Podcast
Questions and Answers
What is the primary reason for the rapid transmission of action potentials in Purkinje fibers?
What is the primary reason for the rapid transmission of action potentials in Purkinje fibers?
Which structure primarily functions as the pacemaker of the heart?
Which structure primarily functions as the pacemaker of the heart?
What is the intrinsic discharge rate of the A-V nodal fibers when not stimulated externally?
What is the intrinsic discharge rate of the A-V nodal fibers when not stimulated externally?
How does the discharge rate of the sinus node compare to that of the Purkinje fibers?
How does the discharge rate of the sinus node compare to that of the Purkinje fibers?
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Why does the sinus node control the heart's beat?
Why does the sinus node control the heart's beat?
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What is the primary substrate the heart uses for ATP during an overnight fasted state?
What is the primary substrate the heart uses for ATP during an overnight fasted state?
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What is the role of the Sinoatrial node in the heart?
What is the role of the Sinoatrial node in the heart?
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Which of the following describes the automaticity of the heart?
Which of the following describes the automaticity of the heart?
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How do the contraction timings between atrial and ventricular muscles differ?
How do the contraction timings between atrial and ventricular muscles differ?
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What primarily influences the resting membrane potential of the sinus nodal fibers?
What primarily influences the resting membrane potential of the sinus nodal fibers?
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Which of the following is NOT a substrate that can be utilized by cardiac myocytes for ATP production?
Which of the following is NOT a substrate that can be utilized by cardiac myocytes for ATP production?
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What is one key characteristic of ventricular muscle fibers compared to sinus nodal fibers?
What is one key characteristic of ventricular muscle fibers compared to sinus nodal fibers?
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Lactate becomes an important substrate for cardiac myocytes during which condition?
Lactate becomes an important substrate for cardiac myocytes during which condition?
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What regulates the entry of calcium into cardiac myocytes during depolarization?
What regulates the entry of calcium into cardiac myocytes during depolarization?
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Which neurotransmitter primarily affects β1-adrenoceptors on the sarcolemma of cardiac myocytes?
Which neurotransmitter primarily affects β1-adrenoceptors on the sarcolemma of cardiac myocytes?
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What is the primary function of intercalated disks in cardiac muscle?
What is the primary function of intercalated disks in cardiac muscle?
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What is the role of cAMP in cardiac myocyte contraction?
What is the role of cAMP in cardiac myocyte contraction?
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How does phosphorylation of the L-type calcium channel affect cardiac myocyte function?
How does phosphorylation of the L-type calcium channel affect cardiac myocyte function?
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What role does protein kinase A play in the signaling pathway of cardiac myocytes?
What role does protein kinase A play in the signaling pathway of cardiac myocytes?
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What unique characteristic do cardiac myocytes have compared to skeletal muscle fibers?
What unique characteristic do cardiac myocytes have compared to skeletal muscle fibers?
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What effect does the binding of epinephrine to β1-adrenoceptors produce in the heart?
What effect does the binding of epinephrine to β1-adrenoceptors produce in the heart?
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Study Notes
Heart Muscle & Signal Transduction
- Cardiac myocytes are a type of striated muscle, microscopically exhibiting crossbands.
- These myocytes share structural and functional similarities with skeletal muscle, with significant differences.
- Cardiac myocytes form a branching network, functioning as a syncytium, formed by cell fusion.
- Individual myocytes connect via specialized cell membranes, called intercalated discs.
- Intercalated discs contain gap junctions for low-resistance electrical transmission between cells
- Gap junctions facilitate electrical (ionic) currents between cells.
- Stimulation of one cardiac myocyte transmits the impulse to all interconnected myocytes.
Regulation of Contraction (Inotropy)
- Calcium entry occurs through L-type calcium channels.
- Calcium is released from the sarcoplasmic reticulum.
- Calcium binds to TN-C.
- Myosin undergoes phosphorylation.
- SERCA (sarcoplasmic/endoplasmic reticulum calcium ATPase) activity is involved.
- Calcium efflux occurs across the sarcolemma
Calcium Entry into Myocytes
- Calcium entry during depolarization is controlled mainly by L-type calcium channel phosphorylation.
- Cyclic adenosine monophosphate (cAMP), related to β-adrenergic receptors, is essential in this regulation.
- Norepinephrine and epinephrine (from sympathetic nerves or adrenal glands) bind primarily to β1-adrenergic receptors on the sarcolemma.
G-Protein regulation
- Gs protein activates adenylyl cyclase, which hydrolyzes ATP creating cAMP.
- cAMP activates protein kinase A (PKA), phosphorylating various sites within the cell.
Regulation of Calcium Efflux
- Key mechanisms include the Na+/Ca++ exchange pump and ATP-dependent calcium pump.
- These pumps move calcium out of the cell, preventing overloading.
- Inhibition of calcium extrusion increases intracellular calcium, and thus, inotropy.
Cardiomyocyte Metabolism
- Cardiac myocytes have a very high metabolic rate, due to their continuous, repetitive contraction.
- Unlike intermittent, short contractions of skeletal muscle, heart muscle contracts one to three times per second throughout life.
- ATP production for contraction is entirely aerobic.
- Accordingly, cardiac myocytes have a large number of mitochondria.
- In oxygen deprivation, contraction is possible for only about a minute.
- Unlike some skeletal muscle fibers, cardiac myocytes have limited anaerobic capacity for ATP.
- They utilize various substrates to produce ATP:
- Primarily fatty acids (approx. 60%) and carbohydrates (approx. 40%) in the overnight fasted state.
- Glucose during high-carbohydrate meals.
- Lactate, especially during exercise, in place of glucose.
- Amino acids and ketones (e.g. acetoacetate) instead of fatty acids.
Myocardial Action Potential
- Cardiac muscle has fast sodium channels, slow sodium-calcium channels, and potassium channels.
- Fast sodium channel opening initiates the rapid upstroke of the action potential.
- The plateau period is primarily due to the opening of slow sodium-calcium channels, lasting around 0.3 seconds.
- Potassium channel opening causes large amounts of positive potassium ions to diffuse outward, returning membrane potential to resting levels.
Self-Excitation of Sinus Nodal Fibers
- The high sodium ion concentration outside sinus nodal fibers, existing sodium channels that tend to leak this positive ion to the inside, allow a slow rise in the resting membrane potential. -Sodium influx happens between heartbeats leading to positive potential shift.
- During the action potential, sodium-calcium channels become inactivated and potassium channels open.
- Influx of positive ions ceases; and significant potassium diffusion outwards occurs, returning the membrane potential to the resting level, which is called hyperpolarization.
Conduction System of the Heart
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The heart generates rhythmic electrical impulses to cause rhythmic contraction of the heart muscle.
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Conduction of these impulses is rapid through the heart, enabling simultaneous contraction in most of the ventricular muscle.
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The sinus (SA node) is the heart's primary pacemaker. -It produces impulses faster than other parts of the heart, leading to coordinated heart contractions.
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The atrial conductive system is organized so that the cardiac impulse does not travel too rapidly from the atria into the ventricles, creating a delay that allows the atria to empty their contents into the ventricles before ventricular contraction starts.
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The conduction velocity in the atria is about 0.3 m/sec, while in the atrial fibers, it is considerably faster, at roughly 1 m/sec.
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The anterior interatrial band acts as a conducting pathway from the atria to the left atrium. Additional pathways also help in efficient atrial impulse conduction.
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The AV node delays the impulse transmission to the ventricles, roughly 0.13 seconds after the propagation through internodal pathways.
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The delay is mainly because there are fewer gap junctions between the successive cells, causing resistance during conduction.
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Following the A–V node, the impulse is transmitted through the penetrating bundle, which is composed of small fascicles.
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Specialized Purkinje fibers transmit the action potential quickly throughout the ventricles, creating almost instantaneous transmission throughout the ventricular muscle.
Extrinsic Control of Heart
- The heart rhythm is controlled by both intrinsic and extrinsic factors. Intrinsic factors are natural inherent components of the heart's conduction system, generating electrical signals for heartbeats.
- Extrinsic factors typically originate outside the heart, such as nerve impulses, hormones, or drugs, impacting rhythm.
Clinical Relevance
- Digitalis and similar glycosides inhibit Na+/K+-ATPase, increasing intracellular sodium.
- This subsequently increases intracellular calcium, leading to enhanced inotropy.
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Description
Explore the intricate world of cardiac myocytes and their role in signal transduction. This quiz delves into the structure, function, and regulatory mechanisms of heart muscle contraction, highlighting the importance of calcium signaling and intercalated discs. Test your knowledge on how these factors contribute to the heart's rhythmic contractions.
