The control of heart rate (SNAB)

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Control of Heart Rate
The heart can beat without any input from the nervous system ­ it is myogenic ­ it can contract without external nervous stimulation.
Contraction of cardiac muscle is initiated by small changes in the electrical charge of cardiac muscle cells ­ a change in polarity spreads like a
wave from cell to cell, and causes the cells to contract.
1. Depolarisation starts at the sinoatrial node (SAN) (a small area of specialised muscle fibres located in the wall of the right atrium.) It
generates an electrical impulse which spreads across the right and left atrium, causing them to contract at the same time.
2. The impulses pass to the ventricles via the atrioventricular node (AVN). Here the impulse is slightly delayed so that the atria have
finished contracting, and that the ventricles have filled with blood before they contract.
3. After the delay, the impulse reaches the Purkinje fibres. These are large, specialised muscle fibres that conduct impulses rapidly to the
apex of the ventricles. There are right and left bundles of fibres, they are collectively called the bundle of His.
4. The impulse spread up through the ventricle walls causing contraction from the apex upwards (the cells at the apex are the first to be
depolarised.) Blood is squeezed into the arteries.
Measuring electrical activity
The electrical activity of the heart can be detected and displayed on an electrocardiogram (ECG), a graphic record of the electrical activity
during the cardiac cycle.
Electrodes are attached to the person's chest and limbs to record the electrical currents produced during the cardiac cycle. When there is a
change in polarisation of the cardiac muscle, a small electrical current can be detected at the skins surface.
If disease disrupts the heart's normal conduction pathways changes will occur in the ECG pattern which can be used for diagnosis of
cardiovascular disease.

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