Control of heart rate (A-level only)
AQA
A-Level
Cardiac muscle is myogenic: it contracts rhythmically without nerve impulses. The intrinsic rhythm originates in the sinoatrial node (SAN) in the right atrium, which depolarises spontaneously to set the basic heart rate. A wave of depolarisation spreads across both atria, causing atrial systole. Cardiac muscle cells are electrically coupled via gap junctions, allowing ions to pass directly. A band of non-conducting connective tissue between the atria and ventricles prevents the wave passing straight down. Consequently, ventricular contraction only begins after the wave is routed through the atrioventricular node (AVN).
9
Objectives
7
Exam Tips
11
Pitfalls
18
Key Terms
16
Mark Points
Subtopics in this area
Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity.
The roles of the sinoatrial node (SAN), atrioventricular node (AVN) and Purkyne tissue in the bundle of His.
The roles and locations of chemoreceptors and pressure receptors and the roles of the autonomic nervous system and effectors in controlling heart rate.
Control of heart rate (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Define myogenic and use it to explain why a heart removed from the body, or a transplanted heart, continues to beat.
- Trace the wave of depolarisation from the SAN across both atria and explain why the atria contract before the ventricles.
- Explain why a band of non-conducting tissue between the atria and the ventricles is essential for efficient pumping.
- Sequence the route of the wave of depolarisation from SAN to AVN to bundle of His to Purkyne tissue, stating what each structure does.
- Explain why the delay at the AVN is essential, referring to ventricular filling and atrial emptying.
- Explain why losing Purkyne tissue, or the gap junction proteins between cardiac muscle cells, would weaken ventricular emptying.
- Trace the full pathway from chemoreceptor to SAN for a rise in blood carbon dioxide, naming the division of the autonomic nervous system used.
- Predict how sympathetic and parasympathetic impulse frequency change when blood pressure rises, and justify the prediction.
- Explain why exercise raises heart rate through carbon dioxide detection rather than through oxygen detection.
Marking Points
Key points examiners look for in your answers
- Cardiac muscle is myogenic, contracting without stimulation by nerve impulses.
- The sinoatrial node acts as the pacemaker, originating the wave of electrical activity.
- A wave of depolarisation spreads across both atria, stimulating them to contract.
- Non-conducting tissue between atria and ventricles prevents the wave passing directly downwards.
- The autonomic nervous system alters the rate of the heartbeat rather than initiating it.
- The SAN initiates the wave of depolarisation, causing the atria to contract.
- The AVN delays the wave, allowing the atria to empty and ventricles to fill before ventricular systole.
- The bundle of His conducts the wave down the septum to the apex of the heart.
- Purkyne tissue carries the wave up the ventricle walls, causing contraction from the apex upwards.
- Gap junctions allow ions to move between cardiac muscle cells, distinct from the main conducting tissue.
- An increase in carbon dioxide concentration, or a fall in pH, is detected by chemoreceptors in the aortic and carotid bodies.
- Impulses are sent to the cardiac or cardiovascular centre in the medulla oblongata.
- More impulses along the sympathetic neurones, or fewer impulses along the parasympathetic or vagus neurones, change the frequency of impulses reaching the SAN.
- The impulses reach the SAN as the effector, which sets the heart rate.
- Pressure receptors (baroreceptors) in the aortic arch and carotid sinus detect a change in blood pressure and produce the opposite response: high pressure increases parasympathetic activity, low pressure increases sympathetic activity.
- For a slowing response, mirror the chain exactly: receptor, medulla, parasympathetic or vagus, SAN.
Examiner Tips
Expert advice for maximising your marks
- 💡Define myogenic clearly as contracting without nerve stimulation to secure foundational marks.
- 💡Use precise terminology such as 'wave of depolarisation' and avoid vague terms like 'signal'.
- 💡Learn the pathway as distinct steps: SAN, atria, AVN delay, bundle of His, and Purkyne tissue.
- 💡Always explain the purpose of the AVN delay by explicitly mentioning atrial emptying and ventricular filling.
- 💡Write 'more impulses along the sympathetic' or 'fewer along the parasympathetic'; the mark is on frequency, not on the nerve firing.
- 💡Always finish the chain at the SAN, because it is a separate mark and is the one most often left out.
- 💡For a slowing response, mirror the chain exactly: receptor, medulla, parasympathetic or vagus, SAN.
Common Mistakes
Pitfalls to avoid in your exam answers
- Using vague terms like 'message' or 'signal' instead of 'wave of depolarisation' or 'impulses'.
- Saying the brain initiates each heartbeat, rather than modifying the baseline myogenic rate.
- Describing the wave as travelling along nerves inside the heart rather than passing between muscle cells.
- Using vague terms like 'signals' or 'messages' instead of 'impulses' or 'waves of depolarisation'.
- Writing that the AVN speeds the impulse up, when its crucial function is to introduce a delay.
- Saying the ventricles contract from the top downwards, which would trap blood at the apex.
- Saying receptors detect a fall in oxygen, when the credited stimulus is a rise in carbon dioxide or a fall in pH.
- Writing about 'an impulse' or 'one impulse' when describing sympathetic or parasympathetic changes; the change is in the frequency of impulses, not the presence or absence of a single impulse.
- Using 'signals' or 'messages' in place of 'impulses' or 'action potentials'.
- Describing a hormonal route through adrenaline when the question asks for the nervous route through the medulla.
- Sending the sympathetic neurones to the AVN or to the ventricle walls rather than to the SAN.