Nervous Control of the Heart

Heart rate is controlled by the cardiovascular control centre in the medulla oblongata. This centre receives information from receptors and sends impulses along two nerves of the autonomic nervous system to the sinoatrial node (SAN).• The accelerator nerve is part of the sympathetic nervous system. Its neurones release noradrenaline at the SAN, which makes the SAN fire more often, so heart rate increases. Sympathetic neurones also supply the ventricle walls and increase the force of contraction, raising stroke volume.
• The vagus nerve is part of the parasympathetic nervous system. Its neurones release acetylcholine at the SAN, which makes the SAN fire less often, so heart rate decreases.These two nerves are antagonistic. The heart rate at any moment depends on the balance of impulses along them. Because both are active all the time, the control centre can raise heart rate by increasing sympathetic impulses, by decreasing vagal impulses, or both.The control centre changes its output in response to sensory information:receptors → sensory neurones → cardiovascular control centre (medulla) → accelerator or vagus nerve → SAN → change in heart rateThe main receptors involved are baroreceptors, which detect blood pressure, and chemoreceptors, which detect the carbon dioxide concentration, pH and oxygen concentration of the blood. Higher brain centres also affect the control centre; for example, heart rate can rise just before a race starts, in anticipation of exercise.The nerves do not make the heart beat, because cardiac muscle is myogenic. They only change how often the SAN produces waves of excitation.