Overview
This lecture covers the cardiac conduction (nodal) system, its key components, how electrical signals regulate heartbeats, and factors influencing heart rate and blood pressure, ending with basics of EKG interpretation.
Cardiac Conduction System Overview
- The cardiac conduction system (nodal system) coordinates heartbeats to ensure all parts contract in unison.
- Specialized muscle tissue, not nerve, conducts depolarization waves throughout the myocardium.
- Key components include the sinoatrial (SA) node, junctional fibers, atrioventricular (AV) node, AV bundle (bundle of His), bundle branches, and Purkinje fibers.
SA Node and Cardiac Pacing
- The SA node, called the "pacemaker," initiates each heartbeat by depolarizing fastest (about 60-80 beats per minute).
- It is located in the right atrial wall near the superior vena cava.
- Heart rate can vary due to sympathetic stimulation (fight-or-flight) or parasympathetic (vagus nerve) regulation.
Signal Pathway Through the Heart
- The SA node sends signals to both atria via junctional fibers, causing simultaneous contraction.
- The AV node, located in the lower right atrium, delays the signal, allowing the atria to contract before the ventricles.
- The AV bundle forks into bundle branches to ensure both ventricles receive the contraction signal.
- Purkinje fibers carry the signal from the apex upward, producing efficient ventricular contraction.
Alternate Pacemakers and Heart Efficiency
- If the SA node fails, the AV node can act as a backup pacemaker, but with reduced heart efficiency (~30% loss).
- Artificial pacemakers may be surgically implanted when nodal function is inadequate.
Regulation of Heart Rate and Blood Pressure
- Heart rate and contraction are influenced by exercise, temperature, and ion (calcium and potassium) concentrations.
- Hypercalcemia: prolonged contraction; hypocalcemia: weak contraction.
- Hyperkalemia: weaker contractions; hypokalemia: possible cardiac failure.
- Sympathetic stimulation increases heart rate and contraction; parasympathetic (vagus nerve) slows it down.
Blood Pressure Control and Baroreceptor Reflex
- Baroreceptors in the carotid artery and aorta detect blood pressure changes and regulate heart rate via the medulla oblongata.
- Vasoconstriction increases blood pressure; vasodilation decreases it.
EKG/ECG (Electrocardiogram) Basics
- EKG measures the heart's electrical activity, not the mechanical contraction.
- P wave: atrial depolarization (atria contract).
- QRS complex: ventricular depolarization (ventricles contract).
- T wave: ventricular repolarization (ventricles relax).
- Atrial repolarization is hidden by the QRS complex.
Key Terms & Definitions
- Depolarization — decrease in cell negativity, initiating muscle contraction.
- SA node — sinoatrial node, primary pacemaker of the heart.
- AV node — atrioventricular node, delays and relays signal to ventricles.
- Purkinje fibers — fibers spreading the impulse through ventricles.
- Baroreceptor — pressure sensor regulating blood pressure.
- EKG/ECG — test measuring electrical activity of the heart.
Action Items / Next Steps
- Know the order and function of each conduction system component.
- Memorize what the P wave, QRS complex, and T wave represent on an EKG.
- Be able to explain effects of calcium and potassium imbalances on cardiac function.