Heart rate node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. For endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm. The "maximum heart rate" (HR) is the highest heart rate an individual can achieve without severe problems through exercise stress, and generally decreases with age. Since HR varies by individual, the most accurate way of measuring any single person's HR is via a cardiac stress test. In this test, a person is subjected to controlled physiologic stress (generally by
Cardiac excitation-contraction coupling Ca Uniport system, which pumps Ca into the mitochondia. Heart rate is affected by nerves. Sympathetic nerves, coming from the spinal cord, increase heart rate, whereas parasympathetic nerves (for example the vagus nerves) work to decrease it. Sympathetic nerves work by releasing a protein (neurotransmitter) called noradrenaline which binds to a specific receptor (beta 1 adrenoceptor) located in the sarcolemma and the t-tubule membrane of cardiac cells. This activates a protein, called a G-protein and results in a series of reactions (known as a cyclic AMP pathway) that leads to the production of a molecule called cAMP (cyclic adenosine monophosphate).