditions with their corresponding descriptions:
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What effect does the sympathetic nervous system have on the velocity of AV node conduction?
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The amplitude of the waves on an ECG measures the heart rate.
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What is the role of acetylcholine in the parasympathetic control of the heart?
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Study Notes
Cardiac Muscle Composition
- Myocardium is composed of two types of muscle cells: contractile and autorhythmic
- 99% of cardiac cells are contractile cells responsible for contraction.
- Action potentials are required for contraction of contractile cells.
- Autorhythmic cells are modified, non-contractile cells found in specific regions of the heart.
- Autorhythmic cells spontaneously generate action potentials, also known as the pacemaker of the heart.
Electrical Conduction System
- Autorhythmic cells generate electrical activity in the heart.
- SA node: primary pacemaker of the heart and determines the heart rate. The SA node exhibits rhythmical self-excitation due to the leakage of Na+ and reduced diffusion of K+ outside the cell.
- AV node: also exhibits autorhythmic activity. It is slower than the SA node and is under the control of the SA node. The AV node serves as a gateway for electrical conduction between atria and ventricles.
- The Bundle of His and Purkinje fibers: rapidly propagate electrical activity from the AV node to the ventricles.
Significance of the Electrical Activity
- The electrical "system" maintains a proper heart rate, coordinates contraction of atria and ventricles, and coordinates contraction of each chamber.
- Electrocardiogram (ECG): used to determine heart rhythm.
- Problems with conduction can cause an abnormal heart rhythm, known as an arrhythmia.
Sequence of Excitation
- SA node self-excitation generates action potentials.
- Action potentials propagate through atria, causing atrial contraction.
- The AV node is activated by the AP wave and transmits electrical activity to the Bundle of His and Purkinje fibers with a slight delay, allowing for blood packing in the ventricles and closure of AV valves.
- Electrical activity propagates through ventricles causing ventricular contraction.
Generation of AP in the SA Node
- SA node cells gradually depolarize due to Na+ leakage inside the cell and reduced diffusion of K+ outside the cell.
- When the threshold potential is reached, an action potential is generated.
- The cycle repeats, creating a pacemaker potential.
- The autonomic nervous system controls the pacemaker potential by sympathetic and parasympathetic innervation.
- Sympathetic fibers reduce the time required to reach the threshold, leading to a faster heart rate.
- Parasympathetic fibers prolong the time required to reach the threshold, resulting in a slower heart rate.
Cardiac Muscle Contraction
- Action potentials from autorhythmic cells propagate to contractile cells.
- Action potentials in contractile cells trigger muscle contraction.
- Due to gap junctions, the heart can be considered a "functional syncytium," allowing for coordinated contraction.
- Contraction follows the "all or none rule."
Action Potential of Contractile Cells
- Differences from action potentials in autorhythmic cells:
- Stable resting membrane potential (no drift).
- Differences from skeletal muscle action potentials:
- Duration: Skeletal muscle APs are very short (milliseconds), while cardiac muscle APs are long (100's of milliseconds).
- Length of refractory period: Skeletal muscle has a shorter refractory period compared to cardiac muscle.
- Functional impact of these differences:
- Cardiac output (blood pumped by each half of the heart per minute) is regulated.
Phases of Cardiac Muscle Action Potential
- Phase 1: resting membrane potential
- Phase 2: rapid depolarization due to the influx of Na+ into the cell.
- Phase 3: short repolarization phase, loss of K+ from the cell.
- Phase 4: plateau phase due to Ca2+ influx into the cell.
- Phase 5: repolarization phase, outward movement of K+ from the cell.
Comparison Between Contractile and Autorhythmic Cells
- Contractile cells generate action potentials that trigger contraction.
- Autorhythmic cells spontaneously generate action potentials that set the pace of the heart.
Excitation-Contraction Coupling
- Calcium-stimulated-calcium-release (CS-CR) mechanism:
- Depolarization opens voltage-gated Ca2+ channels.
- Ca2+ entry triggers an avalanche of Ca2+ release from the sarcoplasmic reticulum (SR), increasing intracellular Ca2+ 10-fold.
- Increased intracellular Ca2+ triggers muscle contraction.
- Ca2+ is pumped back into its pre-stimulation compartment.
Control of Heart Contraction
- Effect of the Sympathetic Nervous System:
- Increases heart rate by stimulating SA node firing and the velocity of AV node conduction, shortening the delay.
- Increases contraction force by increasing Ca2+ release from the SR.
- Reduces contraction time by accelerating Ca2+ transport, reducing the plateau length.
- Sympathetic actions are mediated by epinephrine/norepinephrine on β-adrenergic receptors found in all cardiac cells.
Effect of the Parasympathetic Nervous System (Vagus Nerve)
- Decreases heart rate by reducing SA node firing and decreasing the velocity of AV node conduction, increasing the delay.
- Parasympathetic actions are mediated by acetylcholine binding to muscarinic receptors in autorhythmic cells, increasing K+ permeability and hyperpolarization, increasing the time needed to reach AP threshold.
Electrocardiogram (ECG)
- ECG measures the heart's electrical activity, as simultaneous firing of cardiac cells generates a strong current that is conducted through body fluid to the skin.
- Electrodes placed at different locations on the skin read the progression of the electrical current wave.
Standard ECG Waves
- P wave signifies atrial depolarization.
- QRS wave signifies ventricular depolarization.
- T wave signifies ventricular repolarization.
Base-Apex Lead Configuration
- One lead on the left chest, and a second lead over the neck (jugular groove) are used to measure electrical activity.
- The normal rate and rhythm can be determined by ECG results.
Uses of the ECG
- Assessing heart rate and rhythm:
- Heart rate: intervals between cycles.
- Contraction force: amplitude of waves.
- Rhythm: intervals between each wave.
- Detects abnormalities:
- Heart rate (under resting conditions):
- Bradycardia: a slower heart rate.
- Tachycardia: a faster heart rate.
- Rhythm:
- Abnormally long P-Q interval: indicates AV conduction problem.
- Ectopic beats (extrasystole): action potential generates independently of SA node resulting in extra-contraction.
- Heart rate (under resting conditions):
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Description
Explore the intricate composition of cardiac muscle, focusing on contractile and autorhythmic cells. This quiz also delves into the electrical conduction system of the heart, highlighting the roles of the SA node and AV node in regulating heart rhythms. Test your knowledge on how these components work together to maintain cardiac function.
