Skip to Main Content

Human Anatomy & Physiology Laboratory OER

Materials to accompany KINS 2511 and KINS 2512 Human Anatomy and Physiology labs.


The Electrical Conduction System of the Heart

  1. Explain how the electrical conduction system functions.
  2. List the parts of the electrical conduction system of the heart in the correct sequence for one contraction.
  3. Explain why the SA node normally paces the heart.
  4. Explain how the cardiac conduction system produces coordinated heart chamber contractions. 
  5. Name the waveforms in a normal electrocardiogram (ECG or EKG) and explain the electrical events represented by each waveform.



The functioning of the conduction system of the heart

The components of the cardiac conduction system include (in correct sequence) the sinoatrial (SA) node, the atrioventricular (AV) node, the atrioventricular (AV) bundle, the atrioventricular (AV) bundle branches, and the Purkinje fibers. Cardiac pacemaker cells that form this system do not have a stable resting membrane potential like skeletal muscles or neurons.

  • They spontaneously fire at regular intervals and self-generate an action potential (electrical signal). Their membrane potential starts at about -60 mV and drifts upward. This upward drift is the depolarization phase and it results from a slow inflow of sodium ions without a compensating outflow of potassium ions. In repolarization, the inflow of sodium ions stops and potassium ions exit the cell to return the cell to the original membrane potential (review these electrical changes discussed in API, if needed). It is very important to understand that cardiac pacemaker cells do not need external stimulation by nerve cells to generate action potentials. They will "fire" even if all the nerves to the heart are cut and keep the heart contracting. 

The signal from the pacemaker cells is then transmitted to contractile myocardial cells as these cells are stimulated to generate an action potential. 

  • These cells have a resting membrane potential and the action potentials generated by contractile myocardial cells are similar to those in skeletal muscle and neurons. As these cells depolarize, they contract and pump blood into circulation. As they repolarize, they relax and fill with blood. 

All components of the conduction system of the heart function similarly. However, each individual component has certain unique characteristics. For example, the SA node is considered the pacemaker of the heart because it initiates the heart rhythm known as the sinus rhythm, while the Purkinje fibers have the fastest speed of signal conduction. You can watch Conducting the Signal to see a visual for the conduction of the electric signal. 

The Electrocardiogram (ECG or EKG)
We can detect electrical currents in the heart by means of electrodes (leads) applied to the skin. An instrument called the electrocardiograph amplifies these signals and produces a record, usually on a moving paper chart, called an electrocardiogram (ECG), also commonly abbreviated as EKG (K coming from "kardiology", from the German term for cardiology). To record an ECG, electrodes are typically attached to the wrists, ankles, and six locations on the chest. Simultaneous recordings can be made from electrodes at different distances from the heart; collectively, they provide a comprehensive image of the heart's electrical activity. An ECG is a composite recording of ALL action potentials produced by the nodal and myocardial cells—it should not be construed as a tracing of a single action potential. The EKG has very important diagnostic value: it can help us diagnose problems in the conduction system, heart attacks, heart enlargement and hormone/electrolyte imbalances. In a normal ECG tracing each component, segment, and interval is labeled and corresponds to important electrical events, demonstrating the relationship between these events and the cardiac cycle (contraction and relaxation of the heart).

The ECG is made up of a straight baseline and waves. The waves can either move over or under the baseline. 
A segment is the area between two waves. 
An interval is a straight line and one or more waves. 
A complex is more than one wave appearing in succession of one another. 

ECG Component Duration in Seconds Corresponding Significance
P wave 0.06-0.11 Depolarization of atrial fibers at SA node
P-R interval 0.12-0.20 Time from beginning of depolarization of atrial fibers to beginning of depolarization of the ventricles
P-Q segment 0.08 Time for cardiac impulse to pass through the AV node
QRS complex < 0.12 Depolarization of ventricular fibers
S-T segment 0.12 Time for ventricles to contract
Q-T interval 0.36-0.44 Time from ventricular depolarization to end of ventricular repolarization
T wave 0.16 Repolarization of ventricular fibers (ends pattern)
T-P segment <0.50 Time of relaxation and filling of the heart between beats


McGraw Hill Connect Online Lab 1: EKG