Cardiac Muscle, Conduction System, Electrocardiography, and Cardiac Cycle

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Module 4 part 2

Cardiac Muscle Structure and Function

Characteristics of Cardiac Muscle Cells

Cardiac muscle cells, or myocytes, are specialized for continuous rhythmic contraction and are structurally distinct from skeletal muscle fibers.

Striated Appearance: Cardiac muscle is striated due to the arrangement of actin and myosin filaments, similar to skeletal muscle.
Cell Shape: Cardiac cells are short, fat, branched, and interconnected, unlike the long, cylindrical, multinucleated skeletal muscle fibers.
Nuclei: Each cardiac cell contains one or two centrally located nuclei.
Endomysium: The intercellular spaces are filled with a loose connective tissue matrix (endomysium) containing numerous capillaries, which connects to the dense fibrous skeleton of the heart.
Intercalated Discs: Cardiac cells interconnect at junctions called intercalated discs, which contain anchoring desmosomes and gap junctions.
Desmosomes: Prevent separation of adjacent cells during contraction.
Gap Junctions: Allow ions to pass from cell to cell, enabling direct transmission of the depolarizing impulse across the heart, so the myocardium acts as a functional syncytium.

Anatomy of the heart and cardiac muscle cells with intercalated discs Structure of cardiac muscle cell showing intercalated disc, mitochondria, and sarcoplasmic reticulum

Automaticity and Rhythmicity

Cardiac muscle cells possess unique properties that allow the heart to function independently and rhythmically.

Automaticity: Some cardiac cells are self-excitable and can initiate their own depolarization due to leaky plasma membranes, allowing sodium and calcium ions to enter easily.
Rhythmicity: Cardiac cells contract in a definite rhythm, typically set by the pacemaker (SA node).Sinoatrial Node
Resting Membrane Potential: Cardiac contractile cells remain at rest until excited by the pacemaker, with a resting membrane potential of -90mV.
Refractory Period: The refractory period is longer in cardiac muscle than in skeletal muscle, preventing tetanic contractions and ensuring effective pumping.

Mechanism and Events of Cardiac Muscle Contraction

The contraction of cardiac muscle follows a sequence similar to skeletal muscle but with key differences.

Action Potential Initiation: Influx of sodium ions initiates the action potential by opening voltage-regulated sodium channels.
Calcium Entry: Depolarization down T-tubules causes calcium entry into the sarcoplasm. Calcium is required for crossbridge activation and couples depolarization to myofilament sliding (excitation-contraction coupling).
Calcium Sources: Cardiac muscle relies on both extracellular calcium entry and release from the sarcoplasmic reticulum.
Repolarization: Calcium ions are pumped back into the sarcoplasmic reticulum and extracellular space during repolarization.

Energy Requirements

Cardiac muscle is highly dependent on aerobic metabolism.

Mitochondria: Cardiac muscle has more mitochondria than skeletal muscle, reflecting its reliance on oxygen.
Fuel Preference: Cardiac muscle uses fatty acids more effectively for ATP production, while skeletal muscle prefers glucose during activity.

Intrinsic Cardiac Conduction System

Overview of the Conduction System

The heart's ability to depolarize and contract is intrinsic, relying on specialized cells and gap junctions.

Components: The conduction system includes the SA node, AV node, AV bundle (bundle of His), right and left bundle branches, and Purkinje fibers.
Function: These components ensure orderly, sequential depolarization and contraction from atria to ventricles.

Heart conduction system showing SA node, AV node, bundle branches, and Purkinje fibers Diagram of the cardiac conduction pathway Conduction system of the heart with labeled nodes and fibers

Key Components of the Conduction System

Sinoatrial (SA) Node: Located in the right atrial wall, the SA node is the pacemaker, spontaneously depolarizing 70-80 times per minute and setting the sinus rhythm.
- Atrioventricular (AV) Node: Located in the interatrial septum, the AV node delays the impulse, allowing atrial contracf vgjnhkm,l./tion before ventricular excitation.
AV Bundle (Bundle of His): Transmits the impulse from the AV node to the bundle branches.
Bundle Branches: Carry the impulse through the interventricular septum.
Purkinje Fibers: Penetrate the ventricular myocardium, stimulating contraction of the papillary muscles and ventricles.

Electrocardiography (ECG)
Principles of Electrocardiography
Electrocardiography records the electrical activity of the heart using electrodes placed on the body surface.
- ECG: A recording of the heart's electrical events, useful for diagnosing arrhythmias and conduction abnormalities.
- Leads: A 12-lead ECG provides 12 views of the heart using 10 electrodes.
- Lead II: Commonly used for monitoring, views the inferior aspect of the left myocardium.

ECG electrodes placed on the chest ECG electrode placement diagram ECG tracing with multiple leads Diagram of ECG lead views

ECG Waves and Intervals

A typical ECG consists of three main waves and several intervals.

P Wave: Represents atrial depolarization (contraction), lasts about 0.08 seconds.
QRS Complex: Represents ventricular depolarization (contraction), lasts about 0.08 seconds, and masks atrial repolarization.
T Wave: Represents ventricular repolarization (relaxation), lasts about 0.16 seconds.
P-R Interval: Time from beginning of atrial excitation to beginning of ventricular excitation (~0.16 seconds).
Q-T Interval: Time from beginning of ventricular depolarization through repolarization (~0.36 seconds).

ECG tracing showing P, QRS, and T waves ECG tracing with labeled waves and intervals

ECG Analysis and Arrhythmias

Normal Sinus Rhythm (NSR): P wave, QRS complex, and T wave are present and consistent.
Arrhythmias: Abnormal patterns of cardiac activity, including bradycardia, tachycardia, heart blocks, and fibrillation.
Clinical Significance: Changes in ECG pattern or timing may indicate myocardial infarct or conduction system problems.

Normal sinus rhythm ECG strip Sinus bradycardia ECG strip Sinus tachycardia ECG strip

ECG Patterns in Heart Blocks

Partial AV Block: Every other P wave fails to evoke QRST.
Complete AV Block: P waves and QRST occur independently.

Normal ECG with P, QRS, and T waves Abnormal ECG: partial AV block Abnormal ECG: complete AV block

Cardiac Cycle

Overview of the Cardiac Cycle

The cardiac cycle is the sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.

Systole: Contraction phase, blood is ejected from the chambers.
Diastole: Relaxation phase, chambers refill with blood.
Pressure Changes: Blood flows along a pressure gradient, causing opening and closing of heart valves.

Phases of the Cardiac Cycle

Ventricular Filling (mid-to-late diastole): Blood flows passively into ventricles, AV valves open, semilunar valves closed. About 70% of ventricular filling occurs during this phase.
Atrial Systole: Atria contract, propelling remaining blood into ventricles.
Isovolumetric Ventricular Contraction: Ventricles contract, AV valves close, semilunar valves remain closed, pressure rises.
Ventricular Ejection: Semilunar valves open, blood is expelled into aorta and pulmonary trunk.
Isovolumetric Relaxation (early diastole): Ventricles relax, semilunar valves close, AV valves remain closed, atria fill with blood.

Timing of the Cardiac Cycle

Average Heart Rate: 75 beats per minute.
Cycle Length: About 0.8 seconds per cycle.
Atrial Systole: 0.1 second.
Ventricular Systole: 0.3 second.
Quiescent Period (Total Relaxation): 0.4 second.

Heart Sounds

The cardiac cycle produces characteristic heart sounds.

LUBB: First sound, caused by closure of AV valves during ventricular systole.
DUP: Second sound, caused by closure of semilunar valves during ventricular diastole.
Heart Murmurs: Extra sounds caused by improper closure of valves.

Heart sounds and valve locations diagram

Summary Table: Cardiac Cycle Phases

Phase	Valves Open	Valves Closed	Event
Ventricular Filling	AV (mitral, tricuspid)	Semilunar (aortic, pulmonic)	Passive blood flow into ventricles
Atrial Systole	AV (mitral, tricuspid)	Semilunar (aortic, pulmonic)	Atria contract, active filling of ventricles
Isovolumetric Contraction	None	All	Ventricles contract, pressure rises
Ventricular Ejection	Semilunar (aortic, pulmonic)	AV (mitral, tricuspid)	Blood ejected from ventricles
Isovolumetric Relaxation	None	All	Ventricles relax, atria fill

Key Terms and Definitions

Syncytium: A group of cells that function as a single unit due to electrical coupling.
Automaticity: The ability of cardiac cells to initiate their own depolarization.
Arrhythmia: Abnormal heart rhythm.
Systole: Contraction phase of the heart.
Diastole: Relaxation phase of the heart.
ECG: Electrocardiogram, a recording of the heart's electrical activity.

Relevant Equations

Cardiac Output:
Stroke Volume:

Example

ECG Interpretation: A normal ECG shows regular P waves, QRS complexes, and T waves, indicating normal sinus rhythm. Abnormalities such as missing P waves or irregular QRS complexes may indicate arrhythmias or heart blocks.

Additional info: Academic context was added to clarify the mechanisms of cardiac muscle contraction, conduction system, ECG interpretation, and cardiac cycle phases.