Co-ordination of the Cardiac Cycle

Aims•Describe how heart action is coordinated with reference to the sinoatrial node (SAN), the atrioventricular node (AVN) and the Purkyne tissue

•Be able to interpret and explain electrocardiogram (ECG) traces, with reference to normal and abnormal heart activity

April 19, 2023

• The heart is made of cardiac muscle.

• When the cells receive an electrical impulse they contract - causing a heartbeat.

• Cardiac muscle is myogenic - it can contract on its own, without needing nerve impulses.

Sinoatrial node (SA node)

• This specialized node is found on the upper inside wall of the right atrium.

• The SA node is known as the pacemaker of the heart and initiates a heartbeat every 0.85 seconds.

• This signal travels across the atria causing them to contract and load the ventricles with blood.

• Ventricles are electrically insulated from atria - so they don’t contract yet.

Atrio-ventricular node (AV node)• The AV node is located on the bottom surface of

the right atria and is responsible for initiating the contraction of the ventricles.

• Electrical impulse passes to ventricles via AV node and the Bundle of His. They pass the impulse to the base of the ventricles (~ 0.1 s delay).

• The bundle of His is a group of fibres that conduct impulses to Purkyne fibres which carry impulses to left & right ventricles.

• Ventricles then contract from the bottom upwards.

No impulse

• Cardiac muscle relaxes = diastole

Electrocardiograph

P = atrial systole

QRS = ventricular systole

T = ventricular diastole

• Records electrical activity of heart to monitor heart function.

• Pacemaker generates electrical currents in body fluids around the heart - detected by electrodes to produce ECG

• Heart muscle:

depolarises (loses electrical charge) when it contracts

repolarises (regains charge) when it relaxes

Ventricular fibrillation is an abnormal heart

rhythm that is disorganized and irregular.

Ventricular tachycardia is a rapid, regular heart rhythm that originates in the lower

chambers of the heart.

Heart block refers to a delay in the normal flow of electrical impulses that cause the

heart to beat.

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