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These control mechanisms can be classified into two groups:
When the sinus node is stretched, it will respond by initiating more action potentials.
More action potentials will induce more excitations of the heart, and therefore more contractions.
This is useful, as more blood will then be pumped out of the heart.
This rapid pumping will, in turn, reduce atrial filling and the heart rate will then return to normal.
This system is named after the two people who first described this regulatory system (yes, you guessed it … Mr Frank and Mr Starling).
This system is very nice! If, for some reason, more blood flows into the heart, then the end-diastolic volume is increased, and this will (automatically!) induce a stronger contraction that will pump this extra amount of blood into the arteries.
The sinus node stretch influences the frequency of the heart. The Frank-Starling system affects the contraction force.
The major systems are:
The parasympathetic system (the vagus nerve) only innervates the sinus node and the AV-node. It inhibits the firing rate of the sinus node (longer P-P) and delays the propagation though the AV-node (longer P-Q).
An increase in sympathetic activity will:
As an example of how the autonomic nervous system work on the heart, consider this very nice reflex: the Bainbridge reflex, also called the Respiratory Sinus Arrhythmia.
As the pressure increases in the right atrium, possibly due to an increase venous return, the atrial stretch receptors are activated which send their signals to the medullary centre in the brain. This in turn activates the sympathetic system.
As you inhale, the pressure in the thorax decreases, which induces an increase in venous return and an increase in blood flow to the right atrium. This increases the atrial pressure -> increase heart rate.
After inhalation follows expiration, the thoracic pressure increases, the venous return decreases, less blood flows into the atria, the pressure decreases and the heart rate decreases.
