text: Now this is a tough question to answer without using a little physiology lingo. Unlike skeletal muscle, which is under voluntary control, the heart is an involuntary muscle. Most of us cannot just tell our heart to slow down or speed up. The beating frequency (heart rate) is controlled by the balance of stimulation coming from the sympathetic and parasympathetic branches of the Autonomic Nervous System. Both nervous imputs to the heart converge on a small area of tissue on the right atrium called the Sino-atrial node. Parasympathetic (rest and recover) stimulation tends to slow down the rate, while sympathetic (fight or flight) input increases the rate (and the force of contraction). Normally, there is a balance between the two inputs leaning toward the parasympathetic side. However, even without any nervous input, the heart will beat automatically due to some unique features of its membrane physiology. This intrinsic rate is quite slow however (about 20 bpm). A purely parasympathetic stimulation will result in a heart rate of about 30. So the average untrained person has a resting heart rate of about 70 as a result of some constant sympathetic stimulation. With training, the balance between parasympathetic and sympathetic stimulation tends to shift in favour of the parasympathetic, resulting in a slower resting heart rate. Elite endurance athletes may have resting HRs of 35 to 40. \nThe part of the brain behind this and breathing is called the Medulla.\n\nIt is the caudal-most part of the brainstem and contains many sensory (ascending) and motor (descending) tracts. The pyramid is an example of the latter. Structures that are linked with the cerebellum and part of the reticular formation are also in the medulla.\n\nCertain groups of neurons in the reticular formation control respiration, and the cranial nerve important for maintaining an open airway and controlling heart rate is located here.\n\nThus, if the integrity of the medulla is compromised, vital functions are compromised and death often results.