Overview
This lecture explains the heart's intrinsic conduction system, focusing on autorhythmic cells, the sequence of electrical signaling for contraction, and introduces how to interpret EKG waveforms.
Cardiac Muscle Cell Types
- The heart consists of autorhythmic cells (1%) and non-autorhythmic cells (99%).
- Autorhythmic cells form the intrinsic conduction system, generating and conducting electrical signals.
- Non-autorhythmic cells are responsible for physical contraction (contain actin and myosin).
Components of the Intrinsic Conduction System
- Autorhythmic cells cluster in specific regions: SA node, AV node, AV bundle (bundle of His), bundle branches, and Purkinje fibers.
- The SA node (right atrium) is the pacemaker, initiating signals ~75 times per minute.
- The signal travels quickly through the atria, causing both to contract simultaneously.
- The AV node slows conduction, allowing atria to finish contracting before ventricles do.
- After the AV node, the signal moves rapidly through the AV bundle, bundle branches, and Purkinje fibers, prompting ventricles to contract together.
Damage to the Conduction System
- If the SA node fails, the AV node can take over as pacemaker (~50 beats/min).
- Damage to the AV node results in the AV bundle taking over (~30 beats/min), causing uncoordinated atrial and ventricular contractions.
- This lack of coordination is called heart block.
Introduction to EKG (Electrocardiogram)
- An EKG measures the heart's electrical activity, not contraction or valve sounds.
- The EKG waveform has three main parts: P wave (atrial depolarization), QRS complex (ventricular depolarization and atrial repolarization), and T wave (ventricular repolarization).
- The duration and shape of these waves can indicate normal or damaged cardiac tissue.
- Heart rate can be calculated by measuring intervals on the EKG.
Key Terms & Definitions
- Autorhythmic cells — self-rhythmic cells generating the heart's electrical signals.
- Intrinsic conduction system — network of autorhythmic cells controlling heart rhythm.
- SA node — primary pacemaker of the heart located in the right atrium.
- AV node — slows signal conduction, allowing atrial contraction before ventricular contraction.
- AV bundle (bundle of His) — pathway for signals from AV node to ventricles.
- Bundle branches — split pathways carrying signals to both ventricles.
- Purkinje fibers — conduct signals rapidly throughout ventricles.
- Depolarization — electrical activation of heart muscle cells.
- Repolarization — recovery phase of heart muscle cells.
- Heart block — loss of coordination between atrial and ventricular contractions due to conduction pathway damage.
- EKG/ECG — a record of heart's electrical activity.
Action Items / Next Steps
- Complete the EKG interpretation activity provided for this week.