Heart Action Potentials and Pacemaker Cells
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Questions and Answers

What direct effect does the binding of calcium ions to troponin C have on the actin-myosin interaction in cardiomyocytes?

  • It hydrolyzes ATP, providing the energy needed for myosin to bind to actin.
  • It strengthens the bond between tropomyosin and actin, preventing myosin binding.
  • It causes tropomyosin to shift, exposing the myosin binding sites on actin. (correct)
  • It directly phosphorylates myosin heads, increasing their affinity for actin.

During the cardiac excitation-contraction coupling, what is the primary role of ATP in the myosin-actin interaction?

  • ATP is used to transport calcium ions into the sarcoplasmic reticulum.
  • ATP hydrolysis powers the 'power stroke' of the myosin head, sliding actin and myosin filaments past each other. (correct)
  • ATP is used to block the binding sites on actin, preventing premature muscle contraction.
  • ATP facilitates the initial binding of calcium ions to troponin C.

How do calcium ions facilitate the interaction between actin and myosin filaments in cardiac muscle contraction?

  • Calcium ions directly bind to myosin, activating its ATPase activity.
  • Calcium ions bind to troponin C, initiating a conformational change that exposes myosin-binding sites on actin. (correct)
  • Calcium ions bind to tropomyosin, promoting its interaction with actin.
  • Calcium ions phosphorylate actin, enhancing its binding affinity for myosin.

What is the consequence of removing calcium ions from the sarcoplasm in cardiac muscle cells?

<p>Tropomyosin re-covers the actin binding sites, preventing further cross-bridge formation. (A)</p> Signup and view all the answers

What is the ultimate result of the cyclical binding, sliding, and reattaching of myosin heads on actin filaments in cardiac muscle?

<p>Shortening of the muscle, leading to contraction. (A)</p> Signup and view all the answers

Which characteristic distinguishes pacemaker cells from the majority of other heart cells (cardiomyocytes)?

<p>Pacemaker cells can spontaneously generate action potentials. (B)</p> Signup and view all the answers

How does the arrangement of pacemaker cells and cardiomyocytes contribute to the efficient function of the heart?

<p>Pacemaker cells act as high-speed pathways, while cardiomyocytes conduct the action potential more slowly, ensuring synchronized contraction. (A)</p> Signup and view all the answers

What is the significance of the heart functioning as a 'functional syncytium'?

<p>It ensures that all heart cells contract in a coordinated manner, maximizing the efficiency of each heartbeat. (D)</p> Signup and view all the answers

If the sinoatrial (SA) node fails, what is the most likely immediate consequence for the heart's electrical activity?

<p>The atrioventricular (AV) node or other pacemaker cells will likely take over, setting a new, possibly slower, heart rate. (A)</p> Signup and view all the answers

How would a drug that selectively blocks the function of the Purkinje fibers likely affect heart function?

<p>It would cause uncoordinated contraction of the ventricles. (A)</p> Signup and view all the answers

What cellular process is essential for initiating an action potential in a pacemaker cell?

<p>Depolarization of the cell membrane. (C)</p> Signup and view all the answers

What distinguishes 'automaticity' in the heart?

<p>The ability of pacemaker cells to self-generate action potentials. (B)</p> Signup and view all the answers

How would increasing the resistance in the internodal tracts affect the timing of heart contractions?

<p>It would slow down the transmission of action potentials between nodes. (A)</p> Signup and view all the answers

What is the primary determinant of changes in membrane potential?

<p>The movement of ions across the membrane. (B)</p> Signup and view all the answers

What is the effect of a depolarization wave moving through the heart?

<p>It causes heart muscle contraction. (D)</p> Signup and view all the answers

If depolarization waves occur approximately twice per second, what is the heart rate in beats per minute?

<p>120 beats per minute (B)</p> Signup and view all the answers

During Phase 4 of a pacemaker cell action potential, what is the primary event that occurs?

<p>Slow influx of sodium ions through HCN channels. (A)</p> Signup and view all the answers

What is the 'funny current' in pacemaker cells primarily caused by?

<p>The flow of sodium ions into the cell through HCN channels. (B)</p> Signup and view all the answers

What voltage triggers Phase 0 in pacemaker cells?

<p>-50mV (B)</p> Signup and view all the answers

What ion is responsible for the rapid depolarization in Phase 0 of pacemaker cells?

<p>Calcium (D)</p> Signup and view all the answers

Why do pacemaker cells not have a Phase 1 or Phase 2 during their action potential?

<p>They immediately repolarize after depolarizing. (B)</p> Signup and view all the answers

What occurs during Phase 3 of the pacemaker cell action potential?

<p>Potassium channels open, calcium channels close, leading to repolarization. (A)</p> Signup and view all the answers

How do pacemaker cells influence the rest of the heart?

<p>By generating action potentials that spread to myocytes. (B)</p> Signup and view all the answers

What is the function of myocytes?

<p>To contract and pump blood. (A)</p> Signup and view all the answers

What differentiates cardiac myocytes from skeletal muscle cells in terms of action potential signals?

<p>Cardiac myocytes receive signals from pacemaker cells, while skeletal muscle cells receive signals directly from neurons. (A)</p> Signup and view all the answers

What is the resting membrane potential of a myocyte in Phase 4?

<p>-90 mV (B)</p> Signup and view all the answers

Which of the following best describes the role of HCN channels in pacemaker cells?

<p>They allow for the slow influx of sodium ions, contributing to the pacemaker potential. (B)</p> Signup and view all the answers

How does the relative number of potassium ion channels compared to HCN channels impact the membrane potential during Phase 3 of the pacemaker cell action potential?

<p>More potassium channels cause a net outward positive current, leading to repolarization. (B)</p> Signup and view all the answers

What is the primary role of gap junctions in cardiac myocytes?

<p>Allowing the passage of ions, such as calcium, between adjacent cells, facilitating depolarization. (D)</p> Signup and view all the answers

The influx of which ion is primarily responsible for the rapid depolarization (Phase 0) of a cardiac myocyte action potential?

<p>Sodium (A)</p> Signup and view all the answers

What is the 'threshold potential' in the context of cardiac myocyte action potentials, and why is it significant?

<p>The membrane potential at which voltage-gated sodium channels open, initiating depolarization. (B)</p> Signup and view all the answers

Why is the action potential in cardiac myocytes described as an 'all-or-none' process?

<p>Because the myocyte either fully depolarizes upon reaching threshold or does not depolarize at all. (B)</p> Signup and view all the answers

During which phase of the cardiac myocyte action potential does the 'plateau' occur, and what ionic events characterize this phase?

<p>Phase 2; calcium influx balances potassium efflux, maintaining a stable membrane potential. (D)</p> Signup and view all the answers

What is the primary role of calcium influx during the plateau phase (Phase 2) of the cardiac myocyte action potential?

<p>To initiate the contraction of the myocyte by activating contractile proteins. (D)</p> Signup and view all the answers

During which phase of the cardiac myocyte action potential does repolarization primarily occur, and what ionic movement is responsible?

<p>Phase 3; efflux of potassium ions. (D)</p> Signup and view all the answers

How do T-tubules contribute to the process of excitation-contraction coupling in cardiac myocytes?

<p>They facilitate the rapid spread of depolarization and calcium influx into the cell interior. (C)</p> Signup and view all the answers

What is the function of the sarcoplasmic reticulum (SR) in cardiac myocytes?

<p>To store and release calcium ions, which trigger myocyte contraction. (A)</p> Signup and view all the answers

What is 'calcium-induced calcium release' in the context of cardiac excitation-contraction coupling?

<p>The release of calcium from the sarcoplasmic reticulum triggered by the influx of extracellular calcium. (C)</p> Signup and view all the answers

Which proteins are directly responsible for the mechanical contraction of cardiac myocytes?

<p>Actin and myosin (A)</p> Signup and view all the answers

What structural feature physically connects adjacent cardiomyocytes, providing mechanical stability during contraction?

<p>Desmosomes (B)</p> Signup and view all the answers

How does the coordinated function of cardiomyocytes as a 'functional syncytium' contribute to the heart's overall function?

<p>It ensures rapid, coordinated contraction of the heart muscle for efficient pumping. (D)</p> Signup and view all the answers

If the voltage-gated calcium channels were blocked during Phase 2 of a cardiac myocyte's action potential, what would be the most likely consequence?

<p>The myocyte would fail to contract effectively. (C)</p> Signup and view all the answers

What would happen if the gap junctions between cardiomyocytes were non-functional?

<p>The coordinated contraction of the heart muscle would be disrupted. (D)</p> Signup and view all the answers

Flashcards

Troponin C

Binds calcium, causing tropomyosin to move and expose actin binding sites.

Tropomyosin

Covers actin binding sites, preventing myosin from binding and causing contraction.

Myosin Head

Forms cross-bridges with actin, powered by ATP, to shorten the muscle during contraction.

Cardiac Excitation-Contraction Coupling

The process where an electrical signal (action potential) becomes mechanical work (muscle contraction).

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Actin

Forms bridges with myosin using ATP, enabling the 'power stroke' that pulls the filaments together.

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Action Potentials

Rapid electrical changes across cell membranes, propagating to adjacent cells.

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Pacemaker Cells

Specialized cells that initiate heartbeats by generating action potentials.

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Automaticity

The ability of pacemaker cells to spontaneously generate action potentials.

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Sinoatrial (SA) Node

A cluster of pacemaker in the right atria that sets the normal heart rhythm.

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Electrical Conduction System

Includes the SA node, AV node, Bundle of His, and Purkinje fibers.

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Cardiomyocytes

Heart muscle cells that receive action potentials from pacemaker cells.

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Functional Syncytium

The concept that heart cells act as a single unit due to electrical connections.

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Depolarization

Reversal of polarization, making the cell less negative inside.

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Membrane Potential

The electrical potential difference across a cell's membrane.

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Depolarization Wave

A wave of depolarization moving through cells.

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Phase 4 (Pacemaker)

Phase 4 is the pacemaker potential phase, and it starts when the pacemaker cell is just sort of hanging out with an overall charge or membrane potential of -65mV.

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HCN Channels

Ion channels in pacemaker cells that open when the membrane potential gets very negative, allowing sodium ions to enter.

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Funny Current

The electric current caused by sodium ions rushing into the cell through HCN channels.

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Phase 0 (Pacemaker)

Phase 0 is the depolarization phase.

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Calcium Channels (Pacemaker)

Voltage-gated channels that open during phase 0, allowing calcium to flow into the cell.

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Phase 3 (Pacemaker)

Phase 3 is the repolarization.

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Myocytes

Cells that make up the myocardium and contract to pump blood.

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Myocardium

The muscular middle layer of the heart.

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Phase 4 (Myocyte)

Phase 4 is the resting membrane potential phase.

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Membrane potential polarity

Describes the inside of the cell relative to the outside.

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Gap Junctions

Openings between adjacent myocytes allowing ion flow.

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Threshold Potential

The membrane potential at which voltage-gated sodium channels open, triggering depolarization (about -70mV).

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Phase 0: Depolarization

The initial phase of the action potential where sodium rushes into the cell, causing a rapid increase in membrane potential.

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All-or-None Process

The principle that an action potential either occurs fully or not at all, depending on whether the threshold potential is reached.

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Phase 1: Initial Repolarization

The phase where sodium channels close and potassium channels open, causing a slight drop in membrane potential.

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Phase 2: Plateau Phase

The phase where calcium channels open, and calcium influx counterbalances potassium efflux, maintaining stable membrane potential.

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Sarcoplasmic Reticulum

The organelle that stores intracellular calcium in cardiomyocytes.

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Desmosomes

Proteins that provide structural support to the myocardium.

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T-Tubules

Extensions of the cell membrane that increase surface area and help transmit electrical signals deep into the cell

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Phase 3: Repolarization

The phase where calcium channels close but potassium channels remain open, resulting in a net outward current and a return to resting membrane potential.

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Phase 4: Resting Phase

The resting phase where myocytes are polarized and awaiting the next action potential.

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Calcium-induced Calcium Release

The process where extracellular is released from the sarcoplasmic reticulum.

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Actin and Myosin

Contractile proteins responsible for muscle contraction.

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Study Notes

  • Action potentials are rapid electrical changes across cell membranes, propagating to adjacent cells, vital for heart communication.
  • Pacemaker cells, about 1% of heart cells, initiate heartbeat rhythm by generating action potentials conducted to the other 99% of heart cells.
  • Pacemaker cells exhibit automaticity, launching their own action potentials if signals from neighboring cells are absent.

Pacemaker Cell Locations

  • Sinoatrial node (SA node) in the right atria sets the pace.
  • Internodal tracts connect nodes.
  • Atrioventricular (AV) node delays the signal.
  • Bundle of His transmits impulses to ventricles.
  • Purkinje fibers distribute impulses throughout the ventricles.

Functional Syncytium

  • Heart muscle cells or cardiomyocytes receive action potentials slower than pacemaker cells.
  • Pacemaker cells act as highways for fast action potential transmission.
  • Muscle cells are the slower side roads that help all myocytes contract simultaneously.
  • This system enables cells to act as one unit due to mechanical, chemical, and electrical connections.

Action Potential Chemistry

  • Action potentials are initiated by depolarization, the opposite of polarization.
  • Polarization involves a higher negative charge inside the cell versus outside, defining membrane potential.
  • Membrane potential changes are determined by ion movement, specifically which ions move and membrane permeability.
  • Depolarization occurs when ions shift, making the membrane potential less negative, potentially positive.
  • Depolarization waves in neighboring cells trigger further depolarization, leading to muscle contraction and setting the heart rate.
  • Waves that occur once per second result in 60 beats per minute.

Pacemaker Cell Action Potential Phases

  • Phase 4: Pacemaker potential phase starts at -65mV.
  • HCN channels open when the membrane potential is negative, allowing sodium ions to flow in.
  • Inward rush of sodium ions creates the "funny current."
  • Sodium influx through HCN channels slowly depolarizes the membrane to -50mV.
  • Phase 0: Depolarization phase occurs when the cell reaches -50mV.
  • Voltage-gated calcium channels open allowing calcium to flow into the cell.
  • Sodium and calcium influx raises the membrane potential past 0mV to +10mV.
  • This phase concludes in 0.5 milliseconds
  • Phase 3: Repolarization phase as calcium channels start to close at +10mV and potassium channels open.
  • Outward positive current from potassium leaving the cell repolarizes the membrane.
  • Membrane potential returns to -65 mV, restarting the cycle.

Myocytes

  • Myocytes receive signals from pacemaker cells causing them to contract.
  • Myocytes make up the myocardium, the muscular middle layer of the heart.
  • Myocytes are also called contractile cells because they contract to allow the heart to pump blood.
  • Unlike skeletal muscle cells, cardiac myocytes receive signals from pacemaker cells.

Myocyte Action Potential Phases

  • Phase 4: Resting phase with membrane potential at -90 mV.
  • Gap junctions allow calcium ions to leak in from neighboring cells, raising the membrane potential to -70 mV (threshold potential).
  • Phase 0: Depolarization phase commences at the -70mV threshold.
  • Voltage-gated sodium channels open, causing a rapid influx of sodium ions.
  • Sodium influx raises the membrane potential to +20mV.
  • Action potentials follow an all-or-none principle; threshold must be reached for depolarization.
  • Phase 1: Initial repolarization happens as the membrane potential rises above -70 mV to +20 mV.
  • Sodium channels close, and voltage-gated potassium channels open.
  • Potassium ions exit, reducing the membrane potential causing an outward current and a notch on the graph.
  • Phase 2: Plateau phase begins when voltage-gated calcium channels open allowing calcium ions to flow into the cell.
  • Influx of calcium ions balances potassium outflow, stabilizing the membrane potential.
  • Calcium influx triggers myocyte contraction and influences the action potential length and heartbeat.
  • Phase 3: Repolarization occurs when calcium channels close, but potassium channels remain open.
  • Ion pumps remove calcium ions leading to relaxation.
  • Membrane potential returns to -90 mV, restarting the cycle.

Cardiac Excitation-Contraction Coupling

  • Cardiac excitation-contraction coupling is relationship between electrical signals and mechanical changes that cause heart muscle contraction.

Cardiomyocyte Structure.

  • Cardiomyocytes are branched cells connected by intercalated disks with gap junctions.
  • Gap junctions facilitate ion flow between cells, enabling depolarization of neighboring cells.
  • Desmosomes physically attach cells, maintaining structural integrity during contraction.
  • Transverse tubules (T-tubules) increase surface area and facilitate calcium ion movement deep inside.
  • Sarcoplasmic reticulum stores intracellular calcium.

Process

  • Depolarization spreads via gap junctions, with calcium ions triggering a threshold in neighboring cells.
  • T-tubules enable calcium ions to penetrate deeply into the cell during depolarization.
  • Extracellular calcium binds to ryanodine receptors on the sarcoplasmic reticulum triggering calcium-induced calcium release.
  • Released calcium activates actin and myosin, essential for muscle contraction.
  • Calcium ions bind to troponin C, which moves tropomyosin allowing myosin heads to bind to actin.
  • Myosin heads bind to actin, forming cross-bridges and pulling actin and myosin filaments past one another.
  • This process requires ATP.
  • Calcium ions are removed by ion transporters back into the sarcoplasmic reticulum, extracellular environment, or mitochondria.
  • Removal of calcium causes troponin to revert, blocking actin binding sites and preventing further cross-bridges, leading to muscle relaxation.

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Description

Action potentials facilitate rapid electrical changes across heart cell membranes, crucial for intercellular communication. Pacemaker cells, located in areas such as the SA and AV nodes, initiate and regulate heartbeat rhythm. These cells possess automaticity, ensuring continuous heart function.

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