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The heart can initiate its own electrical activity, while the nervous system adjusts the rate. Separate the origin of a beat from the feedback that modifies it.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Trace SAN, AVN and ventricular conduction
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Myogenic means the heartbeat is initiated:
Within cardiac tissue. The SAN is part of the intrinsic rhythm system.
After watching 2: The AVN delay helps ensure:
Atrial activation precedes ventricular contraction appropriately. Timing coordinates pumping.
What you'll be able to do
Cardiac muscle is myogenic: its rhythmic activation originates within the heart rather than requiring a motor-nerve impulse for every beat. The sinoatrial node, SAN, initiates a wave of excitation across the atria, promoting atrial contraction.
Insulating tissue limits direct spread to ventricles, so excitation is relayed through the atrioventricular node, AVN. A delay allows filling before signals pass through the bundle of His and Purkyne tissue, coordinating ventricular contraction from the lower region upwards towards the outflow vessels. This conduction sequence supports efficient pumping.
The medulla receives receptor information and influences autonomic pathways to the SAN. Sympathetic activity generally increases heart rate, while parasympathetic activity via the vagus generally decreases it. These inputs modify the pacemaker’s activity rather than replacing the myogenic origin of every heartbeat.
Changes in heart rate can alter cardiac output together with stroke volume. The same rate does not guarantee the same output if stroke volume differs. Identify whether a question concerns electrical initiation, contraction strength or the total volume pumped.
Chemoreceptors associated with locations including the carotid and aortic regions monitor relevant blood-chemistry changes; carbon dioxide and pH can indicate altered metabolic demand. Information reaching the cardiovascular centre can increase sympathetic influence during increased demand, raising the delivery of oxygen and removal of carbon dioxide.
Pressure receptors in major arterial regions including the carotid sinus and aortic arch provide information about arterial stretch. A rise in pressure can increase signalling that promotes a rate-lowering response, helping oppose the disturbance. Give a complete feedback chain from detected change to nervous pathway to effector response rather than saying blood “tells the heart to beat”.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.