Times of stress often cause sympathetic excitation. Sympathetic nerve endings form synaptic connections with the heart muscle and when excited release norepinephrine, which acts on the heart’s beta receptors (the heart muscle has beta1 and beta2 receptors, with beta1 receptors predominating, especially in the ventricular muscle, where beta1 receptors account for more than 75% of all beta receptors), causing the heart muscle to contract stronger and the heart to beat faster.