Q3 (10 pts) The largest signal from an electrocardiogram reflects action potenti
ID: 202753 • Letter: Q
Question
Q3 (10 pts) The largest signal from an electrocardiogram reflects action potential propagation through the ventricular muscle, which is the largest structure of the heart. Ventricular conduction is seen as the "QRS" phase of the recording. With properly positioned skin surface electrodes, QRS is a sharp positive voltage declining rapidly to a negative voltage before returning to baseline. Based on what you know about extracellular recording of an action potential traveling down an axon, offer a reasonable explanation for the QRS waveform.Explanation / Answer
In case of neurons, action potential traveling down an axon follows 4 distinct segments:
1) Membrane potential rises rapidly due to rapid depolarization 2) Sodium channels open, causing Sodium ions to enter cytoplasm, reversing the inner membrane charge 3) Inactivation of sodium channels (close) and opening of Potassium channels, allowing cytosolic potassium ions to exit, reverting the membrane charge and initiating repolarization 4) Return to resting membrane potential by closing of potassium channels
In electrocardiogram (ECG), leads placed on skin trace the electrical activity of heart beats, where the heart is depolarized with each beat, to trigger its contraction. The wave of depolarization moves from sinoatrial node (SAN) to atrioventricular node (AVN) and then to the bundle of His. From there it travels to the bundle branches and Purkinje fibres. Then the wave of depolarization is distributed to the ventricular walls. The QRS wave represents depolarization and activation of the left and right ventricles. The large R wave reflects the electrical stimulus as it passes through the ventricle walls, and due to the thick ventricle walls, more voltage is required for ventricular contraction. The small S wave returning the voltage to the baseline represents the continuation of the depolarisation in the Purkinje fibres.
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