Cardiac Physiology Quiz

Cardiac Excitability: Heart Rate and ECG

• Underlying Reason for Heart Contraction: The lecture discusses the mechanisms behind heart cell contraction, focusing on the cause, effect, and how this information is used to study the heart.

Two Types of Cardiac Action Potentials

• Type 1: Non-pacemaker Cells (Myocytes):

  • These cells exhibit "fast response" action potentials, characterized by rapid depolarization in response to an action potential (AP).
  • They're considered "soldiers," needing instructions to fire.
  • These cells make up most of the atrial and ventricular muscle walls.
  • Action potential characteristics are shown graphically, highlighting differences compared to nerve cells.

• Type 2: Pacemaker (Autorhythmic) Cells:

  • Possess unstable resting potentials, which causes spontaneous firing.
  • These "generals" provide firing instructions to the muscular "soldiers".
  • Found in the sinoatrial (SA) and atrioventricular (AV) nodes.
  • The graph showcases the various ion movements that generate action potentials in these cells.

Action Potentials in Cardiac Autorhythmic Cells

• Funny Current Channels (If):
  • Funny current channels (If) contribute to the unstable resting potential.
  • These channels allow both potassium (K⁺) and sodium (Na⁺) permeability.
  • Depolarization happens as pacemaker potential progressively becomes less negative until it reaches a threshold, initiating the action potential.
  • Ion movements during action and pacemaker potentials are visually presented.

Role of Na⁺ and Ca²⁺

• Cardiac Muscle (Non-pacemaker):

  • Sodium (Na⁺) channels are responsible for rapid depolarization.
  • Calcium (Ca²⁺) influx prolongs the action potential and produces the plateau phase.
  • Calcium is involved in the initial depolarization phase.

• Cardiac Pacemaker Cells:

  • A slow depolarizing pacemaker potential (dependent on funny current) facilitates autorhythmic function.

Electrical Conduction to Myocardial Cells

• Autorhythmic Signal Propagation:
  • Depolarizations originating from autorhythmic cells rapidly spread to adjacent contractile cells via gap junctions.

Intrinsic Conduction System

• Autorhythmic Cells: All cells in the intrinsic conduction system are autorhythmic, generating spontaneous action potentials.
• Locations (Wiring): The system includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, right and left bundle branches, and Purkinje fibers, visualized in anatomical diagrams.

Nodes (Control Points)

• SA Node: Sets the heart rate at approximately 70 bpm.
• AV Node: Routes electrical signals, delays transmission of action potentials. Additional nodes (AV node, Purkinje fibers) can act as pacemakers under specific conditions. Specified ranges of beats per minute (bpm) are also noted.

Conduction Fibres

• Conduction Fibres: The specialized conducting fibres are sheathed, except in atrial and ventricular myocyte contact regions.
• Fibrous Barrier: Atrial and ventricular myocyte syncytia are separated by inert fibrous tissue, lacking gap junctions that prevent unwanted propagation between atria and ventricles.

Heart Rate Control (Sympathetic and Parasympathetic Nerves)

• Both sympathetic and parasympathetic nervous systems regulate heart rate.

• Parasympathetic (Vagus Nerve): Lowers heart rate by releasing acetylcholine (ACh) onto muscarinic receptors (M2Rs). Resting heart rate is approximated at 60–80 beats/min.

• Sympathetic Nerves: Increase heart rate by releasing norepinephrine (NE). Catecholamines from the adrenal gland boost this response.

• Specific ion channel activity is involved in the respective responses.

  • Parasympathetic activation triggers a decrease in cAMP levels, lowering activity of HCN and Ca2+ channels.
  • Sympathetic activation increases cAMP, upregulating Ca2+ and HCN (funny current) channel activity.

Modulation of Heart Rate by the Autonomic Nervous System

• Graphs illustrate changes in membrane potential in response to sympathetic and parasympathetic stimulation.

Review Question

• One of the presented options, regarding autorhythmic cells, is the correct answer. The answer is included in the previous notes.

Control of Heart Rate vs Contraction Strength

• Vagus Nerves (Parasympathetic): Regulate heart rate but not contraction strength. They only innervate SA and AV nodes.
• Sympathetic Fibers: Effect both heart rate and contraction strength, also innervating the atria and ventricles.

Additional Discussion Points

• ECG (Electrocardiogram): Study of heart anatomy and electrical properties aid in better understanding health and diseases.
• ECG Wave Relationships and Correlates with Electrical Activity: Explain the correlation between ECG tracings and electrical events within the heart's conduction cycle (e.g., P wave, QRS complex, T wave).
• ECG Analysis Tips: Analysis and interpretation of ECG tracings involve factors such as heart rate, rhythm regularity, presence of normal waves, and comparing P-R and R-R intervals for uniformity.