Control of heart rate - AL only (3.6.1.3)
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Cardiac muscle is myogenic, meaning that it contracts on its own without nervous stimulation. The heartbeat is initiated by electrical activity within the heart muscle itself.
Small changes in the electrical charge (polarity) of cardiac muscle cells spread as a wave of electrical excitation from cell to cell, causing them to contract in a coordinated sequence that produces the rhythmic heartbeat.
The coordination of a heartbeat is a multistep process:
- The Sinoatrial node (SAN) initiates an electrical impulse, causing a wave of depolarisation over the atria (atrial systole).
- The atrioventricular node (AVN) receives the impulse and delays its spread to allow the ventricles to fill with blood.
- The impulse travels down the bundle of His and reaches the apex of the ventricles.
- Purkyne fibres conduct the impulse triggering ventricular contraction (ventricular systole) from the apex upwards, forcing blood into the pulmonary artery and aorta.
The medulla oblongata in the brain controls heart rate via the autonomic nervous system:
- The sympathetic nervous system sends impulses through sympathetic neurones that release noradrenaline at the Sinoatrial node (SAN), increasing heart rate.
- The parasympathetic nervous system sends impulses through parasympathetic neurones that release acetylcholine at the SAN, decreasing heart rate.
The SAN acts as the effector, adjusting heart rate in response to these signals.
Chemoreceptors in the carotid arteries and aorta detect changes in blood chemistry. These chemoreceptors send impulses to the medulla oblongata, which adjusts heart rate accordingly.
If rises and drops, the medulla oblongata increases impulses via the sympathetic nervous system to the SAN, raising heart rate to boost blood flow to the lungs and remove excess
Conversely, if falls and rises, the medulla oblongata increases impulses to the SAN via the parasympathetic nervous system, slowing heart rate back to resting levels.
Baroreceptors (pressure receptors) are stretch-sensitive receptors located in the walls of the carotid arteries and aorta that detect changes in arterial blood pressure. They send impulses to the medulla oblongata to adjust heart rate.
When blood pressure is high, the medulla oblongata increases impulses via the parasympathetic nervous system to the SAN, decreasing heart rate and allowing blood pressure to fall.
When blood pressure is low, the medulla increases impulses via the sympathetic nervous system to the SAN, increasing heart rate and raising blood pressure back to normal.