When blood pressure is higher than normal pressure receptors within the walls of the aorta and carotid arteries detect the change and send impulses to the cardiovascular centre in the medulla.The centre sends impulses via the vagus nerve which is part of the parasympathetic nervous system to the SAN to decrease the heart rate.When blood pressure is lower than normal pressure receptors within the walls of the aorta and carotid arteries detect the change and send impulses to the cardiovascular centre in the medulla.The centre sends impulses via the accelerator nerve which is part of the sympathetic nervous system to the SAN to increase the heart rate.