International A Level Biology / Muscles, movement and the heart

The myogenic heartbeat: SAN, AVN, bundle of His

Edexcel International A Level Biology (YBI11), chapter 21: Muscles, movement and the heart, specification point 7.12. Free lesson.

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The heartbeat is myogenic, so each beat starts in the heart muscle itself. The conducting system then coordinates atrial and ventricular contractions so the ventricles fill before pumping blood out.

What myogenic means

Cardiac muscle is myogenic: it can contract and relax on its own, without a nerve impulse starting each beat. This is why a transplanted heart can beat even when its nerves have not been reconnected. Nerves from the brain can change the heart rate, but they do not start each beat. In an answer, write that cardiac muscle is myogenic, rather than only saying that the heart is myogenic.

How excitation travels through the heart

The heartbeat follows one route in a fixed order. The SAN starts the wave of excitation, the AVN delays it, and the bundle of His and Purkyne fibres carry it through the ventricles. The sinoatrial node (SAN) is a small patch of tissue in the wall of the right atrium. It is the heart's pacemaker. The SAN depolarises. This means the charge across its cell membranes reverses, setting off a wave of excitation. The wave spreads across both atria, causing them to contract. This is atrial systole, which pushes blood into the ventricles. A layer of non-conducting tissue…

Why the AVN delays the excitation

The delay gives the atria time to finish contracting and empty into the ventricles before the ventricles contract. Without the delay, the two sets of chambers could contract together, so the ventricles would not fill properly. The non-conducting tissue prevents the atria and ventricles contracting together by blocking the direct route. If this tissue is damaged and excitation passes straight from the atria to the ventricles, the ventricles contract before the atria have emptied. The ventricles are then less full, so they pump less blood out per beat.

Why the ventricles contract from the apex upwards

Purkyne fibres spread excitation upwards through the ventricle walls. This makes the ventricles contract from the apex upwards, squeezing blood towards the arteries, which leave from the top of the heart. The ventricles squeeze blood into the aorta and pulmonary artery. Contracting from the bottom upwards helps empty the ventricles, like squeezing toothpaste from the closed end.

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