Cardiac Muscle Physiology and Conducting System

Cardiac Muscle Physiology and Conducting System

Review of Skeletal Muscle Contraction

• Action potential from the nervous system releases acetylcholine to the skeletal muscle fiber.
• Acetylcholine opens Na^+ gates, allowing the sarcolemma to reach threshold.
• The action potential travels down the sarcolemma into the T-tubule, causing the release of Ca^{++}.
• Ca^{++} triggers the contraction.

Generation of Action Potentials in the Myocardium

• Cardiac muscle spontaneously depolarizes.
• It doesn't require a nervous system signal to open Na^+ gates.
• Na^+ gates are leaky, allowing gradual influx and depolarization to threshold, triggering an action potential.
• This process occurs in specialized cells within the myocardium, part of the heart's conduction system.
• The heart can beat independently of the brain.

The Conducting System

• Two types of cardiac muscle cells:
  • Conducting system: Controls and coordinates the heartbeat.
  • Contractile cells: Produce contractions that propel blood.
• Specialized cardiac muscle cells initiate and distribute electrical impulses, stimulating contraction.
• Automaticity/Autorhythmicity: Cardiac muscle tissue contracts automatically.

Components of the Conducting System

• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Internodal pathways
• AV bundle
• Bundle branches
• Purkinje fibers

Structures of the Conducting System

• Sinoatrial (SA) node: Located in the wall of the right atrium.
• Atrioventricular (AV) node: Located at the junction between atria and ventricles.
• Conducting cells: Found throughout the myocardium.

Conducting Cells

• Interconnect the SA and AV nodes.
• Distribute the stimulus through the myocardium.
  • Internodal pathways: in the atria.
  • AV bundle and the bundle branches: in the ventricles.

Prepotential Signal

• Also called pacemaker potential.
• Resting potential of conducting cells gradually depolarizes toward threshold.
• SA node depolarizes first, establishing the heart rate.

Establishment of Heart Rate

• Heart Rate
  • SA node generates 80–100 action potentials per minute (intrinsic regulation).
  • AV node generates 40–60 action potentials per minute.
  • Nervous and endocrine systems activate to speed up or slow down heart rate from the pacemaker rate.

Steps in the Conduction System

1. Sinoatrial (SA) Node
   • Located in the posterior wall of the right atrium.
   • Contains pacemaker cells.
   • Connected to the AV node by internodal pathways.
   • Begins atrial activation (depolarization).
2. Atrioventricular (AV) Node
   • Located in the floor of the right atrium.
   • Receives impulse from the SA node.
   • Delays the impulse.
   • Atrial contraction (systole) begins.
   • Ventricles are relaxed (diastole).
3. The AV Bundle
   • In the septum
   • Carries impulse to the left and right bundle branches
     • Which conduct to Purkinje fibers
   • And to the moderator band
     • Which conducts to papillary muscles
4. Purkinje Fibers
   • Distribute the impulse through the ventricles.
   • Atrial contraction is completed (diastole).
   • Ventricular contraction begins (systole).

The Cardiac Cycle

• Begins with an action potential at the SA node.
• Transmitted through the conducting system.
• Produces action potentials in cardiac muscle cells (contractile cells).
• Electrocardiogram (ECG or EKG):
  • Records electrical events in the cardiac cycle.
  • Electrical events are the depolarizing and repolarizing of the atria and ventricles.

Electrocardiogram (ECG or EKG)

• A recording of electrical events in the heart.
• Four major electrical events:
  • Atria depolarize.
  • Atria repolarize.
  • Ventricles depolarize.
  • Ventricles repolarize.
• Obtained by electrodes placed at specific body locations.
• Abnormal patterns diagnose damage.

EKG/ECG Waves

• P wave: Atria depolarize.
• QRS complex: Ventricles depolarize; atria repolarize (hidden).
• T wave: Ventricles repolarize.

Time Intervals Between ECG Waves

• P–R interval: From the start of atrial depolarization to the start of the QRS complex; atria are contracting at the end.
• Q–T interval: From ventricular depolarization to ventricular repolarization; ventricle contracting before the T.

Abnormal Pacemaker Function

• Bradycardia: Abnormally slow heart rate.
• Tachycardia: Abnormally fast heart rate.
• Ectopic pacemaker:
  • Abnormal cells.
  • Generate a high rate of action potentials.
  • Bypass the conducting system.
  • Disrupt ventricular contractions.

ECG/EKG Patterns

• Paroxysmal Atrial Tachycardia (PAT)
• Atrial Fibrillation (AF)
• Ventricular Tachycardia (VT)
• Ventricular Fibrillation (VF)