Electrophysiology of the heart

• What are the cells in the myocardium called

  Cardiomyocytes
• What is fibrillation

  Unregulated contractions
• What is myogenic contractions of the heart

  Spontaneous contractions of the myocardium
• What does cardiac depolarisation result in

  Contraction
• What are the features of the myocardium

  Cardiomyocytes are mononucleated.

  They are striated.

  They are branched.

  They are separated by intercalated discs made of gap junctions.
• What is the role of the gap junctions in the myocardium

  Allows for rapid transmission of ions and action potentials.
• Why are cardiomyocytes branched

  Increases their connectivity.
• What is the result of the features of the myocardium

  It means that is functions as a synctium [as one unit].
• Label the myocardium.

  Desmosomes hold the cells together.

  A) Intercalated discs

  B) Gap junctions

  
• How many syncytia exist in the heart

  2. The atrium and ventricles.
• What is the cardiac resting potential

  -90 mv
• What effect does hypoxia have on cardiac action potentials

  The resting potential is less negative. This inactivates some sodium channels, resulting in arrhythmias.
• What is an arrhythmia

  An abnormal heart beat
• What is the main difference between cardiac Action potentials and neural ones

  Cardiac action potentials have an extended depolarisation and repolarisation period. Which results in an extended refractory period.
• What is the result of the extended refractory period in heart cells


  Reduces the risk of tetany.
• What is tetany

  Repeated stimulation and contraction
• What is the main feature of cardiac action potentials

  They are slow
• What is the role of the extended refractory period

  To allow for filling of the ventricles between contractions.
• Describe the cardiac action potential

  1. Fast sodium channels open [Rapid depolarisation]
  2. Slow calcium channels open
  3. Some potassium channels open
  4. Calcium channels close
  5. Potassium channels open

  A) Rapid depolarisation

  B) Maintained depolarisation

  C) Repolarisation

  D) Depolarisation

  E) Repolarisation

  F) Refractory period

  G) Contraction

  
• What stimulates contraction

  The action potential allows calcium to enter the cell, stimulating excitation contraction coupling.
• Label the control of the heart rate

  The sinoatrial node is the pacemaker.

  The Bundle of His descends through the septum.

  The Purkinje fibres extend through the ventricular walls.

  A) Sinoatrial node

  B) Atrioventricular node

  C) Bundle of His

  D) Purkinje fibers

  E) Sympathetic

  F) Parasympathetic

  
• Describe the cardiac conduction system

  1. The SAN produces regular impulses
  2. This stimulates atria contraction
  3. The annulus fibrosus prevents the depolarisation from spreading to the ventricles
  4. AV node passes depolarisation to the Bundle of His
  5. Purkinje fibres stimulate ventricular contraction
• Describe SAN depolarisation

  Spontaneous and slow. This is termed the pacemaker potential and is due to funny channels. This causes the resting membrane potential to decrease. Once the threshold potential is reached an action potential is stimulated.
• Describe AVN depolarization

  They depolarise spontaneously but slower than the SAN. This means that the SAN is able to trigger them before they have fully depolarised.
• How does the pacemaker potential arise

  The pacemaker potential is mediated by the T-channels and funny channels [Na+ and Ca 2+].

  1. Funny channels allow sodium to enter
  2. T-channels allow calcium to enter
  3. L-channels allow calcium to enter
  4. Potassium enters and repolarises the cell

  A) Funny channels open

  B) T-channels open

  C) Pacemaker potential

  D) L-channels open

  E) Potassium channels open

  
• Describe the neural control of the heart rate

  Baroreceptors and chemoreceptors send information to the medulla oblongata.

  This triggers the autonomic nervous system.
• What effect does muscarinic ACH have on the SA node

  Decreases the SA node firing.
• What effect to Beta-adrenergic receptors have on the SA node

  Increase the firing
• What binds to beta-adrenergic receptors

  Catecholamines
• What effect does Hyperkalaemia have on SA node firing

  Increases
• What effect does hyperthyroidism have on SA firing

  Increases
• What effect does hyperthermia have on SA node firing

  Increases
• What is tachycardia

  When the heart rate is above 100 bpm.
• What is bracycardia 


  When the heart rate is below 60 bpm.
• How are problems with the heart rate treated

  Defibrillation to reset the heart rhythm.
• Label the electrocardiogram

  The P wave represents atrial depolarisation.

  The QRS complex represents ventricular depolarisation.

  The ST segment represents ventricular systole.

  The T wave represents ventricular repolarisation.

  The relaxation wave represents ventricular filling.

  A) P wave

  B) QRS complex

  C) T wave

  D) Relaxation wave

  
• What is an ECG

  An electrocardiogram. It shows the electrical signals through the heart so is used to visualise action potentials.