• Working Myocardium
• Action Potential
• Propagation of Action Potential
• Action Potential and Electrical Voltage
• Action Potential and Contraction of Cardiomyocyte
• Action Potential in the Conduction System
• Action Potential in the SA Node
• I_f Channels and Heart Rate
• Working Myocardium vs. SA Node
• Suppression of the Conduction System
• ECG and Action Potential
• ECG and Cardiac Cycle

Working Myocardium

Cardiomyocytes of the working myocardium

• Cardiomyocytes in working myocardium have cylindrical (elongated) shape
• They are branched, with 2 or more branches at the ends, which connect to other cardiomyocytes
• They are interconnected by intercalated discs
• The intercalated discs contain gap junctions (nexus) through which ions can freely pass
• Thus, action potential spreads very quickly through cardiomyocytes
• Cardiomyocytes form a syncytium
• It is a multi-cellular structure formed by fusion of many cells
• In principle, it is a multi-cellular structure with shared cytoplasm
• They contain troponin, which is released upon their damage - necrosis (in case of myocardial infarction)

Action Potential

• It is a rapid change in electrical voltage of the cardiomyocyte
• It arises due to changes in concentration of intracellular and extracellular ions
• Action potential propagates more rapidly through conduction system than through working myocardium
• It involves depolarization - reversal of membrane polarity
• Subsequently, repolarization occurs - returning to the original state

Depolarization

• When the cardiomyocyte is stimulated by a negative electrical impulse
• From neighboring cells (impulses spontaneously originate in the SA node)
• From the electrode of a cardiac pacemaker
• This opens Na⁺ ion channels
• Followed by influx of Na⁺ into the cell due to the electrical gradient
• Causing a change in electrical voltage
• because intracellular Na⁺ concentration increases
• When the membrane potential reaches the threshold potential (-50mV)
• additional rapid sodium channels open
• The result is depolarization
• It involves a reversal of membrane polarity
• The extracellular space becomes negative
• The intracellular space becomes positive due to Na⁺ influx
• Depolarization spreads from the activation site as a depolarization wave

Repolarization

• At the end of depolarization, there is a movement of ions through channels again
• Mainly involving Ca⁺⁺ and K⁺ ions
• This results in a return to the original polarized state
• The repolarization wave spreads slowly and oppositely to depolarization
• In the myocardium, epicardium begins repolarizing earlier than endocardium
• The extracellular environment again changes to positive

Propagation of Action Potential

Cardiomyocytes of the Working Myocardium

• They are branched and longitudinally connected by intercalated discs
• The cytoplasm of cardiomyocytes is connected through intercalated discs
• The entire muscle fiber behaves like a single long cell (syncytium)
• The depolarization wave propagates throughout the myocardium as if through a single cell

Action Potential and Electrical Voltage

• Volt is the difference in electrical potential between 2 points
• The 1st point is the intracellular environment
• The 2nd point is the extracellular environment
• Resting Membrane Potential
• Has a value of -90mV
• The cell is negative intracellularly
• Depolarized Cardiomyocyte
• Has a value of 15mV
• The cell is positive intracellularly
• During the action potential
• There is a change in ion distribution
• And consequently, in electrical voltage

Action Potential in the Conduction System

• Action potential (impulse) originates spontaneously in the SA node
• with a frequency of approximately 60/min.
• From the SA node, it begins to activate the atrial myocardium
• Spreads like throwing a stone into water
• From the SA node diffuses into the atria (P wave)
• Resulting in atrial systole
• The impulse simultaneously rapidly spreads through the conduction system to the AV node
• From the AV node to the Bundle of His and to the ventricles
• First activates the septum (Q wave), and then the ventricles (R wave)
  
  
• Conduction system
• Spontaneously generates impulses in the SA node
• Does not contract
• Conducts impulse rapidly
• The impulse spreads from the conduction system to the working myocardium
• Working myocardium
• Does not spontaneously generate impulses
• Contracts
• Each impulse triggers systole
• Conducts impulse more slowly than the conduction system

I_f Channels and Heart Rate

• I_f channels in the SA node spontaneously depolarize and determine the heart rate
• Their depolarization rate is influenced by the autonomic nervous system
• Activation of the sympathetic system releases catecholamines (adrenaline, noradrenaline)
• The depolarization rate increases if the slope during the I_f current is steeper
• This allows the curve to reach the threshold value (-50mV) sooner

Inhibition of the SA Node

• For example, certain medications
• The I_f current curve of the action potential becomes flatter
• The 4th phase of the action potential lengthens
• The heart rate decreases

Action Potential in Various Parts of the Heart

• Action potential in different parts of the heart has a different shape
• It varies especially in the conduction system and working myocardium
• Due to distinct properties of cardiomyocytes
• Action potential (impulse) of the working myocardium is captured by an ECG device and results in an ECG waveform

ECG and Action Potential

ECG (lead V6) and Action Potential

• QRS complex represents ventricular depolarization
• During the QRS complex, a depolarization wave spreads through the ventricles
• ST segment is an isoelectric line
• During the second phase of the action potential, the entire myocardial wall is depolarized (-), no electrical vector is formed
• T wave represents ventricular repolarization
• The electrical vector during repolarization has the same direction as during depolarization
• However, repolarization takes longer, hence the T wave is wider than the QRS complex
• QT interval corresponds to 1 electrical cardiac cycle:
• Ventricular depolarization + repolarization
• P wave represents atrial depolarization
• The action potential of the P wave is not shown in the image
• Atrial repolarization is hidden within the QRS complex