Lecture 4.2 - Initiation and control of electrical activity in the heart

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Questions and Answers

During phase 3 of a fast response action potential in contractile cells, which ion channels are closed?

K+ channels

L-type Ca2+ channels (correct)

Voltage-dependent Na+ channels

Sodium-calcium exchanger channels

In contractile cells, which ion is taken back to the extracellular fluid via the sodium-calcium exchanger and calcium proton ATPase pumps?

K+

Na+

Mg2+

Ca2+ (correct)

During which phase of the action potential do K+ channels open and K+ starts exiting the cell?

Phase 1

Phase 3 (correct)

Phase 2

Phase 4

In nodal cells, which type of ion channels determines excitability?

L-type Ca2+ channels Signup and view all the answers

During the refractory period, what limits the generation of a new action potential?

Inactivation of voltage-dependent Na+ channels Signup and view all the answers

What is the result of premature excitation during the refractory period?

Decreased amplitude of the action potential Signup and view all the answers

In contractile cells, what determines the membrane potential during phase 4?

Very little movement of ions Signup and view all the answers

How does the refractory period differ between nodal and contractile cells?

It is longer in nodal cells Signup and view all the answers

What is the primary function of the gap junctions in cardiac muscle cells?

To allow the depolarising current to flow through the cardiac muscle cells from one to another Signup and view all the answers

During which phase of the action potential do the L-type Ca2+ channels open in nodal cells?

Phase 0 (upstroke) Signup and view all the answers

What is the primary function of the ryanodine receptors (RyRs) in cardiac muscle cells?

To open up channels and allow more Ca2+ ions to move out into the cytoplasm Signup and view all the answers

What is the resting membrane potential of contractile cells in the cardiac muscle?

-90mV Signup and view all the answers

What type of ion channels are responsible for the rapid depolarization of contractile cells in the cardiac muscle?

Voltage-gated Na+ channels Signup and view all the answers

During which phase of the action potential do the K+ channels open in contractile cells?

Phase 1 Signup and view all the answers

What is the primary function of the desmosomes in cardiac muscle cells?

To anchor the ends of cardiac muscle fibres together Signup and view all the answers

What is the primary function of the funny Na+ channels in nodal cells?

To generate action potentials spontaneously Signup and view all the answers

What is the primary function of the T-type Ca2+ channels in nodal cells?

To generate action potentials spontaneously Signup and view all the answers

What is the primary function of the intercalated discs in cardiac muscle cells?

To anchor the ends of cardiac muscle fibres together and hold the different cardiomyocytes together Signup and view all the answers

Study Notes

Action Potential Phases

Phase 3 (final repolarization): L-type Ca2+ channels close, membrane potential decreases fast, and Ca2+ is taken back to sarcoplasmic reticulum or outside the cell via sodium-calcium exchanger and calcium proton ATPase pumps.
Phase 4 (resting membrane potential): very little movement of ions, and K+ channels open, allowing K+ to start exiting the cell.

Action Potential Refractory Period

In contractile cells, excitability depends on the availability of voltage-dependent Na+ channels.
The refractory period occurs when the depolarized cell cannot be re-excited until it has repolarized, due to the lack of available Na+ ions.
Reactivation of Na+ channels occurs in phase 3, allowing the next action potential to take place.
Premature excitation can result in a smaller amplitude action potential, leading to lower contraction.

Cardiomyocytes

Elongated, cylindrical, striated cells with a single nucleus.
Contain high numbers of mitochondria to support contraction.
Gap junctions allow depolarizing current to flow between cardiac muscle cells.
Desmosomes anchor ends of cardiac muscle fibers together, allowing efficient contraction.
Gap junctions and desmosomes form intercalated discs.

Automaticity of the Heart

The heart has an intrinsic ability to generate action potentials spontaneously and depolarize contractile myocardial cells, causing them to contract.
The brain does not control the initiation of action potentials, which are generated spontaneously by nodal cells.

Nodal Cells

SA node: located just beneath the superior vena cava, generates action potentials spontaneously, and is non-contractile.
AV node: located just beneath the pulmonary trunk, acts as a connection between atria and ventricles, and has a 0.1s delay in conduction.
AV bundle (Bundle of His) carries action potentials from the AV node to bundle branches.
Right bundle branch (RBB) and left bundle branch (LBB) carry action potentials to the right and left myocardium, respectively.
Purkinje fibers carry action potentials to ventricular muscle.

Slow Response Action Potentials - Nodal Cells

Resting membrane potential (RMP) of nodal cells: -60mV.
SA nodal cells have an unstable RMP that spontaneously depolarizes due to pacemaker potential.
Phase 4: diastole, funny Na+ channels open, and T-type Ca2+ channels open, reaching the threshold potential.
Phase 0: L-type Ca2+ channels open, and the cell depolarizes.
Phase 3: L-type Ca2+ channels inactivate, and K+ channels activate, causing cell repolarization.

Fast Response Action Potentials - Contractile Cells

RMP of contractile cells: -90mV.
Positive ions from nodal cells move into contractile cells via gap junctions, reaching the threshold potential.
Phase 0: voltage-gated Na+ channels open, and the cell depolarizes quickly.
Phase 1: Na+ channels close, and K+ channels open, causing initial repolarization.
Phase 2: K+ moves out, and Ca2+ moves in, causing the membrane potential to plateau.
Phase 3: K+ channels continue to open, and the cell repolarizes.

Excitation-Contraction Coupling

Occurs in phase 2.
Ca2+ ions activate ryanodine receptors (RyRs) in the sarcoplasmic reticulum.
RyRs open up channels, releasing more Ca2+ ions into the cytoplasm.
Ca2+ ions bind to troponin, changing the shape of tropomyosin and allowing myosin head to interact with actin, leading to contraction.
Multiple cardiomyocytes receive signals at the same time, synchronizing their action and contracting together as a unit (functional syncytium).

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Description

Learn about the different phases of an action potential, including final repolarization and resting membrane potential, as well as the refractory period in contractile cells.