Cardiac Excitability and Conductivity

Cardiac Excitability and Conductivity (3)

• Common with other muscle tissues

• Heart muscle cell membrane is an excitable membrane capable of transmitting an action potential

• Depolarization occurs due to the opening of fast sodium channels

Skeletal Muscle (3)

• No plateau phase

• Depolarization is rapid, followed by quick repolarization

• designed for rapid, repetitive contractions

Skeletal Muscle Action Potential (photo!)

Skeletal Muscle Excitation-Contraction Coupling

Relies on calcium from the sarcoplasmic reticulum only!

Skeletal Muscle Notice Time Scale (photo)

Ventricular Muscle (Cardiac Muscle) (4)

• Has a plateau phase due to the prolonged opening of slow calcium channels

  • lasting several hundred milliseconds

• Maintains the potential at a positive level during the action potential

• needs rhythmic, sustained contractions to pump blood effectively

Ventricular muscle Prolonged Depolarization (3)

• Leads to a longer absolute refractory period (ARP)

• Prevents tetanization (continuous contraction)

  • Crucial for proper heart function

Ventricular Muscle Action Potential (photo!)

Ventricular Muscle Functional Importance (2)

• Heart muscle cells require diastole (relaxation phase) to fill with blood

• Prolonged ARP ensures rhythmic contractions and prevents overlap of successive beats, maintaining efficient blood pumping

Ventricular Muscle Excitation-Contraction Coupling (2)

• Calcium influx during the plateau phase triggers a stronger contraction

• Uses extracellular calcium + from the sarcoplasmic reticulum

Ventricular Muscle Notice Time Scale (photo)

Increased Excitability meaning (2)

• Refers to conditions or factors that make it easier for heart muscle cells to reach the threshold for generating an action potential

• Increases the heart's sensitivity to stimuli, potentially leading to arrhythmias or abnormal heart rhythms

Increase Excitability (6)

• Sympathetic stimulation

• Mild hyperkalaemia (partial depolarization)

• Adrenaline

• Mild hypoxia (partial depolarization)

• Ischaemia

• Digitalis increases atrial muscle excitability

Decreased Excitability meaning (2)

• Refers to conditions or factors that make it harder for heart muscle cells to reach the threshold for generating an action potential

• Reduces the heart's ability to respond to stimuli

Decrease Excitability (7)

• Parasympathetic stimulation (atrial muscle only)

• Marked hyperkalaemia (marked depolarization)

• Hypokalaemia (hyperpolarization)

• Hyponatraemia

• Acetylcholine

• Marked hypoxia (marked depolarization)

• Digitalis decreases ventricular muscle excitability

Pathological Conditions of Excitability (6)

• Extrasystoles

• Paroxysmal Tachycardia

• Atrial Flutter

• Atrial Fibrillation

• Ventricular Flutter

• Ventricular Fibrillation

Extrasystoles definition - Pathological Conditions of Excitability

Abnormal systoles occurring during early diastole due to impulses from an irritable ectopic focus other than the SA node

Extrasystoles types (3)

• Atrial Extrasystoles

• Nodal Extrasystoles

• Ventricular Extrasystoles

Atrial Extrasystoles

Arising from the atrial muscle, followed by a normal diastole as the abnormal impulse resets the SA node

Nodal Extrasystoles

Arising from the AV node and AV bundle

Ventricular Extrasystoles

Arising from the ventricular muscle

Nodal Extrasystoles & Ventricular Extrasystoles ‘similarity’ (2)

• Both followed by a "compensatory" pause due to a longer diastole

• The extrasystole does not reset the SA node, thus:

  • Normal impulse is generated by the SA node but cannot be transmitted to the ventricles

  • The ventricular muscle is still in the absolute refractory period (ARP)

Paroxysmal Tachycardia (7) - Pathological Conditions of Excitability

• High heart rate in short attacks (seconds to days)

  • due to Ectopic focus firing faster than SA node

• Rate: Usually regular, atrial or ventricular origin

• Dormant focus between attacks

  • Triggered by anxiety, anger, fear

    • These are factors that increase excitability - trigger condition

• Symptoms: Palpitations or awareness of heartbeats

Tachycardia meaning

abnormally high heart rate

Atrial Flutter (5) - Pathological Conditions of Excitability

• High atrial rate (200-300 beats/min)

• Diminished pumping action of atria

  • due to the long refractory period of the AV node

• AV node refractory period limits transmission to ventricles (max 200 impulses/min)

• Creating Physiological heart block

Atrial Fibrillation (8) - Pathological Conditions of Excitability

• Extremely high atrial rate (>350 beats/min)

• Individual atrial fibres beat asynchronously

• Complete loss of atrial pumping action

• "Bag of worms" feeling of atrial beats

• Heart block present

• Irregular ventricular rhythm

  • Impulses transmitted randomly after reaching the necessary threshold

• Not fatal: Ventricular pumping preserved

Ventricular Flutter (5) - Pathological Conditions of Excitability

• High ventricular rate (200-350 beats/min)

  • Due to nodal/ectopic focus firing at a high rate

• Markedly reduced cardiac output

  • due to shortened diastole

• Fainting usually occurs

Ventricular Fibrillation (4) - Pathological Conditions of Excitability

• Extremely high ventricular rate (>350 beats/min)

• Asynchronous myocardial fibre activity

• Complete loss of ventricular pumping action

• Rapidly fatal (within minutes)

Resting Membrane Potential & Cardiac Impulse Types (4)

• Resting Membrane Potential

• Depolarising Membrane

• Depolarised Membrane

• Repolarising Membrane

Resting Membrane Potential (3)

• The electrical charge across the membrane at rest

• Inside the cell: negative relative to the outside

• Maintained by ion gradients (sodium, potassium)

Depolarising Membrane (3)

• The membrane potential becomes less negative (more positive)

• Sodium ions (Na⁺) rush into the cell

• Leads to the generation of an action potential

Depolarised Membrane (3)

• The membrane potential reaches its peak (positive)

• Action potential is propagated along the muscle

• A phase where the cell is unresponsive to further stimulation (absolute refractory period)

Repolarising Membrane (3)

• The membrane potential returns to resting negative state

• Potassium ions (K⁺) leave the cell

• The cell prepares for the next action potential

Circumstances for Re-entry & Circus Movement - Normal Conditions (2)

• The depolarising wave travels through the muscle and eventually dies out

• It encounters tissues in their absolute refractory period, preventing further action potential propagation

Circumstances for Re-entry & Circus Movement - Abnormal Conditions (2)

• The depolarising wave reaches repolarised tissue (tissue that is no longer in the refractory period)

• The tissue can be depolarised again, allowing the impulse to continue

Circus or Re-entry Movement (4)

• The impulse circulates around the tissue in a circular pattern, rather than dying out

• This creates a self-sustaining loop of electrical activity

• Leads to abnormal heart rhythms or arrhythmias

  • as the continuous re-entry disrupts the normal sequence of cardiac contraction

Conditions that can give rise to Circus Movement (3)

• Decrease in the Velocity of the Impulse

• Length of Pathway

• Shortening of the Refractory Period

Decrease in the Velocity of the Impulse (4)

• Blockage of the Purkinje fibres

• Ischemia of the muscle

• High blood potassium levels

• Results in a decreased rate of conduction

Length of Pathway (3)

Impulse takes a longer pathway, returning to the origin and encountering muscle in the resting phase

• Dilated Hearts

  • Increased size of the heart chambers leading to longer impulse travel time (long pathway)

Shortening of the Refractory Period (2)

Can occur due to factors like:

• Adrenaline release

• Electric AC shock

Importance of the Purkinje Fibres in Preventing Fibrillation (2)

• Rapid Conduction

• Longer Refractory Period

Rapid Conduction meaning

Purkinje fibres conduct impulses within a few 100ths of a second, ensuring that no part of the muscle is out of refractoriness

Longer Refractory Period (4)

• The refractory period of Purkinje fibres is more than 25% longer than that of ventricular muscle fibres

• This delay ensures that the impulse transmitted only after muscle fibre repolarisation

• Prevents continuous or abnormal stimulation

  • could lead to fibrillation

Conductivity definition

Specialised conducting system ensures excitation travels to all heart muscle fibres in a specific pattern

Conducting Pathways (5+1)

• Anterior, Middle, and Posterior Internodal Atrial Bundles

• A-V Node

  • Only pathway from atria to AV bundle; slow conduction, long absolute refractory period

• A-V Bundle

• Right and Left Bundle Branches

• Purkinje Fibres

Heart Block definition

Failure of impulses from the atria to reach the ventricles due to blockage at the AV node or Bundle

Types of Heart Block (2+2)

• Partial

  • Regular

  • Irregular

• Complete

Partial Heart Block

Some impulses fail to pass

Regular Partial Heart Block

Fixed ratio between atrial and ventricular rhythms

Irregular Partial Heart Block

NO fixed ratio between atrial and ventricular rhythms

Complete Heart Block

All atrial impulses fail to reach the ventricles, causing uncoordinated atrial and ventricular rhythms

Heart Block Classification (3)

• 1st Degree: Delay in conduction

• 2nd Degree: Partial block

• 3rd Degree: Complete block