Cardiac pacemaker controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells (AV node) to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SA node. This is the normal conduction of electrical activity in the
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).