The heart muscle is unique. It can generate its own action potential without any external stimulation. However, the timing of heart muscle contractions can be altered by external sources. The heart is primarily influenced by 3 extrinsic (external) systems:
The Parasympathetic Nervous System
The parasympathetic nervous system (also known as the “Rest and Digest” or vagus response) is a division of the autonomic nervous system and controls involuntary physiological functions. The parasympathetic nervous system conserves energy when action is not needed in the form of decreased blood pressure, decreased heart rate, and beginning the process of digestion.
One important parasympathetic nerve that innervates many of the thoracic and abdominal viscera (internal organs) is the vagus nerve. The vagusnerveis important because it stimulates involuntary actions of the cardiovascular (breathing and heart rate slowing) and digestive (stomach acid increase) systems.
The Sympathetic Nervous System
The sympathetic nervous system (also known as the “Fight or Flight” response) is the other division of the autonomic nervous system. The sympathetic nervous system uses energy and prepares the body for action through blood pressure increases, heart rate increases, and digestive slowing.
At rest, the vagus (parasympathetic) response is dominant with a corresponding lower heart rate. As intensity increases toward maximum oxygen uptake, the sympathetic response begins to take over and heart rate increases.
The Endocrine System
During sympathetic stimulation, the hormones norepinephrine and epinephrine (a.k.a. adrenaline and noradrenaline) are released by the adrenal medulla, on the superior aspect of the kidneys. These hormones increase heart rate and prepare other organs as a response to stress.