These contractions of the heart are stimulated by electrical impulses which originate from the nodes that are present in the heart; sino-atrial (SA node) and the atrioventricular (AV node). These nodes are clusters of nerve of cells.
It is the SA node that initiates the heart to contract. Due to the fact the heart is made up predominately of cardiac muscles, it has the ability to contract in a variety of directions unlike striated muscle that can only contract one way.
The SA node is situated at the upper corner of the RA towards the outside of the atrium. The SA node stimulates contraction by the use of electrical impulses. When the SA node stimulates the contraction, the electrical impulses spread from the SA node, through the myocardium of both the atriums.
When these impulses reach the AV node, which is positioned at the top of the RV towards the interventricular septum, the AV node holds the electrical impulse static. This means contraction stops allowing blood to pour into the ventricles. This only lasts for a moment, and then the AV stimulates another electrical impulse. From the AV node the electrical impulses spread along fibres called ‘his fibres’ till they reach the interventricular septum. Here the impulses continue down the his fibres of the septum, till they reach the bottom of the ventricles. The impulses branch out; left and right around the ventricles along different fibres called purkinje fibres. This causes the ventricles to contract, and pumps the blood out of the ventricles. When all the ventricles have contracted the impulse is lost and the ventricles relax again (diastole phase). Until the SA node sends another electrical impulse and the whole process will begin again ‘another heartbeat’.
Image found at http://www.phschool.com/science/biology_place/biocoach/cardio1/electrical.html
The hearts ability to beat with no interference from the CNS (central nervous system) is due to cardiac muscle being myogenic which means it is self stimulating. The heart’s contraction is controlled by the SA node. This is what stimulates contractions, not impulses from the brain. However, sometimes the heart does need to beat faster, such as during exercise. To speed up the heart the CNS has the ability to stimulate the SA node to contract.
During times when the heart needs to speed up, the sympathetic nervous system stimulates the heart to speed up via phrenic nerves. When the heart needs to slow down the parasympathetic nervous system will take control via the vagus nerves. Without intervention from the CNS then the heart will always beat at the same rate. Parker (2007) states “The heart controls its own rhythm but its rate is controlled by the central nervous system”.
