3.6.1.3Responses and HomeostasisCore

Myogenic Rhythm and Control of Heart Rate

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.

35 min Nervous and Muscular Coordination

What you'll be able to do

Trace SAN, AVN and ventricular conduction
Explain autonomic adjustment of the pacemaker
Use receptor information to predict rate changes

Coordinate the heartbeat electrically

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.

Adjust the pacemaker through autonomic input

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.

Explain receptor-based feedback

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”.

Apply the biology

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.

Example 1Explain the functional importance of the AVN delay.

1It separates atrial and ventricular activation in time.

2The atria can complete their contribution to ventricular filling before ventricular systole.

3This improves coordination of blood movement rather than making all chambers contract simultaneously.

AnswerLink conduction timing to filling and ejection.

Example 2Heart rate rises from 60 to 90 min⁻¹ while stroke volume remains 70 cm³. Calculate the output change.

1Initial output =60×70=4200 cm³ min⁻¹.

2Final output =90×70=6300 cm³ min⁻¹.

3Increase =2100 cm³ min⁻¹, or 50%.

AnswerOutput rises from 4.2 to 6.3 dm³ min⁻¹ in this fixed-stroke-volume model.

Common mistakes to avoid

Saying autonomic nerves are required to initiate each normal heartbeat.

Distinguish myogenic generation from modulation.

Inferring cardiac output from heart rate without considering stroke volume.

Use both terms in the output equation.

Key takeaways

Intrinsic conduction coordinates chamber activation.
Autonomic input modifies the pacemaker.
Receptor feedback relates circulation to pressure and metabolic conditions.

Test yourself

Answer a few questions, then review anything you missed.

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