atrial systole

1. atrium contracts and blood is pushed into the ventricles (where the contents are under low pressure) by contraction of the walls of the atrium

2. contraction prevents backflow by blocking off the veins that brought the blood to the heart

atrial diastole

3. the atrium relaxes

ventricular systole

4. the ventricle contracts with a forceful contraction

5. high pressure shuts the atrioventricular valve and opens the semilunar valves, forcing blood into the aorta

6. a pulse is generated

ventricular diastole

7. ventricles relax

each contraction of cardiac muscle is followed by relaxation and elastic recoil

myogenic = originates within the muscle rather than the nervous system, and results in depolarisation

sinoatrial node

1. cells of the SAN depolarise and a wave of excitation spreads rapidly across the atria, causing them to contract simultaneously (atrial systole)

atrioventricular node

2. boundary between atria and ventricles is made of connective tissue which does not conduct these impulses

3. impulses stimulate AVN to pass the ventricles

bundle of His and purkyne tissue

4. AVN is connected to bundle of His - specially modified muscle fibres, and then impulse is passed to finer branching network of purkyne tissue to the base of the ventricles

5. purkyne tissue carries impulses five times faster than surrounding muscle to ensure that the base of the ventricle contracts first in ventricle systole, forcing blood in the right direction

refractory period = period of insensitivity to stimulation - longer in the heart than in most muscles, allows heart muscles to relax to allow refilling and prevents the muscle suffering from fatigue