Coordination of the Heart

The heartbeat is initiated from the heart itself, what is the word to describe this?

The heart is myogenic

Sinoatrial (SA) Node

Located in the right atrium, its a group of specialised cells, it generates electrical impulses acting as the heart's pacemaker.

What does the Sino-Atrial Node do?

The cells in the SAN depolarise (become electrically excited) which triggers a wave of electrical excitation across the atria and causes it to contract - Atrial systole

Atrial Systole

The contraction of the atrial muscles caused by the electrical impulse generated by the SA node.

Why can’t this wave of electrical excitation pass down to the ventricles?

Because separating the atria and the ventricles is a layer of non-conducting tissue and this will not pass the electrical excitation through it

Atrioventricular (AV) Node

Located at the junction of the atria and ventricles, it briefly delays the electrical signal, allowing the atria to fully contract.

What is the AVN connected to?

Connected to conducting fibres called Purkyne fibres

What is the role of the AVN?

It detects the electrical excitation passing over the atria and after a short delay, transmits the electrical excitation down the purkyne fibres which causes ventricles to contract

Why does the AVN ensure a delay before transmitting the electrical excitation down the purkyne fibres?

To ensure the ventricles contract AFTER the atria have contracted.

Bundle of His

Bundle of his are purkyne fibres bundles together. They are a collection of specialized muscle fibres that conducts electrical impulses from the AV node down the septum towards the Purkyne fibers.

Purkyne Fibers

After the bundle of His, these purkyne fibres branches and the purkyne fibres run down the septum to the apex and up the walls of the ventricles

What is the role of the Purkyne fibres?

They transmit the electrical excitation from the AVN down the septum, apex and walls of ventricles which causes the ventricles to contract - Ventricular systole

Why do the ventricles contract from the Apex upwards?

To ensure that the maximum amount of blood is pumped out of the ventricles.

Sequence of Electrical Events in the Heart

1. SA node generates impulse; 2. Atria contract; 3. AV node delays impulse; 4. Bundle of His conducts impulse; 5. Purkyne fibers cause ventricular contraction.