Pacemaker Cells and Action Potentials Quiz

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Questions and Answers

What is the main function of ion channels in pacemaker cells?

They facilitate a selective passage of ions to initiate rhythmic contractions. (correct)

They only allow potassium ions to flow during repolarization.

They provide a non-selective pathway for all ions to pass through.

They generate action potentials independently of other cells.

Which phase of the action potential in pacemaker cells is characterized by the opening of T-type Ca2+ channels?

Depolarization phase

Repolarization phase

Pacemaker potential (correct)

Resting phase

What occurs during the depolarization phase of the pacemaker action potential?

K+ ions exit the cell, causing the membrane to become more negative.

T-type Ca2+ channels close, leading to a decrease in membrane potential.

Na+ and Ca2+ ions flow into the cell, causing rapid depolarization. (correct)

The membrane stabilizes at -40 mV.

What happens to the action potentials as they travel from the right atrium to the left atrium?

They generate a wavefront of excitation resulting in muscular contraction. Signup and view all the answers

Which channel is responsible for the hyperpolarization effect in pacemaker cells?

Funny channel Signup and view all the answers

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

To initiate and regulate electrical impulses for contraction Signup and view all the answers

Which phase of the cardiac cycle is characterized by the contraction and emptying of the heart's chambers?

Systole Signup and view all the answers

What characteristic enables cardiomyocytes to contract independently of central nervous system commands?

Autorhythmicity Signup and view all the answers

Which structure in the cardiac conduction system is directly responsible for initiating the heartbeat?

Sinoatrial (SA) node Signup and view all the answers

During which phase does the heart undergo isovolumetric contraction?

Just before ventricular ejection Signup and view all the answers

Which of the following phases follows ventricular ejection in the cardiac cycle?

Isovolumetric relaxation Signup and view all the answers

What is the primary electrolyte involved in the action potential of cardiomyocytes?

Sodium Signup and view all the answers

Which of the following is essential for the rhythmic contraction of the heart's muscle tissue?

Presence of intercalated discs Signup and view all the answers

What is the primary role of the Funny channel (If) in cardiomyocytes?

Influx of Na+ Signup and view all the answers

Which ion channel primarily contributes to the plateau phase of the cardiac action potential?

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

What structure allows for the transmission of electrical signals between interconnected cardiomyocytes?

Gap junction Signup and view all the answers

Which phase of the cardiac action potential is characterized by rapid depolarization?

Rising phase Signup and view all the answers

How does an increase in intracellular Ca2+ levels affect cardiac muscle contraction?

Triggers the contraction of cardiac muscle Signup and view all the answers

What effect does hyperkalemia have on the cardiac action potential?

Decreases the threshold for action potential generation Signup and view all the answers

What is the function of desmosomes in cardiomyocytes?

Hold cells mechanically together Signup and view all the answers

What characterizes the refractory period in cardiac muscle cells?

Prevents spontaneous depolarization Signup and view all the answers

Which channel is primarily responsible for K+ efflux during the repolarization phase?

Persistent K+ channel Signup and view all the answers

During which phase of the cardiac cycle does the Na+ channel close?

Plateau phase Signup and view all the answers

What is the purpose of electrical defibrillation in cardiac arrest?

To depolarize all parts of the heart simultaneously Signup and view all the answers

Which ion primarily establishes the resting membrane potential in cardiomyocytes?

K+ Signup and view all the answers

In which channel does an influx of Ca2+ primarily occur during the action potential?

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

Study Notes

Human Physiology BIOL3205 - Cardiovascular System I

Course instructor: Prof. Chi Bun Chan
Contact information: [email protected], 39173823
Course details: Cardiovascular system - I
Location: 5N10 Kadoorie Biological Sciences Building

Cardiovascular System

Part I: Heart
Part II: Blood vessel and pressure
- Images of heart anatomy and blood vessel pressure, including labelled images of aorta, arteries, arterioles, capillaries, veins, and venules.
- Measurements of pressure are shown in mmHg (millimeters of mercury).

Lecture Outline

Basic anatomy of the heart
Structure of the cardiomyocyte
Cardiac cycle
Autorhythmicity of pacemaker cells
Generation of action potential in pacemaker cells and cardiomyocytes
Control of cardiac output
Heart failure

What Is the Cardiovascular System?

Cardio (Greek - heart) vascular (vessel)
The cardiovascular system is made of three components:
- Heart (pump that maintains continuous blood flow)
- Blood vessels (passageways for blood)
- Blood (transport medium)
Functions:
- Homeostasis (regulates and maintains a stable internal state)
- Nutrient and oxygen delivery to body parts
- Removal of waste products from body parts

Structure of the Heart

The first organ to become functional during fetal development.
Heart is composed of mainly cardiac muscle (cardiomyocyte).
Divided into right and left halves separated by a septum
The atria and ventricles are separated by atrioventricular (AV) valves
Blood returning to the heart moves through veins, blood moving away through arteries.
Semilunar valves are present (in arteries).
Dual pump function; right and left sides function independently.
Asymmetrical muscle distribution

Circulation

The circulatory system has two divisions: pulmonary and systemic circuits.
Left side of the heart handles oxygenated blood (bright red).
Right side of the heart handles deoxygenated blood (dark red).
Blood circulation is unidirectional.

Cardiac Cycle

Normal heart rate: 60-100 beats/minute.
To pump out blood effectively, the contraction of the heart must be highly synchronized is referred to as the cardiac cycle.
Cardiac Cycle is split into Systole (contraction and emptying) and Diastole (relaxation and filling) phases.
The cycle has 4 phases:
- Ventricular filling
- Isovolumetric contraction
- Ventricular ejection
- Isovolumetric relaxation

Pacemaker Cells and Conduction Fibers

Cardiac muscle contracts by signals originating within the muscle itself known as autorhythmicity.
Pacemaker cells (SA node and AV node) initiate the action potential.
The action potential travels through the heart via conduction fibers (bundle of His and Purkinje fibers)
The speed of the conduction through the AV node is slower.

Heartbeats (Highly CoOrdinated Work)

Action potential starts in the SA node.
Impulses travel through the atria (spreads as a wave).
Atria contraction occurs.
Signal reaches the AV node and is delayed.
Signal travels through the bundle of His and the Purkinje fibers.
Ventricle contraction occurs.

Electrical Activity in Pacemaker Cells

Pacemaker potential: A slow drift of the membrane potential until it reaches the threshold (-40 mV).
Three phases (sequential openings of channels):
- Pacemaker potential: Opening of funny and T-type Ca2+ channels
- Depolarization: Opening of L-type Ca2+ channels
- Repolarization: Opening of K+ channels

Ion Channels in Pacemaker Cells

Ion channels are protein channels that span the cell membrane, which allow the selective passage of ions.
Types of ion channels on cardiomyocytes:
Na+, K+, Ca2+ (T-Type, L-Type, Funny)

Generation of Action Potential in Pacemaker Cells

Depicted as a graph of action potential in pacemaker cells.
Shows the order of channel activation during an action potential.

Impulse Transmission Between Cardiomyocytes and Pacemaker Cells

Intercalated discs connect cardiomyocytes and pacemaker cells.
Two types of membrane junctions
- Desmosomes: mechanically hold cells together.
- Gap junctions: allow cell-to-cell spread of electrical signals.

Electrical Activity in Cardiomyocytes

The action potential for cardiomyocytes is more prolonged compared to that of pacemaker cells.
Cardiomyocytes have 5 phases of depolarization:
- Rising
- Brief repolarization
- Plateau
- Repolarization
- Resting

Ion Channels Activities in Cardiac Action Potential

Depicted as diagrams illustrating the activity of different ion channels
Shows the channel openings/closings on a graph representing the cardiac action potential

Cardiac Action Potential

A more detailed description of the cardiac action potential.

Coupling of Electrical Signal and Contraction

Contraction of cardiac muscle is triggered by calcium influx (increase in intracellular calcium level).
Ca2+ binds to the troponin-tropomyosin complex, myosin crosses-bridge with actin, thin myofilament inward, shorten the sarcomere.

Electrical Activity and Contraction

Cardiac muscle cannot be re-stimulated until the contraction is almost over.
No summation of contraction (tetanus).
Protective mechanism to ensure complete filling and emptying of the heart.
Long closure of Na+ channels.

Refractory Period and Summation

Depicts refractory period and summation diagrams differentiating skeletal and cardiac musculature.
Skeletal muscle refractory period is shorter (faster depolarization).
Cardiac muscle action potential has a prolonged refractory period which prevents summation.

Ion Balance and Cardiac Health

Potassium gradient across the cell membrane is vital for establishing the cardiomyocyte membrane potential.
Potassium (K+) imbalance (hyperkalemia) negatively impacts cardiac electrical potential, potentially leading to fatal abnormal heart rhythms (arrhythmias).
Causes are often related to kidney disease.

Cardiac Defibrillation

Fibrillation is a random, uncoordinated excitation and contraction of cardiac cells.
Can be corrected by electrical defibrillation that depolarizes all parts of the heart simultaneously.
AED (Automated External Defibrillator) can perform electrical defibrillation.

Can Heart Beats Be Controlled?

Ways to help control heart beats, showing an example of target zones for training based on intensity and duration.

Cardiac Output and Its Control

Heart rate (HR) and stroke volume (SV) can influence cardiac output (CO), which measures the volume of blood pumped each minute by the ventricles
CO is a critical indicator of the heart's efficiency.
- Factors affecting HR (e.g., autonomic innervation, hormones, fitness) & Factors affecting SV (e.g., heart size, fitness, contractility).
Intrinsic control (end diastolic volume (EDV))
Extrinsic control(parasympathetic & sympathetic nerves. Hormones)

Intrinsic Control of Cardiac Output

End-diastolic volume (EDV) and end-systolic volume (ESV), stroke volume (SV).
EDV is the intrinsic factor that controls the stroke force.
Contraction varies in response to ventricle stretch or venous return.
Blood is equalized between the right and left sides of the heart.
Prevents blood buildup in the heart.

Frank-Starling Law of the Heart

Starling's Law: Ventricle's contractile force is proportional to its diastolic volume (stretch)
The relationship between end-diastolic volume and stroke volume is shown in a graphical representation (Starling curve).
The curve reflects that the cardiac muscle is always operating at lengths less than optimum.

Extrinsic Control of Cardiac Output

*Sympathetic nerves: * dominate in stressful/emergencies by increasing, HR, SV, and contractility
*Parasympathetic nerves: * dominate during quiet/relaxed situations by decreasing, HR, and contractility.
*Hormones: * (e.g., epinephrine)
- Increase heart rate & SV
Overall, nervous system & hormonal systems can change heart rate and stroke volume.

Sympathetic Control of Cardiac Activity

Thoracic nerve to the atrium (SA node, AV node), and ventricles.
Release norepinephrine at the neuromuscular junction.
Increases heart rate: speeding up SA node's depolarization (increase Na+ and Ca2+ channels activities)-->increase the frequency of action potential.
Increases contractile strength (prolonged opening of L-type Ca2+ channel) in both the atrium and ventricle

Parasympathetic Control of Cardiac Activity

Vagus nerve to the atrium (SA and AV nodes)
Decrease heart rate by hyperpolarizing the SA node (increase K+ channel).
Decrease frequency of action potentials
Reduce contractile strength (shorten plateau phase) of the atrium
Release acetylcholine (ACh) at the neuromuscular junction.

Comparison of Sympathetic and Parasympathetic Controls

Shows a table of how the sympathetic and parasympathetic systems differ in their effects on the heart.

Integrative Control of Cardiac Output

Both heart rate and stroke volume can be changed simultaneously.
Balance of control factors from sympathetic and parasympathetic signals.
Disruption of this system leads to heart failure.

Heart Failure

Heart failure (HF) is the inability of cardiac output to keep pace with the body's demand for supply and waste removal.
Types of heart failure (HF): Systolic HF & Congestive HF.
Causes: Damage to heart muscle (ischemia), prolonged high blood pressure, ventricle hypertrophy, dyspnea, edema.

After the Lecture

Basic anatomy of the heart
Structure of the cardiomyocyte
Cardiac cycle
Autorhythmicity of pacemaker cells
Generation of action potential in pacemaker cells and cardiomyocytes
Electrocardiogram
Control of cardiac output
Heart failure

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Test your knowledge on the functions of ion channels in pacemaker cells and the phases of their action potentials. This quiz covers crucial concepts such as depolarization, T-type Ca2+ channels, and the transmission of action potentials across the atria. Prepare to deepen your understanding of cardiac physiology.