Cardiac Muscle Physiology Quiz

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Questions and Answers

What is the primary action of the vagal tone at rest concerning the heart?

It has no influence on heart rate.

It directly stimulates the cardiac muscle.

It decreases the heart rate. (correct)

It increases the heart rate significantly.

Which neurotransmitter binds to β1 receptors on the SA node membranes to facilitate sympathetic stimulation?

Serotonin

Acetylcholine

Noradrenaline (correct)

Dopamine

Which effect does the autonomic nervous system NOT have on the heart?

Regenerating heart tissue (correct)

Modulating the heart rate

Altering the rate of impulse conduction

Changing the force of contraction

What characterizes the relative refractory period in cardiac action potentials?

Some Na channels are in a state to reopen. Signup and view all the answers

What term describes the effect of sympathetic action on the force of contraction in the heart?

Inotropic effect Signup and view all the answers

Which receptor type is associated with acetylcholine in the parasympathetic stimulation of the heart?

Muscarinic receptors Signup and view all the answers

What physiological change occurs when there is a positive chronotropic effect on the heart?

Increase in heart rate Signup and view all the answers

Which term describes the effect on the rate of impulse conduction through the AV node?

Dromotropic effect Signup and view all the answers

Which statement best differentiates between sympathetic and parasympathetic influences on the cardiac system?

Parasympathetic stimulation increases Ca2+ conductivity. Signup and view all the answers

Which effect is associated with increased vagal tone on the cardiac system?

Decreased heart rate Signup and view all the answers

What percentage of heart muscle is composed of contractile cells?

99% Signup and view all the answers

Which type of cardiac cell does NOT contribute to contractile force?

Auto-rhythmic cells Signup and view all the answers

What specialized structure connects cardiac muscle cells and facilitates electrical activity?

Gap junctions Signup and view all the answers

How do contractile cells in the heart become activated?

By change in membrane potential Signup and view all the answers

What type of channels are specialized in the sarcolemma of cardiac muscle cells?

Voltage-gated Ca2+ channels Signup and view all the answers

Which feature is characteristic of myocardial contractile cells?

Striated and branched fibers Signup and view all the answers

What is the primary role of auto-rhythmic cells within the heart?

To initiate and distribute electrical activity Signup and view all the answers

What allows cardiac muscle fibers to achieve greater shortening and lengthening?

Fibers not anchored at ends Signup and view all the answers

What is the primary role of the sinoatrial (SA) node in the heart's electrical conduction system?

To initiate the heart rate Signup and view all the answers

What is true regarding the function of the cardiac auto-rhythmic cells?

They control and coordinate the heartbeat. Signup and view all the answers

Which structure is responsible for isolating the electrical activity of the atria from the ventricles?

Fibrous cardiac skeleton Signup and view all the answers

Which of the following statements about cardiac muscle cells is incorrect?

All cardiac muscle cells have multiple nuclei. Signup and view all the answers

What occurs during the delay in conduction at the atrioventricular (AV) node?

Complete atrial contraction occurs before ventricular depolarization Signup and view all the answers

What promotes the rapid conduction of the electrical signal throughout the ventricles?

Purkinje fibres Signup and view all the answers

What is primarily responsible for the slow conduction at the AV node?

Small diameter of cells Signup and view all the answers

Why is there a refractory period in myocytes?

To prevent overstimulation of the heart Signup and view all the answers

How does the fibrous skeleton assist in the heart's electrical activity?

By electrically isolating the atria from the ventricles Signup and view all the answers

What is the effect of bundle branch delays on the heart's contraction sequence?

Allows atrial contraction to occur before ventricular contraction Signup and view all the answers

What function does the sodium channel in the SA node serve during the cardiac cycle?

It prepares for rapid depolarization Signup and view all the answers

What prevents excessively fast atrial contraction rates from resulting in rapid ventricular rates?

Delay at the AV node Signup and view all the answers

What is the primary role of autorhythmic cells in the heart?

They initiate action potentials throughout the heart. Signup and view all the answers

What distinguishes cardiac muscle action potentials from skeletal muscle action potentials?

Cardiac muscle action potentials are wider. Signup and view all the answers

What occurs immediately after calcium ions (Ca2+) bind to troponin in cardiac muscle contraction?

Tropomyosin shifts to expose myosin binding sites. Signup and view all the answers

Which of the following statements is true regarding the refractory period in cardiac muscle?

It prevents tetanus during sustained contractions. Signup and view all the answers

Which structure initiates the electrical signal that coordinates heart contractions?

Sinoatrial node Signup and view all the answers

How does calcium contribute to the contraction of cardiac muscle cells?

By facilitating the release of calcium from the sarcoplasmic reticulum. Signup and view all the answers

What prevents cardiac muscle from undergoing tetanus?

The long refractory period. Signup and view all the answers

What is the function of Na+/Ca2+ exchangers in cardiac muscle cells?

To remove excess calcium after contraction. Signup and view all the answers

Why do cardiac muscle cells have gap junctions?

To enable quick transmission of action potentials between cells. Signup and view all the answers

Which component directly causes the exposure of myosin binding sites during cardiac muscle contraction?

Calcium binding to troponin. Signup and view all the answers

Study Notes

Cardiac Cells

There are two main types of cardiac muscle cells:
- Contractile cells: responsible for generating force and contraction.
- Autorhythmic cells: initiate and transmit electrical impulses, coordinating the heartbeat.

Myocardial Contractile Cells

Structure:
- Striated, short, branched.
- Single, central nucleus.
- Connected by gap junctions and intercalated discs.
- Sarcolemma contains specialized voltage-gated calcium channels absent in skeletal muscle.
- Fibers are not anchored at the ends, allowing for greater sarcomere shortening and lengthening.

Cardiac Muscle Action Potential

Action potential:
- Spreads from autorhythmic cells to contractile cells through gap junctions.
- Travels along the plasma membrane to T-tubules.
- Opens calcium channels in plasma membrane and sarcoplasmic reticulum (SR).

Calcium and Contraction

Calcium-induced calcium release: calcium influx triggers the release of stored calcium from the SR.
Calcium binding: calcium binds to troponin, shifting tropomyosin and exposing myosin binding sites.
Cross-bridge cycling: initiates muscle contraction.

Cardiac Muscle Refractory Period

Refractory period: prevents summation and tetanus in cardiac muscle, unlike skeletal muscle.

Autorhythmic Cardiac Cells

Found in the conducting system of the heart:
- Sinoatrial (SA) node: considered the heart's pacemaker.
- Atrioventricular (AV) node: delays signal transmission to the ventricles.
- Bundle of His: conducts impulses to the ventricles.
- Purkinje fibers: rapidly carry signals throughout the ventricle walls.

Action Potential in a Pacemaker Cell

Pacemaker Potential: a gradual but automatic depolarization due to the slow inward movement of sodium and calcium ions.
Depolarization: caused by rapid influx of calcium ions.
Repolarization: due to the outward movement of potassium ions.

Internal Factors Affecting Electrical Activity in the Heart

Fibrous skeleton: electrically isolates atria from ventricles to prevent simultaneous contraction, enabling efficient blood flow.
SA node: fires most frequently and sets the heart rate as it has the shortest cycle of repolarization and depolarization.
AV node: provides a delay in conduction to ensure complete atrial contraction and emptying before ventricular depolarization.

External Factors Affecting Electrical Activity in the Heart

Autonomic Nervous System (ANS): modulates heart rate and force of contraction.
- Sympathetic Stimulation: increases heart rate (positive chronotropic effect) and force of contraction (positive inotropic effect).
- Parasympathetic Stimulation (vagal tone): decreases heart rate (negative chronotropic effect).

Influence of the Autonomic Nervous System on the Heart

Sympathetic stimulation:
- Neurotransmitter: norepinephrine.
- Receptor: β1 adrenergic receptors on SA node membranes.
Parasympathetic stimulation:
- Neurotransmitter: acetylcholine.
- Receptor: muscarinic receptors on nodal membranes.
Inotropic effect: Sympathetic stimulation increases the force of contraction in the ventricles.
Dromotropic effect: Sympathetic stimulation increases the rate of conduction through the AV node.
Bathmotropic effect: Sympathetic stimulation increases the rate of attainment of threshold for action potential generation in the AV node and ventricles.

Refractory Periods

Absolute refractory period: the myocyte cannot be stimulated as all sodium channels are inactivated.
Relative refractory period: the myocyte can be stimulated, but only with a stronger stimulus as some sodium channels have returned to the closed state.

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Description

Test your knowledge on cardiac muscle cells, including contractile and autorhythmic cells. Explore the structure, action potentials, and the role of calcium in muscle contraction. This quiz covers essential concepts in cardiac physiology.